diff --git a/sessions/02.md b/sessions/02.md
new file mode 100644
index 0000000..f43c3b5
--- /dev/null
+++ b/sessions/02.md
@@ -0,0 +1,1753 @@
+../neo/neo 
+YOU: update repl to support function definitions
+## USER NO ITEMS
+1: update repl to support function definitions - IN PROGRESS assistant false
+-- AGENT START WORKING 1
+--- STOP_REASON: tool_use
+AI: 
+
+I'll start by researching the current state of the project to understand the codebase and the REPL implementation.
+CALLING TOOL: dir_list {"name":"."}
+$$$ {"entries":[{"name":".git","is_dir":true},{"name":"README.md","is_dir":false},{"name":"ast","is_dir":true},{"name":"cmd","is_dir":true},{"name":"docs","is_dir":true},{"name":"evaluator","is_dir":true},{"name":"go.mod","is_dir":false},{"name":"integration_test.go","is_dir":false},{"name":"lexer","is_dir":true},{"name":"matheval","is_dir":false},{"name":"parser","is_dir":true},{"name":"repl","is_dir":true},{"name":"sessions","is_dir":true},{"name":"token","is_dir":true}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"README.md"}
+$$$ "# matheval\n\nA math expression evaluator with an interactive REPL, written in Go.\n\n## Features\n\n- Arithmetic operators: `+`, `-`, `*`, `/`\n- Parentheses for grouping\n- Floating point numbers (including `.5` syntax)\n- Correct operator precedence (`*` and `/` bind tighter than `+` and `-`)\n- Left-to-right associativity\n- Clear error messages with position reporting\n\n## Build\n\n```sh\ngo build -o matheval ./cmd/matheval\n```\n\n## Usage\n\nRun the REPL:\n\n```sh\n./matheval\n```\n\nThen type expressions:\n\n```\n\u003e\u003e 2 + 3 * 4\n14\n\u003e\u003e (2 + 3) * 4\n20\n\u003e\u003e 7 / 2\n3.5\n\u003e\u003e 1 / 0\nerror: division by zero\n```\n\nPress `Ctrl+D` (EOF) to exit.\n\nYou can also pipe input:\n\n```sh\necho \"2 + 3\" | ./matheval\n```\n\n## Architecture\n\n```\nInput string → Lexer → Parser → AST → Evaluator → Result\n```\n\n| Package     | Responsibility                        |\n|-------------|---------------------------------------|\n| `token`     | Token types and data structures       |\n| `lexer`     | Tokenizes input string                |\n| `ast`       | AST node types (`NumberLit`, `BinaryExpr`) |\n| `parser`    | Recursive-descent parser              |\n| `evaluator` | Walks AST and computes result         |\n| `repl`      | Read-eval-print loop                  |\n\n## Grammar\n\n```\nexpr   → term (('+' | '-') term)*\nterm   → factor (('*' | '/') factor)*\nfactor → NUMBER | '(' expr ')'\n```\n\n## Tests\n\n```sh\ngo test ./...\n```\n\nThis runs unit tests for each package plus integration tests covering the full pipeline.\n"
+
+CALLING TOOL: dir_list {"name":"ast"}
+$$$ {"entries":[{"name":"ast.go","is_dir":false},{"name":"ast_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"cmd"}
+$$$ {"entries":[{"name":"matheval","is_dir":true}]}
+
+CALLING TOOL: dir_list {"name":"evaluator"}
+$$$ {"entries":[{"name":"evaluator.go","is_dir":false},{"name":"evaluator_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"lexer"}
+$$$ {"entries":[{"name":"lexer.go","is_dir":false},{"name":"lexer_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"parser"}
+$$$ {"entries":[{"name":"parser.go","is_dir":false},{"name":"parser_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"repl"}
+$$$ {"entries":[{"name":"repl.go","is_dir":false},{"name":"repl_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"token"}
+$$$ {"entries":[{"name":"token.go","is_dir":false},{"name":"token_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"docs"}
+$$$ {"entries":[{"name":"design.md","is_dir":false},{"name":"plan.md","is_dir":false}]}
+
+CALLING TOOL: file_read {"path":"go.mod"}
+$$$ "module matheval\n\ngo 1.23.1\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast.go"}
+$$$ "package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer.go"}
+$$$ "package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i \u003c len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch \u003e= '0' \u0026\u0026 ch \u003c= '9'\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser.go"}
+$$$ "package parser\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strconv\"\n)\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions.\nfunc Parse(tokens []token.Token) (ast.Node, error) {\n\tp := \u0026parser{tokens: tokens}\n\tnode, err := p.expr()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\t// Ensure all tokens have been consumed (only EOF remains).\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n\treturn node, nil\n}\n\n// parser holds the state for a single parse operation.\ntype parser struct {\n\ttokens []token.Token\n\tpos    int\n}\n\n// current returns the token at the current position.\nfunc (p *parser) current() token.Token {\n\tif p.pos \u003e= len(p.tokens) {\n\t\treturn token.Token{Type: token.EOF}\n\t}\n\treturn p.tokens[p.pos]\n}\n\n// advance moves to the next token and returns the previous one.\nfunc (p *parser) advance() token.Token {\n\ttok := p.current()\n\tp.pos++\n\treturn tok\n}\n\n// expect consumes a token of the given type or returns an error.\nfunc (p *parser) expect(typ token.Type) (token.Token, error) {\n\ttok := p.current()\n\tif tok.Type != typ {\n\t\treturn tok, fmt.Errorf(\"expected %v but got %v at position %d\", typ, tok.Type, tok.Pos)\n\t}\n\tp.advance()\n\treturn tok, nil\n}\n\n// expr → term (('+' | '-') term)*\nfunc (p *parser) expr() (ast.Node, error) {\n\tleft, err := p.term()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Plus || p.current().Type == token.Minus {\n\t\top := p.advance()\n\t\tright, err := p.term()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// term → factor (('*' | '/') factor)*\nfunc (p *parser) term() (ast.Node, error) {\n\tleft, err := p.factor()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Star || p.current().Type == token.Slash {\n\t\top := p.advance()\n\t\tright, err := p.factor()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// factor → NUMBER | '(' expr ')'\nfunc (p *parser) factor() (ast.Node, error) {\n\ttok := p.current()\n\tswitch tok.Type {\n\tcase token.Number:\n\t\tp.advance()\n\t\tval, err := strconv.ParseFloat(tok.Literal, 64)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"invalid number %q at position %d: %w\", tok.Literal, tok.Pos, err)\n\t\t}\n\t\treturn \u0026ast.NumberLit{Value: val}, nil\n\n\tcase token.LParen:\n\t\tp.advance() // consume '('\n\t\tnode, err := p.expr()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\treturn nil, fmt.Errorf(\"missing closing parenthesis at position %d\", p.current().Pos)\n\t\t}\n\t\treturn node, nil\n\n\tdefault:\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator.go"}
+$$$ "package evaluator\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n)\n\n// Eval evaluates an AST node and returns the result.\n// Returns an error on division by zero.\nfunc Eval(node ast.Node) (float64, error) {\n\tswitch n := node.(type) {\n\tcase *ast.NumberLit:\n\t\treturn n.Value, nil\n\n\tcase *ast.BinaryExpr:\n\t\tleft, err := Eval(n.Left)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\t\tright, err := Eval(n.Right)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\n\t\tswitch n.Op {\n\t\tcase token.Plus:\n\t\t\treturn left + right, nil\n\t\tcase token.Minus:\n\t\t\treturn left - right, nil\n\t\tcase token.Star:\n\t\t\treturn left * right, nil\n\t\tcase token.Slash:\n\t\t\tif right == 0 {\n\t\t\t\treturn 0, fmt.Errorf(\"division by zero\")\n\t\t\t}\n\t\t\treturn left / right, nil\n\t\tdefault:\n\t\t\treturn 0, fmt.Errorf(\"unknown operator: %v\", n.Op)\n\t\t}\n\n\tdefault:\n\t\treturn 0, fmt.Errorf(\"unknown node type: %T\", node)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl.go"}
+$$$ "package repl\n\nimport (\n\t\"bufio\"\n\t\"fmt\"\n\t\"io\"\n\t\"math\"\n\t\"strings\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\nconst prompt = \"\u003e\u003e \"\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer) {\n\tscanner := bufio.NewScanner(r)\n\tfmt.Fprint(w, prompt)\n\n\tfor scanner.Scan() {\n\t\tline := strings.TrimSpace(scanner.Text())\n\t\tif line == \"\" {\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\tresult, err := evalLine(line)\n\t\tif err != nil {\n\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t} else {\n\t\t\tfmt.Fprintln(w, formatResult(result))\n\t\t}\n\n\t\tfmt.Fprint(w, prompt)\n\t}\n}\n\n// evalLine tokenizes, parses, and evaluates a single expression string.\nfunc evalLine(line string) (float64, error) {\n\ttokens, err := lexer.Tokenize(line)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\treturn evaluator.Eval(tree)\n}\n\n// formatResult formats a float64 for display.\n// Whole numbers are printed without decimal points.\nfunc formatResult(val float64) string {\n\tif val == math.Trunc(val) \u0026\u0026 !math.IsInf(val, 0) \u0026\u0026 !math.IsNaN(val) {\n\t\treturn fmt.Sprintf(\"%g\", val)\n\t}\n\treturn fmt.Sprintf(\"%g\", val)\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me read the test files and existing docs:
+CALLING TOOL: file_read {"path":"integration_test.go"}
+$$$ "package matheval_test\n\nimport (\n\t\"math\"\n\t\"strings\"\n\t\"testing\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\n// eval is a helper that runs the full pipeline: lexer → parser → evaluator.\nfunc eval(t *testing.T, input string) float64 {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\tt.Fatalf(\"lexer error for %q: %v\", input, err)\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"parser error for %q: %v\", input, err)\n\t}\n\tresult, err := evaluator.Eval(tree)\n\tif err != nil {\n\t\tt.Fatalf(\"evaluator error for %q: %v\", input, err)\n\t}\n\treturn result\n}\n\n// evalErr is a helper that expects the full pipeline to return an error.\nfunc evalErr(t *testing.T, input string) error {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\treturn err\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn err\n\t}\n\t_, err = evaluator.Eval(tree)\n\treturn err\n}\n\nfunc assertApprox(t *testing.T, input string, expected, got float64) {\n\tt.Helper()\n\tif math.Abs(expected-got) \u003e 1e-9 {\n\t\tt.Errorf(\"%q: expected %v, got %v\", input, expected, got)\n\t}\n}\n\n// --- Basic arithmetic ---\n\nfunc TestIntegration_SingleNumber(t *testing.T) {\n\tassertApprox(t, \"42\", 42, eval(t, \"42\"))\n}\n\nfunc TestIntegration_DecimalNumber(t *testing.T) {\n\tassertApprox(t, \"3.14\", 3.14, eval(t, \"3.14\"))\n}\n\nfunc TestIntegration_LeadingDot(t *testing.T) {\n\tassertApprox(t, \".5\", 0.5, eval(t, \".5\"))\n}\n\nfunc TestIntegration_Addition(t *testing.T) {\n\tassertApprox(t, \"1 + 2\", 3, eval(t, \"1 + 2\"))\n}\n\nfunc TestIntegration_Subtraction(t *testing.T) {\n\tassertApprox(t, \"10 - 4\", 6, eval(t, \"10 - 4\"))\n}\n\nfunc TestIntegration_Multiplication(t *testing.T) {\n\tassertApprox(t, \"3 * 7\", 21, eval(t, \"3 * 7\"))\n}\n\nfunc TestIntegration_Division(t *testing.T) {\n\tassertApprox(t, \"10 / 4\", 2.5, eval(t, \"10 / 4\"))\n}\n\n// --- Precedence and associativity ---\n\nfunc TestIntegration_PrecedenceMulOverAdd(t *testing.T) {\n\t// 2 + 3 * 4 = 2 + 12 = 14\n\tassertApprox(t, \"2 + 3 * 4\", 14, eval(t, \"2 + 3 * 4\"))\n}\n\nfunc TestIntegration_PrecedenceDivOverSub(t *testing.T) {\n\t// 10 - 6 / 3 = 10 - 2 = 8\n\tassertApprox(t, \"10 - 6 / 3\", 8, eval(t, \"10 - 6 / 3\"))\n}\n\nfunc TestIntegration_LeftAssociativitySub(t *testing.T) {\n\t// 10 - 3 - 2 = (10 - 3) - 2 = 5\n\tassertApprox(t, \"10 - 3 - 2\", 5, eval(t, \"10 - 3 - 2\"))\n}\n\nfunc TestIntegration_LeftAssociativityDiv(t *testing.T) {\n\t// 24 / 4 / 3 = (24 / 4) / 3 = 2\n\tassertApprox(t, \"24 / 4 / 3\", 2, eval(t, \"24 / 4 / 3\"))\n}\n\n// --- Parentheses ---\n\nfunc TestIntegration_ParensOverridePrecedence(t *testing.T) {\n\t// (2 + 3) * 4 = 20\n\tassertApprox(t, \"(2 + 3) * 4\", 20, eval(t, \"(2 + 3) * 4\"))\n}\n\nfunc TestIntegration_NestedParens(t *testing.T) {\n\t// ((1 + 2)) = 3\n\tassertApprox(t, \"((1 + 2))\", 3, eval(t, \"((1 + 2))\"))\n}\n\nfunc TestIntegration_DeeplyNestedParens(t *testing.T) {\n\t// ((((((1 + 2)))))) = 3\n\tassertApprox(t, \"((((((1 + 2))))))\", 3, eval(t, \"((((((1 + 2))))))\"))\n}\n\nfunc TestIntegration_ParensOnBothSides(t *testing.T) {\n\t// (1 + 2) * (3 + 4) = 3 * 7 = 21\n\tassertApprox(t, \"(1 + 2) * (3 + 4)\", 21, eval(t, \"(1 + 2) * (3 + 4)\"))\n}\n\nfunc TestIntegration_ParensNestedComplex(t *testing.T) {\n\t// ((2 + 3) * (4 - 1)) / 5 = (5 * 3) / 5 = 3\n\tassertApprox(t, \"((2 + 3) * (4 - 1)) / 5\", 3, eval(t, \"((2 + 3) * (4 - 1)) / 5\"))\n}\n\n// --- Complex expressions ---\n\nfunc TestIntegration_AllOperators(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2 = 1 + 6 - 2 = 5\n\tassertApprox(t, \"1 + 2 * 3 - 4 / 2\", 5, eval(t, \"1 + 2 * 3 - 4 / 2\"))\n}\n\nfunc TestIntegration_LongChainedAddition(t *testing.T) {\n\t// 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55\n\tassertApprox(t, \"1+2+3+4+5+6+7+8+9+10\", 55, eval(t, \"1+2+3+4+5+6+7+8+9+10\"))\n}\n\nfunc TestIntegration_LongChainedMixed(t *testing.T) {\n\t// 2 * 3 + 4 * 5 - 6 / 2 + 1 = 6 + 20 - 3 + 1 = 24\n\tassertApprox(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\", 24, eval(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\"))\n}\n\n// --- Floating point ---\n\nfunc TestIntegration_FloatArithmetic(t *testing.T) {\n\t// 1.5 + 2.5 = 4.0\n\tassertApprox(t, \"1.5 + 2.5\", 4.0, eval(t, \"1.5 + 2.5\"))\n}\n\nfunc TestIntegration_FloatDivision(t *testing.T) {\n\t// 7 / 2 = 3.5\n\tassertApprox(t, \"7 / 2\", 3.5, eval(t, \"7 / 2\"))\n}\n\nfunc TestIntegration_FloatPrecision(t *testing.T) {\n\t// 0.1 + 0.2 ≈ 0.3 (within tolerance)\n\tassertApprox(t, \"0.1 + 0.2\", 0.3, eval(t, \"0.1 + 0.2\"))\n}\n\n// --- Whitespace variations ---\n\nfunc TestIntegration_NoSpaces(t *testing.T) {\n\tassertApprox(t, \"1+2*3\", 7, eval(t, \"1+2*3\"))\n}\n\nfunc TestIntegration_ExtraSpaces(t *testing.T) {\n\tassertApprox(t, \"  1  +  2  \", 3, eval(t, \"  1  +  2  \"))\n}\n\nfunc TestIntegration_TabsAndSpaces(t *testing.T) {\n\tassertApprox(t, \"1\\t+\\t2\", 3, eval(t, \"1\\t+\\t2\"))\n}\n\n// --- Error cases ---\n\nfunc TestIntegration_DivisionByZero(t *testing.T) {\n\terr := evalErr(t, \"1 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n\tif !strings.Contains(err.Error(), \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_DivisionByZeroInSubExpr(t *testing.T) {\n\terr := evalErr(t, \"1 + 2 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestIntegration_InvalidCharacter(t *testing.T) {\n\terr := evalErr(t, \"1 @ 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenLeft(t *testing.T) {\n\terr := evalErr(t, \"(1 + 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing closing paren\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenRight(t *testing.T) {\n\terr := evalErr(t, \"1 + 2)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected closing paren\")\n\t}\n}\n\nfunc TestIntegration_EmptyParens(t *testing.T) {\n\terr := evalErr(t, \"()\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\nfunc TestIntegration_TrailingOperator(t *testing.T) {\n\terr := evalErr(t, \"1 +\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestIntegration_LeadingOperator(t *testing.T) {\n\terr := evalErr(t, \"* 1\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for leading operator\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveOperators(t *testing.T) {\n\terr := evalErr(t, \"1 + * 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestIntegration_EmptyInput(t *testing.T) {\n\t// Empty string should produce only EOF, parser should error\n\terr := evalErr(t, \"\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveNumbers(t *testing.T) {\n\terr := evalErr(t, \"1 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive numbers without operator\")\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl_test.go"}
+$$$ "package repl\n\nimport (\n\t\"bytes\"\n\t\"strings\"\n\t\"testing\"\n)\n\nfunc TestRun_SingleExpression(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n}\n\nfunc TestRun_MultipleExpressions(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n3 * 4\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"12\") {\n\t\tt.Errorf(\"expected output to contain '12', got %q\", got)\n\t}\n}\n\nfunc TestRun_ErrorContinues(t *testing.T) {\n\t// First line has error, second is valid.\n\tin := strings.NewReader(\"1 +\\n2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"5\") {\n\t\tt.Errorf(\"expected output to contain '5' after error recovery, got %q\", got)\n\t}\n}\n\nfunc TestRun_DivisionByZero(t *testing.T) {\n\tin := strings.NewReader(\"1 / 0\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyLine(t *testing.T) {\n\t// Empty lines should be skipped, not cause errors.\n\tin := strings.NewReader(\"\\n1 + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"2\") {\n\t\tt.Errorf(\"expected output to contain '2', got %q\", got)\n\t}\n\t// Should not contain any error.\n\tif strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"empty line should not produce error, got %q\", got)\n\t}\n}\n\nfunc TestRun_Prompt(t *testing.T) {\n\tin := strings.NewReader(\"42\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected prompt '\u003e\u003e ' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_FloatResult(t *testing.T) {\n\tin := strings.NewReader(\"7 / 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3.5\") {\n\t\tt.Errorf(\"expected output to contain '3.5', got %q\", got)\n\t}\n}\n\nfunc TestRun_InvalidCharacter(t *testing.T) {\n\tin := strings.NewReader(\"1 @ 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyInput(t *testing.T) {\n\t// No input at all — just EOF.\n\tin := strings.NewReader(\"\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Should just show the prompt and exit gracefully.\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected at least one prompt, got %q\", got)\n\t}\n}\n\nfunc TestRun_WholeIntegerNoTrailingZeros(t *testing.T) {\n\t// 2 + 3 = 5, should print \"5\" not \"5.000000\".\n\tin := strings.NewReader(\"2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Result line is \"5\\n\" (between prompts).\n\tif !strings.Contains(got, \"5\\n\") {\n\t\tt.Errorf(\"expected '5\\\\n' in output, got %q\", got)\n\t}\n\t// Should not contain \"5.0\"\n\tif strings.Contains(got, \"5.0\") {\n\t\tt.Errorf(\"expected no trailing zeros, got %q\", got)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator_test.go"}
+$$$ "package evaluator\n\nimport (\n\t\"math\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestEvalNumberLit(t *testing.T) {\n\tresult, err := Eval(\u0026ast.NumberLit{Value: 42.5})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 42.5 {\n\t\tt.Fatalf(\"expected 42.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalAddition(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 3 {\n\t\tt.Fatalf(\"expected 3, got %v\", result)\n\t}\n}\n\nfunc TestEvalSubtraction(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Minus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalMultiplication(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Star,\n\t\tLeft:  \u0026ast.NumberLit{Value: 3},\n\t\tRight: \u0026ast.NumberLit{Value: 7},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 21 {\n\t\tt.Fatalf(\"expected 21, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivision(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 2.5 {\n\t\tt.Fatalf(\"expected 2.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZero(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 5},\n\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestEvalNestedExpr(t *testing.T) {\n\t// (1 + 2) * (8 / 4) = 3 * 2 = 6\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 8},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalDeeplyNested(t *testing.T) {\n\t// ((2 + 3) * 4) - (10 / 5) = 20 - 2 = 18\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Minus,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp: token.Star,\n\t\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\t\tOp:    token.Plus,\n\t\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\t\tRight: \u0026ast.NumberLit{Value: 3},\n\t\t\t},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\t\tRight: \u0026ast.NumberLit{Value: 5},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 18 {\n\t\tt.Fatalf(\"expected 18, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZeroInSubExpr(t *testing.T) {\n\t// 1 + (2 / 0) — error should propagate\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:   token.Plus,\n\t\tLeft: \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t\t},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error from sub-expression\")\n\t}\n}\n\nfunc TestEvalFloatingPoint(t *testing.T) {\n\t// 1.5 + 2.3 = 3.8\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1.5},\n\t\tRight: \u0026ast.NumberLit{Value: 2.3},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif math.Abs(result-3.8) \u003e 1e-12 {\n\t\tt.Fatalf(\"expected 3.8, got %v\", result)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser_test.go"}
+$$$ "package parser\n\nimport (\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// helper: tokenize inline for concise tests\nfunc tokens(toks ...token.Token) []token.Token {\n\treturn toks\n}\n\nfunc tok(typ token.Type, lit string, pos int) token.Token {\n\treturn token.Token{Type: typ, Literal: lit, Pos: pos}\n}\n\n// --- Success cases ---\n\nfunc TestParseSingleNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"42\", 0),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 42 {\n\t\tt.Fatalf(\"expected 42, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseDecimalNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"3.14\", 0),\n\t\ttok(token.EOF, \"\", 4),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 3.14 {\n\t\tt.Fatalf(\"expected 3.14, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseAddition(t *testing.T) {\n\t// 1 + 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseSubtraction(t *testing.T) {\n\t// 5 - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"5\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 5)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseMultiplication(t *testing.T) {\n\t// 2 * 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 2)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseDivision(t *testing.T) {\n\t// 6 / 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"6\", 0),\n\t\ttok(token.Slash, \"/\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 6)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParsePrecedence(t *testing.T) {\n\t// 1 + 2 * 3  →  1 + (2 * 3)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root should be Plus\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\t// Right should be Star\n\tright, ok := expr.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", expr.Right)\n\t}\n\tif right.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", right.Op)\n\t}\n\tassertNumber(t, right.Left, 2)\n\tassertNumber(t, right.Right, 3)\n}\n\nfunc TestParsePrecedenceMulFirst(t *testing.T) {\n\t// 2 * 3 + 1  →  (2 * 3) + 1\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.Plus, \"+\", 6),\n\t\ttok(token.Number, \"1\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 2)\n\tassertNumber(t, left.Right, 3)\n\tassertNumber(t, expr.Right, 1)\n}\n\nfunc TestParseLeftAssociativity(t *testing.T) {\n\t// 1 - 2 - 3  →  (1 - 2) - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Minus, \"-\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 - 2) - 3\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseParentheses(t *testing.T) {\n\t// (1 + 2) * 3\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.RParen, \")\", 6),\n\t\ttok(token.Star, \"*\", 8),\n\t\ttok(token.Number, \"3\", 10),\n\t\ttok(token.EOF, \"\", 11),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseNestedParentheses(t *testing.T) {\n\t// ((1 + 2))\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.LParen, \"(\", 1),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.Plus, \"+\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.RParen, \")\", 7),\n\t\ttok(token.RParen, \")\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseComplexExpression(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2  →  (1 + (2*3)) - (4/2)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 5),\n\t\ttok(token.Number, \"3\", 6),\n\t\ttok(token.Minus, \"-\", 8),\n\t\ttok(token.Number, \"4\", 10),\n\t\ttok(token.Slash, \"/\", 11),\n\t\ttok(token.Number, \"2\", 12),\n\t\ttok(token.EOF, \"\", 13),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 + (2*3)) - (4/2)\n\troot, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif root.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", root.Op)\n\t}\n\t// Left: 1 + (2*3)\n\tleft, ok := root.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", root.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tmul, ok := left.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", left.Right)\n\t}\n\tif mul.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", mul.Op)\n\t}\n\tassertNumber(t, mul.Left, 2)\n\tassertNumber(t, mul.Right, 3)\n\t// Right: 4/2\n\tdiv, ok := root.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", root.Right)\n\t}\n\tif div.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", div.Op)\n\t}\n\tassertNumber(t, div.Left, 4)\n\tassertNumber(t, div.Right, 2)\n}\n\n// --- Error cases ---\n\nfunc TestParseEmptyInput(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.EOF, \"\", 0),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestParseMissingRParen(t *testing.T) {\n\t// (1 + 2\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.EOF, \"\", 6),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing right paren\")\n\t}\n}\n\nfunc TestParseUnexpectedRParen(t *testing.T) {\n\t// ) 1\n\ttoks := tokens(\n\t\ttok(token.RParen, \")\", 0),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected right paren\")\n\t}\n}\n\nfunc TestParseTrailingOperator(t *testing.T) {\n\t// 1 +\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestParseTrailingTokens(t *testing.T) {\n\t// 1 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Number, \"2\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing tokens\")\n\t}\n}\n\nfunc TestParseConsecutiveOperators(t *testing.T) {\n\t// 1 + * 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Star, \"*\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.EOF, \"\", 7),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestParseEmptyParens(t *testing.T) {\n\t// ()\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.RParen, \")\", 1),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\n// --- Helper ---\n\nfunc assertNumber(t *testing.T, node ast.Node, expected float64) {\n\tt.Helper()\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != expected {\n\t\tt.Fatalf(\"expected %f, got %f\", expected, num.Value)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer_test.go"}
+$$$ "package lexer\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestTokenizeEmpty(t *testing.T) {\n\ttokens, err := Tokenize(\"\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeWhitespaceOnly(t *testing.T) {\n\ttokens, err := Tokenize(\"   \\t\\n  \")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeSingleNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"42\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeDecimalNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"3.14\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"3.14\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeLeadingDotNumber(t *testing.T) {\n\ttokens, err := Tokenize(\".5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \".5\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeOperators(t *testing.T) {\n\ttokens, err := Tokenize(\"+-*/\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 0},\n\t\t{Type: token.Minus, Literal: \"-\", Pos: 1},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 2},\n\t\t{Type: token.Slash, Literal: \"/\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeParens(t *testing.T) {\n\ttokens, err := Tokenize(\"()\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 1},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFullExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"(1 + 2.5) * 3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 1},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2.5\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 10},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeNoSpaces(t *testing.T) {\n\ttokens, err := Tokenize(\"1+2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeInvalidCharacter(t *testing.T) {\n\t_, err := Tokenize(\"1 + @\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleInvalidCharacters(t *testing.T) {\n\t_, err := Tokenize(\"1 \u0026 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleDecimals(t *testing.T) {\n\t// \"1.2.3\" — the lexer should read \"1.2\" as a number, then \".3\" as another number\n\ttokens, err := Tokenize(\"1.2.3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1.2\", Pos: 0},\n\t\t{Type: token.Number, Literal: \".3\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\n// assertTokens is a test helper that compares two token slices.\nfunc assertTokens(t *testing.T, want, got []token.Token) {\n\tt.Helper()\n\tif len(want) != len(got) {\n\t\tt.Fatalf(\"token count: want %d, got %d\\nwant: %v\\ngot:  %v\", len(want), len(got), want, got)\n\t}\n\tfor i := range want {\n\t\tif want[i].Type != got[i].Type {\n\t\t\tt.Errorf(\"token[%d].Type: want %v, got %v\", i, want[i].Type, got[i].Type)\n\t\t}\n\t\tif want[i].Literal != got[i].Literal {\n\t\t\tt.Errorf(\"token[%d].Literal: want %q, got %q\", i, want[i].Literal, got[i].Literal)\n\t\t}\n\t\tif want[i].Pos != got[i].Pos {\n\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"token/token_test.go"}
+$$$ "package token\n\nimport \"testing\"\n\nfunc TestTypeString(t *testing.T) {\n\ttests := []struct {\n\t\ttyp  Type\n\t\twant string\n\t}{\n\t\t{Number, \"Number\"},\n\t\t{Plus, \"+\"},\n\t\t{Minus, \"-\"},\n\t\t{Star, \"*\"},\n\t\t{Slash, \"/\"},\n\t\t{LParen, \"(\"},\n\t\t{RParen, \")\"},\n\t\t{EOF, \"EOF\"},\n\t\t{Type(99), \"Unknown(99)\"},\n\t}\n\tfor _, tc := range tests {\n\t\tif got := tc.typ.String(); got != tc.want {\n\t\t\tt.Errorf(\"Type(%d).String() = %q, want %q\", int(tc.typ), got, tc.want)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast_test.go"}
+$$$ "package ast\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// Compile-time check: both types satisfy Node.\nvar (\n\t_ Node = (*NumberLit)(nil)\n\t_ Node = (*BinaryExpr)(nil)\n)\n\nfunc TestNumberLit(t *testing.T) {\n\tn := \u0026NumberLit{Value: 3.14}\n\tif n.Value != 3.14 {\n\t\tt.Fatalf(\"expected 3.14, got %f\", n.Value)\n\t}\n}\n\nfunc TestBinaryExpr(t *testing.T) {\n\tleft := \u0026NumberLit{Value: 1}\n\tright := \u0026NumberLit{Value: 2}\n\texpr := \u0026BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  left,\n\t\tRight: right,\n\t}\n\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tif expr.Left != left {\n\t\tt.Fatal(\"Left child mismatch\")\n\t}\n\tif expr.Right != right {\n\t\tt.Fatal(\"Right child mismatch\")\n\t}\n}\n\nfunc TestNestedBinaryExpr(t *testing.T) {\n\t// Represents: (1 + 2) * 3\n\ttree := \u0026BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: \u0026BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  \u0026NumberLit{Value: 1},\n\t\t\tRight: \u0026NumberLit{Value: 2},\n\t\t},\n\t\tRight: \u0026NumberLit{Value: 3},\n\t}\n\n\tif tree.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", tree.Op)\n\t}\n\n\tinner, ok := tree.Left.(*BinaryExpr)\n\tif !ok {\n\t\tt.Fatal(\"Left should be *BinaryExpr\")\n\t}\n\tif inner.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", inner.Op)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"docs/design.md"}
+$$$ "# Math Expression Evaluator — Design Document\n\n## Requirements Summary\n- Language: Go\n- Operators: `+`, `-`, `*`, `/` with parentheses\n- Numbers: floating point (e.g. `3.14`, `42`, `0.5`)\n- Variables: none\n- Interface: CLI REPL\n- Error handling: print error message, continue REPL\n\n## Approaches Considered\n\n### 1. Recursive-Descent with AST (chosen)\n- **Lexer → Parser → AST → Evaluator → REPL**\n- Clean separation: each stage is independently testable\n- AST is a reusable intermediate representation\n- Easy to extend (new operators, pretty-printing, optimization)\n- Well-suited for 2 precedence levels + parentheses\n\n### 2. Recursive-Descent with Direct Evaluation\n- Parser evaluates inline — no AST\n- Fewer types, less code\n- Couples parsing and evaluation — harder to test, extend\n\n### 3. Shunting-Yard Algorithm\n- Converts to RPN then evaluates\n- Good for many precedence levels; overkill here\n- Harder to produce clear error messages\n\n**Decision:** Approach 1. The AST adds minimal overhead but provides clean boundaries.\n\n## Architecture\n\n```\nInput string\n    │\n    ▼\n ┌───────┐\n │ Lexer │  string → []Token\n └───┬───┘\n     │\n     ▼\n ┌────────┐\n │ Parser │  []Token → AST (Node)\n └───┬────┘\n     │\n     ▼\n ┌───────────┐\n │ Evaluator │  Node → float64\n └───┬───────┘\n     │\n     ▼\n ┌──────┐\n │ REPL │  read line → eval → print result or error\n └──────┘\n```\n\n## Component Interfaces\n\n### Token (data type)\n\n```go\npackage token\n\ntype Type int\n\nconst (\n    Number Type = iota\n    Plus        // +\n    Minus       // -\n    Star        // *\n    Slash       // /\n    LParen      // (\n    RParen      // )\n    EOF\n)\n\ntype Token struct {\n    Type    Type\n    Literal string  // raw text, e.g. \"3.14\", \"+\"\n    Pos     int     // position in input (for error messages)\n}\n```\n\n### Lexer\n\n```go\npackage lexer\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error)\n```\n\n### AST (data types)\n\n```go\npackage ast\n\n// Node is the interface all AST nodes implement.\ntype Node interface {\n    node() // sealed marker method\n}\n\n// NumberLit represents a numeric literal.\ntype NumberLit struct {\n    Value float64\n}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n    Op    token.Type  // Plus, Minus, Star, Slash\n    Left  Node\n    Right Node\n}\n```\n\n### Parser\n\n```go\npackage parser\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions (mismatched parens, etc.).\nfunc Parse(tokens []token.Token) (ast.Node, error)\n```\n\nGrammar (recursive-descent):\n```\nexpr   → term (('+' | '-') term)*\nterm   → factor (('*' | '/') factor)*\nfactor → NUMBER | '(' expr ')'\n```\n\n### Evaluator\n\n```go\npackage evaluator\n\n// Eval evaluates an AST node and returns the result.\n// Returns an error on division by zero.\nfunc Eval(node ast.Node) (float64, error)\n```\n\n### REPL\n\n```go\npackage repl\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer)\n```\n\n## Package Layout\n\n```\nmatheval/\n├── cmd/\n│   └── matheval/\n│       └── main.go          # entry point, calls repl.Run\n├── token/\n│   └── token.go             # Token type and constants\n├── lexer/\n│   ├── lexer.go             # Tokenize function\n│   └── lexer_test.go\n├── ast/\n│   └── ast.go               # AST node types\n├── parser/\n│   ├── parser.go            # Parse function\n│   └── parser_test.go\n├── evaluator/\n│   ├── evaluator.go         # Eval function\n│   └── evaluator_test.go\n├── repl/\n│   ├── repl.go              # REPL loop\n│   └── repl_test.go\n├── docs/\n│   ├── design.md\n│   └── plan.md\n├── go.mod\n└── README.md\n```\n\n## Error Handling\n- Lexer: returns error for invalid characters (e.g. `@`, `#`)\n- Parser: returns error for syntax errors (unexpected token, mismatched parens)\n- Evaluator: returns error for division by zero\n- REPL: catches any error, prints it, prompts for next input\n\n## Key Design Decisions\n1. **Functional API over structs** — `Tokenize()`, `Parse()`, `Eval()` are stateless functions. No need for struct receivers since there's no configuration or state to carry.\n2. **Sealed AST interface** — unexported marker method prevents external implementations, keeping the node set closed.\n3. **Position tracking in tokens** — enables precise error messages (\"error at position 5\").\n4. **REPL takes io.Reader/io.Writer** — makes it testable without stdin/stdout.\n"
+
+CALLING TOOL: file_read {"path":"docs/plan.md"}
+$$$ "# Math Expression Evaluator — Implementation Plan\n\n## Phase: Implement\n\nSteps are ordered. Each step includes writing the code and its unit tests (TDD).\n\n### Step 1: Project Skeleton\n- `go mod init matheval`\n- Create directory structure: `cmd/matheval/`, `token/`, `lexer/`, `ast/`, `parser/`, `evaluator/`, `repl/`\n- Create placeholder `main.go`\n\n### Step 2: Token Package\n- Define `Type` enum constants\n- Define `Token` struct\n- Add `String()` method on `Type` for debugging\n\n### Step 3: Lexer\n- Implement `Tokenize(input string) ([]Token, error)`\n- Handle: whitespace skipping, number literals (integers and decimals), operators `+-*/`, parentheses `()`, EOF, invalid characters\n- **Tests:** valid expressions, decimal numbers, invalid chars, empty input, whitespace-only\n\n### Step 4: AST Package\n- Define `Node` interface with sealed marker\n- Define `NumberLit` struct\n- Define `BinaryExpr` struct\n\n### Step 5: Parser\n- Implement recursive-descent parser following grammar:\n  - `expr → term (('+' | '-') term)*`\n  - `term → factor (('*' | '/') factor)*`\n  - `factor → NUMBER | '(' expr ')'`\n- Internal parser struct to track position in token slice\n- Return error on: unexpected token, mismatched parens, trailing tokens\n- **Tests:** single number, simple binary, precedence, parentheses, nested parens, error cases\n\n### Step 6: Evaluator\n- Implement `Eval(node ast.Node) (float64, error)`\n- Recursively walk AST\n- Return error on division by zero\n- **Tests:** literals, all 4 operators, nested expressions, division by zero\n\n### Step 7: REPL\n- Implement `Run(r io.Reader, w io.Writer)`\n- Read line, tokenize, parse, evaluate, print result or error\n- Loop until EOF\n- **Tests:** successful expression, error expression, multi-line session\n\n### Step 8: main.go\n- Wire `repl.Run(os.Stdin, os.Stdout)`\n\n### Step 9: Integration Test\n- End-to-end test: feed expression string through all stages, verify result\n- Test edge cases: deeply nested parens, long expressions\n\n### Step 10: Final Commit \u0026 README\n- Write README.md with usage instructions\n- Final commit\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: dir_list {"name":"cmd/matheval"}
+$$$ {"entries":[{"name":"main.go","is_dir":false}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"cmd/matheval/main.go"}
+$$$ "package main\n\nimport (\n\t\"os\"\n\n\t\"matheval/repl\"\n)\n\nfunc main() {\n\trepl.Run(os.Stdin, os.Stdout)\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now I have a thorough understanding of the codebase. Let me verify existing tests pass:
+!!!!! {"command":"cd /root && go test ./... 2>&1"}: you are already in project root
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1"}: ok
+--- STOP_REASON: tool_use
+AI: Good, all tests pass. Now I have a complete understanding of the project. Let me create the Research phase TODO items.
+CALLING TOOL: todo_item_add {"items":[{"title":"Research: Understand requirements for function definitions","description":"The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.","assignedTo":"assistant","parentId":"1","items":[{"title":"What syntax should function definitions use?","description":"e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.","assignedTo":"user"},{"title":"Should functions support multiple parameters?","description":"e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).","assignedTo":"user"},{"title":"How should function calls look in expressions?","description":"e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?","assignedTo":"user"},{"title":"Should function definitions persist across REPL lines?","description":"e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).","assignedTo":"user"},{"title":"Can functions call other user-defined functions?","description":"e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).","assignedTo":"user"},{"title":"Should there be any built-in functions?","description":"e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.","assignedTo":"user"},{"title":"Can functions be redefined?","description":"e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?","assignedTo":"user"},{"title":"What should happen when defining a function — should it print anything?","description":"e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?","assignedTo":"user"}],"parallel":true}]}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: I've created the Research phase TODO items with clarification questions. These are all assigned to you since they require your input on the requirements. The questions cover:
+
+1. **Syntax** for function definitions
+2. **Multiple parameters** support
+3. **Function call syntax** in expressions
+4. **State persistence** across REPL lines
+5. **Inter-function calls** (functions calling other functions)
+6. **Built-in functions** scope
+7. **Redefinition** behavior
+8. **REPL output** on definition
+
+These can be answered in parallel — they are independent questions. Please respond to each so I can proceed to the Design phase.
+## AGENT NO ITEMS
+1: update repl to support function definitions - IN PROGRESS user false
+	1.1: Research: Understand requirements for function definitions - IN PROGRESS assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+		1.1.1: What syntax should function definitions use? - IN PROGRESS user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		1.1.2: Should functions support multiple parameters? - IN PROGRESS user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		1.1.3: How should function calls look in expressions? - IN PROGRESS user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		1.1.4: Should function definitions persist across REPL lines? - IN PROGRESS user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		1.1.5: Can functions call other user-defined functions? - IN PROGRESS user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		1.1.6: Should there be any built-in functions? - IN PROGRESS user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		1.1.7: Can functions be redefined? - IN PROGRESS user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		1.1.8: What should happen when defining a function — should it print anything? - IN PROGRESS user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+-- YOU START WORKING 8
+YOU 1.1.1 What syntax should function definitions use?: f(x) = x + 1
+YOU 1.1.2 Should functions support multiple parameters?: yes
+YOU 1.1.3 How should function calls look in expressions?: yes and function arguments can be any arbitrary valid expression
+YOU 1.1.4 Should function definitions persist across REPL lines?: yes
+YOU 1.1.5 Can functions call other user-defined functions?: yes
+YOU 1.1.6 Should there be any built-in functions?: no
+YOU 1.1.7 Can functions be redefined?: no
+YOU 1.1.8 What should happen when defining a function — should it print anything?: print that new function with given name was defined
+-- YOU END WORKING
+## USER NO ITEMS
+1: update repl to support function definitions - IN PROGRESS user false
+	1.1: Research: Understand requirements for function definitions - IN PROGRESS assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+		1.1.1: What syntax should function definitions use? - IN PROGRESS assistant false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - IN PROGRESS assistant false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - IN PROGRESS assistant false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - IN PROGRESS assistant false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - IN PROGRESS assistant false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - IN PROGRESS assistant false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+			 - user: no
+		1.1.7: Can functions be redefined? - IN PROGRESS assistant false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - IN PROGRESS assistant false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+			 - user: print that new function with given name was defined
+-- AGENT START WORKING 1.1.1, 1.1.2, 1.1.3, 1.1.4, 1.1.5, 1.1.6, 1.1.7, 1.1.8
+--- STOP_REASON: tool_use
+AI: 
+
+I'll mark all the clarification items as done since all questions have been answered, and then summarize the findings on the parent item.
+CALLING TOOL: todo_item_mark_done {"id":"1.1.1","summary":"Syntax: `f(x) = x + 1`"}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.2","summary":"Yes, multiple parameters supported: `f(x, y) = x + y`"}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.3","summary":"Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.4","summary":"Yes, definitions persist across REPL lines. REPL needs stateful function registry."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.5","summary":"Yes, functions can call other user-defined functions. Implies late binding during evaluation."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.6","summary":"No built-in functions. Only user-defined functions."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.7","summary":"No, functions cannot be redefined. Attempting to redefine should be an error."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.1.8","summary":"Print confirmation message when a function is defined (e.g. \"defined f\")."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me explore the current codebase to complete the research phase.
+CALLING TOOL: dir_list {"name":"."}
+$$$ {"entries":[{"name":".git","is_dir":true},{"name":"README.md","is_dir":false},{"name":"ast","is_dir":true},{"name":"cmd","is_dir":true},{"name":"docs","is_dir":true},{"name":"evaluator","is_dir":true},{"name":"go.mod","is_dir":false},{"name":"integration_test.go","is_dir":false},{"name":"lexer","is_dir":true},{"name":"matheval","is_dir":false},{"name":"parser","is_dir":true},{"name":"repl","is_dir":true},{"name":"sessions","is_dir":true},{"name":"token","is_dir":true}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: dir_list {"name":"ast"}
+$$$ {"entries":[{"name":"ast.go","is_dir":false},{"name":"ast_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"cmd"}
+$$$ {"entries":[{"name":"matheval","is_dir":true}]}
+
+CALLING TOOL: dir_list {"name":"evaluator"}
+$$$ {"entries":[{"name":"evaluator.go","is_dir":false},{"name":"evaluator_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"lexer"}
+$$$ {"entries":[{"name":"lexer.go","is_dir":false},{"name":"lexer_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"parser"}
+$$$ {"entries":[{"name":"parser.go","is_dir":false},{"name":"parser_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"repl"}
+$$$ {"entries":[{"name":"repl.go","is_dir":false},{"name":"repl_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"token"}
+$$$ {"entries":[{"name":"token.go","is_dir":false},{"name":"token_test.go","is_dir":false}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast.go"}
+$$$ "package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer.go"}
+$$$ "package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i \u003c len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch \u003e= '0' \u0026\u0026 ch \u003c= '9'\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser.go"}
+$$$ "package parser\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strconv\"\n)\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions.\nfunc Parse(tokens []token.Token) (ast.Node, error) {\n\tp := \u0026parser{tokens: tokens}\n\tnode, err := p.expr()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\t// Ensure all tokens have been consumed (only EOF remains).\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n\treturn node, nil\n}\n\n// parser holds the state for a single parse operation.\ntype parser struct {\n\ttokens []token.Token\n\tpos    int\n}\n\n// current returns the token at the current position.\nfunc (p *parser) current() token.Token {\n\tif p.pos \u003e= len(p.tokens) {\n\t\treturn token.Token{Type: token.EOF}\n\t}\n\treturn p.tokens[p.pos]\n}\n\n// advance moves to the next token and returns the previous one.\nfunc (p *parser) advance() token.Token {\n\ttok := p.current()\n\tp.pos++\n\treturn tok\n}\n\n// expect consumes a token of the given type or returns an error.\nfunc (p *parser) expect(typ token.Type) (token.Token, error) {\n\ttok := p.current()\n\tif tok.Type != typ {\n\t\treturn tok, fmt.Errorf(\"expected %v but got %v at position %d\", typ, tok.Type, tok.Pos)\n\t}\n\tp.advance()\n\treturn tok, nil\n}\n\n// expr → term (('+' | '-') term)*\nfunc (p *parser) expr() (ast.Node, error) {\n\tleft, err := p.term()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Plus || p.current().Type == token.Minus {\n\t\top := p.advance()\n\t\tright, err := p.term()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// term → factor (('*' | '/') factor)*\nfunc (p *parser) term() (ast.Node, error) {\n\tleft, err := p.factor()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Star || p.current().Type == token.Slash {\n\t\top := p.advance()\n\t\tright, err := p.factor()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// factor → NUMBER | '(' expr ')'\nfunc (p *parser) factor() (ast.Node, error) {\n\ttok := p.current()\n\tswitch tok.Type {\n\tcase token.Number:\n\t\tp.advance()\n\t\tval, err := strconv.ParseFloat(tok.Literal, 64)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"invalid number %q at position %d: %w\", tok.Literal, tok.Pos, err)\n\t\t}\n\t\treturn \u0026ast.NumberLit{Value: val}, nil\n\n\tcase token.LParen:\n\t\tp.advance() // consume '('\n\t\tnode, err := p.expr()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\treturn nil, fmt.Errorf(\"missing closing parenthesis at position %d\", p.current().Pos)\n\t\t}\n\t\treturn node, nil\n\n\tdefault:\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator.go"}
+$$$ "package evaluator\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n)\n\n// Eval evaluates an AST node and returns the result.\n// Returns an error on division by zero.\nfunc Eval(node ast.Node) (float64, error) {\n\tswitch n := node.(type) {\n\tcase *ast.NumberLit:\n\t\treturn n.Value, nil\n\n\tcase *ast.BinaryExpr:\n\t\tleft, err := Eval(n.Left)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\t\tright, err := Eval(n.Right)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\n\t\tswitch n.Op {\n\t\tcase token.Plus:\n\t\t\treturn left + right, nil\n\t\tcase token.Minus:\n\t\t\treturn left - right, nil\n\t\tcase token.Star:\n\t\t\treturn left * right, nil\n\t\tcase token.Slash:\n\t\t\tif right == 0 {\n\t\t\t\treturn 0, fmt.Errorf(\"division by zero\")\n\t\t\t}\n\t\t\treturn left / right, nil\n\t\tdefault:\n\t\t\treturn 0, fmt.Errorf(\"unknown operator: %v\", n.Op)\n\t\t}\n\n\tdefault:\n\t\treturn 0, fmt.Errorf(\"unknown node type: %T\", node)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl.go"}
+$$$ "package repl\n\nimport (\n\t\"bufio\"\n\t\"fmt\"\n\t\"io\"\n\t\"math\"\n\t\"strings\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\nconst prompt = \"\u003e\u003e \"\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer) {\n\tscanner := bufio.NewScanner(r)\n\tfmt.Fprint(w, prompt)\n\n\tfor scanner.Scan() {\n\t\tline := strings.TrimSpace(scanner.Text())\n\t\tif line == \"\" {\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\tresult, err := evalLine(line)\n\t\tif err != nil {\n\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t} else {\n\t\t\tfmt.Fprintln(w, formatResult(result))\n\t\t}\n\n\t\tfmt.Fprint(w, prompt)\n\t}\n}\n\n// evalLine tokenizes, parses, and evaluates a single expression string.\nfunc evalLine(line string) (float64, error) {\n\ttokens, err := lexer.Tokenize(line)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\treturn evaluator.Eval(tree)\n}\n\n// formatResult formats a float64 for display.\n// Whole numbers are printed without decimal points.\nfunc formatResult(val float64) string {\n\tif val == math.Trunc(val) \u0026\u0026 !math.IsInf(val, 0) \u0026\u0026 !math.IsNaN(val) {\n\t\treturn fmt.Sprintf(\"%g\", val)\n\t}\n\treturn fmt.Sprintf(\"%g\", val)\n}\n"
+
+CALLING TOOL: file_read {"path":"go.mod"}
+$$$ "module matheval\n\ngo 1.23.1\n"
+
+CALLING TOOL: file_read {"path":"integration_test.go"}
+$$$ "package matheval_test\n\nimport (\n\t\"math\"\n\t\"strings\"\n\t\"testing\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\n// eval is a helper that runs the full pipeline: lexer → parser → evaluator.\nfunc eval(t *testing.T, input string) float64 {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\tt.Fatalf(\"lexer error for %q: %v\", input, err)\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"parser error for %q: %v\", input, err)\n\t}\n\tresult, err := evaluator.Eval(tree)\n\tif err != nil {\n\t\tt.Fatalf(\"evaluator error for %q: %v\", input, err)\n\t}\n\treturn result\n}\n\n// evalErr is a helper that expects the full pipeline to return an error.\nfunc evalErr(t *testing.T, input string) error {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\treturn err\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn err\n\t}\n\t_, err = evaluator.Eval(tree)\n\treturn err\n}\n\nfunc assertApprox(t *testing.T, input string, expected, got float64) {\n\tt.Helper()\n\tif math.Abs(expected-got) \u003e 1e-9 {\n\t\tt.Errorf(\"%q: expected %v, got %v\", input, expected, got)\n\t}\n}\n\n// --- Basic arithmetic ---\n\nfunc TestIntegration_SingleNumber(t *testing.T) {\n\tassertApprox(t, \"42\", 42, eval(t, \"42\"))\n}\n\nfunc TestIntegration_DecimalNumber(t *testing.T) {\n\tassertApprox(t, \"3.14\", 3.14, eval(t, \"3.14\"))\n}\n\nfunc TestIntegration_LeadingDot(t *testing.T) {\n\tassertApprox(t, \".5\", 0.5, eval(t, \".5\"))\n}\n\nfunc TestIntegration_Addition(t *testing.T) {\n\tassertApprox(t, \"1 + 2\", 3, eval(t, \"1 + 2\"))\n}\n\nfunc TestIntegration_Subtraction(t *testing.T) {\n\tassertApprox(t, \"10 - 4\", 6, eval(t, \"10 - 4\"))\n}\n\nfunc TestIntegration_Multiplication(t *testing.T) {\n\tassertApprox(t, \"3 * 7\", 21, eval(t, \"3 * 7\"))\n}\n\nfunc TestIntegration_Division(t *testing.T) {\n\tassertApprox(t, \"10 / 4\", 2.5, eval(t, \"10 / 4\"))\n}\n\n// --- Precedence and associativity ---\n\nfunc TestIntegration_PrecedenceMulOverAdd(t *testing.T) {\n\t// 2 + 3 * 4 = 2 + 12 = 14\n\tassertApprox(t, \"2 + 3 * 4\", 14, eval(t, \"2 + 3 * 4\"))\n}\n\nfunc TestIntegration_PrecedenceDivOverSub(t *testing.T) {\n\t// 10 - 6 / 3 = 10 - 2 = 8\n\tassertApprox(t, \"10 - 6 / 3\", 8, eval(t, \"10 - 6 / 3\"))\n}\n\nfunc TestIntegration_LeftAssociativitySub(t *testing.T) {\n\t// 10 - 3 - 2 = (10 - 3) - 2 = 5\n\tassertApprox(t, \"10 - 3 - 2\", 5, eval(t, \"10 - 3 - 2\"))\n}\n\nfunc TestIntegration_LeftAssociativityDiv(t *testing.T) {\n\t// 24 / 4 / 3 = (24 / 4) / 3 = 2\n\tassertApprox(t, \"24 / 4 / 3\", 2, eval(t, \"24 / 4 / 3\"))\n}\n\n// --- Parentheses ---\n\nfunc TestIntegration_ParensOverridePrecedence(t *testing.T) {\n\t// (2 + 3) * 4 = 20\n\tassertApprox(t, \"(2 + 3) * 4\", 20, eval(t, \"(2 + 3) * 4\"))\n}\n\nfunc TestIntegration_NestedParens(t *testing.T) {\n\t// ((1 + 2)) = 3\n\tassertApprox(t, \"((1 + 2))\", 3, eval(t, \"((1 + 2))\"))\n}\n\nfunc TestIntegration_DeeplyNestedParens(t *testing.T) {\n\t// ((((((1 + 2)))))) = 3\n\tassertApprox(t, \"((((((1 + 2))))))\", 3, eval(t, \"((((((1 + 2))))))\"))\n}\n\nfunc TestIntegration_ParensOnBothSides(t *testing.T) {\n\t// (1 + 2) * (3 + 4) = 3 * 7 = 21\n\tassertApprox(t, \"(1 + 2) * (3 + 4)\", 21, eval(t, \"(1 + 2) * (3 + 4)\"))\n}\n\nfunc TestIntegration_ParensNestedComplex(t *testing.T) {\n\t// ((2 + 3) * (4 - 1)) / 5 = (5 * 3) / 5 = 3\n\tassertApprox(t, \"((2 + 3) * (4 - 1)) / 5\", 3, eval(t, \"((2 + 3) * (4 - 1)) / 5\"))\n}\n\n// --- Complex expressions ---\n\nfunc TestIntegration_AllOperators(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2 = 1 + 6 - 2 = 5\n\tassertApprox(t, \"1 + 2 * 3 - 4 / 2\", 5, eval(t, \"1 + 2 * 3 - 4 / 2\"))\n}\n\nfunc TestIntegration_LongChainedAddition(t *testing.T) {\n\t// 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55\n\tassertApprox(t, \"1+2+3+4+5+6+7+8+9+10\", 55, eval(t, \"1+2+3+4+5+6+7+8+9+10\"))\n}\n\nfunc TestIntegration_LongChainedMixed(t *testing.T) {\n\t// 2 * 3 + 4 * 5 - 6 / 2 + 1 = 6 + 20 - 3 + 1 = 24\n\tassertApprox(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\", 24, eval(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\"))\n}\n\n// --- Floating point ---\n\nfunc TestIntegration_FloatArithmetic(t *testing.T) {\n\t// 1.5 + 2.5 = 4.0\n\tassertApprox(t, \"1.5 + 2.5\", 4.0, eval(t, \"1.5 + 2.5\"))\n}\n\nfunc TestIntegration_FloatDivision(t *testing.T) {\n\t// 7 / 2 = 3.5\n\tassertApprox(t, \"7 / 2\", 3.5, eval(t, \"7 / 2\"))\n}\n\nfunc TestIntegration_FloatPrecision(t *testing.T) {\n\t// 0.1 + 0.2 ≈ 0.3 (within tolerance)\n\tassertApprox(t, \"0.1 + 0.2\", 0.3, eval(t, \"0.1 + 0.2\"))\n}\n\n// --- Whitespace variations ---\n\nfunc TestIntegration_NoSpaces(t *testing.T) {\n\tassertApprox(t, \"1+2*3\", 7, eval(t, \"1+2*3\"))\n}\n\nfunc TestIntegration_ExtraSpaces(t *testing.T) {\n\tassertApprox(t, \"  1  +  2  \", 3, eval(t, \"  1  +  2  \"))\n}\n\nfunc TestIntegration_TabsAndSpaces(t *testing.T) {\n\tassertApprox(t, \"1\\t+\\t2\", 3, eval(t, \"1\\t+\\t2\"))\n}\n\n// --- Error cases ---\n\nfunc TestIntegration_DivisionByZero(t *testing.T) {\n\terr := evalErr(t, \"1 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n\tif !strings.Contains(err.Error(), \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_DivisionByZeroInSubExpr(t *testing.T) {\n\terr := evalErr(t, \"1 + 2 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestIntegration_InvalidCharacter(t *testing.T) {\n\terr := evalErr(t, \"1 @ 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenLeft(t *testing.T) {\n\terr := evalErr(t, \"(1 + 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing closing paren\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenRight(t *testing.T) {\n\terr := evalErr(t, \"1 + 2)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected closing paren\")\n\t}\n}\n\nfunc TestIntegration_EmptyParens(t *testing.T) {\n\terr := evalErr(t, \"()\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\nfunc TestIntegration_TrailingOperator(t *testing.T) {\n\terr := evalErr(t, \"1 +\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestIntegration_LeadingOperator(t *testing.T) {\n\terr := evalErr(t, \"* 1\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for leading operator\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveOperators(t *testing.T) {\n\terr := evalErr(t, \"1 + * 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestIntegration_EmptyInput(t *testing.T) {\n\t// Empty string should produce only EOF, parser should error\n\terr := evalErr(t, \"\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveNumbers(t *testing.T) {\n\terr := evalErr(t, \"1 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive numbers without operator\")\n\t}\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me also read the test files for each component:
+CALLING TOOL: file_read {"path":"token/token_test.go"}
+$$$ "package token\n\nimport \"testing\"\n\nfunc TestTypeString(t *testing.T) {\n\ttests := []struct {\n\t\ttyp  Type\n\t\twant string\n\t}{\n\t\t{Number, \"Number\"},\n\t\t{Plus, \"+\"},\n\t\t{Minus, \"-\"},\n\t\t{Star, \"*\"},\n\t\t{Slash, \"/\"},\n\t\t{LParen, \"(\"},\n\t\t{RParen, \")\"},\n\t\t{EOF, \"EOF\"},\n\t\t{Type(99), \"Unknown(99)\"},\n\t}\n\tfor _, tc := range tests {\n\t\tif got := tc.typ.String(); got != tc.want {\n\t\t\tt.Errorf(\"Type(%d).String() = %q, want %q\", int(tc.typ), got, tc.want)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast_test.go"}
+$$$ "package ast\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// Compile-time check: both types satisfy Node.\nvar (\n\t_ Node = (*NumberLit)(nil)\n\t_ Node = (*BinaryExpr)(nil)\n)\n\nfunc TestNumberLit(t *testing.T) {\n\tn := \u0026NumberLit{Value: 3.14}\n\tif n.Value != 3.14 {\n\t\tt.Fatalf(\"expected 3.14, got %f\", n.Value)\n\t}\n}\n\nfunc TestBinaryExpr(t *testing.T) {\n\tleft := \u0026NumberLit{Value: 1}\n\tright := \u0026NumberLit{Value: 2}\n\texpr := \u0026BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  left,\n\t\tRight: right,\n\t}\n\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tif expr.Left != left {\n\t\tt.Fatal(\"Left child mismatch\")\n\t}\n\tif expr.Right != right {\n\t\tt.Fatal(\"Right child mismatch\")\n\t}\n}\n\nfunc TestNestedBinaryExpr(t *testing.T) {\n\t// Represents: (1 + 2) * 3\n\ttree := \u0026BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: \u0026BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  \u0026NumberLit{Value: 1},\n\t\t\tRight: \u0026NumberLit{Value: 2},\n\t\t},\n\t\tRight: \u0026NumberLit{Value: 3},\n\t}\n\n\tif tree.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", tree.Op)\n\t}\n\n\tinner, ok := tree.Left.(*BinaryExpr)\n\tif !ok {\n\t\tt.Fatal(\"Left should be *BinaryExpr\")\n\t}\n\tif inner.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", inner.Op)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer_test.go"}
+$$$ "package lexer\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestTokenizeEmpty(t *testing.T) {\n\ttokens, err := Tokenize(\"\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeWhitespaceOnly(t *testing.T) {\n\ttokens, err := Tokenize(\"   \\t\\n  \")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeSingleNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"42\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeDecimalNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"3.14\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"3.14\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeLeadingDotNumber(t *testing.T) {\n\ttokens, err := Tokenize(\".5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \".5\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeOperators(t *testing.T) {\n\ttokens, err := Tokenize(\"+-*/\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 0},\n\t\t{Type: token.Minus, Literal: \"-\", Pos: 1},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 2},\n\t\t{Type: token.Slash, Literal: \"/\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeParens(t *testing.T) {\n\ttokens, err := Tokenize(\"()\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 1},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFullExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"(1 + 2.5) * 3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 1},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2.5\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 10},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeNoSpaces(t *testing.T) {\n\ttokens, err := Tokenize(\"1+2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeInvalidCharacter(t *testing.T) {\n\t_, err := Tokenize(\"1 + @\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleInvalidCharacters(t *testing.T) {\n\t_, err := Tokenize(\"1 \u0026 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleDecimals(t *testing.T) {\n\t// \"1.2.3\" — the lexer should read \"1.2\" as a number, then \".3\" as another number\n\ttokens, err := Tokenize(\"1.2.3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1.2\", Pos: 0},\n\t\t{Type: token.Number, Literal: \".3\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\n// assertTokens is a test helper that compares two token slices.\nfunc assertTokens(t *testing.T, want, got []token.Token) {\n\tt.Helper()\n\tif len(want) != len(got) {\n\t\tt.Fatalf(\"token count: want %d, got %d\\nwant: %v\\ngot:  %v\", len(want), len(got), want, got)\n\t}\n\tfor i := range want {\n\t\tif want[i].Type != got[i].Type {\n\t\t\tt.Errorf(\"token[%d].Type: want %v, got %v\", i, want[i].Type, got[i].Type)\n\t\t}\n\t\tif want[i].Literal != got[i].Literal {\n\t\t\tt.Errorf(\"token[%d].Literal: want %q, got %q\", i, want[i].Literal, got[i].Literal)\n\t\t}\n\t\tif want[i].Pos != got[i].Pos {\n\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser_test.go"}
+$$$ "package parser\n\nimport (\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// helper: tokenize inline for concise tests\nfunc tokens(toks ...token.Token) []token.Token {\n\treturn toks\n}\n\nfunc tok(typ token.Type, lit string, pos int) token.Token {\n\treturn token.Token{Type: typ, Literal: lit, Pos: pos}\n}\n\n// --- Success cases ---\n\nfunc TestParseSingleNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"42\", 0),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 42 {\n\t\tt.Fatalf(\"expected 42, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseDecimalNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"3.14\", 0),\n\t\ttok(token.EOF, \"\", 4),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 3.14 {\n\t\tt.Fatalf(\"expected 3.14, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseAddition(t *testing.T) {\n\t// 1 + 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseSubtraction(t *testing.T) {\n\t// 5 - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"5\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 5)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseMultiplication(t *testing.T) {\n\t// 2 * 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 2)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseDivision(t *testing.T) {\n\t// 6 / 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"6\", 0),\n\t\ttok(token.Slash, \"/\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 6)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParsePrecedence(t *testing.T) {\n\t// 1 + 2 * 3  →  1 + (2 * 3)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root should be Plus\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\t// Right should be Star\n\tright, ok := expr.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", expr.Right)\n\t}\n\tif right.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", right.Op)\n\t}\n\tassertNumber(t, right.Left, 2)\n\tassertNumber(t, right.Right, 3)\n}\n\nfunc TestParsePrecedenceMulFirst(t *testing.T) {\n\t// 2 * 3 + 1  →  (2 * 3) + 1\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.Plus, \"+\", 6),\n\t\ttok(token.Number, \"1\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 2)\n\tassertNumber(t, left.Right, 3)\n\tassertNumber(t, expr.Right, 1)\n}\n\nfunc TestParseLeftAssociativity(t *testing.T) {\n\t// 1 - 2 - 3  →  (1 - 2) - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Minus, \"-\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 - 2) - 3\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseParentheses(t *testing.T) {\n\t// (1 + 2) * 3\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.RParen, \")\", 6),\n\t\ttok(token.Star, \"*\", 8),\n\t\ttok(token.Number, \"3\", 10),\n\t\ttok(token.EOF, \"\", 11),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseNestedParentheses(t *testing.T) {\n\t// ((1 + 2))\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.LParen, \"(\", 1),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.Plus, \"+\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.RParen, \")\", 7),\n\t\ttok(token.RParen, \")\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseComplexExpression(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2  →  (1 + (2*3)) - (4/2)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 5),\n\t\ttok(token.Number, \"3\", 6),\n\t\ttok(token.Minus, \"-\", 8),\n\t\ttok(token.Number, \"4\", 10),\n\t\ttok(token.Slash, \"/\", 11),\n\t\ttok(token.Number, \"2\", 12),\n\t\ttok(token.EOF, \"\", 13),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 + (2*3)) - (4/2)\n\troot, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif root.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", root.Op)\n\t}\n\t// Left: 1 + (2*3)\n\tleft, ok := root.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", root.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tmul, ok := left.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", left.Right)\n\t}\n\tif mul.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", mul.Op)\n\t}\n\tassertNumber(t, mul.Left, 2)\n\tassertNumber(t, mul.Right, 3)\n\t// Right: 4/2\n\tdiv, ok := root.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", root.Right)\n\t}\n\tif div.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", div.Op)\n\t}\n\tassertNumber(t, div.Left, 4)\n\tassertNumber(t, div.Right, 2)\n}\n\n// --- Error cases ---\n\nfunc TestParseEmptyInput(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.EOF, \"\", 0),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestParseMissingRParen(t *testing.T) {\n\t// (1 + 2\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.EOF, \"\", 6),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing right paren\")\n\t}\n}\n\nfunc TestParseUnexpectedRParen(t *testing.T) {\n\t// ) 1\n\ttoks := tokens(\n\t\ttok(token.RParen, \")\", 0),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected right paren\")\n\t}\n}\n\nfunc TestParseTrailingOperator(t *testing.T) {\n\t// 1 +\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestParseTrailingTokens(t *testing.T) {\n\t// 1 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Number, \"2\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing tokens\")\n\t}\n}\n\nfunc TestParseConsecutiveOperators(t *testing.T) {\n\t// 1 + * 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Star, \"*\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.EOF, \"\", 7),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestParseEmptyParens(t *testing.T) {\n\t// ()\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.RParen, \")\", 1),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\n// --- Helper ---\n\nfunc assertNumber(t *testing.T, node ast.Node, expected float64) {\n\tt.Helper()\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != expected {\n\t\tt.Fatalf(\"expected %f, got %f\", expected, num.Value)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator_test.go"}
+$$$ "package evaluator\n\nimport (\n\t\"math\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestEvalNumberLit(t *testing.T) {\n\tresult, err := Eval(\u0026ast.NumberLit{Value: 42.5})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 42.5 {\n\t\tt.Fatalf(\"expected 42.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalAddition(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 3 {\n\t\tt.Fatalf(\"expected 3, got %v\", result)\n\t}\n}\n\nfunc TestEvalSubtraction(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Minus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalMultiplication(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Star,\n\t\tLeft:  \u0026ast.NumberLit{Value: 3},\n\t\tRight: \u0026ast.NumberLit{Value: 7},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 21 {\n\t\tt.Fatalf(\"expected 21, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivision(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 2.5 {\n\t\tt.Fatalf(\"expected 2.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZero(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 5},\n\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestEvalNestedExpr(t *testing.T) {\n\t// (1 + 2) * (8 / 4) = 3 * 2 = 6\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 8},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalDeeplyNested(t *testing.T) {\n\t// ((2 + 3) * 4) - (10 / 5) = 20 - 2 = 18\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Minus,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp: token.Star,\n\t\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\t\tOp:    token.Plus,\n\t\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\t\tRight: \u0026ast.NumberLit{Value: 3},\n\t\t\t},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\t\tRight: \u0026ast.NumberLit{Value: 5},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 18 {\n\t\tt.Fatalf(\"expected 18, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZeroInSubExpr(t *testing.T) {\n\t// 1 + (2 / 0) — error should propagate\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:   token.Plus,\n\t\tLeft: \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t\t},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error from sub-expression\")\n\t}\n}\n\nfunc TestEvalFloatingPoint(t *testing.T) {\n\t// 1.5 + 2.3 = 3.8\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1.5},\n\t\tRight: \u0026ast.NumberLit{Value: 2.3},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif math.Abs(result-3.8) \u003e 1e-12 {\n\t\tt.Fatalf(\"expected 3.8, got %v\", result)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl_test.go"}
+$$$ "package repl\n\nimport (\n\t\"bytes\"\n\t\"strings\"\n\t\"testing\"\n)\n\nfunc TestRun_SingleExpression(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n}\n\nfunc TestRun_MultipleExpressions(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n3 * 4\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"12\") {\n\t\tt.Errorf(\"expected output to contain '12', got %q\", got)\n\t}\n}\n\nfunc TestRun_ErrorContinues(t *testing.T) {\n\t// First line has error, second is valid.\n\tin := strings.NewReader(\"1 +\\n2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"5\") {\n\t\tt.Errorf(\"expected output to contain '5' after error recovery, got %q\", got)\n\t}\n}\n\nfunc TestRun_DivisionByZero(t *testing.T) {\n\tin := strings.NewReader(\"1 / 0\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyLine(t *testing.T) {\n\t// Empty lines should be skipped, not cause errors.\n\tin := strings.NewReader(\"\\n1 + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"2\") {\n\t\tt.Errorf(\"expected output to contain '2', got %q\", got)\n\t}\n\t// Should not contain any error.\n\tif strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"empty line should not produce error, got %q\", got)\n\t}\n}\n\nfunc TestRun_Prompt(t *testing.T) {\n\tin := strings.NewReader(\"42\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected prompt '\u003e\u003e ' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_FloatResult(t *testing.T) {\n\tin := strings.NewReader(\"7 / 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3.5\") {\n\t\tt.Errorf(\"expected output to contain '3.5', got %q\", got)\n\t}\n}\n\nfunc TestRun_InvalidCharacter(t *testing.T) {\n\tin := strings.NewReader(\"1 @ 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyInput(t *testing.T) {\n\t// No input at all — just EOF.\n\tin := strings.NewReader(\"\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Should just show the prompt and exit gracefully.\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected at least one prompt, got %q\", got)\n\t}\n}\n\nfunc TestRun_WholeIntegerNoTrailingZeros(t *testing.T) {\n\t// 2 + 3 = 5, should print \"5\" not \"5.000000\".\n\tin := strings.NewReader(\"2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Result line is \"5\\n\" (between prompts).\n\tif !strings.Contains(got, \"5\\n\") {\n\t\tt.Errorf(\"expected '5\\\\n' in output, got %q\", got)\n\t}\n\t// Should not contain \"5.0\"\n\tif strings.Contains(got, \"5.0\") {\n\t\tt.Errorf(\"expected no trailing zeros, got %q\", got)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"cmd/matheval/main.go"}
+$$$ "package main\n\nimport (\n\t\"os\"\n\n\t\"matheval/repl\"\n)\n\nfunc main() {\n\trepl.Run(os.Stdin, os.Stdout)\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now I have full understanding of the codebase. Let me verify all tests pass currently:
+!!!!! {"command":"cd /root && go test ./... 2>&1"}: you are already in project dir
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t(cached)\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t(cached)\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t(cached)\nok  \tmatheval/token\t(cached)\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass. Now let me mark the research parent item as done with a comprehensive summary.
+CALLING TOOL: todo_item_mark_done {"id":"1.1","summary":"**Requirements gathered:**\n\n1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression\n2. **Multiple params:** Yes — `f(x, y) = x + y`\n3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`\n4. **Persistence:** Definitions persist across REPL lines (stateful function registry)\n5. **Cross-calling:** Functions can call other user-defined functions (late binding)\n6. **Built-ins:** None\n7. **Redefinition:** Not allowed — attempting to redefine is an error\n8. **Output on define:** Print confirmation like \"defined f\"\n\n**Current architecture (5 packages + main):**\n- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF\n- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support\n- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`\n- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)\n- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless\n- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper\n\n**Key gaps to fill:**\n- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types\n- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes\n- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists\n- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context\n- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me create the Design phase TODO:
+CALLING TOOL: todo_item_add {"items":[{"title":"Design: Solution architecture for function definitions","description":"Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.","assignedTo":"assistant","parentId":"1","items":[{"title":"Design token layer changes","description":"New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.","assignedTo":"assistant"},{"title":"Design AST layer changes","description":"New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.","assignedTo":"assistant"},{"title":"Design lexer changes","description":"Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.","assignedTo":"assistant"},{"title":"Design parser changes","description":"Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.","assignedTo":"assistant"},{"title":"Design evaluator changes","description":"Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.","assignedTo":"assistant"},{"title":"Design REPL changes","description":"REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.","assignedTo":"assistant"}],"parallel":true}]}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me work on the design. I'll think through each component:
+CALLING TOOL: todo_item_add_comment {"id":"1.2","comment":"Starting design work. Will work through each sub-component and then present a unified design document.","assignTo":"assistant"}
+ERR: You shall never initiate a discussion, if you want to clarify something create a TODO item for it.
+--- STOP_REASON: tool_use
+AI: Let me now design each component. I'll work through the design systematically.
+
+### Token Layer Design
+
+New token types:
+- `Ident` — identifier (e.g., `f`, `x`, `myFunc`)
+- `Comma` — `,`
+- `Equals` — `=`
+
+### AST Layer Design
+
+New node types:
+- `Ident` — variable reference in expressions: `{Name string}`
+- `FuncCall` — function call in expressions: `{Name string, Args []Node}`
+- `FuncDef` — function definition (top-level statement): `{Name string, Params []string, Body Node}`
+
+The parser currently returns `ast.Node`. A function definition is not an expression that evaluates to a float64. Two approaches:
+
+**Approach A:** Add a new top-level type `Statement` (interface) with two variants: `ExprStmt` wrapping a `Node`, and `FuncDef`. Parser returns `Statement` instead of `Node`.
+
+**Approach B:** Keep `FuncDef` as an `ast.Node`. Parser returns `Node`. Evaluator/REPL checks the type to decide behavior.
+
+**Approach A** is cleaner — it separates the concerns of "what kind of input line is this?" from "what expression does it contain?" But it's a bigger refactor.
+
+**Approach B** is simpler but muddies the AST — `FuncDef` isn't really an expression node, yet it implements `Node`.
+
+I'll go with **Approach A** since it's cleaner and the refactoring cost is minimal (the `Parse` function signature changes, and the REPL adapts).
+
+### Lexer Design
+
+The lexer needs to recognize identifiers: sequences of letters (and potentially digits after the first letter). An identifier starts with a letter (`a-z`, `A-Z`) and continues with letters or digits.
+
+New single-char tokens: `,` → `Comma`, `=` → `Equals`.
+
+### Parser Design
+
+The parser needs a new top-level entry point:
+```
+ParseLine(tokens) → Statement
+```
+
+Where `Statement` is either:
+- `ExprStmt{Expr Node}` — an expression to evaluate
+- `FuncDef{Name, Params, Body}` — a function definition
+
+**Distinguishing definition from expression:** 
+A definition looks like `Ident LParen Ident (Comma Ident)* RParen Equals expr`. The parser can look ahead: if the first tokens are `Ident LParen` and eventually `RParen Equals`, it's a definition. But this lookahead is complex.
+
+Simpler approach: If line starts with `Ident LParen` and we can find a matching `RParen` followed by `Equals`, parse as definition. Otherwise parse as expression.
+
+Even simpler: Try to detect `=` in the token stream. If there's an `=`, it's a definition. If not, it's an expression. But this is fragile.
+
+Best approach: Use a two-token lookahead. If first token is `Ident` and second is `LParen`, try parsing as a function definition (speculatively). If the speculative parse fails (no `=` after closing paren), backtrack and parse as expression. Or: just check if the token stream contains `=` at the appropriate position.
+
+Actually, the simplest correct approach: Parse the beginning. If we see `Ident(Ident....) =`, it's a definition. The key discriminator is finding `=` after a well-formed parameter list. We can use the parser's position tracking to backtrack.
+
+Or even simpler: since `=` is not used anywhere in expressions, we can scan the tokens for `Equals`. If present, parse as definition. If not, parse as expression.
+
+**Factor changes for function calls:** In `factor()`, when we see an `Ident` token followed by `LParen`, it's a function call. Otherwise it's a variable reference.
+
+### Evaluator Design
+
+The evaluator needs:
+1. **FuncRegistry** — stores defined functions: `map[string]*ast.FuncDef`
+2. **Environment** — variable bindings for function parameters: `map[string]float64`
+
+New signature approach:
+```go
+type Evaluator struct {
+    funcs map[string]*FuncDef  // function registry
+}
+
+func New() *Evaluator
+func (e *Evaluator) Define(def *ast.FuncDef) error  // returns error if already defined
+func (e *Evaluator) Eval(node ast.Node, env map[string]float64) (float64, error)
+```
+
+The `Eval` function stays pure for expression evaluation but now accepts an environment for variable lookup. The `Evaluator` struct holds the function registry.
+
+For function calls: lookup the function in the registry, evaluate each argument, bind param names to argument values in a new environment, then evaluate the function body with that environment.
+
+### REPL Design
+
+The REPL creates an `Evaluator` at startup. For each line:
+1. Tokenize
+2. Parse (returns `Statement`)
+3. If `FuncDef`: call `evaluator.Define()`, print "defined <name>"
+4. If `ExprStmt`: call `evaluator.Eval()`, print result
+
+Let me now mark the design sub-items as done and write the design document.
+CALLING TOOL: todo_item_mark_done {"id":"1.2.1","summary":"New token types: Ident, Comma, Equals. No changes to existing tokens."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.2.2","summary":"New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.2.3","summary":"Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.2.4","summary":"Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.2.5","summary":"Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call."}
+$$$ "done"
+
+CALLING TOOL: todo_item_mark_done {"id":"1.2.6","summary":"REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me write the design document:
+!!!!! {"command":"mkdir -p docs"}: docs folder should already be there. make sure to update existing `docs/design.md`
+--- STOP_REASON: tool_use
+CALLING TOOL: dir_list {"name":"docs"}
+$$$ {"entries":[{"name":"design.md","is_dir":false},{"name":"plan.md","is_dir":false}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"docs/design.md"}
+$$$ "# Math Expression Evaluator — Design Document\n\n## Requirements Summary\n- Language: Go\n- Operators: `+`, `-`, `*`, `/` with parentheses\n- Numbers: floating point (e.g. `3.14`, `42`, `0.5`)\n- Variables: none\n- Interface: CLI REPL\n- Error handling: print error message, continue REPL\n\n## Approaches Considered\n\n### 1. Recursive-Descent with AST (chosen)\n- **Lexer → Parser → AST → Evaluator → REPL**\n- Clean separation: each stage is independently testable\n- AST is a reusable intermediate representation\n- Easy to extend (new operators, pretty-printing, optimization)\n- Well-suited for 2 precedence levels + parentheses\n\n### 2. Recursive-Descent with Direct Evaluation\n- Parser evaluates inline — no AST\n- Fewer types, less code\n- Couples parsing and evaluation — harder to test, extend\n\n### 3. Shunting-Yard Algorithm\n- Converts to RPN then evaluates\n- Good for many precedence levels; overkill here\n- Harder to produce clear error messages\n\n**Decision:** Approach 1. The AST adds minimal overhead but provides clean boundaries.\n\n## Architecture\n\n```\nInput string\n    │\n    ▼\n ┌───────┐\n │ Lexer │  string → []Token\n └───┬───┘\n     │\n     ▼\n ┌────────┐\n │ Parser │  []Token → AST (Node)\n └───┬────┘\n     │\n     ▼\n ┌───────────┐\n │ Evaluator │  Node → float64\n └───┬───────┘\n     │\n     ▼\n ┌──────┐\n │ REPL │  read line → eval → print result or error\n └──────┘\n```\n\n## Component Interfaces\n\n### Token (data type)\n\n```go\npackage token\n\ntype Type int\n\nconst (\n    Number Type = iota\n    Plus        // +\n    Minus       // -\n    Star        // *\n    Slash       // /\n    LParen      // (\n    RParen      // )\n    EOF\n)\n\ntype Token struct {\n    Type    Type\n    Literal string  // raw text, e.g. \"3.14\", \"+\"\n    Pos     int     // position in input (for error messages)\n}\n```\n\n### Lexer\n\n```go\npackage lexer\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error)\n```\n\n### AST (data types)\n\n```go\npackage ast\n\n// Node is the interface all AST nodes implement.\ntype Node interface {\n    node() // sealed marker method\n}\n\n// NumberLit represents a numeric literal.\ntype NumberLit struct {\n    Value float64\n}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n    Op    token.Type  // Plus, Minus, Star, Slash\n    Left  Node\n    Right Node\n}\n```\n\n### Parser\n\n```go\npackage parser\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions (mismatched parens, etc.).\nfunc Parse(tokens []token.Token) (ast.Node, error)\n```\n\nGrammar (recursive-descent):\n```\nexpr   → term (('+' | '-') term)*\nterm   → factor (('*' | '/') factor)*\nfactor → NUMBER | '(' expr ')'\n```\n\n### Evaluator\n\n```go\npackage evaluator\n\n// Eval evaluates an AST node and returns the result.\n// Returns an error on division by zero.\nfunc Eval(node ast.Node) (float64, error)\n```\n\n### REPL\n\n```go\npackage repl\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer)\n```\n\n## Package Layout\n\n```\nmatheval/\n├── cmd/\n│   └── matheval/\n│       └── main.go          # entry point, calls repl.Run\n├── token/\n│   └── token.go             # Token type and constants\n├── lexer/\n│   ├── lexer.go             # Tokenize function\n│   └── lexer_test.go\n├── ast/\n│   └── ast.go               # AST node types\n├── parser/\n│   ├── parser.go            # Parse function\n│   └── parser_test.go\n├── evaluator/\n│   ├── evaluator.go         # Eval function\n│   └── evaluator_test.go\n├── repl/\n│   ├── repl.go              # REPL loop\n│   └── repl_test.go\n├── docs/\n│   ├── design.md\n│   └── plan.md\n├── go.mod\n└── README.md\n```\n\n## Error Handling\n- Lexer: returns error for invalid characters (e.g. `@`, `#`)\n- Parser: returns error for syntax errors (unexpected token, mismatched parens)\n- Evaluator: returns error for division by zero\n- REPL: catches any error, prints it, prompts for next input\n\n## Key Design Decisions\n1. **Functional API over structs** — `Tokenize()`, `Parse()`, `Eval()` are stateless functions. No need for struct receivers since there's no configuration or state to carry.\n2. **Sealed AST interface** — unexported marker method prevents external implementations, keeping the node set closed.\n3. **Position tracking in tokens** — enables precise error messages (\"error at position 5\").\n4. **REPL takes io.Reader/io.Writer** — makes it testable without stdin/stdout.\n"
+
+CALLING TOOL: file_read {"path":"docs/plan.md"}
+$$$ "# Math Expression Evaluator — Implementation Plan\n\n## Phase: Implement\n\nSteps are ordered. Each step includes writing the code and its unit tests (TDD).\n\n### Step 1: Project Skeleton\n- `go mod init matheval`\n- Create directory structure: `cmd/matheval/`, `token/`, `lexer/`, `ast/`, `parser/`, `evaluator/`, `repl/`\n- Create placeholder `main.go`\n\n### Step 2: Token Package\n- Define `Type` enum constants\n- Define `Token` struct\n- Add `String()` method on `Type` for debugging\n\n### Step 3: Lexer\n- Implement `Tokenize(input string) ([]Token, error)`\n- Handle: whitespace skipping, number literals (integers and decimals), operators `+-*/`, parentheses `()`, EOF, invalid characters\n- **Tests:** valid expressions, decimal numbers, invalid chars, empty input, whitespace-only\n\n### Step 4: AST Package\n- Define `Node` interface with sealed marker\n- Define `NumberLit` struct\n- Define `BinaryExpr` struct\n\n### Step 5: Parser\n- Implement recursive-descent parser following grammar:\n  - `expr → term (('+' | '-') term)*`\n  - `term → factor (('*' | '/') factor)*`\n  - `factor → NUMBER | '(' expr ')'`\n- Internal parser struct to track position in token slice\n- Return error on: unexpected token, mismatched parens, trailing tokens\n- **Tests:** single number, simple binary, precedence, parentheses, nested parens, error cases\n\n### Step 6: Evaluator\n- Implement `Eval(node ast.Node) (float64, error)`\n- Recursively walk AST\n- Return error on division by zero\n- **Tests:** literals, all 4 operators, nested expressions, division by zero\n\n### Step 7: REPL\n- Implement `Run(r io.Reader, w io.Writer)`\n- Read line, tokenize, parse, evaluate, print result or error\n- Loop until EOF\n- **Tests:** successful expression, error expression, multi-line session\n\n### Step 8: main.go\n- Wire `repl.Run(os.Stdin, os.Stdout)`\n\n### Step 9: Integration Test\n- End-to-end test: feed expression string through all stages, verify result\n- Test edge cases: deeply nested parens, long expressions\n\n### Step 10: Final Commit \u0026 README\n- Write README.md with usage instructions\n- Final commit\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me update the design document with the function definitions feature:
+CALLING TOOL: file_write {"path":"docs/design.md","contents":"# Math Expression Evaluator — Design Document\n\n## Requirements Summary\n- Language: Go\n- Operators: `+`, `-`, `*`, `/` with parentheses\n- Numbers: floating point (e.g. `3.14`, `42`, `0.5`)\n- **Functions: user-defined with `f(x) = x + 1` syntax**\n- Interface: CLI REPL\n- Error handling: print error message, continue REPL\n\n### Function Definition Requirements\n- **Syntax:** `f(x) = x + 1` — name, parenthesized params, `=`, body expression\n- **Multiple params:** `f(x, y) = x + y`\n- **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions\n- **Persistence:** Definitions persist across REPL lines\n- **Cross-calling:** Functions can call other user-defined functions (late binding)\n- **Built-ins:** None\n- **Redefinition:** Not allowed (error)\n- **Output on define:** Print \"defined <name>\"\n\n## Approaches Considered\n\n### 1. Recursive-Descent with AST (chosen)\n- **Lexer → Parser → AST → Evaluator → REPL**\n- Clean separation: each stage is independently testable\n- AST is a reusable intermediate representation\n- Easy to extend (new operators, pretty-printing, optimization)\n- Well-suited for 2 precedence levels + parentheses\n\n### 2. Recursive-Descent with Direct Evaluation\n- Parser evaluates inline — no AST\n- Fewer types, less code\n- Couples parsing and evaluation — harder to test, extend\n\n### 3. Shunting-Yard Algorithm\n- Converts to RPN then evaluates\n- Good for many precedence levels; overkill here\n- Harder to produce clear error messages\n\n**Decision:** Approach 1. The AST adds minimal overhead but provides clean boundaries.\n\n## Architecture\n\n```\nInput string\n    │\n    ▼\n ┌───────┐\n │ Lexer │  string → []Token\n └───┬───┘\n     │\n     ▼\n ┌────────┐\n │ Parser │  []Token → Statement (ExprStmt | FuncDef)\n └───┬────┘\n     │\n     ▼\n ┌───────────┐\n │ Evaluator │  stateful: function registry + expression evaluation\n └───┬───────┘\n     │\n     ▼\n ┌──────┐\n │ REPL │  read line → parse → route (define or eval) → print\n └──────┘\n```\n\n## Component Interfaces\n\n### Token (data type)\n\n```go\npackage token\n\ntype Type int\n\nconst (\n    Number Type = iota // numeric literal\n    Plus               // +\n    Minus              // -\n    Star               // *\n    Slash              // /\n    LParen             // (\n    RParen             // )\n    Ident              // identifier (e.g. f, x, myFunc)\n    Comma              // ,\n    Equals             // =\n    EOF                // end of input\n)\n\ntype Token struct {\n    Type    Type\n    Literal string  // raw text, e.g. \"3.14\", \"+\", \"f\"\n    Pos     int     // position in input (for error messages)\n}\n```\n\n### Lexer\n\n```go\npackage lexer\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\n// Recognizes: numbers, operators, parens, identifiers, comma, equals.\nfunc Tokenize(input string) ([]token.Token, error)\n```\n\n### AST (data types)\n\n```go\npackage ast\n\n// Node is the interface all expression AST nodes implement.\ntype Node interface {\n    node() // sealed marker method\n}\n\n// NumberLit represents a numeric literal.\ntype NumberLit struct {\n    Value float64\n}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n    Op    token.Type  // Plus, Minus, Star, Slash\n    Left  Node\n    Right Node\n}\n\n// Ident represents a variable reference (function parameter).\ntype Ident struct {\n    Name string\n}\n\n// FuncCall represents a function call (e.g. f(1+2, 3)).\ntype FuncCall struct {\n    Name string\n    Args []Node\n}\n\n// Statement is the interface for top-level parsed constructs.\ntype Statement interface {\n    stmt() // sealed marker method\n}\n\n// ExprStmt wraps an expression used as a statement.\ntype ExprStmt struct {\n    Expr Node\n}\n\n// FuncDef represents a function definition: name(params) = body\ntype FuncDef struct {\n    Name   string\n    Params []string\n    Body   Node\n}\n```\n\n### Parser\n\n```go\npackage parser\n\n// Parse converts a slice of tokens into an expression AST.\n// Kept for backward compatibility.\nfunc Parse(tokens []token.Token) (ast.Node, error)\n\n// ParseLine converts a slice of tokens into a Statement.\n// Distinguishes function definitions from expressions.\nfunc ParseLine(tokens []token.Token) (ast.Statement, error)\n```\n\nGrammar (extended):\n```\nline     → funcdef | expr\nfuncdef  → IDENT '(' params ')' '=' expr\nparams   → IDENT (',' IDENT)*\nexpr     → term (('+' | '-') term)*\nterm     → factor (('*' | '/') factor)*\nfactor   → NUMBER | IDENT '(' args ')' | IDENT | '(' expr ')'\nargs     → expr (',' expr)*\n```\n\n**Definition detection:** Scan token stream for `Equals` token. If present → parse as function definition. If absent → parse as expression. This works because `=` is not valid in expressions.\n\n### Evaluator\n\n```go\npackage evaluator\n\n// Evaluator holds function definitions and evaluates expressions.\ntype Evaluator struct {\n    funcs map[string]*ast.FuncDef\n}\n\n// New creates a new Evaluator with an empty function registry.\nfunc New() *Evaluator\n\n// Define registers a function definition.\n// Returns an error if a function with the same name is already defined.\nfunc (e *Evaluator) Define(def *ast.FuncDef) error\n\n// Eval evaluates an expression AST node.\n// env provides variable bindings (function parameters).\n// Pass nil for top-level evaluation.\nfunc (e *Evaluator) Eval(node ast.Node, env map[string]float64) (float64, error)\n```\n\n**Function call evaluation:**\n1. Look up function name in registry\n2. Evaluate each argument expression in caller's environment\n3. Check argument count matches parameter count\n4. Create new environment: `param[i] → argValue[i]`\n5. Evaluate function body in new environment\n\n**Late binding:** Function body references are resolved at call time, not definition time. This naturally supports cross-function calls as long as the called function is defined before the call is evaluated.\n\n### REPL\n\n```go\npackage repl\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\n// Maintains function registry across lines.\nfunc Run(r io.Reader, w io.Writer)\n```\n\n**Line processing flow:**\n1. Tokenize line\n2. `ParseLine()` → `Statement`\n3. Switch on statement type:\n   - `*ast.FuncDef` → `evaluator.Define(def)`, print \"defined <name>\"\n   - `*ast.ExprStmt` → `evaluator.Eval(expr, nil)`, print result\n\n## Package Layout\n\n```\nmatheval/\n├── cmd/\n│   └── matheval/\n│       └── main.go          # entry point, calls repl.Run\n├── token/\n│   └── token.go             # Token type and constants\n├── lexer/\n│   ├── lexer.go             # Tokenize function\n│   └── lexer_test.go\n├── ast/\n│   └── ast.go               # AST node types + Statement types\n├── parser/\n│   ├── parser.go            # Parse + ParseLine functions\n│   └── parser_test.go\n├── evaluator/\n│   ├── evaluator.go         # Evaluator struct with Define + Eval\n│   └── evaluator_test.go\n├── repl/\n│   ├── repl.go              # REPL loop with state\n│   └── repl_test.go\n├── docs/\n│   ├── design.md\n│   └── plan.md\n├── go.mod\n└── README.md\n```\n\n## Error Handling\n- Lexer: returns error for invalid characters (e.g. `@`, `#`)\n- Parser: returns error for syntax errors (unexpected token, mismatched parens, malformed definitions)\n- Evaluator: returns error for division by zero, undefined function, undefined variable, argument count mismatch, function redefinition\n- REPL: catches any error, prints it, prompts for next input\n\n## Key Design Decisions\n1. **Statement vs Node separation** — `Statement` interface separates top-level constructs (definitions vs expressions) from expression nodes. This keeps the expression evaluator clean.\n2. **Stateful Evaluator struct** — replaces the previous stateless `Eval()` function. Required to hold the function registry. The `Eval` method still takes an explicit environment for testability.\n3. **Late binding** — function bodies reference other functions by name; resolved at call time. Simple and correct.\n4. **Backward-compatible Parse()** — existing `Parse()` function kept. New `ParseLine()` added for the REPL.\n5. **Sealed AST interface** — unexported marker method prevents external implementations, keeping the node set closed.\n6. **Position tracking in tokens** — enables precise error messages.\n7. **REPL takes io.Reader/io.Writer** — makes it testable without stdin/stdout.\n8. **Definition detection via Equals scan** — simple and unambiguous since `=` cannot appear in expressions.\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.2","summary":"Design complete. Key decisions:\n\n1. **Token layer:** Add Ident, Comma, Equals token types.\n2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.\n3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.\n4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.\n5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.\n6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.\n\nDesign document updated at docs/design.md. Ready for Planning phase upon approval."}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: The **Research** and **Design** phases are complete. Here's a summary of the design:
+
+### Design Summary
+
+**6 layers need changes:**
+
+| Layer | Change |
+|-------|--------|
+| **Token** | Add `Ident`, `Comma`, `Equals` token types |
+| **Lexer** | Recognize identifiers, `,`, `=` |
+| **AST** | Add `Ident`, `FuncCall` expression nodes; add `Statement` interface with `ExprStmt` and `FuncDef` |
+| **Parser** | New `ParseLine()` → `Statement`; detect definitions by `=` token; extend `factor()` for identifiers and function calls |
+| **Evaluator** | Stateful `Evaluator` struct with function registry; `Define()` + `Eval(node, env)` methods |
+| **REPL** | Maintain `Evaluator` across lines; route definitions vs expressions |
+
+**Key design choice:** Introduced a `Statement` interface to cleanly separate "what kind of input line is this?" from expression evaluation. This avoids polluting the expression `Node` tree with definition constructs.
+
+Full design document is at `docs/design.md`. Shall I proceed to the **Planning** phase?
+## AGENT NO ITEMS
+1: update repl to support function definitions - IN PROGRESS user false
+	1.1: Research: Understand requirements for function definitions - DONE assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+	SUMMARY: **Requirements gathered:**
+	
+	1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression
+	2. **Multiple params:** Yes — `f(x, y) = x + y`
+	3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`
+	4. **Persistence:** Definitions persist across REPL lines (stateful function registry)
+	5. **Cross-calling:** Functions can call other user-defined functions (late binding)
+	6. **Built-ins:** None
+	7. **Redefinition:** Not allowed — attempting to redefine is an error
+	8. **Output on define:** Print confirmation like "defined f"
+	
+	**Current architecture (5 packages + main):**
+	- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF
+	- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support
+	- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`
+	- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)
+	- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless
+	- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper
+	
+	**Key gaps to fill:**
+	- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types
+	- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes
+	- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists
+	- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context
+	- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output
+		1.1.1: What syntax should function definitions use? - DONE user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		SUMMARY: Syntax: `f(x) = x + 1`
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - DONE user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		SUMMARY: Yes, multiple parameters supported: `f(x, y) = x + y`
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - DONE user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		SUMMARY: Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression.
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - DONE user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		SUMMARY: Yes, definitions persist across REPL lines. REPL needs stateful function registry.
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - DONE user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		SUMMARY: Yes, functions can call other user-defined functions. Implies late binding during evaluation.
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - DONE user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		SUMMARY: No built-in functions. Only user-defined functions.
+			 - user: no
+		1.1.7: Can functions be redefined? - DONE user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		SUMMARY: No, functions cannot be redefined. Attempting to redefine should be an error.
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - DONE user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+		SUMMARY: Print confirmation message when a function is defined (e.g. "defined f").
+			 - user: print that new function with given name was defined
+	1.2: Design: Solution architecture for function definitions - DONE assistant true
+	Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.
+	SUMMARY: Design complete. Key decisions:
+	
+	1. **Token layer:** Add Ident, Comma, Equals token types.
+	2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.
+	3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.
+	4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.
+	5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.
+	6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.
+	
+	Design document updated at docs/design.md. Ready for Planning phase upon approval.
+		1.2.1: Design token layer changes - DONE assistant false
+		New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.
+		SUMMARY: New token types: Ident, Comma, Equals. No changes to existing tokens.
+		1.2.2: Design AST layer changes - DONE assistant false
+		New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.
+		SUMMARY: New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes.
+		1.2.3: Design lexer changes - DONE assistant false
+		Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.
+		SUMMARY: Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals.
+		1.2.4: Design parser changes - DONE assistant false
+		Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.
+		SUMMARY: Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility.
+		1.2.5: Design evaluator changes - DONE assistant false
+		Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.
+		SUMMARY: Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call.
+		1.2.6: Design REPL changes - DONE assistant false
+		REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.
+		SUMMARY: REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print.
+-- YOU START WORKING 1
+YOU 1 update repl to support function definitions: looks good
+-- YOU END WORKING
+## USER NO ITEMS
+1: update repl to support function definitions - IN PROGRESS assistant false
+	 - user: looks good
+	1.1: Research: Understand requirements for function definitions - DONE assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+	SUMMARY: **Requirements gathered:**
+	
+	1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression
+	2. **Multiple params:** Yes — `f(x, y) = x + y`
+	3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`
+	4. **Persistence:** Definitions persist across REPL lines (stateful function registry)
+	5. **Cross-calling:** Functions can call other user-defined functions (late binding)
+	6. **Built-ins:** None
+	7. **Redefinition:** Not allowed — attempting to redefine is an error
+	8. **Output on define:** Print confirmation like "defined f"
+	
+	**Current architecture (5 packages + main):**
+	- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF
+	- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support
+	- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`
+	- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)
+	- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless
+	- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper
+	
+	**Key gaps to fill:**
+	- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types
+	- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes
+	- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists
+	- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context
+	- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output
+		1.1.1: What syntax should function definitions use? - DONE user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		SUMMARY: Syntax: `f(x) = x + 1`
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - DONE user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		SUMMARY: Yes, multiple parameters supported: `f(x, y) = x + y`
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - DONE user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		SUMMARY: Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression.
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - DONE user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		SUMMARY: Yes, definitions persist across REPL lines. REPL needs stateful function registry.
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - DONE user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		SUMMARY: Yes, functions can call other user-defined functions. Implies late binding during evaluation.
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - DONE user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		SUMMARY: No built-in functions. Only user-defined functions.
+			 - user: no
+		1.1.7: Can functions be redefined? - DONE user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		SUMMARY: No, functions cannot be redefined. Attempting to redefine should be an error.
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - DONE user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+		SUMMARY: Print confirmation message when a function is defined (e.g. "defined f").
+			 - user: print that new function with given name was defined
+	1.2: Design: Solution architecture for function definitions - DONE assistant true
+	Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.
+	SUMMARY: Design complete. Key decisions:
+	
+	1. **Token layer:** Add Ident, Comma, Equals token types.
+	2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.
+	3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.
+	4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.
+	5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.
+	6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.
+	
+	Design document updated at docs/design.md. Ready for Planning phase upon approval.
+		1.2.1: Design token layer changes - DONE assistant false
+		New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.
+		SUMMARY: New token types: Ident, Comma, Equals. No changes to existing tokens.
+		1.2.2: Design AST layer changes - DONE assistant false
+		New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.
+		SUMMARY: New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes.
+		1.2.3: Design lexer changes - DONE assistant false
+		Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.
+		SUMMARY: Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals.
+		1.2.4: Design parser changes - DONE assistant false
+		Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.
+		SUMMARY: Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility.
+		1.2.5: Design evaluator changes - DONE assistant false
+		Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.
+		SUMMARY: Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call.
+		1.2.6: Design REPL changes - DONE assistant false
+		REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.
+		SUMMARY: REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print.
+-- AGENT START WORKING 1
+--- STOP_REASON: tool_use
+AI: 
+
+I see the Design phase is complete and approved. Let me review the design document and current codebase to create the detailed implementation plan.
+CALLING TOOL: file_read {"path":"docs/design.md"}
+$$$ "# Math Expression Evaluator — Design Document\n\n## Requirements Summary\n- Language: Go\n- Operators: `+`, `-`, `*`, `/` with parentheses\n- Numbers: floating point (e.g. `3.14`, `42`, `0.5`)\n- **Functions: user-defined with `f(x) = x + 1` syntax**\n- Interface: CLI REPL\n- Error handling: print error message, continue REPL\n\n### Function Definition Requirements\n- **Syntax:** `f(x) = x + 1` — name, parenthesized params, `=`, body expression\n- **Multiple params:** `f(x, y) = x + y`\n- **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions\n- **Persistence:** Definitions persist across REPL lines\n- **Cross-calling:** Functions can call other user-defined functions (late binding)\n- **Built-ins:** None\n- **Redefinition:** Not allowed (error)\n- **Output on define:** Print \"defined \u003cname\u003e\"\n\n## Approaches Considered\n\n### 1. Recursive-Descent with AST (chosen)\n- **Lexer → Parser → AST → Evaluator → REPL**\n- Clean separation: each stage is independently testable\n- AST is a reusable intermediate representation\n- Easy to extend (new operators, pretty-printing, optimization)\n- Well-suited for 2 precedence levels + parentheses\n\n### 2. Recursive-Descent with Direct Evaluation\n- Parser evaluates inline — no AST\n- Fewer types, less code\n- Couples parsing and evaluation — harder to test, extend\n\n### 3. Shunting-Yard Algorithm\n- Converts to RPN then evaluates\n- Good for many precedence levels; overkill here\n- Harder to produce clear error messages\n\n**Decision:** Approach 1. The AST adds minimal overhead but provides clean boundaries.\n\n## Architecture\n\n```\nInput string\n    │\n    ▼\n ┌───────┐\n │ Lexer │  string → []Token\n └───┬───┘\n     │\n     ▼\n ┌────────┐\n │ Parser │  []Token → Statement (ExprStmt | FuncDef)\n └───┬────┘\n     │\n     ▼\n ┌───────────┐\n │ Evaluator │  stateful: function registry + expression evaluation\n └───┬───────┘\n     │\n     ▼\n ┌──────┐\n │ REPL │  read line → parse → route (define or eval) → print\n └──────┘\n```\n\n## Component Interfaces\n\n### Token (data type)\n\n```go\npackage token\n\ntype Type int\n\nconst (\n    Number Type = iota // numeric literal\n    Plus               // +\n    Minus              // -\n    Star               // *\n    Slash              // /\n    LParen             // (\n    RParen             // )\n    Ident              // identifier (e.g. f, x, myFunc)\n    Comma              // ,\n    Equals             // =\n    EOF                // end of input\n)\n\ntype Token struct {\n    Type    Type\n    Literal string  // raw text, e.g. \"3.14\", \"+\", \"f\"\n    Pos     int     // position in input (for error messages)\n}\n```\n\n### Lexer\n\n```go\npackage lexer\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\n// Recognizes: numbers, operators, parens, identifiers, comma, equals.\nfunc Tokenize(input string) ([]token.Token, error)\n```\n\n### AST (data types)\n\n```go\npackage ast\n\n// Node is the interface all expression AST nodes implement.\ntype Node interface {\n    node() // sealed marker method\n}\n\n// NumberLit represents a numeric literal.\ntype NumberLit struct {\n    Value float64\n}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n    Op    token.Type  // Plus, Minus, Star, Slash\n    Left  Node\n    Right Node\n}\n\n// Ident represents a variable reference (function parameter).\ntype Ident struct {\n    Name string\n}\n\n// FuncCall represents a function call (e.g. f(1+2, 3)).\ntype FuncCall struct {\n    Name string\n    Args []Node\n}\n\n// Statement is the interface for top-level parsed constructs.\ntype Statement interface {\n    stmt() // sealed marker method\n}\n\n// ExprStmt wraps an expression used as a statement.\ntype ExprStmt struct {\n    Expr Node\n}\n\n// FuncDef represents a function definition: name(params) = body\ntype FuncDef struct {\n    Name   string\n    Params []string\n    Body   Node\n}\n```\n\n### Parser\n\n```go\npackage parser\n\n// Parse converts a slice of tokens into an expression AST.\n// Kept for backward compatibility.\nfunc Parse(tokens []token.Token) (ast.Node, error)\n\n// ParseLine converts a slice of tokens into a Statement.\n// Distinguishes function definitions from expressions.\nfunc ParseLine(tokens []token.Token) (ast.Statement, error)\n```\n\nGrammar (extended):\n```\nline     → funcdef | expr\nfuncdef  → IDENT '(' params ')' '=' expr\nparams   → IDENT (',' IDENT)*\nexpr     → term (('+' | '-') term)*\nterm     → factor (('*' | '/') factor)*\nfactor   → NUMBER | IDENT '(' args ')' | IDENT | '(' expr ')'\nargs     → expr (',' expr)*\n```\n\n**Definition detection:** Scan token stream for `Equals` token. If present → parse as function definition. If absent → parse as expression. This works because `=` is not valid in expressions.\n\n### Evaluator\n\n```go\npackage evaluator\n\n// Evaluator holds function definitions and evaluates expressions.\ntype Evaluator struct {\n    funcs map[string]*ast.FuncDef\n}\n\n// New creates a new Evaluator with an empty function registry.\nfunc New() *Evaluator\n\n// Define registers a function definition.\n// Returns an error if a function with the same name is already defined.\nfunc (e *Evaluator) Define(def *ast.FuncDef) error\n\n// Eval evaluates an expression AST node.\n// env provides variable bindings (function parameters).\n// Pass nil for top-level evaluation.\nfunc (e *Evaluator) Eval(node ast.Node, env map[string]float64) (float64, error)\n```\n\n**Function call evaluation:**\n1. Look up function name in registry\n2. Evaluate each argument expression in caller's environment\n3. Check argument count matches parameter count\n4. Create new environment: `param[i] → argValue[i]`\n5. Evaluate function body in new environment\n\n**Late binding:** Function body references are resolved at call time, not definition time. This naturally supports cross-function calls as long as the called function is defined before the call is evaluated.\n\n### REPL\n\n```go\npackage repl\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\n// Maintains function registry across lines.\nfunc Run(r io.Reader, w io.Writer)\n```\n\n**Line processing flow:**\n1. Tokenize line\n2. `ParseLine()` → `Statement`\n3. Switch on statement type:\n   - `*ast.FuncDef` → `evaluator.Define(def)`, print \"defined \u003cname\u003e\"\n   - `*ast.ExprStmt` → `evaluator.Eval(expr, nil)`, print result\n\n## Package Layout\n\n```\nmatheval/\n├── cmd/\n│   └── matheval/\n│       └── main.go          # entry point, calls repl.Run\n├── token/\n│   └── token.go             # Token type and constants\n├── lexer/\n│   ├── lexer.go             # Tokenize function\n│   └── lexer_test.go\n├── ast/\n│   └── ast.go               # AST node types + Statement types\n├── parser/\n│   ├── parser.go            # Parse + ParseLine functions\n│   └── parser_test.go\n├── evaluator/\n│   ├── evaluator.go         # Evaluator struct with Define + Eval\n│   └── evaluator_test.go\n├── repl/\n│   ├── repl.go              # REPL loop with state\n│   └── repl_test.go\n├── docs/\n│   ├── design.md\n│   └── plan.md\n├── go.mod\n└── README.md\n```\n\n## Error Handling\n- Lexer: returns error for invalid characters (e.g. `@`, `#`)\n- Parser: returns error for syntax errors (unexpected token, mismatched parens, malformed definitions)\n- Evaluator: returns error for division by zero, undefined function, undefined variable, argument count mismatch, function redefinition\n- REPL: catches any error, prints it, prompts for next input\n\n## Key Design Decisions\n1. **Statement vs Node separation** — `Statement` interface separates top-level constructs (definitions vs expressions) from expression nodes. This keeps the expression evaluator clean.\n2. **Stateful Evaluator struct** — replaces the previous stateless `Eval()` function. Required to hold the function registry. The `Eval` method still takes an explicit environment for testability.\n3. **Late binding** — function bodies reference other functions by name; resolved at call time. Simple and correct.\n4. **Backward-compatible Parse()** — existing `Parse()` function kept. New `ParseLine()` added for the REPL.\n5. **Sealed AST interface** — unexported marker method prevents external implementations, keeping the node set closed.\n6. **Position tracking in tokens** — enables precise error messages.\n7. **REPL takes io.Reader/io.Writer** — makes it testable without stdin/stdout.\n8. **Definition detection via Equals scan** — simple and unambiguous since `=` cannot appear in expressions.\n"
+
+CALLING TOOL: dir_list {"name":"."}
+$$$ {"entries":[{"name":".git","is_dir":true},{"name":"README.md","is_dir":false},{"name":"ast","is_dir":true},{"name":"cmd","is_dir":true},{"name":"docs","is_dir":true},{"name":"evaluator","is_dir":true},{"name":"go.mod","is_dir":false},{"name":"integration_test.go","is_dir":false},{"name":"lexer","is_dir":true},{"name":"matheval","is_dir":false},{"name":"parser","is_dir":true},{"name":"repl","is_dir":true},{"name":"sessions","is_dir":true},{"name":"token","is_dir":true}]}
+
+--- STOP_REASON: tool_use
+AI: Let me read all current source files to understand the existing implementation:
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer.go"}
+$$$ "package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i \u003c len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch \u003e= '0' \u0026\u0026 ch \u003c= '9'\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast.go"}
+$$$ "package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser.go"}
+$$$ "package parser\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strconv\"\n)\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions.\nfunc Parse(tokens []token.Token) (ast.Node, error) {\n\tp := \u0026parser{tokens: tokens}\n\tnode, err := p.expr()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\t// Ensure all tokens have been consumed (only EOF remains).\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n\treturn node, nil\n}\n\n// parser holds the state for a single parse operation.\ntype parser struct {\n\ttokens []token.Token\n\tpos    int\n}\n\n// current returns the token at the current position.\nfunc (p *parser) current() token.Token {\n\tif p.pos \u003e= len(p.tokens) {\n\t\treturn token.Token{Type: token.EOF}\n\t}\n\treturn p.tokens[p.pos]\n}\n\n// advance moves to the next token and returns the previous one.\nfunc (p *parser) advance() token.Token {\n\ttok := p.current()\n\tp.pos++\n\treturn tok\n}\n\n// expect consumes a token of the given type or returns an error.\nfunc (p *parser) expect(typ token.Type) (token.Token, error) {\n\ttok := p.current()\n\tif tok.Type != typ {\n\t\treturn tok, fmt.Errorf(\"expected %v but got %v at position %d\", typ, tok.Type, tok.Pos)\n\t}\n\tp.advance()\n\treturn tok, nil\n}\n\n// expr → term (('+' | '-') term)*\nfunc (p *parser) expr() (ast.Node, error) {\n\tleft, err := p.term()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Plus || p.current().Type == token.Minus {\n\t\top := p.advance()\n\t\tright, err := p.term()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// term → factor (('*' | '/') factor)*\nfunc (p *parser) term() (ast.Node, error) {\n\tleft, err := p.factor()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Star || p.current().Type == token.Slash {\n\t\top := p.advance()\n\t\tright, err := p.factor()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// factor → NUMBER | '(' expr ')'\nfunc (p *parser) factor() (ast.Node, error) {\n\ttok := p.current()\n\tswitch tok.Type {\n\tcase token.Number:\n\t\tp.advance()\n\t\tval, err := strconv.ParseFloat(tok.Literal, 64)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"invalid number %q at position %d: %w\", tok.Literal, tok.Pos, err)\n\t\t}\n\t\treturn \u0026ast.NumberLit{Value: val}, nil\n\n\tcase token.LParen:\n\t\tp.advance() // consume '('\n\t\tnode, err := p.expr()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\treturn nil, fmt.Errorf(\"missing closing parenthesis at position %d\", p.current().Pos)\n\t\t}\n\t\treturn node, nil\n\n\tdefault:\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator.go"}
+$$$ "package evaluator\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n)\n\n// Eval evaluates an AST node and returns the result.\n// Returns an error on division by zero.\nfunc Eval(node ast.Node) (float64, error) {\n\tswitch n := node.(type) {\n\tcase *ast.NumberLit:\n\t\treturn n.Value, nil\n\n\tcase *ast.BinaryExpr:\n\t\tleft, err := Eval(n.Left)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\t\tright, err := Eval(n.Right)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\n\t\tswitch n.Op {\n\t\tcase token.Plus:\n\t\t\treturn left + right, nil\n\t\tcase token.Minus:\n\t\t\treturn left - right, nil\n\t\tcase token.Star:\n\t\t\treturn left * right, nil\n\t\tcase token.Slash:\n\t\t\tif right == 0 {\n\t\t\t\treturn 0, fmt.Errorf(\"division by zero\")\n\t\t\t}\n\t\t\treturn left / right, nil\n\t\tdefault:\n\t\t\treturn 0, fmt.Errorf(\"unknown operator: %v\", n.Op)\n\t\t}\n\n\tdefault:\n\t\treturn 0, fmt.Errorf(\"unknown node type: %T\", node)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl.go"}
+$$$ "package repl\n\nimport (\n\t\"bufio\"\n\t\"fmt\"\n\t\"io\"\n\t\"math\"\n\t\"strings\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\nconst prompt = \"\u003e\u003e \"\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer) {\n\tscanner := bufio.NewScanner(r)\n\tfmt.Fprint(w, prompt)\n\n\tfor scanner.Scan() {\n\t\tline := strings.TrimSpace(scanner.Text())\n\t\tif line == \"\" {\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\tresult, err := evalLine(line)\n\t\tif err != nil {\n\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t} else {\n\t\t\tfmt.Fprintln(w, formatResult(result))\n\t\t}\n\n\t\tfmt.Fprint(w, prompt)\n\t}\n}\n\n// evalLine tokenizes, parses, and evaluates a single expression string.\nfunc evalLine(line string) (float64, error) {\n\ttokens, err := lexer.Tokenize(line)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\treturn evaluator.Eval(tree)\n}\n\n// formatResult formats a float64 for display.\n// Whole numbers are printed without decimal points.\nfunc formatResult(val float64) string {\n\tif val == math.Trunc(val) \u0026\u0026 !math.IsInf(val, 0) \u0026\u0026 !math.IsNaN(val) {\n\t\treturn fmt.Sprintf(\"%g\", val)\n\t}\n\treturn fmt.Sprintf(\"%g\", val)\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me read the existing tests:
+CALLING TOOL: file_read {"path":"lexer/lexer_test.go"}
+$$$ "package lexer\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestTokenizeEmpty(t *testing.T) {\n\ttokens, err := Tokenize(\"\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeWhitespaceOnly(t *testing.T) {\n\ttokens, err := Tokenize(\"   \\t\\n  \")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeSingleNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"42\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeDecimalNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"3.14\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"3.14\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeLeadingDotNumber(t *testing.T) {\n\ttokens, err := Tokenize(\".5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \".5\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeOperators(t *testing.T) {\n\ttokens, err := Tokenize(\"+-*/\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 0},\n\t\t{Type: token.Minus, Literal: \"-\", Pos: 1},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 2},\n\t\t{Type: token.Slash, Literal: \"/\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeParens(t *testing.T) {\n\ttokens, err := Tokenize(\"()\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 1},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFullExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"(1 + 2.5) * 3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 1},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2.5\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 10},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeNoSpaces(t *testing.T) {\n\ttokens, err := Tokenize(\"1+2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeInvalidCharacter(t *testing.T) {\n\t_, err := Tokenize(\"1 + @\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleInvalidCharacters(t *testing.T) {\n\t_, err := Tokenize(\"1 \u0026 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleDecimals(t *testing.T) {\n\t// \"1.2.3\" — the lexer should read \"1.2\" as a number, then \".3\" as another number\n\ttokens, err := Tokenize(\"1.2.3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1.2\", Pos: 0},\n\t\t{Type: token.Number, Literal: \".3\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\n// assertTokens is a test helper that compares two token slices.\nfunc assertTokens(t *testing.T, want, got []token.Token) {\n\tt.Helper()\n\tif len(want) != len(got) {\n\t\tt.Fatalf(\"token count: want %d, got %d\\nwant: %v\\ngot:  %v\", len(want), len(got), want, got)\n\t}\n\tfor i := range want {\n\t\tif want[i].Type != got[i].Type {\n\t\t\tt.Errorf(\"token[%d].Type: want %v, got %v\", i, want[i].Type, got[i].Type)\n\t\t}\n\t\tif want[i].Literal != got[i].Literal {\n\t\t\tt.Errorf(\"token[%d].Literal: want %q, got %q\", i, want[i].Literal, got[i].Literal)\n\t\t}\n\t\tif want[i].Pos != got[i].Pos {\n\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser_test.go"}
+$$$ "package parser\n\nimport (\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// helper: tokenize inline for concise tests\nfunc tokens(toks ...token.Token) []token.Token {\n\treturn toks\n}\n\nfunc tok(typ token.Type, lit string, pos int) token.Token {\n\treturn token.Token{Type: typ, Literal: lit, Pos: pos}\n}\n\n// --- Success cases ---\n\nfunc TestParseSingleNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"42\", 0),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 42 {\n\t\tt.Fatalf(\"expected 42, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseDecimalNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"3.14\", 0),\n\t\ttok(token.EOF, \"\", 4),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 3.14 {\n\t\tt.Fatalf(\"expected 3.14, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseAddition(t *testing.T) {\n\t// 1 + 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseSubtraction(t *testing.T) {\n\t// 5 - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"5\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 5)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseMultiplication(t *testing.T) {\n\t// 2 * 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 2)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseDivision(t *testing.T) {\n\t// 6 / 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"6\", 0),\n\t\ttok(token.Slash, \"/\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 6)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParsePrecedence(t *testing.T) {\n\t// 1 + 2 * 3  →  1 + (2 * 3)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root should be Plus\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\t// Right should be Star\n\tright, ok := expr.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", expr.Right)\n\t}\n\tif right.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", right.Op)\n\t}\n\tassertNumber(t, right.Left, 2)\n\tassertNumber(t, right.Right, 3)\n}\n\nfunc TestParsePrecedenceMulFirst(t *testing.T) {\n\t// 2 * 3 + 1  →  (2 * 3) + 1\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.Plus, \"+\", 6),\n\t\ttok(token.Number, \"1\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 2)\n\tassertNumber(t, left.Right, 3)\n\tassertNumber(t, expr.Right, 1)\n}\n\nfunc TestParseLeftAssociativity(t *testing.T) {\n\t// 1 - 2 - 3  →  (1 - 2) - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Minus, \"-\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 - 2) - 3\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseParentheses(t *testing.T) {\n\t// (1 + 2) * 3\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.RParen, \")\", 6),\n\t\ttok(token.Star, \"*\", 8),\n\t\ttok(token.Number, \"3\", 10),\n\t\ttok(token.EOF, \"\", 11),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseNestedParentheses(t *testing.T) {\n\t// ((1 + 2))\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.LParen, \"(\", 1),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.Plus, \"+\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.RParen, \")\", 7),\n\t\ttok(token.RParen, \")\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseComplexExpression(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2  →  (1 + (2*3)) - (4/2)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 5),\n\t\ttok(token.Number, \"3\", 6),\n\t\ttok(token.Minus, \"-\", 8),\n\t\ttok(token.Number, \"4\", 10),\n\t\ttok(token.Slash, \"/\", 11),\n\t\ttok(token.Number, \"2\", 12),\n\t\ttok(token.EOF, \"\", 13),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 + (2*3)) - (4/2)\n\troot, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif root.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", root.Op)\n\t}\n\t// Left: 1 + (2*3)\n\tleft, ok := root.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", root.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tmul, ok := left.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", left.Right)\n\t}\n\tif mul.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", mul.Op)\n\t}\n\tassertNumber(t, mul.Left, 2)\n\tassertNumber(t, mul.Right, 3)\n\t// Right: 4/2\n\tdiv, ok := root.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", root.Right)\n\t}\n\tif div.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", div.Op)\n\t}\n\tassertNumber(t, div.Left, 4)\n\tassertNumber(t, div.Right, 2)\n}\n\n// --- Error cases ---\n\nfunc TestParseEmptyInput(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.EOF, \"\", 0),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestParseMissingRParen(t *testing.T) {\n\t// (1 + 2\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.EOF, \"\", 6),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing right paren\")\n\t}\n}\n\nfunc TestParseUnexpectedRParen(t *testing.T) {\n\t// ) 1\n\ttoks := tokens(\n\t\ttok(token.RParen, \")\", 0),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected right paren\")\n\t}\n}\n\nfunc TestParseTrailingOperator(t *testing.T) {\n\t// 1 +\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestParseTrailingTokens(t *testing.T) {\n\t// 1 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Number, \"2\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing tokens\")\n\t}\n}\n\nfunc TestParseConsecutiveOperators(t *testing.T) {\n\t// 1 + * 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Star, \"*\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.EOF, \"\", 7),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestParseEmptyParens(t *testing.T) {\n\t// ()\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.RParen, \")\", 1),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\n// --- Helper ---\n\nfunc assertNumber(t *testing.T, node ast.Node, expected float64) {\n\tt.Helper()\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != expected {\n\t\tt.Fatalf(\"expected %f, got %f\", expected, num.Value)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator_test.go"}
+$$$ "package evaluator\n\nimport (\n\t\"math\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestEvalNumberLit(t *testing.T) {\n\tresult, err := Eval(\u0026ast.NumberLit{Value: 42.5})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 42.5 {\n\t\tt.Fatalf(\"expected 42.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalAddition(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 3 {\n\t\tt.Fatalf(\"expected 3, got %v\", result)\n\t}\n}\n\nfunc TestEvalSubtraction(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Minus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalMultiplication(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Star,\n\t\tLeft:  \u0026ast.NumberLit{Value: 3},\n\t\tRight: \u0026ast.NumberLit{Value: 7},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 21 {\n\t\tt.Fatalf(\"expected 21, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivision(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 2.5 {\n\t\tt.Fatalf(\"expected 2.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZero(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 5},\n\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestEvalNestedExpr(t *testing.T) {\n\t// (1 + 2) * (8 / 4) = 3 * 2 = 6\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 8},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalDeeplyNested(t *testing.T) {\n\t// ((2 + 3) * 4) - (10 / 5) = 20 - 2 = 18\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Minus,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp: token.Star,\n\t\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\t\tOp:    token.Plus,\n\t\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\t\tRight: \u0026ast.NumberLit{Value: 3},\n\t\t\t},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\t\tRight: \u0026ast.NumberLit{Value: 5},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 18 {\n\t\tt.Fatalf(\"expected 18, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZeroInSubExpr(t *testing.T) {\n\t// 1 + (2 / 0) — error should propagate\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:   token.Plus,\n\t\tLeft: \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t\t},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error from sub-expression\")\n\t}\n}\n\nfunc TestEvalFloatingPoint(t *testing.T) {\n\t// 1.5 + 2.3 = 3.8\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1.5},\n\t\tRight: \u0026ast.NumberLit{Value: 2.3},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif math.Abs(result-3.8) \u003e 1e-12 {\n\t\tt.Fatalf(\"expected 3.8, got %v\", result)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl_test.go"}
+$$$ "package repl\n\nimport (\n\t\"bytes\"\n\t\"strings\"\n\t\"testing\"\n)\n\nfunc TestRun_SingleExpression(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n}\n\nfunc TestRun_MultipleExpressions(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n3 * 4\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"12\") {\n\t\tt.Errorf(\"expected output to contain '12', got %q\", got)\n\t}\n}\n\nfunc TestRun_ErrorContinues(t *testing.T) {\n\t// First line has error, second is valid.\n\tin := strings.NewReader(\"1 +\\n2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"5\") {\n\t\tt.Errorf(\"expected output to contain '5' after error recovery, got %q\", got)\n\t}\n}\n\nfunc TestRun_DivisionByZero(t *testing.T) {\n\tin := strings.NewReader(\"1 / 0\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyLine(t *testing.T) {\n\t// Empty lines should be skipped, not cause errors.\n\tin := strings.NewReader(\"\\n1 + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"2\") {\n\t\tt.Errorf(\"expected output to contain '2', got %q\", got)\n\t}\n\t// Should not contain any error.\n\tif strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"empty line should not produce error, got %q\", got)\n\t}\n}\n\nfunc TestRun_Prompt(t *testing.T) {\n\tin := strings.NewReader(\"42\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected prompt '\u003e\u003e ' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_FloatResult(t *testing.T) {\n\tin := strings.NewReader(\"7 / 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3.5\") {\n\t\tt.Errorf(\"expected output to contain '3.5', got %q\", got)\n\t}\n}\n\nfunc TestRun_InvalidCharacter(t *testing.T) {\n\tin := strings.NewReader(\"1 @ 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyInput(t *testing.T) {\n\t// No input at all — just EOF.\n\tin := strings.NewReader(\"\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Should just show the prompt and exit gracefully.\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected at least one prompt, got %q\", got)\n\t}\n}\n\nfunc TestRun_WholeIntegerNoTrailingZeros(t *testing.T) {\n\t// 2 + 3 = 5, should print \"5\" not \"5.000000\".\n\tin := strings.NewReader(\"2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Result line is \"5\\n\" (between prompts).\n\tif !strings.Contains(got, \"5\\n\") {\n\t\tt.Errorf(\"expected '5\\\\n' in output, got %q\", got)\n\t}\n\t// Should not contain \"5.0\"\n\tif strings.Contains(got, \"5.0\") {\n\t\tt.Errorf(\"expected no trailing zeros, got %q\", got)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"integration_test.go"}
+$$$ "package matheval_test\n\nimport (\n\t\"math\"\n\t\"strings\"\n\t\"testing\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\n// eval is a helper that runs the full pipeline: lexer → parser → evaluator.\nfunc eval(t *testing.T, input string) float64 {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\tt.Fatalf(\"lexer error for %q: %v\", input, err)\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"parser error for %q: %v\", input, err)\n\t}\n\tresult, err := evaluator.Eval(tree)\n\tif err != nil {\n\t\tt.Fatalf(\"evaluator error for %q: %v\", input, err)\n\t}\n\treturn result\n}\n\n// evalErr is a helper that expects the full pipeline to return an error.\nfunc evalErr(t *testing.T, input string) error {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\treturn err\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn err\n\t}\n\t_, err = evaluator.Eval(tree)\n\treturn err\n}\n\nfunc assertApprox(t *testing.T, input string, expected, got float64) {\n\tt.Helper()\n\tif math.Abs(expected-got) \u003e 1e-9 {\n\t\tt.Errorf(\"%q: expected %v, got %v\", input, expected, got)\n\t}\n}\n\n// --- Basic arithmetic ---\n\nfunc TestIntegration_SingleNumber(t *testing.T) {\n\tassertApprox(t, \"42\", 42, eval(t, \"42\"))\n}\n\nfunc TestIntegration_DecimalNumber(t *testing.T) {\n\tassertApprox(t, \"3.14\", 3.14, eval(t, \"3.14\"))\n}\n\nfunc TestIntegration_LeadingDot(t *testing.T) {\n\tassertApprox(t, \".5\", 0.5, eval(t, \".5\"))\n}\n\nfunc TestIntegration_Addition(t *testing.T) {\n\tassertApprox(t, \"1 + 2\", 3, eval(t, \"1 + 2\"))\n}\n\nfunc TestIntegration_Subtraction(t *testing.T) {\n\tassertApprox(t, \"10 - 4\", 6, eval(t, \"10 - 4\"))\n}\n\nfunc TestIntegration_Multiplication(t *testing.T) {\n\tassertApprox(t, \"3 * 7\", 21, eval(t, \"3 * 7\"))\n}\n\nfunc TestIntegration_Division(t *testing.T) {\n\tassertApprox(t, \"10 / 4\", 2.5, eval(t, \"10 / 4\"))\n}\n\n// --- Precedence and associativity ---\n\nfunc TestIntegration_PrecedenceMulOverAdd(t *testing.T) {\n\t// 2 + 3 * 4 = 2 + 12 = 14\n\tassertApprox(t, \"2 + 3 * 4\", 14, eval(t, \"2 + 3 * 4\"))\n}\n\nfunc TestIntegration_PrecedenceDivOverSub(t *testing.T) {\n\t// 10 - 6 / 3 = 10 - 2 = 8\n\tassertApprox(t, \"10 - 6 / 3\", 8, eval(t, \"10 - 6 / 3\"))\n}\n\nfunc TestIntegration_LeftAssociativitySub(t *testing.T) {\n\t// 10 - 3 - 2 = (10 - 3) - 2 = 5\n\tassertApprox(t, \"10 - 3 - 2\", 5, eval(t, \"10 - 3 - 2\"))\n}\n\nfunc TestIntegration_LeftAssociativityDiv(t *testing.T) {\n\t// 24 / 4 / 3 = (24 / 4) / 3 = 2\n\tassertApprox(t, \"24 / 4 / 3\", 2, eval(t, \"24 / 4 / 3\"))\n}\n\n// --- Parentheses ---\n\nfunc TestIntegration_ParensOverridePrecedence(t *testing.T) {\n\t// (2 + 3) * 4 = 20\n\tassertApprox(t, \"(2 + 3) * 4\", 20, eval(t, \"(2 + 3) * 4\"))\n}\n\nfunc TestIntegration_NestedParens(t *testing.T) {\n\t// ((1 + 2)) = 3\n\tassertApprox(t, \"((1 + 2))\", 3, eval(t, \"((1 + 2))\"))\n}\n\nfunc TestIntegration_DeeplyNestedParens(t *testing.T) {\n\t// ((((((1 + 2)))))) = 3\n\tassertApprox(t, \"((((((1 + 2))))))\", 3, eval(t, \"((((((1 + 2))))))\"))\n}\n\nfunc TestIntegration_ParensOnBothSides(t *testing.T) {\n\t// (1 + 2) * (3 + 4) = 3 * 7 = 21\n\tassertApprox(t, \"(1 + 2) * (3 + 4)\", 21, eval(t, \"(1 + 2) * (3 + 4)\"))\n}\n\nfunc TestIntegration_ParensNestedComplex(t *testing.T) {\n\t// ((2 + 3) * (4 - 1)) / 5 = (5 * 3) / 5 = 3\n\tassertApprox(t, \"((2 + 3) * (4 - 1)) / 5\", 3, eval(t, \"((2 + 3) * (4 - 1)) / 5\"))\n}\n\n// --- Complex expressions ---\n\nfunc TestIntegration_AllOperators(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2 = 1 + 6 - 2 = 5\n\tassertApprox(t, \"1 + 2 * 3 - 4 / 2\", 5, eval(t, \"1 + 2 * 3 - 4 / 2\"))\n}\n\nfunc TestIntegration_LongChainedAddition(t *testing.T) {\n\t// 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55\n\tassertApprox(t, \"1+2+3+4+5+6+7+8+9+10\", 55, eval(t, \"1+2+3+4+5+6+7+8+9+10\"))\n}\n\nfunc TestIntegration_LongChainedMixed(t *testing.T) {\n\t// 2 * 3 + 4 * 5 - 6 / 2 + 1 = 6 + 20 - 3 + 1 = 24\n\tassertApprox(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\", 24, eval(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\"))\n}\n\n// --- Floating point ---\n\nfunc TestIntegration_FloatArithmetic(t *testing.T) {\n\t// 1.5 + 2.5 = 4.0\n\tassertApprox(t, \"1.5 + 2.5\", 4.0, eval(t, \"1.5 + 2.5\"))\n}\n\nfunc TestIntegration_FloatDivision(t *testing.T) {\n\t// 7 / 2 = 3.5\n\tassertApprox(t, \"7 / 2\", 3.5, eval(t, \"7 / 2\"))\n}\n\nfunc TestIntegration_FloatPrecision(t *testing.T) {\n\t// 0.1 + 0.2 ≈ 0.3 (within tolerance)\n\tassertApprox(t, \"0.1 + 0.2\", 0.3, eval(t, \"0.1 + 0.2\"))\n}\n\n// --- Whitespace variations ---\n\nfunc TestIntegration_NoSpaces(t *testing.T) {\n\tassertApprox(t, \"1+2*3\", 7, eval(t, \"1+2*3\"))\n}\n\nfunc TestIntegration_ExtraSpaces(t *testing.T) {\n\tassertApprox(t, \"  1  +  2  \", 3, eval(t, \"  1  +  2  \"))\n}\n\nfunc TestIntegration_TabsAndSpaces(t *testing.T) {\n\tassertApprox(t, \"1\\t+\\t2\", 3, eval(t, \"1\\t+\\t2\"))\n}\n\n// --- Error cases ---\n\nfunc TestIntegration_DivisionByZero(t *testing.T) {\n\terr := evalErr(t, \"1 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n\tif !strings.Contains(err.Error(), \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_DivisionByZeroInSubExpr(t *testing.T) {\n\terr := evalErr(t, \"1 + 2 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestIntegration_InvalidCharacter(t *testing.T) {\n\terr := evalErr(t, \"1 @ 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenLeft(t *testing.T) {\n\terr := evalErr(t, \"(1 + 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing closing paren\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenRight(t *testing.T) {\n\terr := evalErr(t, \"1 + 2)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected closing paren\")\n\t}\n}\n\nfunc TestIntegration_EmptyParens(t *testing.T) {\n\terr := evalErr(t, \"()\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\nfunc TestIntegration_TrailingOperator(t *testing.T) {\n\terr := evalErr(t, \"1 +\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestIntegration_LeadingOperator(t *testing.T) {\n\terr := evalErr(t, \"* 1\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for leading operator\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveOperators(t *testing.T) {\n\terr := evalErr(t, \"1 + * 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestIntegration_EmptyInput(t *testing.T) {\n\t// Empty string should produce only EOF, parser should error\n\terr := evalErr(t, \"\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveNumbers(t *testing.T) {\n\terr := evalErr(t, \"1 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive numbers without operator\")\n\t}\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me check the main.go file:
+CALLING TOOL: file_read {"path":"cmd/matheval/main.go"}
+$$$ "package main\n\nimport (\n\t\"os\"\n\n\t\"matheval/repl\"\n)\n\nfunc main() {\n\trepl.Run(os.Stdin, os.Stdout)\n}\n"
+
+CALLING TOOL: file_read {"path":"go.mod"}
+$$$ "module matheval\n\ngo 1.23.1\n"
+
+--- STOP_REASON: tool_use
+AI: Good. I now have full context. Let me verify all existing tests pass before planning:
+!!!!! {"command":"cd /root && go test ./... 2>&1"}: you are already in project dir
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t(cached)\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t(cached)\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t(cached)\nok  \tmatheval/token\t(cached)\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass. Now I have complete understanding. Let me create the Plan phase.
+CALLING TOOL: todo_item_add {"items":[{"title":"Plan: Detailed implementation plan for function definitions","description":"Create a step-by-step implementation plan following TDD approach. Changes go bottom-up through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step is small, testable, and maintains backward compatibility.","assignedTo":"assistant","parentId":"1","items":[{"title":"Step 1: Add new token types (Ident, Comma, Equals)","description":"Add Ident, Comma, Equals to token/token.go. Update String() method. No tests needed — pure data types.","assignedTo":"assistant"},{"title":"Step 2: Add new AST nodes (Ident, FuncCall, Statement, ExprStmt, FuncDef)","description":"Add to ast/ast.go: Ident{Name}, FuncCall{Name, Args}, Statement interface with stmt() marker, ExprStmt{Expr}, FuncDef{Name, Params, Body}. Seal the interfaces.","assignedTo":"assistant"},{"title":"Step 3: Update lexer to recognize identifiers, comma, equals","description":"In lexer/lexer.go: add isLetter() helper, add identifier scanning (letter then alphanumeric), add comma and equals to single-char switch. Write tests for: identifiers, comma, equals, mixed expressions like 'f(x) = x + 1', 'f(1, 2)'.","assignedTo":"assistant"},{"title":"Step 4: Update parser — add ParseLine, extend factor for Ident/FuncCall","description":"In parser/parser.go: (1) Extend factor() to handle Ident token — if followed by LParen, parse as FuncCall with comma-separated args, else parse as Ident. (2) Add ParseLine() that detects Equals token to distinguish FuncDef from expression, returns Statement. (3) Keep Parse() backward-compatible. Write tests for: ParseLine with func defs, func calls, ident references, error cases.","assignedTo":"assistant"},{"title":"Step 5: Refactor evaluator to stateful Evaluator struct","description":"In evaluator/evaluator.go: (1) Create Evaluator struct with funcs map. (2) New() constructor. (3) Eval(node, env) method handling NumberLit, BinaryExpr, Ident, FuncCall. (4) Define(def) method with redefinition error. (5) Keep old Eval() function as wrapper for backward compat (calls New().Eval(node,nil)). Update evaluator tests. Add new tests for Ident, FuncCall, Define, error cases.","assignedTo":"assistant"},{"title":"Step 6: Update REPL to use stateful evaluator and ParseLine","description":"In repl/repl.go: (1) Create Evaluator in Run(). (2) Replace Parse() with ParseLine(). (3) Route FuncDef to Define+print, ExprStmt to Eval+print. Update REPL tests to cover function definitions and calls across lines.","assignedTo":"assistant"},{"title":"Step 7: Add integration tests for function features","description":"In integration_test.go: Add tests for full pipeline with functions: define and call, multi-param, cross-function calls, error cases (undefined func, wrong arg count, redefine). Update eval/evalErr helpers to use stateful Evaluator.","assignedTo":"assistant"},{"title":"Step 8: Write plan document to docs/plan.md","description":"Write the detailed plan document to docs/plan.md for reference.","assignedTo":"assistant"}],"parallel":false}]}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me write the plan document:
+CALLING TOOL: file_write {"path":"docs/plan.md","contents":"# Implementation Plan: Function Definitions\n\n## Overview\nBottom-up implementation through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step maintains backward compatibility and follows TDD.\n\n## Steps\n\n### Step 1: Token layer (`token/token.go`)\n- Add `Ident`, `Comma`, `Equals` constants to `Type` enum\n- Update `String()` for new types\n- No tests needed — pure data types\n\n### Step 2: AST layer (`ast/ast.go`)\n- Add `Ident` struct: `Name string`; implements `Node`\n- Add `FuncCall` struct: `Name string`, `Args []Node`; implements `Node`\n- Add `Statement` interface with sealed `stmt()` marker\n- Add `ExprStmt` struct: `Expr Node`; implements `Statement`\n- Add `FuncDef` struct: `Name string`, `Params []string`, `Body Node`; implements `Statement`\n- No tests needed — pure data types\n\n### Step 3: Lexer (`lexer/lexer.go`)\n- Add `isLetter(ch byte) bool` helper\n- Before the single-char switch, add branch: if `isLetter(ch)`, scan identifier (letter then letters/digits), emit `Ident` token\n- Add `','` → `Comma` and `'='` → `Equals` to single-char switch\n- **Tests:** identifiers (`x`, `foo`, `f1`), comma, equals, full definition `f(x) = x + 1`, call `f(1, 2)`, mixed with numbers\n\n### Step 4: Parser (`parser/parser.go`)\n- Extend `factor()`:\n  - `Ident` followed by `LParen` → parse `FuncCall`: consume `(`, parse args as comma-separated exprs, consume `)`\n  - `Ident` not followed by `LParen` → return `&ast.Ident{Name}`\n- Add `parseFuncDef()`: expects `Ident(` params `) = expr`\n- Add `ParseLine(tokens) (Statement, error)`:\n  - Scan for `Equals` token (not inside parens)\n  - If found → `parseFuncDef()` → `*ast.FuncDef`\n  - If not → `expr()` → `*ast.ExprStmt{Expr}`\n- Keep `Parse()` unchanged for backward compat\n- **Tests:** ParseLine for defs and exprs, factor for ident and func call, error cases\n\n### Step 5: Evaluator (`evaluator/evaluator.go`)\n- Add `Evaluator` struct with `funcs map[string]*ast.FuncDef`\n- `New() *Evaluator`\n- `Define(def *ast.FuncDef) error` — error on redefinition\n- `Eval(node ast.Node, env map[string]float64) (float64, error)`:\n  - `*ast.NumberLit` → return value\n  - `*ast.BinaryExpr` → recurse left/right with same env\n  - `*ast.Ident` → lookup in env, error if not found\n  - `*ast.FuncCall` → lookup func, eval args in caller env, bind params, eval body in new env\n- Keep package-level `Eval(node) (float64, error)` as backward-compat wrapper\n- **Tests:** all existing tests still pass, new tests for Ident, FuncCall, Define, errors\n\n### Step 6: REPL (`repl/repl.go`)\n- In `Run()`: create `evaluator.New()` before loop\n- Replace `evalLine()` with inline logic using `ParseLine()`\n- `*ast.FuncDef` → `ev.Define(def)`, print `\"defined <name>\"`\n- `*ast.ExprStmt` → `ev.Eval(stmt.Expr, nil)`, print result\n- **Tests:** define + call across lines, redefine error, undefined func error\n\n### Step 7: Integration tests (`integration_test.go`)\n- Update `eval()`/`evalErr()` helpers to use `Evaluator` struct\n- Add tests:\n  - Define and call single-param function\n  - Define and call multi-param function\n  - Cross-function calls\n  - Nested function calls in expressions\n  - Error: undefined function\n  - Error: wrong argument count\n  - Error: function redefinition\n  - Error: undefined variable\n\n## Backward Compatibility\n- `Parse()` remains unchanged — returns `ast.Node`\n- Package-level `Eval()` remains — wraps `New().Eval(node, nil)`\n- Existing tests must continue to pass at every step\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.3.8","summary":"Plan document written to docs/plan.md"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me present this to the user for approval before moving to implementation.
+CALLING TOOL: todo_item_add_comment {"id":"1","comment":"Plan phase complete. See docs/plan.md for full details. Summary of 7 implementation steps:\n\n1. **Token**: Add Ident, Comma, Equals types\n2. **AST**: Add Ident, FuncCall nodes + Statement interface (ExprStmt, FuncDef)\n3. **Lexer**: Recognize identifiers, comma, equals + tests\n4. **Parser**: Extend factor() for Ident/FuncCall, add ParseLine() for definition detection + tests\n5. **Evaluator**: Refactor to stateful Evaluator struct with Define() and Eval(node, env) + tests\n6. **REPL**: Use stateful evaluator and ParseLine, route defs vs exprs + tests\n7. **Integration tests**: Full pipeline tests for function features\n\nAll steps maintain backward compatibility. Ready to implement upon approval.","assignTo":"user"}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: The **Plan phase** is complete. Here's the summary:
+
+**7 implementation steps**, bottom-up through the stack:
+
+| Step | Layer | Changes |
+|------|-------|---------|
+| 1 | Token | Add `Ident`, `Comma`, `Equals` types |
+| 2 | AST | Add `Ident`, `FuncCall` nodes + `Statement` interface (`ExprStmt`, `FuncDef`) |
+| 3 | Lexer | Recognize identifiers, comma, equals + tests |
+| 4 | Parser | Extend `factor()` for Ident/FuncCall, add `ParseLine()` + tests |
+| 5 | Evaluator | Refactor to stateful `Evaluator` struct with `Define()` and `Eval(node, env)` + tests |
+| 6 | REPL | Use stateful evaluator + `ParseLine`, route defs vs exprs + tests |
+| 7 | Integration | Full pipeline tests for function features |
+
+All steps maintain backward compatibility (existing `Parse()` and `Eval()` functions preserved as wrappers).
+
+Full details in `docs/plan.md`. Shall I proceed to the **Implement** phase?
+1: update repl to support function definitions - IN PROGRESS user false
+	 - user: looks good
+	 - assistant: Plan phase complete. See docs/plan.md for full details. Summary of 7 implementation steps:
+
+1. **Token**: Add Ident, Comma, Equals types
+2. **AST**: Add Ident, FuncCall nodes + Statement interface (ExprStmt, FuncDef)
+3. **Lexer**: Recognize identifiers, comma, equals + tests
+4. **Parser**: Extend factor() for Ident/FuncCall, add ParseLine() for definition detection + tests
+5. **Evaluator**: Refactor to stateful Evaluator struct with Define() and Eval(node, env) + tests
+6. **REPL**: Use stateful evaluator and ParseLine, route defs vs exprs + tests
+7. **Integration tests**: Full pipeline tests for function features
+
+All steps maintain backward compatibility. Ready to implement upon approval.
+	1.1: Research: Understand requirements for function definitions - DONE assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+	SUMMARY: **Requirements gathered:**
+	
+	1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression
+	2. **Multiple params:** Yes — `f(x, y) = x + y`
+	3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`
+	4. **Persistence:** Definitions persist across REPL lines (stateful function registry)
+	5. **Cross-calling:** Functions can call other user-defined functions (late binding)
+	6. **Built-ins:** None
+	7. **Redefinition:** Not allowed — attempting to redefine is an error
+	8. **Output on define:** Print confirmation like "defined f"
+	
+	**Current architecture (5 packages + main):**
+	- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF
+	- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support
+	- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`
+	- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)
+	- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless
+	- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper
+	
+	**Key gaps to fill:**
+	- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types
+	- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes
+	- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists
+	- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context
+	- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output
+		1.1.1: What syntax should function definitions use? - DONE user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		SUMMARY: Syntax: `f(x) = x + 1`
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - DONE user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		SUMMARY: Yes, multiple parameters supported: `f(x, y) = x + y`
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - DONE user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		SUMMARY: Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression.
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - DONE user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		SUMMARY: Yes, definitions persist across REPL lines. REPL needs stateful function registry.
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - DONE user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		SUMMARY: Yes, functions can call other user-defined functions. Implies late binding during evaluation.
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - DONE user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		SUMMARY: No built-in functions. Only user-defined functions.
+			 - user: no
+		1.1.7: Can functions be redefined? - DONE user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		SUMMARY: No, functions cannot be redefined. Attempting to redefine should be an error.
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - DONE user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+		SUMMARY: Print confirmation message when a function is defined (e.g. "defined f").
+			 - user: print that new function with given name was defined
+	1.2: Design: Solution architecture for function definitions - DONE assistant true
+	Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.
+	SUMMARY: Design complete. Key decisions:
+	
+	1. **Token layer:** Add Ident, Comma, Equals token types.
+	2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.
+	3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.
+	4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.
+	5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.
+	6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.
+	
+	Design document updated at docs/design.md. Ready for Planning phase upon approval.
+		1.2.1: Design token layer changes - DONE assistant false
+		New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.
+		SUMMARY: New token types: Ident, Comma, Equals. No changes to existing tokens.
+		1.2.2: Design AST layer changes - DONE assistant false
+		New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.
+		SUMMARY: New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes.
+		1.2.3: Design lexer changes - DONE assistant false
+		Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.
+		SUMMARY: Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals.
+		1.2.4: Design parser changes - DONE assistant false
+		Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.
+		SUMMARY: Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility.
+		1.2.5: Design evaluator changes - DONE assistant false
+		Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.
+		SUMMARY: Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call.
+		1.2.6: Design REPL changes - DONE assistant false
+		REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.
+		SUMMARY: REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print.
+	1.3: Plan: Detailed implementation plan for function definitions - IN PROGRESS assistant false
+	Create a step-by-step implementation plan following TDD approach. Changes go bottom-up through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step is small, testable, and maintains backward compatibility.
+		1.3.1: Step 1: Add new token types (Ident, Comma, Equals) - IN PROGRESS assistant false
+		Add Ident, Comma, Equals to token/token.go. Update String() method. No tests needed — pure data types.
+		1.3.2: Step 2: Add new AST nodes (Ident, FuncCall, Statement, ExprStmt, FuncDef) - IN PROGRESS assistant false
+		Add to ast/ast.go: Ident{Name}, FuncCall{Name, Args}, Statement interface with stmt() marker, ExprStmt{Expr}, FuncDef{Name, Params, Body}. Seal the interfaces.
+		1.3.3: Step 3: Update lexer to recognize identifiers, comma, equals - IN PROGRESS assistant false
+		In lexer/lexer.go: add isLetter() helper, add identifier scanning (letter then alphanumeric), add comma and equals to single-char switch. Write tests for: identifiers, comma, equals, mixed expressions like 'f(x) = x + 1', 'f(1, 2)'.
+		1.3.4: Step 4: Update parser — add ParseLine, extend factor for Ident/FuncCall - IN PROGRESS assistant false
+		In parser/parser.go: (1) Extend factor() to handle Ident token — if followed by LParen, parse as FuncCall with comma-separated args, else parse as Ident. (2) Add ParseLine() that detects Equals token to distinguish FuncDef from expression, returns Statement. (3) Keep Parse() backward-compatible. Write tests for: ParseLine with func defs, func calls, ident references, error cases.
+		1.3.5: Step 5: Refactor evaluator to stateful Evaluator struct - IN PROGRESS assistant false
+		In evaluator/evaluator.go: (1) Create Evaluator struct with funcs map. (2) New() constructor. (3) Eval(node, env) method handling NumberLit, BinaryExpr, Ident, FuncCall. (4) Define(def) method with redefinition error. (5) Keep old Eval() function as wrapper for backward compat (calls New().Eval(node,nil)). Update evaluator tests. Add new tests for Ident, FuncCall, Define, error cases.
+		1.3.6: Step 6: Update REPL to use stateful evaluator and ParseLine - IN PROGRESS assistant false
+		In repl/repl.go: (1) Create Evaluator in Run(). (2) Replace Parse() with ParseLine(). (3) Route FuncDef to Define+print, ExprStmt to Eval+print. Update REPL tests to cover function definitions and calls across lines.
+		1.3.7: Step 7: Add integration tests for function features - IN PROGRESS assistant false
+		In integration_test.go: Add tests for full pipeline with functions: define and call, multi-param, cross-function calls, error cases (undefined func, wrong arg count, redefine). Update eval/evalErr helpers to use stateful Evaluator.
+		1.3.8: Step 8: Write plan document to docs/plan.md - DONE assistant false
+		Write the detailed plan document to docs/plan.md for reference.
+		SUMMARY: Plan document written to docs/plan.md
+-- AGENT START WORKING 1.3.1
+--- STOP_REASON: tool_use
+AI: 
+
+I'll start by examining the current token package to understand the existing structure.
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Small change. I'll add the three new token types: `Ident`, `Comma`, `Equals`.
+CALLING TOOL: file_write {"path":"token/token.go","contents":"package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tIdent              // identifier (e.g. \"f\", \"x\")\n\tComma              // ,\n\tEquals             // =\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase Ident:\n\t\treturn \"Ident\"\n\tcase Comma:\n\t\treturn \",\"\n\tcase Equals:\n\t\treturn \"=\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Let me verify existing tests still pass:
+!!!!! {"command":"cd \"$(pwd)\" && go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"cd \"$(pwd)\" && go test ./... 2>&1"}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t2.095s\nok  \tmatheval/ast\t0.777s\nok  \tmatheval/evaluator\t1.311s\nok  \tmatheval/lexer\t1.698s\nok  \tmatheval/parser\t2.554s\nok  \tmatheval/repl\t2.918s\nok  \tmatheval/token\t3.369s\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass. Let me commit:
+!!!!! {"command":"cd \"$(pwd)\" && git add -A && git commit -m \"token: add Ident, Comma, Equals token types\""}: OK
+CALLING TOOL: bash_command {"command":"cd \"$(pwd)\" && git add -A && git commit -m \"token: add Ident, Comma, Equals token types\""}
+$$$ "[master c3ec07d] token: add Ident, Comma, Equals token types\n 3 files changed, 201 insertions(+), 90 deletions(-)\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.3.1","summary":"Added three new token types to token/token.go: Ident (identifiers), Comma, Equals. Updated String() method. All existing tests pass. Committed."}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: Done. Added `Ident`, `Comma`, `Equals` to the token type enum with corresponding `String()` cases. All existing tests pass. Committed as `c3ec07d`.
+## USER NO ITEMS
+1: update repl to support function definitions - IN PROGRESS user false
+	 - user: looks good
+	 - assistant: Plan phase complete. See docs/plan.md for full details. Summary of 7 implementation steps:
+
+1. **Token**: Add Ident, Comma, Equals types
+2. **AST**: Add Ident, FuncCall nodes + Statement interface (ExprStmt, FuncDef)
+3. **Lexer**: Recognize identifiers, comma, equals + tests
+4. **Parser**: Extend factor() for Ident/FuncCall, add ParseLine() for definition detection + tests
+5. **Evaluator**: Refactor to stateful Evaluator struct with Define() and Eval(node, env) + tests
+6. **REPL**: Use stateful evaluator and ParseLine, route defs vs exprs + tests
+7. **Integration tests**: Full pipeline tests for function features
+
+All steps maintain backward compatibility. Ready to implement upon approval.
+	1.1: Research: Understand requirements for function definitions - DONE assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+	SUMMARY: **Requirements gathered:**
+	
+	1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression
+	2. **Multiple params:** Yes — `f(x, y) = x + y`
+	3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`
+	4. **Persistence:** Definitions persist across REPL lines (stateful function registry)
+	5. **Cross-calling:** Functions can call other user-defined functions (late binding)
+	6. **Built-ins:** None
+	7. **Redefinition:** Not allowed — attempting to redefine is an error
+	8. **Output on define:** Print confirmation like "defined f"
+	
+	**Current architecture (5 packages + main):**
+	- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF
+	- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support
+	- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`
+	- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)
+	- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless
+	- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper
+	
+	**Key gaps to fill:**
+	- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types
+	- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes
+	- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists
+	- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context
+	- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output
+		1.1.1: What syntax should function definitions use? - DONE user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		SUMMARY: Syntax: `f(x) = x + 1`
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - DONE user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		SUMMARY: Yes, multiple parameters supported: `f(x, y) = x + y`
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - DONE user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		SUMMARY: Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression.
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - DONE user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		SUMMARY: Yes, definitions persist across REPL lines. REPL needs stateful function registry.
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - DONE user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		SUMMARY: Yes, functions can call other user-defined functions. Implies late binding during evaluation.
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - DONE user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		SUMMARY: No built-in functions. Only user-defined functions.
+			 - user: no
+		1.1.7: Can functions be redefined? - DONE user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		SUMMARY: No, functions cannot be redefined. Attempting to redefine should be an error.
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - DONE user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+		SUMMARY: Print confirmation message when a function is defined (e.g. "defined f").
+			 - user: print that new function with given name was defined
+	1.2: Design: Solution architecture for function definitions - DONE assistant true
+	Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.
+	SUMMARY: Design complete. Key decisions:
+	
+	1. **Token layer:** Add Ident, Comma, Equals token types.
+	2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.
+	3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.
+	4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.
+	5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.
+	6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.
+	
+	Design document updated at docs/design.md. Ready for Planning phase upon approval.
+		1.2.1: Design token layer changes - DONE assistant false
+		New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.
+		SUMMARY: New token types: Ident, Comma, Equals. No changes to existing tokens.
+		1.2.2: Design AST layer changes - DONE assistant false
+		New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.
+		SUMMARY: New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes.
+		1.2.3: Design lexer changes - DONE assistant false
+		Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.
+		SUMMARY: Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals.
+		1.2.4: Design parser changes - DONE assistant false
+		Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.
+		SUMMARY: Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility.
+		1.2.5: Design evaluator changes - DONE assistant false
+		Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.
+		SUMMARY: Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call.
+		1.2.6: Design REPL changes - DONE assistant false
+		REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.
+		SUMMARY: REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print.
+	1.3: Plan: Detailed implementation plan for function definitions - IN PROGRESS assistant false
+	Create a step-by-step implementation plan following TDD approach. Changes go bottom-up through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step is small, testable, and maintains backward compatibility.
+		1.3.1: Step 1: Add new token types (Ident, Comma, Equals) - DONE user false
+		Add Ident, Comma, Equals to token/token.go. Update String() method. No tests needed — pure data types.
+		SUMMARY: Added three new token types to token/token.go: Ident (identifiers), Comma, Equals. Updated String() method. All existing tests pass. Committed.
+		1.3.2: Step 2: Add new AST nodes (Ident, FuncCall, Statement, ExprStmt, FuncDef) - IN PROGRESS assistant false
+		Add to ast/ast.go: Ident{Name}, FuncCall{Name, Args}, Statement interface with stmt() marker, ExprStmt{Expr}, FuncDef{Name, Params, Body}. Seal the interfaces.
+		1.3.3: Step 3: Update lexer to recognize identifiers, comma, equals - IN PROGRESS assistant false
+		In lexer/lexer.go: add isLetter() helper, add identifier scanning (letter then alphanumeric), add comma and equals to single-char switch. Write tests for: identifiers, comma, equals, mixed expressions like 'f(x) = x + 1', 'f(1, 2)'.
+		1.3.4: Step 4: Update parser — add ParseLine, extend factor for Ident/FuncCall - IN PROGRESS assistant false
+		In parser/parser.go: (1) Extend factor() to handle Ident token — if followed by LParen, parse as FuncCall with comma-separated args, else parse as Ident. (2) Add ParseLine() that detects Equals token to distinguish FuncDef from expression, returns Statement. (3) Keep Parse() backward-compatible. Write tests for: ParseLine with func defs, func calls, ident references, error cases.
+		1.3.5: Step 5: Refactor evaluator to stateful Evaluator struct - IN PROGRESS assistant false
+		In evaluator/evaluator.go: (1) Create Evaluator struct with funcs map. (2) New() constructor. (3) Eval(node, env) method handling NumberLit, BinaryExpr, Ident, FuncCall. (4) Define(def) method with redefinition error. (5) Keep old Eval() function as wrapper for backward compat (calls New().Eval(node,nil)). Update evaluator tests. Add new tests for Ident, FuncCall, Define, error cases.
+		1.3.6: Step 6: Update REPL to use stateful evaluator and ParseLine - IN PROGRESS assistant false
+		In repl/repl.go: (1) Create Evaluator in Run(). (2) Replace Parse() with ParseLine(). (3) Route FuncDef to Define+print, ExprStmt to Eval+print. Update REPL tests to cover function definitions and calls across lines.
+		1.3.7: Step 7: Add integration tests for function features - IN PROGRESS assistant false
+		In integration_test.go: Add tests for full pipeline with functions: define and call, multi-param, cross-function calls, error cases (undefined func, wrong arg count, redefine). Update eval/evalErr helpers to use stateful Evaluator.
+		1.3.8: Step 8: Write plan document to docs/plan.md - DONE assistant false
+		Write the detailed plan document to docs/plan.md for reference.
+		SUMMARY: Plan document written to docs/plan.md
+-- AGENT START WORKING 1.3.2
+--- STOP_REASON: tool_use
+AI: 
+
+I'll start by examining the current AST code to understand the existing structure.
+CALLING TOOL: file_read {"path":"ast/ast.go"}
+$$$ "package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n"
+
+--- STOP_REASON: tool_use
+AI: The structure is clear. I need to add:
+1. `Ident` — variable reference node (implements `Node`)
+2. `FuncCall` — function call node (implements `Node`)
+3. `Statement` — sealed interface for top-level REPL input
+4. `ExprStmt` — wraps an expression as a statement
+5. `FuncDef` — function definition statement
+CALLING TOOL: file_write {"path":"ast/ast.go","contents":"package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n\n// Ident represents a variable reference (e.g. x, y).\ntype Ident struct {\n\tName string\n}\n\nfunc (*Ident) node() {}\n\n// FuncCall represents a function call (e.g. f(1, 2+3)).\ntype FuncCall struct {\n\tName string\n\tArgs []Node\n}\n\nfunc (*FuncCall) node() {}\n\n// Statement is the interface for top-level REPL input.\n// A line is either an expression to evaluate or a function definition.\n// The unexported marker method seals the interface.\ntype Statement interface {\n\tstmt() // sealed marker\n}\n\n// ExprStmt wraps an expression as a top-level statement.\ntype ExprStmt struct {\n\tExpr Node\n}\n\nfunc (*ExprStmt) stmt() {}\n\n// FuncDef represents a function definition (e.g. f(x, y) = x + y).\ntype FuncDef struct {\n\tName   string\n\tParams []string\n\tBody   Node\n}\n\nfunc (*FuncDef) stmt() {}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me verify everything compiles and existing tests still pass:
+!!!!! {"command":"cd /home/user && go build ./... 2>&1"}: you are already in project dir
+--- STOP_REASON: tool_use
+!!!!! {"command":"go build ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go build ./... 2>&1"}
+$$$ ""
+
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t(cached)\nok  \tmatheval/ast\t0.627s\nok  \tmatheval/evaluator\t1.007s\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t1.383s\nok  \tmatheval/repl\t(cached)\nok  \tmatheval/token\t(cached)\n"
+
+--- STOP_REASON: tool_use
+AI: All packages build and all tests pass. Let me commit:
+!!!!! {"command":"git add -A && git commit -m \"ast: add Ident, FuncCall nodes and Statement interface (ExprStmt, FuncDef)\n\n- Ident{Name}: variable reference node (sealed, implements Node)\n- FuncCall{Name, Args}: function call node (sealed, implements Node)\n- Statement interface: sealed, for top-level REPL input\n- ExprStmt{Expr}: wraps expression as statement\n- FuncDef{Name, Params, Body}: function definition statement\" 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"ast: add Ident, FuncCall nodes and Statement interface (ExprStmt, FuncDef)\n\n- Ident{Name}: variable reference node (sealed, implements Node)\n- FuncCall{Name, Args}: function call node (sealed, implements Node)\n- Statement interface: sealed, for top-level REPL input\n- ExprStmt{Expr}: wraps expression as statement\n- FuncDef{Name, Params, Body}: function definition statement\" 2>&1"}
+$$$ "[master 5337c2b] ast: add Ident, FuncCall nodes and Statement interface (ExprStmt, FuncDef)\n 1 file changed, 38 insertions(+)\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.3.2","summary":"Added 5 new types to ast/ast.go: Ident{Name} and FuncCall{Name, Args} as sealed Node implementations for expression-level use; Statement sealed interface with ExprStmt{Expr} and FuncDef{Name, Params, Body} for top-level REPL input routing. All existing tests pass. Committed."}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: Done. Added to `ast/ast.go`:
+
+- **`Ident{Name}`** — variable reference node, implements `Node` (sealed)
+- **`FuncCall{Name, Args}`** — function call node, implements `Node` (sealed)
+- **`Statement`** — sealed interface for top-level REPL input
+- **`ExprStmt{Expr}`** — wraps an expression as a statement
+- **`FuncDef{Name, Params, Body}`** — function definition statement
+
+All existing tests pass. Committed. Ready for step 1.3.3 (lexer changes).
+1: update repl to support function definitions - IN PROGRESS user false
+	 - user: looks good
+	 - assistant: Plan phase complete. See docs/plan.md for full details. Summary of 7 implementation steps:
+
+1. **Token**: Add Ident, Comma, Equals types
+2. **AST**: Add Ident, FuncCall nodes + Statement interface (ExprStmt, FuncDef)
+3. **Lexer**: Recognize identifiers, comma, equals + tests
+4. **Parser**: Extend factor() for Ident/FuncCall, add ParseLine() for definition detection + tests
+5. **Evaluator**: Refactor to stateful Evaluator struct with Define() and Eval(node, env) + tests
+6. **REPL**: Use stateful evaluator and ParseLine, route defs vs exprs + tests
+7. **Integration tests**: Full pipeline tests for function features
+
+All steps maintain backward compatibility. Ready to implement upon approval.
+	1.1: Research: Understand requirements for function definitions - DONE assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+	SUMMARY: **Requirements gathered:**
+	
+	1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression
+	2. **Multiple params:** Yes — `f(x, y) = x + y`
+	3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`
+	4. **Persistence:** Definitions persist across REPL lines (stateful function registry)
+	5. **Cross-calling:** Functions can call other user-defined functions (late binding)
+	6. **Built-ins:** None
+	7. **Redefinition:** Not allowed — attempting to redefine is an error
+	8. **Output on define:** Print confirmation like "defined f"
+	
+	**Current architecture (5 packages + main):**
+	- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF
+	- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support
+	- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`
+	- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)
+	- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless
+	- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper
+	
+	**Key gaps to fill:**
+	- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types
+	- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes
+	- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists
+	- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context
+	- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output
+		1.1.1: What syntax should function definitions use? - DONE user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		SUMMARY: Syntax: `f(x) = x + 1`
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - DONE user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		SUMMARY: Yes, multiple parameters supported: `f(x, y) = x + y`
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - DONE user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		SUMMARY: Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression.
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - DONE user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		SUMMARY: Yes, definitions persist across REPL lines. REPL needs stateful function registry.
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - DONE user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		SUMMARY: Yes, functions can call other user-defined functions. Implies late binding during evaluation.
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - DONE user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		SUMMARY: No built-in functions. Only user-defined functions.
+			 - user: no
+		1.1.7: Can functions be redefined? - DONE user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		SUMMARY: No, functions cannot be redefined. Attempting to redefine should be an error.
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - DONE user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+		SUMMARY: Print confirmation message when a function is defined (e.g. "defined f").
+			 - user: print that new function with given name was defined
+	1.2: Design: Solution architecture for function definitions - DONE assistant true
+	Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.
+	SUMMARY: Design complete. Key decisions:
+	
+	1. **Token layer:** Add Ident, Comma, Equals token types.
+	2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.
+	3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.
+	4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.
+	5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.
+	6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.
+	
+	Design document updated at docs/design.md. Ready for Planning phase upon approval.
+		1.2.1: Design token layer changes - DONE assistant false
+		New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.
+		SUMMARY: New token types: Ident, Comma, Equals. No changes to existing tokens.
+		1.2.2: Design AST layer changes - DONE assistant false
+		New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.
+		SUMMARY: New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes.
+		1.2.3: Design lexer changes - DONE assistant false
+		Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.
+		SUMMARY: Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals.
+		1.2.4: Design parser changes - DONE assistant false
+		Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.
+		SUMMARY: Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility.
+		1.2.5: Design evaluator changes - DONE assistant false
+		Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.
+		SUMMARY: Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call.
+		1.2.6: Design REPL changes - DONE assistant false
+		REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.
+		SUMMARY: REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print.
+	1.3: Plan: Detailed implementation plan for function definitions - IN PROGRESS assistant false
+	Create a step-by-step implementation plan following TDD approach. Changes go bottom-up through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step is small, testable, and maintains backward compatibility.
+		1.3.1: Step 1: Add new token types (Ident, Comma, Equals) - DONE user false
+		Add Ident, Comma, Equals to token/token.go. Update String() method. No tests needed — pure data types.
+		SUMMARY: Added three new token types to token/token.go: Ident (identifiers), Comma, Equals. Updated String() method. All existing tests pass. Committed.
+		1.3.2: Step 2: Add new AST nodes (Ident, FuncCall, Statement, ExprStmt, FuncDef) - DONE user false
+		Add to ast/ast.go: Ident{Name}, FuncCall{Name, Args}, Statement interface with stmt() marker, ExprStmt{Expr}, FuncDef{Name, Params, Body}. Seal the interfaces.
+		SUMMARY: Added 5 new types to ast/ast.go: Ident{Name} and FuncCall{Name, Args} as sealed Node implementations for expression-level use; Statement sealed interface with ExprStmt{Expr} and FuncDef{Name, Params, Body} for top-level REPL input routing. All existing tests pass. Committed.
+		1.3.3: Step 3: Update lexer to recognize identifiers, comma, equals - IN PROGRESS assistant false
+		In lexer/lexer.go: add isLetter() helper, add identifier scanning (letter then alphanumeric), add comma and equals to single-char switch. Write tests for: identifiers, comma, equals, mixed expressions like 'f(x) = x + 1', 'f(1, 2)'.
+		1.3.4: Step 4: Update parser — add ParseLine, extend factor for Ident/FuncCall - IN PROGRESS assistant false
+		In parser/parser.go: (1) Extend factor() to handle Ident token — if followed by LParen, parse as FuncCall with comma-separated args, else parse as Ident. (2) Add ParseLine() that detects Equals token to distinguish FuncDef from expression, returns Statement. (3) Keep Parse() backward-compatible. Write tests for: ParseLine with func defs, func calls, ident references, error cases.
+		1.3.5: Step 5: Refactor evaluator to stateful Evaluator struct - IN PROGRESS assistant false
+		In evaluator/evaluator.go: (1) Create Evaluator struct with funcs map. (2) New() constructor. (3) Eval(node, env) method handling NumberLit, BinaryExpr, Ident, FuncCall. (4) Define(def) method with redefinition error. (5) Keep old Eval() function as wrapper for backward compat (calls New().Eval(node,nil)). Update evaluator tests. Add new tests for Ident, FuncCall, Define, error cases.
+		1.3.6: Step 6: Update REPL to use stateful evaluator and ParseLine - IN PROGRESS assistant false
+		In repl/repl.go: (1) Create Evaluator in Run(). (2) Replace Parse() with ParseLine(). (3) Route FuncDef to Define+print, ExprStmt to Eval+print. Update REPL tests to cover function definitions and calls across lines.
+		1.3.7: Step 7: Add integration tests for function features - IN PROGRESS assistant false
+		In integration_test.go: Add tests for full pipeline with functions: define and call, multi-param, cross-function calls, error cases (undefined func, wrong arg count, redefine). Update eval/evalErr helpers to use stateful Evaluator.
+		1.3.8: Step 8: Write plan document to docs/plan.md - DONE assistant false
+		Write the detailed plan document to docs/plan.md for reference.
+		SUMMARY: Plan document written to docs/plan.md
+-- AGENT START WORKING 1.3.3
+--- STOP_REASON: tool_use
+AI: 
+
+I'll start by examining the current lexer implementation and tests to understand what needs to change.
+CALLING TOOL: file_read {"path":"lexer/lexer.go"}
+$$$ "package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i \u003c len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch \u003e= '0' \u0026\u0026 ch \u003c= '9'\n}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer_test.go"}
+$$$ "package lexer\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestTokenizeEmpty(t *testing.T) {\n\ttokens, err := Tokenize(\"\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeWhitespaceOnly(t *testing.T) {\n\ttokens, err := Tokenize(\"   \\t\\n  \")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeSingleNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"42\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeDecimalNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"3.14\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"3.14\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeLeadingDotNumber(t *testing.T) {\n\ttokens, err := Tokenize(\".5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \".5\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeOperators(t *testing.T) {\n\ttokens, err := Tokenize(\"+-*/\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 0},\n\t\t{Type: token.Minus, Literal: \"-\", Pos: 1},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 2},\n\t\t{Type: token.Slash, Literal: \"/\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeParens(t *testing.T) {\n\ttokens, err := Tokenize(\"()\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 1},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFullExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"(1 + 2.5) * 3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 1},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2.5\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 10},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeNoSpaces(t *testing.T) {\n\ttokens, err := Tokenize(\"1+2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeInvalidCharacter(t *testing.T) {\n\t_, err := Tokenize(\"1 + @\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleInvalidCharacters(t *testing.T) {\n\t_, err := Tokenize(\"1 \u0026 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleDecimals(t *testing.T) {\n\t// \"1.2.3\" — the lexer should read \"1.2\" as a number, then \".3\" as another number\n\ttokens, err := Tokenize(\"1.2.3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1.2\", Pos: 0},\n\t\t{Type: token.Number, Literal: \".3\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\n// assertTokens is a test helper that compares two token slices.\nfunc assertTokens(t *testing.T, want, got []token.Token) {\n\tt.Helper()\n\tif len(want) != len(got) {\n\t\tt.Fatalf(\"token count: want %d, got %d\\nwant: %v\\ngot:  %v\", len(want), len(got), want, got)\n\t}\n\tfor i := range want {\n\t\tif want[i].Type != got[i].Type {\n\t\t\tt.Errorf(\"token[%d].Type: want %v, got %v\", i, want[i].Type, got[i].Type)\n\t\t}\n\t\tif want[i].Literal != got[i].Literal {\n\t\t\tt.Errorf(\"token[%d].Literal: want %q, got %q\", i, want[i].Literal, got[i].Literal)\n\t\t}\n\t\tif want[i].Pos != got[i].Pos {\n\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tIdent              // identifier (e.g. \"f\", \"x\")\n\tComma              // ,\n\tEquals             // =\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase Ident:\n\t\treturn \"Ident\"\n\tcase Comma:\n\t\treturn \",\"\n\tcase Equals:\n\t\treturn \"=\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Good. I have full context. The changes are small and straightforward. I'll:
+
+1. Add `isLetter` helper
+2. Add identifier scanning (starts with letter, continues with letters/digits)
+3. Add comma and equals to the single-char switch
+4. Write tests for all new token cases
+
+Let me implement the lexer changes first, then write the tests.
+CALLING TOOL: file_write {"path":"lexer/lexer.go","contents":"package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i < len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i < len(input) && (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Identifier: starts with letter, continues with letters/digits.\n\t\tif isLetter(ch) {\n\t\t\tstart := i\n\t\t\tfor i < len(input) && (isLetter(input[i]) || isDigit(input[i])) {\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Ident,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tcase ',':\n\t\t\ttyp = token.Comma\n\t\tcase '=':\n\t\t\ttyp = token.Equals\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch >= '0' && ch <= '9'\n}\n\nfunc isLetter(ch byte) bool {\n\treturn (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') || ch == '_'\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now add the new tests:
+CALLING TOOL: file_write {"path":"lexer/lexer_test.go","contents":"package lexer\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestTokenizeEmpty(t *testing.T) {\n\ttokens, err := Tokenize(\"\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeWhitespaceOnly(t *testing.T) {\n\ttokens, err := Tokenize(\"   \\t\\n  \")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeSingleNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"42\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeDecimalNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"3.14\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"3.14\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeLeadingDotNumber(t *testing.T) {\n\ttokens, err := Tokenize(\".5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \".5\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeOperators(t *testing.T) {\n\ttokens, err := Tokenize(\"+-*/\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 0},\n\t\t{Type: token.Minus, Literal: \"-\", Pos: 1},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 2},\n\t\t{Type: token.Slash, Literal: \"/\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeParens(t *testing.T) {\n\ttokens, err := Tokenize(\"()\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 1},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFullExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"(1 + 2.5) * 3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 1},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2.5\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 10},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeNoSpaces(t *testing.T) {\n\ttokens, err := Tokenize(\"1+2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeInvalidCharacter(t *testing.T) {\n\t_, err := Tokenize(\"1 + @\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleInvalidCharacters(t *testing.T) {\n\t_, err := Tokenize(\"1 & 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleDecimals(t *testing.T) {\n\t// \"1.2.3\" — the lexer should read \"1.2\" as a number, then \".3\" as another number\n\ttokens, err := Tokenize(\"1.2.3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1.2\", Pos: 0},\n\t\t{Type: token.Number, Literal: \".3\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeSingleIdent(t *testing.T) {\n\ttokens, err := Tokenize(\"x\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeMultiCharIdent(t *testing.T) {\n\ttokens, err := Tokenize(\"foo\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"foo\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeIdentWithDigits(t *testing.T) {\n\ttokens, err := Tokenize(\"x2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"x2\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeIdentWithUnderscore(t *testing.T) {\n\ttokens, err := Tokenize(\"my_func\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"my_func\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 7},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeComma(t *testing.T) {\n\ttokens, err := Tokenize(\",\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Comma, Literal: \",\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeEquals(t *testing.T) {\n\ttokens, err := Tokenize(\"=\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFuncDefinition(t *testing.T) {\n\ttokens, err := Tokenize(\"f(x) = x + 1\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 7},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 9},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 11},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFuncCallWithArgs(t *testing.T) {\n\ttokens, err := Tokenize(\"f(1, 2)\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 6},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 7},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeMultiParamFuncDef(t *testing.T) {\n\ttokens, err := Tokenize(\"add(x, y) = x + y\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"add\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 3},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 4},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"y\", Pos: 7},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 10},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 12},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 14},\n\t\t{Type: token.Ident, Literal: \"y\", Pos: 16},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 18},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFuncCallInExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"f(1+2, 3*4) + 5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 4},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 7},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 8},\n\t\t{Type: token.Number, Literal: \"4\", Pos: 9},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 10},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 12},\n\t\t{Type: token.Number, Literal: \"5\", Pos: 14},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 16},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\n// assertTokens is a test helper that compares two token slices.\nfunc assertTokens(t *testing.T, want, got []token.Token) {\n\tt.Helper()\n\tif len(want) != len(got) {\n\t\tt.Fatalf(\"token count: want %d, got %d\\nwant: %v\\ngot:  %v\", len(want), len(got), want, got)\n\t}\n\tfor i := range want {\n\t\tif want[i].Type != got[i].Type {\n\t\t\tt.Errorf(\"token[%d].Type: want %v, got %v\", i, want[i].Type, got[i].Type)\n\t\t}\n\t\tif want[i].Literal != got[i].Literal {\n\t\t\tt.Errorf(\"token[%d].Literal: want %q, got %q\", i, want[i].Literal, got[i].Literal)\n\t\t}\n\t\tif want[i].Pos != got[i].Pos {\n\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n\t\t}\n\t}\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now run the tests:
+!!!!! {"command":"cd /home/user/repos/matheval && go test ./lexer/ -v 2>&1"}: you are already in project dir
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./lexer/ -v 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./lexer/ -v 2>&1"}
+ERR: exit status 1
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./lexer/ -v"}: OK
+CALLING TOOL: bash_command {"command":"go test ./lexer/ -v"}
+ERR: exit status 1
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./lexer/ -v 2>&1; echo \"EXIT: $?\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./lexer/ -v 2>&1; echo \"EXIT: $?\""}
+$$$ "=== RUN   TestTokenizeEmpty\n--- PASS: TestTokenizeEmpty (0.00s)\n=== RUN   TestTokenizeWhitespaceOnly\n--- PASS: TestTokenizeWhitespaceOnly (0.00s)\n=== RUN   TestTokenizeSingleNumber\n--- PASS: TestTokenizeSingleNumber (0.00s)\n=== RUN   TestTokenizeDecimalNumber\n--- PASS: TestTokenizeDecimalNumber (0.00s)\n=== RUN   TestTokenizeLeadingDotNumber\n--- PASS: TestTokenizeLeadingDotNumber (0.00s)\n=== RUN   TestTokenizeOperators\n--- PASS: TestTokenizeOperators (0.00s)\n=== RUN   TestTokenizeParens\n--- PASS: TestTokenizeParens (0.00s)\n=== RUN   TestTokenizeFullExpression\n--- PASS: TestTokenizeFullExpression (0.00s)\n=== RUN   TestTokenizeNoSpaces\n--- PASS: TestTokenizeNoSpaces (0.00s)\n=== RUN   TestTokenizeInvalidCharacter\n--- PASS: TestTokenizeInvalidCharacter (0.00s)\n=== RUN   TestTokenizeMultipleInvalidCharacters\n--- PASS: TestTokenizeMultipleInvalidCharacters (0.00s)\n=== RUN   TestTokenizeMultipleDecimals\n--- PASS: TestTokenizeMultipleDecimals (0.00s)\n=== RUN   TestTokenizeSingleIdent\n--- PASS: TestTokenizeSingleIdent (0.00s)\n=== RUN   TestTokenizeMultiCharIdent\n--- PASS: TestTokenizeMultiCharIdent (0.00s)\n=== RUN   TestTokenizeIdentWithDigits\n--- PASS: TestTokenizeIdentWithDigits (0.00s)\n=== RUN   TestTokenizeIdentWithUnderscore\n--- PASS: TestTokenizeIdentWithUnderscore (0.00s)\n=== RUN   TestTokenizeComma\n--- PASS: TestTokenizeComma (0.00s)\n=== RUN   TestTokenizeEquals\n--- PASS: TestTokenizeEquals (0.00s)\n=== RUN   TestTokenizeFuncDefinition\n    lexer_test.go:240: token[8].Pos: want 13, got 12\n--- FAIL: TestTokenizeFuncDefinition (0.00s)\n=== RUN   TestTokenizeFuncCallWithArgs\n--- PASS: TestTokenizeFuncCallWithArgs (0.00s)\n=== RUN   TestTokenizeMultiParamFuncDef\n    lexer_test.go:278: token[10].Pos: want 18, got 17\n--- FAIL: TestTokenizeMultiParamFuncDef (0.00s)\n=== RUN   TestTokenizeFuncCallInExpression\n    lexer_test.go:301: token[12].Pos: want 16, got 15\n--- FAIL: TestTokenizeFuncCallInExpression (0.00s)\nFAIL\nFAIL\tmatheval/lexer\t0.258s\nFAIL\nEXIT: 1\n"
+
+--- STOP_REASON: tool_use
+AI: A few EOF position mismatches — I miscounted the string lengths. Let me fix the expected positions:
+
+- `"f(x) = x + 1"` → length 13, so EOF at 13. But test says 13, error says got 12. Let me count: `f(x) = x + 1` → f=0, (=1, x=2, )=3, ' '=4, ==5, ' '=6, x=7, ' '=8, +=9, ' '=10, 1=11 → length 12, EOF at 12.
+!!!!! {"command":"echo -n \"f(x) = x + 1\" | wc -c; echo -n \"add(x, y) = x + y\" | wc -c; echo -n \"f(1+2, 3*4) + 5\" | wc -c"}: OK
+CALLING TOOL: bash_command {"command":"echo -n \"f(x) = x + 1\" | wc -c; echo -n \"add(x, y) = x + y\" | wc -c; echo -n \"f(1+2, 3*4) + 5\" | wc -c"}
+$$$ "      12\n      17\n      15\n"
+
+--- STOP_REASON: tool_use
+AI: Fix the EOF positions in the tests:
+!!!!! {"command":"sed -i 's/{Type: token.EOF, Literal: \"\", Pos: 13},/{Type: token.EOF, Literal: \"\", Pos: 12},/' lexer/lexer_test.go\nsed -i 's/{Type: token.EOF, Literal: \"\", Pos: 18},/{Type: token.EOF, Literal: \"\", Pos: 17},/' lexer/lexer_test.go\nsed -i 's/{Type: token.EOF, Literal: \"\", Pos: 16},/{Type: token.EOF, Literal: \"\", Pos: 15},/' lexer/lexer_test.go"}: OK
+CALLING TOOL: bash_command {"command":"sed -i 's/{Type: token.EOF, Literal: \"\", Pos: 13},/{Type: token.EOF, Literal: \"\", Pos: 12},/' lexer/lexer_test.go\nsed -i 's/{Type: token.EOF, Literal: \"\", Pos: 18},/{Type: token.EOF, Literal: \"\", Pos: 17},/' lexer/lexer_test.go\nsed -i 's/{Type: token.EOF, Literal: \"\", Pos: 16},/{Type: token.EOF, Literal: \"\", Pos: 15},/' lexer/lexer_test.go"}
+$$$ ""
+
+--- STOP_REASON: tool_use
+AI: But wait — there are other tests with Pos 13 (the full expression test). Let me be more surgical. Let me check what the sed did:
+!!!!! {"command":"grep -n \"Pos: 12}\" lexer/lexer_test.go; grep -n \"Pos: 13}\" lexer/lexer_test.go"}: OK
+CALLING TOOL: bash_command {"command":"grep -n \"Pos: 12}\" lexer/lexer_test.go; grep -n \"Pos: 13}\" lexer/lexer_test.go"}
+ERR: exit status 1
+--- STOP_REASON: tool_use
+!!!!! {"command":"grep -n \"Pos:\" lexer/lexer_test.go | tail -30"}: OK
+CALLING TOOL: bash_command {"command":"grep -n \"Pos:\" lexer/lexer_test.go | tail -30"}
+$$$ "251:\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n252:\t\t{Type: token.Comma, Literal: \",\", Pos: 3},\n253:\t\t{Type: token.Number, Literal: \"2\", Pos: 5},\n254:\t\t{Type: token.RParen, Literal: \")\", Pos: 6},\n255:\t\t{Type: token.EOF, Literal: \"\", Pos: 7},\n266:\t\t{Type: token.Ident, Literal: \"add\", Pos: 0},\n267:\t\t{Type: token.LParen, Literal: \"(\", Pos: 3},\n268:\t\t{Type: token.Ident, Literal: \"x\", Pos: 4},\n269:\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n270:\t\t{Type: token.Ident, Literal: \"y\", Pos: 7},\n271:\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n272:\t\t{Type: token.Equals, Literal: \"=\", Pos: 10},\n273:\t\t{Type: token.Ident, Literal: \"x\", Pos: 12},\n274:\t\t{Type: token.Plus, Literal: \"+\", Pos: 14},\n275:\t\t{Type: token.Ident, Literal: \"y\", Pos: 16},\n276:\t\t{Type: token.EOF, Literal: \"\", Pos: 17},\n287:\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n288:\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n289:\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n290:\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n291:\t\t{Type: token.Number, Literal: \"2\", Pos: 4},\n292:\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n293:\t\t{Type: token.Number, Literal: \"3\", Pos: 7},\n294:\t\t{Type: token.Star, Literal: \"*\", Pos: 8},\n295:\t\t{Type: token.Number, Literal: \"4\", Pos: 9},\n296:\t\t{Type: token.RParen, Literal: \")\", Pos: 10},\n297:\t\t{Type: token.Plus, Literal: \"+\", Pos: 12},\n298:\t\t{Type: token.Number, Literal: \"5\", Pos: 14},\n299:\t\t{Type: token.EOF, Literal: \"\", Pos: 15},\n318:\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n"
+
+--- STOP_REASON: tool_use
+AI: The sed already fixed them correctly. Let me also verify the full expression test wasn't broken:
+!!!!! {"command":"grep -n \"Pos: 1[23]}\" lexer/lexer_test.go"}: OK
+CALLING TOOL: bash_command {"command":"grep -n \"Pos: 1[23]}\" lexer/lexer_test.go"}
+$$$ "104:\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n105:\t\t{Type: token.EOF, Literal: \"\", Pos: 12},\n238:\t\t{Type: token.EOF, Literal: \"\", Pos: 12},\n273:\t\t{Type: token.Ident, Literal: \"x\", Pos: 12},\n297:\t\t{Type: token.Plus, Literal: \"+\", Pos: 12},\n"
+
+--- STOP_REASON: tool_use
+AI: The `TestTokenizeFullExpression` test at line 105 now has `Pos: 12` but the string `"(1 + 2.5) * 3"` has length 14, so EOF should be at 13. The sed replaced too broadly. Let me check the original test:
+!!!!! {"command":"echo -n \"(1 + 2.5) * 3\" | wc -c"}: OK
+CALLING TOOL: bash_command {"command":"echo -n \"(1 + 2.5) * 3\" | wc -c"}
+$$$ "      13\n"
+
+--- STOP_REASON: tool_use
+AI: Right, the full expression test should have EOF at 13 but the sed changed it to 12. Let me fix this properly by rewriting the specific test section:
+!!!!! {"command":"sed -i '105s/Pos: 12}/Pos: 13}/' lexer/lexer_test.go"}: OK
+CALLING TOOL: bash_command {"command":"sed -i '105s/Pos: 12}/Pos: 13}/' lexer/lexer_test.go"}
+$$$ ""
+
+--- STOP_REASON: tool_use
+AI: Now run all tests:
+!!!!! {"command":"go test ./lexer/ -v 2>&1; echo \"EXIT: $?\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./lexer/ -v 2>&1; echo \"EXIT: $?\""}
+$$$ "=== RUN   TestTokenizeEmpty\n--- PASS: TestTokenizeEmpty (0.00s)\n=== RUN   TestTokenizeWhitespaceOnly\n--- PASS: TestTokenizeWhitespaceOnly (0.00s)\n=== RUN   TestTokenizeSingleNumber\n--- PASS: TestTokenizeSingleNumber (0.00s)\n=== RUN   TestTokenizeDecimalNumber\n--- PASS: TestTokenizeDecimalNumber (0.00s)\n=== RUN   TestTokenizeLeadingDotNumber\n--- PASS: TestTokenizeLeadingDotNumber (0.00s)\n=== RUN   TestTokenizeOperators\n--- PASS: TestTokenizeOperators (0.00s)\n=== RUN   TestTokenizeParens\n--- PASS: TestTokenizeParens (0.00s)\n=== RUN   TestTokenizeFullExpression\n--- PASS: TestTokenizeFullExpression (0.00s)\n=== RUN   TestTokenizeNoSpaces\n--- PASS: TestTokenizeNoSpaces (0.00s)\n=== RUN   TestTokenizeInvalidCharacter\n--- PASS: TestTokenizeInvalidCharacter (0.00s)\n=== RUN   TestTokenizeMultipleInvalidCharacters\n--- PASS: TestTokenizeMultipleInvalidCharacters (0.00s)\n=== RUN   TestTokenizeMultipleDecimals\n--- PASS: TestTokenizeMultipleDecimals (0.00s)\n=== RUN   TestTokenizeSingleIdent\n--- PASS: TestTokenizeSingleIdent (0.00s)\n=== RUN   TestTokenizeMultiCharIdent\n--- PASS: TestTokenizeMultiCharIdent (0.00s)\n=== RUN   TestTokenizeIdentWithDigits\n--- PASS: TestTokenizeIdentWithDigits (0.00s)\n=== RUN   TestTokenizeIdentWithUnderscore\n--- PASS: TestTokenizeIdentWithUnderscore (0.00s)\n=== RUN   TestTokenizeComma\n--- PASS: TestTokenizeComma (0.00s)\n=== RUN   TestTokenizeEquals\n--- PASS: TestTokenizeEquals (0.00s)\n=== RUN   TestTokenizeFuncDefinition\n--- PASS: TestTokenizeFuncDefinition (0.00s)\n=== RUN   TestTokenizeFuncCallWithArgs\n--- PASS: TestTokenizeFuncCallWithArgs (0.00s)\n=== RUN   TestTokenizeMultiParamFuncDef\n--- PASS: TestTokenizeMultiParamFuncDef (0.00s)\n=== RUN   TestTokenizeFuncCallInExpression\n--- PASS: TestTokenizeFuncCallInExpression (0.00s)\nPASS\nok  \tmatheval/lexer\t0.615s\nEXIT: 0\n"
+
+--- STOP_REASON: tool_use
+AI: All 22 tests pass. Let me also ensure all existing tests across the project still pass:
+!!!!! {"command":"go test ./... 2>&1; echo \"EXIT: $?\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1; echo \"EXIT: $?\""}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t0.403s\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t(cached)\nok  \tmatheval/lexer\t0.620s\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t0.979s\nok  \tmatheval/token\t(cached)\nEXIT: 0\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass. Let me commit:
+!!!!! {"command":"git add -A && git commit -m \"lexer: recognize identifiers, comma, and equals tokens\n\n- Add identifier scanning: starts with letter/underscore, continues with\n  letters/digits/underscores. Produces Ident tokens.\n- Add comma and equals to single-char token switch.\n- Add isLetter() helper.\n- Add 9 new tests covering: single ident, multi-char ident, ident with\n  digits, ident with underscore, comma, equals, function definition\n  syntax, function call with args, multi-param func def, func call in\n  expression.\""}: OK
+CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"lexer: recognize identifiers, comma, and equals tokens\n\n- Add identifier scanning: starts with letter/underscore, continues with\n  letters/digits/underscores. Produces Ident tokens.\n- Add comma and equals to single-char token switch.\n- Add isLetter() helper.\n- Add 9 new tests covering: single ident, multi-char ident, ident with\n  digits, ident with underscore, comma, equals, function definition\n  syntax, function call with args, multi-param func def, func call in\n  expression.\""}
+$$$ "[master dc6d8f6] lexer: recognize identifiers, comma, and equals tokens\n 2 files changed, 174 insertions(+)\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.3.3","summary":"Updated lexer to recognize 3 new token types: Ident (identifiers starting with letter/underscore, continuing with letters/digits/underscores), Comma, Equals. Added isLetter() helper. Added 9 new tests covering identifiers, comma, equals, function definition syntax, function calls with arguments, multi-param definitions, and function calls in expressions. All 22 lexer tests pass. All project tests pass. Committed."}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: Step 3 complete. Summary of changes:
+
+**lexer/lexer.go:**
+- Added `isLetter()` helper (`a-z`, `A-Z`, `_`)
+- Added identifier scanning block: starts with letter, continues with letters/digits → emits `Ident` token
+- Added `','` → `Comma` and `'='` → `Equals` to the single-char switch
+
+**lexer/lexer_test.go:**
+- 9 new tests: `TestTokenizeSingleIdent`, `TestTokenizeMultiCharIdent`, `TestTokenizeIdentWithDigits`, `TestTokenizeIdentWithUnderscore`, `TestTokenizeComma`, `TestTokenizeEquals`, `TestTokenizeFuncDefinition`, `TestTokenizeFuncCallWithArgs`, `TestTokenizeMultiParamFuncDef`, `TestTokenizeFuncCallInExpression`
+- All 22 tests pass, full project green
+## USER NO ITEMS
+1: update repl to support function definitions - IN PROGRESS user false
+	 - user: looks good
+	 - assistant: Plan phase complete. See docs/plan.md for full details. Summary of 7 implementation steps:
+
+1. **Token**: Add Ident, Comma, Equals types
+2. **AST**: Add Ident, FuncCall nodes + Statement interface (ExprStmt, FuncDef)
+3. **Lexer**: Recognize identifiers, comma, equals + tests
+4. **Parser**: Extend factor() for Ident/FuncCall, add ParseLine() for definition detection + tests
+5. **Evaluator**: Refactor to stateful Evaluator struct with Define() and Eval(node, env) + tests
+6. **REPL**: Use stateful evaluator and ParseLine, route defs vs exprs + tests
+7. **Integration tests**: Full pipeline tests for function features
+
+All steps maintain backward compatibility. Ready to implement upon approval.
+	1.1: Research: Understand requirements for function definitions - DONE assistant true
+	The task is to update the REPL to support function definitions. Before designing a solution, we need to clarify what exactly 'function definitions' means in this context. This is a math expression evaluator that currently supports +, -, *, /, parentheses, and floating point numbers. We need to understand the expected syntax, semantics, and scope of this feature.
+	SUMMARY: **Requirements gathered:**
+	
+	1. **Syntax:** `f(x) = x + 1` — name followed by parenthesized params, `=`, then body expression
+	2. **Multiple params:** Yes — `f(x, y) = x + y`
+	3. **Function calls:** Allowed anywhere a number can appear; arguments are arbitrary expressions — `f(1+2, 3*4)`
+	4. **Persistence:** Definitions persist across REPL lines (stateful function registry)
+	5. **Cross-calling:** Functions can call other user-defined functions (late binding)
+	6. **Built-ins:** None
+	7. **Redefinition:** Not allowed — attempting to redefine is an error
+	8. **Output on define:** Print confirmation like "defined f"
+	
+	**Current architecture (5 packages + main):**
+	- `token` — Token types: Number, Plus, Minus, Star, Slash, LParen, RParen, EOF
+	- `lexer` — `Tokenize(string) ([]Token, error)` — character-level scanning, no identifier support
+	- `ast` — Sealed `Node` interface with `NumberLit` and `BinaryExpr`
+	- `parser` — `Parse([]Token) (Node, error)` — recursive descent (expr→term→factor)
+	- `evaluator` — `Eval(Node) (float64, error)` — tree-walking evaluator, stateless
+	- `repl` — `Run(io.Reader, io.Writer)` — line-by-line REPL, stateless `evalLine` helper
+	
+	**Key gaps to fill:**
+	- Lexer: needs `Ident` (identifiers), `Comma`, `Equals` token types
+	- AST: needs `FuncDef`, `FuncCall`, `Ident` (variable reference) nodes
+	- Parser: needs to distinguish expression vs function definition; parse function calls in factor position; parse comma-separated argument lists
+	- Evaluator: needs function registry and variable scope/environment; `Eval` signature must change to accept environment context
+	- REPL: needs to maintain state (function registry) across lines; distinguish definition output from expression output
+		1.1.1: What syntax should function definitions use? - DONE user false
+		e.g. `f(x) = x + 1`, `def f(x) = x * 2`, `fn f(x) = ...`? This fundamentally affects lexer, parser, and AST design.
+		SUMMARY: Syntax: `f(x) = x + 1`
+			 - user: f(x) = x + 1
+		1.1.2: Should functions support multiple parameters? - DONE user false
+		e.g. `f(x, y) = x + y`. Affects parser (comma-separated param lists) and evaluator (environment/scope).
+		SUMMARY: Yes, multiple parameters supported: `f(x, y) = x + y`
+			 - user: yes
+		1.1.3: How should function calls look in expressions? - DONE user false
+		e.g. `f(3)`, `f(1+2, 3*4)`. Should calls be allowed anywhere a number can appear?
+		SUMMARY: Function calls allowed anywhere a number can appear. Arguments can be any arbitrary valid expression.
+			 - user: yes and function arguments can be any arbitrary valid expression
+		1.1.4: Should function definitions persist across REPL lines? - DONE user false
+		e.g. line 1: `f(x) = x + 1`, line 2: `f(5)` → 6. This implies the REPL needs state (an environment/registry of defined functions).
+		SUMMARY: Yes, definitions persist across REPL lines. REPL needs stateful function registry.
+			 - user: yes
+		1.1.5: Can functions call other user-defined functions? - DONE user false
+		e.g. `f(x) = x + 1` then `g(x) = f(x) * 2`. Affects evaluation strategy (late binding vs early binding).
+		SUMMARY: Yes, functions can call other user-defined functions. Implies late binding during evaluation.
+			 - user: yes
+		1.1.6: Should there be any built-in functions? - DONE user false
+		e.g. `sin(x)`, `cos(x)`, `sqrt(x)`. This could be a separate concern but worth clarifying scope.
+		SUMMARY: No built-in functions. Only user-defined functions.
+			 - user: no
+		1.1.7: Can functions be redefined? - DONE user false
+		e.g. define `f(x) = x + 1`, then later `f(x) = x * 2`. Should the second definition replace the first?
+		SUMMARY: No, functions cannot be redefined. Attempting to redefine should be an error.
+			 - user: no
+		1.1.8: What should happen when defining a function — should it print anything? - DONE user false
+		e.g. `f(x) = x + 1` — should REPL print 'defined f' or nothing or the definition back?
+		SUMMARY: Print confirmation message when a function is defined (e.g. "defined f").
+			 - user: print that new function with given name was defined
+	1.2: Design: Solution architecture for function definitions - DONE assistant true
+	Design the changes needed across all packages to support function definitions and calls. Consider multiple approaches, present component interfaces, and get approval before planning.
+	SUMMARY: Design complete. Key decisions:
+	
+	1. **Token layer:** Add Ident, Comma, Equals token types.
+	2. **AST layer:** Add Ident, FuncCall nodes for expressions. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at top level.
+	3. **Lexer:** Recognize identifiers (letter+alphanumeric), comma, equals.
+	4. **Parser:** New ParseLine() → Statement. Detect definitions by presence of Equals token. Extend factor() for Ident (variable ref) and FuncCall.
+	5. **Evaluator:** Becomes stateful struct with function registry. New() constructor, Define() for registration, Eval(node, env) for evaluation with variable scope.
+	6. **REPL:** Creates Evaluator at startup, maintains across lines. Routes FuncDef→Define+print, ExprStmt→Eval+print.
+	
+	Design document updated at docs/design.md. Ready for Planning phase upon approval.
+		1.2.1: Design token layer changes - DONE assistant false
+		New token types needed: Ident (identifiers like 'f', 'x'), Comma, Equals. Evaluate if any existing tokens need modification.
+		SUMMARY: New token types: Ident, Comma, Equals. No changes to existing tokens.
+		1.2.2: Design AST layer changes - DONE assistant false
+		New AST node types: FuncDef (name, params, body), FuncCall (name, args), Ident (variable reference). Consider how these fit with the sealed Node interface.
+		SUMMARY: New AST nodes: Ident{Name}, FuncCall{Name, Args[]Node}, FuncDef{Name, Params[]string, Body Node}. Introduce Statement interface (ExprStmt, FuncDef) to separate definitions from expressions at the top level. Node interface stays for expression-level nodes.
+		1.2.3: Design lexer changes - DONE assistant false
+		Lexer must recognize identifiers (letter sequences) and new single-char tokens (comma, equals). Must distinguish identifiers from numbers.
+		SUMMARY: Lexer recognizes identifiers (start with letter, continue with letters/digits) and new single-char tokens: comma, equals.
+		1.2.4: Design parser changes - DONE assistant false
+		Parser must: (1) distinguish function definitions from expressions at top level, (2) parse function calls in factor position (Ident followed by LParen), (3) handle Ident as variable reference when not followed by LParen. Consider if Parse returns a different top-level type.
+		SUMMARY: Parser gets new top-level ParseLine() returning Statement. Detects definitions by scanning for Equals token. factor() extended to handle Ident (variable ref) and Ident+LParen (function call with comma-separated args). Existing Parse() kept for backward compatibility.
+		1.2.5: Design evaluator changes - DONE assistant false
+		Evaluator needs: (1) function registry to store defined functions, (2) variable scope/environment for parameter binding during calls, (3) late binding for cross-function calls. Current Eval(Node) signature must change to accept environment. Consider approach: Environment struct vs passing maps.
+		SUMMARY: Evaluator becomes stateful struct with function registry. New Evaluator struct with New(), Define(FuncDef) error, Eval(Node, env) (float64, error). Late binding for cross-function calls. Fresh environment per call.
+		1.2.6: Design REPL changes - DONE assistant false
+		REPL must: (1) maintain function registry across lines, (2) distinguish definition lines from expression lines (different output behavior), (3) pass registry to evaluator.
+		SUMMARY: REPL creates Evaluator at startup, maintains it across lines. Uses ParseLine() to get Statement. Routes FuncDef to Define() with confirmation print, ExprStmt to Eval() with result print.
+	1.3: Plan: Detailed implementation plan for function definitions - IN PROGRESS assistant false
+	Create a step-by-step implementation plan following TDD approach. Changes go bottom-up through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step is small, testable, and maintains backward compatibility.
+		1.3.1: Step 1: Add new token types (Ident, Comma, Equals) - DONE user false
+		Add Ident, Comma, Equals to token/token.go. Update String() method. No tests needed — pure data types.
+		SUMMARY: Added three new token types to token/token.go: Ident (identifiers), Comma, Equals. Updated String() method. All existing tests pass. Committed.
+		1.3.2: Step 2: Add new AST nodes (Ident, FuncCall, Statement, ExprStmt, FuncDef) - DONE user false
+		Add to ast/ast.go: Ident{Name}, FuncCall{Name, Args}, Statement interface with stmt() marker, ExprStmt{Expr}, FuncDef{Name, Params, Body}. Seal the interfaces.
+		SUMMARY: Added 5 new types to ast/ast.go: Ident{Name} and FuncCall{Name, Args} as sealed Node implementations for expression-level use; Statement sealed interface with ExprStmt{Expr} and FuncDef{Name, Params, Body} for top-level REPL input routing. All existing tests pass. Committed.
+		1.3.3: Step 3: Update lexer to recognize identifiers, comma, equals - DONE user false
+		In lexer/lexer.go: add isLetter() helper, add identifier scanning (letter then alphanumeric), add comma and equals to single-char switch. Write tests for: identifiers, comma, equals, mixed expressions like 'f(x) = x + 1', 'f(1, 2)'.
+		SUMMARY: Updated lexer to recognize 3 new token types: Ident (identifiers starting with letter/underscore, continuing with letters/digits/underscores), Comma, Equals. Added isLetter() helper. Added 9 new tests covering identifiers, comma, equals, function definition syntax, function calls with arguments, multi-param definitions, and function calls in expressions. All 22 lexer tests pass. All project tests pass. Committed.
+		1.3.4: Step 4: Update parser — add ParseLine, extend factor for Ident/FuncCall - IN PROGRESS assistant false
+		In parser/parser.go: (1) Extend factor() to handle Ident token — if followed by LParen, parse as FuncCall with comma-separated args, else parse as Ident. (2) Add ParseLine() that detects Equals token to distinguish FuncDef from expression, returns Statement. (3) Keep Parse() backward-compatible. Write tests for: ParseLine with func defs, func calls, ident references, error cases.
+		1.3.5: Step 5: Refactor evaluator to stateful Evaluator struct - IN PROGRESS assistant false
+		In evaluator/evaluator.go: (1) Create Evaluator struct with funcs map. (2) New() constructor. (3) Eval(node, env) method handling NumberLit, BinaryExpr, Ident, FuncCall. (4) Define(def) method with redefinition error. (5) Keep old Eval() function as wrapper for backward compat (calls New().Eval(node,nil)). Update evaluator tests. Add new tests for Ident, FuncCall, Define, error cases.
+		1.3.6: Step 6: Update REPL to use stateful evaluator and ParseLine - IN PROGRESS assistant false
+		In repl/repl.go: (1) Create Evaluator in Run(). (2) Replace Parse() with ParseLine(). (3) Route FuncDef to Define+print, ExprStmt to Eval+print. Update REPL tests to cover function definitions and calls across lines.
+		1.3.7: Step 7: Add integration tests for function features - IN PROGRESS assistant false
+		In integration_test.go: Add tests for full pipeline with functions: define and call, multi-param, cross-function calls, error cases (undefined func, wrong arg count, redefine). Update eval/evalErr helpers to use stateful Evaluator.
+		1.3.8: Step 8: Write plan document to docs/plan.md - DONE assistant false
+		Write the detailed plan document to docs/plan.md for reference.
+		SUMMARY: Plan document written to docs/plan.md
+-- AGENT START WORKING 1.3.4
+--- STOP_REASON: tool_use
+AI: 
+
+I'll start by reading the current parser code and tests to understand the existing implementation before making changes.
+CALLING TOOL: file_read {"path":"parser/parser.go"}
+$$$ "package parser\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strconv\"\n)\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions.\nfunc Parse(tokens []token.Token) (ast.Node, error) {\n\tp := \u0026parser{tokens: tokens}\n\tnode, err := p.expr()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\t// Ensure all tokens have been consumed (only EOF remains).\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n\treturn node, nil\n}\n\n// parser holds the state for a single parse operation.\ntype parser struct {\n\ttokens []token.Token\n\tpos    int\n}\n\n// current returns the token at the current position.\nfunc (p *parser) current() token.Token {\n\tif p.pos \u003e= len(p.tokens) {\n\t\treturn token.Token{Type: token.EOF}\n\t}\n\treturn p.tokens[p.pos]\n}\n\n// advance moves to the next token and returns the previous one.\nfunc (p *parser) advance() token.Token {\n\ttok := p.current()\n\tp.pos++\n\treturn tok\n}\n\n// expect consumes a token of the given type or returns an error.\nfunc (p *parser) expect(typ token.Type) (token.Token, error) {\n\ttok := p.current()\n\tif tok.Type != typ {\n\t\treturn tok, fmt.Errorf(\"expected %v but got %v at position %d\", typ, tok.Type, tok.Pos)\n\t}\n\tp.advance()\n\treturn tok, nil\n}\n\n// expr → term (('+' | '-') term)*\nfunc (p *parser) expr() (ast.Node, error) {\n\tleft, err := p.term()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Plus || p.current().Type == token.Minus {\n\t\top := p.advance()\n\t\tright, err := p.term()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// term → factor (('*' | '/') factor)*\nfunc (p *parser) term() (ast.Node, error) {\n\tleft, err := p.factor()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Star || p.current().Type == token.Slash {\n\t\top := p.advance()\n\t\tright, err := p.factor()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// factor → NUMBER | '(' expr ')'\nfunc (p *parser) factor() (ast.Node, error) {\n\ttok := p.current()\n\tswitch tok.Type {\n\tcase token.Number:\n\t\tp.advance()\n\t\tval, err := strconv.ParseFloat(tok.Literal, 64)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"invalid number %q at position %d: %w\", tok.Literal, tok.Pos, err)\n\t\t}\n\t\treturn \u0026ast.NumberLit{Value: val}, nil\n\n\tcase token.LParen:\n\t\tp.advance() // consume '('\n\t\tnode, err := p.expr()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\treturn nil, fmt.Errorf(\"missing closing parenthesis at position %d\", p.current().Pos)\n\t\t}\n\t\treturn node, nil\n\n\tdefault:\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser_test.go"}
+$$$ "package parser\n\nimport (\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// helper: tokenize inline for concise tests\nfunc tokens(toks ...token.Token) []token.Token {\n\treturn toks\n}\n\nfunc tok(typ token.Type, lit string, pos int) token.Token {\n\treturn token.Token{Type: typ, Literal: lit, Pos: pos}\n}\n\n// --- Success cases ---\n\nfunc TestParseSingleNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"42\", 0),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 42 {\n\t\tt.Fatalf(\"expected 42, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseDecimalNumber(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.Number, \"3.14\", 0),\n\t\ttok(token.EOF, \"\", 4),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 3.14 {\n\t\tt.Fatalf(\"expected 3.14, got %f\", num.Value)\n\t}\n}\n\nfunc TestParseAddition(t *testing.T) {\n\t// 1 + 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseSubtraction(t *testing.T) {\n\t// 5 - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"5\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 5)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseMultiplication(t *testing.T) {\n\t// 2 * 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 2)\n\tassertNumber(t, expr.Right, 3)\n}\n\nfunc TestParseDivision(t *testing.T) {\n\t// 6 / 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"6\", 0),\n\t\ttok(token.Slash, \"/\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.EOF, \"\", 5),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 6)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParsePrecedence(t *testing.T) {\n\t// 1 + 2 * 3  →  1 + (2 * 3)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root should be Plus\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\t// Right should be Star\n\tright, ok := expr.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", expr.Right)\n\t}\n\tif right.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", right.Op)\n\t}\n\tassertNumber(t, right.Left, 2)\n\tassertNumber(t, right.Right, 3)\n}\n\nfunc TestParsePrecedenceMulFirst(t *testing.T) {\n\t// 2 * 3 + 1  →  (2 * 3) + 1\n\ttoks := tokens(\n\t\ttok(token.Number, \"2\", 0),\n\t\ttok(token.Star, \"*\", 2),\n\t\ttok(token.Number, \"3\", 4),\n\t\ttok(token.Plus, \"+\", 6),\n\t\ttok(token.Number, \"1\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus at root, got %v\", expr.Op)\n\t}\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 2)\n\tassertNumber(t, left.Right, 3)\n\tassertNumber(t, expr.Right, 1)\n}\n\nfunc TestParseLeftAssociativity(t *testing.T) {\n\t// 1 - 2 - 3  →  (1 - 2) - 3\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Minus, \"-\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Minus, \"-\", 6),\n\t\ttok(token.Number, \"3\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 - 2) - 3\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseParentheses(t *testing.T) {\n\t// (1 + 2) * 3\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.RParen, \")\", 6),\n\t\ttok(token.Star, \"*\", 8),\n\t\ttok(token.Number, \"3\", 10),\n\t\ttok(token.EOF, \"\", 11),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Star {\n\t\tt.Fatalf(\"expected Star at root, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Right, 3)\n\tleft, ok := expr.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", expr.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tassertNumber(t, left.Right, 2)\n}\n\nfunc TestParseNestedParentheses(t *testing.T) {\n\t// ((1 + 2))\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.LParen, \"(\", 1),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.Plus, \"+\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.RParen, \")\", 7),\n\t\ttok(token.RParen, \")\", 8),\n\t\ttok(token.EOF, \"\", 9),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpr, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif expr.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", expr.Op)\n\t}\n\tassertNumber(t, expr.Left, 1)\n\tassertNumber(t, expr.Right, 2)\n}\n\nfunc TestParseComplexExpression(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2  →  (1 + (2*3)) - (4/2)\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Number, \"2\", 4),\n\t\ttok(token.Star, \"*\", 5),\n\t\ttok(token.Number, \"3\", 6),\n\t\ttok(token.Minus, \"-\", 8),\n\t\ttok(token.Number, \"4\", 10),\n\t\ttok(token.Slash, \"/\", 11),\n\t\ttok(token.Number, \"2\", 12),\n\t\ttok(token.EOF, \"\", 13),\n\t)\n\tnode, err := Parse(toks)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// Root: (1 + (2*3)) - (4/2)\n\troot, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif root.Op != token.Minus {\n\t\tt.Fatalf(\"expected Minus at root, got %v\", root.Op)\n\t}\n\t// Left: 1 + (2*3)\n\tleft, ok := root.Left.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.BinaryExpr, got %T\", root.Left)\n\t}\n\tif left.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", left.Op)\n\t}\n\tassertNumber(t, left.Left, 1)\n\tmul, ok := left.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", left.Right)\n\t}\n\tif mul.Op != token.Star {\n\t\tt.Fatalf(\"expected Star, got %v\", mul.Op)\n\t}\n\tassertNumber(t, mul.Left, 2)\n\tassertNumber(t, mul.Right, 3)\n\t// Right: 4/2\n\tdiv, ok := root.Right.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected right to be *ast.BinaryExpr, got %T\", root.Right)\n\t}\n\tif div.Op != token.Slash {\n\t\tt.Fatalf(\"expected Slash, got %v\", div.Op)\n\t}\n\tassertNumber(t, div.Left, 4)\n\tassertNumber(t, div.Right, 2)\n}\n\n// --- Error cases ---\n\nfunc TestParseEmptyInput(t *testing.T) {\n\ttoks := tokens(\n\t\ttok(token.EOF, \"\", 0),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestParseMissingRParen(t *testing.T) {\n\t// (1 + 2\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.Number, \"1\", 1),\n\t\ttok(token.Plus, \"+\", 3),\n\t\ttok(token.Number, \"2\", 5),\n\t\ttok(token.EOF, \"\", 6),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing right paren\")\n\t}\n}\n\nfunc TestParseUnexpectedRParen(t *testing.T) {\n\t// ) 1\n\ttoks := tokens(\n\t\ttok(token.RParen, \")\", 0),\n\t\ttok(token.Number, \"1\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected right paren\")\n\t}\n}\n\nfunc TestParseTrailingOperator(t *testing.T) {\n\t// 1 +\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestParseTrailingTokens(t *testing.T) {\n\t// 1 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Number, \"2\", 2),\n\t\ttok(token.EOF, \"\", 3),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing tokens\")\n\t}\n}\n\nfunc TestParseConsecutiveOperators(t *testing.T) {\n\t// 1 + * 2\n\ttoks := tokens(\n\t\ttok(token.Number, \"1\", 0),\n\t\ttok(token.Plus, \"+\", 2),\n\t\ttok(token.Star, \"*\", 4),\n\t\ttok(token.Number, \"2\", 6),\n\t\ttok(token.EOF, \"\", 7),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestParseEmptyParens(t *testing.T) {\n\t// ()\n\ttoks := tokens(\n\t\ttok(token.LParen, \"(\", 0),\n\t\ttok(token.RParen, \")\", 1),\n\t\ttok(token.EOF, \"\", 2),\n\t)\n\t_, err := Parse(toks)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\n// --- Helper ---\n\nfunc assertNumber(t *testing.T, node ast.Node, expected float64) {\n\tt.Helper()\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != expected {\n\t\tt.Fatalf(\"expected %f, got %f\", expected, num.Value)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast.go"}
+$$$ "package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n\n// Ident represents a variable reference (e.g. x, y).\ntype Ident struct {\n\tName string\n}\n\nfunc (*Ident) node() {}\n\n// FuncCall represents a function call (e.g. f(1, 2+3)).\ntype FuncCall struct {\n\tName string\n\tArgs []Node\n}\n\nfunc (*FuncCall) node() {}\n\n// Statement is the interface for top-level REPL input.\n// A line is either an expression to evaluate or a function definition.\n// The unexported marker method seals the interface.\ntype Statement interface {\n\tstmt() // sealed marker\n}\n\n// ExprStmt wraps an expression as a top-level statement.\ntype ExprStmt struct {\n\tExpr Node\n}\n\nfunc (*ExprStmt) stmt() {}\n\n// FuncDef represents a function definition (e.g. f(x, y) = x + y).\ntype FuncDef struct {\n\tName   string\n\tParams []string\n\tBody   Node\n}\n\nfunc (*FuncDef) stmt() {}\n"
+
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tIdent              // identifier (e.g. \"f\", \"x\")\n\tComma              // ,\n\tEquals             // =\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase Ident:\n\t\treturn \"Ident\"\n\tcase Comma:\n\t\treturn \",\"\n\tcase Equals:\n\t\treturn \"=\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Good. I have full context. Now let me check the lexer for completeness:
+CALLING TOOL: file_read {"path":"lexer/lexer.go"}
+$$$ "package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i \u003c len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Identifier: starts with letter, continues with letters/digits.\n\t\tif isLetter(ch) {\n\t\t\tstart := i\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isLetter(input[i]) || isDigit(input[i])) {\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Ident,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tcase ',':\n\t\t\ttyp = token.Comma\n\t\tcase '=':\n\t\t\ttyp = token.Equals\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch \u003e= '0' \u0026\u0026 ch \u003c= '9'\n}\n\nfunc isLetter(ch byte) bool {\n\treturn (ch \u003e= 'a' \u0026\u0026 ch \u003c= 'z') || (ch \u003e= 'A' \u0026\u0026 ch \u003c= 'Z') || ch == '_'\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me verify all tests pass before making changes:
+!!!!! {"command":"cd /home/user/project && go test ./... 2>&1"}: you are already in project dir
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
+$$$ "ok  \tmatheval\t(cached)\n?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t(cached)\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t(cached)\nok  \tmatheval/token\t(cached)\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass. Now I have full context. The task is clear and manageable — I'll implement it directly.
+
+**Changes needed in `parser/parser.go`:**
+
+1. **Extend `factor()`** to handle `Ident` token:
+   - If followed by `LParen` → parse as `FuncCall` with comma-separated args
+   - Otherwise → parse as `Ident` (variable reference)
+
+2. **Add `ParseLine()`** that returns `ast.Statement`:
+   - Scan tokens for `Equals` to detect function definition
+   - If definition: parse `name(param1, param2, ...) = body`
+   - Otherwise: delegate to `Parse()` and wrap in `ExprStmt`
+
+Let me implement this:
+CALLING TOOL: file_write {"path":"parser/parser.go","contents":"package parser\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strconv\"\n)\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions.\nfunc Parse(tokens []token.Token) (ast.Node, error) {\n\tp := &parser{tokens: tokens}\n\tnode, err := p.expr()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\t// Ensure all tokens have been consumed (only EOF remains).\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n\treturn node, nil\n}\n\n// ParseLine parses a full REPL line, returning either a function definition\n// or an expression statement.\nfunc ParseLine(tokens []token.Token) (ast.Statement, error) {\n\tif len(tokens) == 0 || tokens[0].Type == token.EOF {\n\t\treturn nil, fmt.Errorf(\"empty input\")\n\t}\n\n\t// Detect function definition: look for Equals token.\n\t// A function definition has the form: Ident LParen params RParen Equals body\n\tif isFuncDef(tokens) {\n\t\treturn parseFuncDef(tokens)\n\t}\n\n\t// Otherwise, parse as expression.\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\treturn &ast.ExprStmt{Expr: node}, nil\n}\n\n// isFuncDef checks if the token stream looks like a function definition.\n// Pattern: Ident LParen ... RParen Equals ...\nfunc isFuncDef(tokens []token.Token) bool {\n\tif len(tokens) < 5 {\n\t\treturn false\n\t}\n\tif tokens[0].Type != token.Ident {\n\t\treturn false\n\t}\n\tif tokens[1].Type != token.LParen {\n\t\treturn false\n\t}\n\t// Find matching RParen, then check for Equals.\n\tdepth := 0\n\tfor i := 1; i < len(tokens); i++ {\n\t\tswitch tokens[i].Type {\n\t\tcase token.LParen:\n\t\t\tdepth++\n\t\tcase token.RParen:\n\t\t\tdepth--\n\t\t\tif depth == 0 {\n\t\t\t\t// Next token must be Equals for this to be a func def.\n\t\t\t\tif i+1 < len(tokens) && tokens[i+1].Type == token.Equals {\n\t\t\t\t\treturn true\n\t\t\t\t}\n\t\t\t\treturn false\n\t\t\t}\n\t\tcase token.EOF:\n\t\t\treturn false\n\t\t}\n\t}\n\treturn false\n}\n\n// parseFuncDef parses: Ident LParen param1, param2, ... RParen Equals body\nfunc parseFuncDef(tokens []token.Token) (*ast.FuncDef, error) {\n\tp := &parser{tokens: tokens}\n\n\t// Function name.\n\tnameTok, err := p.expect(token.Ident)\n\tif err != nil {\n\t\treturn nil, fmt.Errorf(\"expected function name: %w\", err)\n\t}\n\tname := nameTok.Literal\n\n\t// Opening paren.\n\tif _, err := p.expect(token.LParen); err != nil {\n\t\treturn nil, fmt.Errorf(\"expected '(' after function name: %w\", err)\n\t}\n\n\t// Parameters: comma-separated identifiers.\n\tvar params []string\n\tif p.current().Type != token.RParen {\n\t\tparamTok, err := p.expect(token.Ident)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"expected parameter name: %w\", err)\n\t\t}\n\t\tparams = append(params, paramTok.Literal)\n\n\t\tfor p.current().Type == token.Comma {\n\t\t\tp.advance() // consume comma\n\t\t\tparamTok, err := p.expect(token.Ident)\n\t\t\tif err != nil {\n\t\t\t\treturn nil, fmt.Errorf(\"expected parameter name after ',': %w\", err)\n\t\t\t}\n\t\t\tparams = append(params, paramTok.Literal)\n\t\t}\n\t}\n\n\t// Closing paren.\n\tif _, err := p.expect(token.RParen); err != nil {\n\t\treturn nil, fmt.Errorf(\"expected ')' after parameters: %w\", err)\n\t}\n\n\t// Equals sign.\n\tif _, err := p.expect(token.Equals); err != nil {\n\t\treturn nil, fmt.Errorf(\"expected '=' in function definition: %w\", err)\n\t}\n\n\t// Body expression.\n\tbody, err := p.expr()\n\tif err != nil {\n\t\treturn nil, fmt.Errorf(\"error in function body: %w\", err)\n\t}\n\n\t// Ensure all tokens consumed.\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d after function body\", tok.Type, tok.Pos)\n\t}\n\n\treturn &ast.FuncDef{\n\t\tName:   name,\n\t\tParams: params,\n\t\tBody:   body,\n\t}, nil\n}\n\n// parser holds the state for a single parse operation.\ntype parser struct {\n\ttokens []token.Token\n\tpos    int\n}\n\n// current returns the token at the current position.\nfunc (p *parser) current() token.Token {\n\tif p.pos >= len(p.tokens) {\n\t\treturn token.Token{Type: token.EOF}\n\t}\n\treturn p.tokens[p.pos]\n}\n\n// advance moves to the next token and returns the previous one.\nfunc (p *parser) advance() token.Token {\n\ttok := p.current()\n\tp.pos++\n\treturn tok\n}\n\n// expect consumes a token of the given type or returns an error.\nfunc (p *parser) expect(typ token.Type) (token.Token, error) {\n\ttok := p.current()\n\tif tok.Type != typ {\n\t\treturn tok, fmt.Errorf(\"expected %v but got %v at position %d\", typ, tok.Type, tok.Pos)\n\t}\n\tp.advance()\n\treturn tok, nil\n}\n\n// expr → term (('+' | '-') term)*\nfunc (p *parser) expr() (ast.Node, error) {\n\tleft, err := p.term()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Plus || p.current().Type == token.Minus {\n\t\top := p.advance()\n\t\tright, err := p.term()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = &ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// term → factor (('*' | '/') factor)*\nfunc (p *parser) term() (ast.Node, error) {\n\tleft, err := p.factor()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Star || p.current().Type == token.Slash {\n\t\top := p.advance()\n\t\tright, err := p.factor()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = &ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// factor → NUMBER | IDENT | IDENT '(' args ')' | '(' expr ')'\nfunc (p *parser) factor() (ast.Node, error) {\n\ttok := p.current()\n\tswitch tok.Type {\n\tcase token.Number:\n\t\tp.advance()\n\t\tval, err := strconv.ParseFloat(tok.Literal, 64)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"invalid number %q at position %d: %w\", tok.Literal, tok.Pos, err)\n\t\t}\n\t\treturn &ast.NumberLit{Value: val}, nil\n\n\tcase token.Ident:\n\t\tp.advance()\n\t\t// If followed by '(', this is a function call.\n\t\tif p.current().Type == token.LParen {\n\t\t\tp.advance() // consume '('\n\t\t\tvar args []ast.Node\n\t\t\tif p.current().Type != token.RParen {\n\t\t\t\targ, err := p.expr()\n\t\t\t\tif err != nil {\n\t\t\t\t\treturn nil, err\n\t\t\t\t}\n\t\t\t\targs = append(args, arg)\n\t\t\t\tfor p.current().Type == token.Comma {\n\t\t\t\t\tp.advance() // consume ','\n\t\t\t\t\targ, err := p.expr()\n\t\t\t\t\tif err != nil {\n\t\t\t\t\t\treturn nil, err\n\t\t\t\t\t}\n\t\t\t\t\targs = append(args, arg)\n\t\t\t\t}\n\t\t\t}\n\t\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\t\treturn nil, fmt.Errorf(\"expected ')' after function arguments at position %d\", p.current().Pos)\n\t\t\t}\n\t\t\treturn &ast.FuncCall{Name: tok.Literal, Args: args}, nil\n\t\t}\n\t\t// Otherwise, it's a variable reference.\n\t\treturn &ast.Ident{Name: tok.Literal}, nil\n\n\tcase token.LParen:\n\t\tp.advance() // consume '('\n\t\tnode, err := p.expr()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\treturn nil, fmt.Errorf(\"missing closing parenthesis at position %d\", p.current().Pos)\n\t\t}\n\t\treturn node, nil\n\n\tdefault:\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let's verify existing tests still pass:
+!!!!! {"command":"go test ./parser/ -v 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./parser/ -v 2>&1"}
+$$$ "=== RUN   TestParseSingleNumber\n--- PASS: TestParseSingleNumber (0.00s)\n=== RUN   TestParseDecimalNumber\n--- PASS: TestParseDecimalNumber (0.00s)\n=== RUN   TestParseAddition\n--- PASS: TestParseAddition (0.00s)\n=== RUN   TestParseSubtraction\n--- PASS: TestParseSubtraction (0.00s)\n=== RUN   TestParseMultiplication\n--- PASS: TestParseMultiplication (0.00s)\n=== RUN   TestParseDivision\n--- PASS: TestParseDivision (0.00s)\n=== RUN   TestParsePrecedence\n--- PASS: TestParsePrecedence (0.00s)\n=== RUN   TestParsePrecedenceMulFirst\n--- PASS: TestParsePrecedenceMulFirst (0.00s)\n=== RUN   TestParseLeftAssociativity\n--- PASS: TestParseLeftAssociativity (0.00s)\n=== RUN   TestParseParentheses\n--- PASS: TestParseParentheses (0.00s)\n=== RUN   TestParseNestedParentheses\n--- PASS: TestParseNestedParentheses (0.00s)\n=== RUN   TestParseComplexExpression\n--- PASS: TestParseComplexExpression (0.00s)\n=== RUN   TestParseEmptyInput\n--- PASS: TestParseEmptyInput (0.00s)\n=== RUN   TestParseMissingRParen\n--- PASS: TestParseMissingRParen (0.00s)\n=== RUN   TestParseUnexpectedRParen\n--- PASS: TestParseUnexpectedRParen (0.00s)\n=== RUN   TestParseTrailingOperator\n--- PASS: TestParseTrailingOperator (0.00s)\n=== RUN   TestParseTrailingTokens\n--- PASS: TestParseTrailingTokens (0.00s)\n=== RUN   TestParseConsecutiveOperators\n--- PASS: TestParseConsecutiveOperators (0.00s)\n=== RUN   TestParseEmptyParens\n--- PASS: TestParseEmptyParens (0.00s)\nPASS\nok  \tmatheval/parser\t0.586s\n"
+
+--- STOP_REASON: max_tokens
+AI: All 19 existing tests pass. Now let me add comprehensive tests for the new functionality:
+CALLING TOOL: file_write {"path":"parser/parser_test.go"}
+$$$ "done"
+
+--- STOP_REASON: max_tokens
+CALLING TOOL: file_write {"path":"parser/parser_test.go"}
+$$$ "done"
+
+interrupt: 2
+~/dev/src/math $ ../neo/neo 
+YOU: read docs/plan.md and continue implementing function definition support
+## USER NO ITEMS
+1: read docs/plan.md and continue implementing function definition support - IN PROGRESS assistant false
+-- AGENT START WORKING 1
+--- STOP_REASON: tool_use
+AI: 
+
+I'll start by reading the plan document to understand the current state and what needs to be implemented.
+CALLING TOOL: file_read {"path":"docs/plan.md"}
+$$$ "# Implementation Plan: Function Definitions\n\n## Overview\nBottom-up implementation through the stack: token → ast → lexer → parser → evaluator → repl → integration tests. Each step maintains backward compatibility and follows TDD.\n\n## Steps\n\n### Step 1: Token layer (`token/token.go`)\n- Add `Ident`, `Comma`, `Equals` constants to `Type` enum\n- Update `String()` for new types\n- No tests needed — pure data types\n\n### Step 2: AST layer (`ast/ast.go`)\n- Add `Ident` struct: `Name string`; implements `Node`\n- Add `FuncCall` struct: `Name string`, `Args []Node`; implements `Node`\n- Add `Statement` interface with sealed `stmt()` marker\n- Add `ExprStmt` struct: `Expr Node`; implements `Statement`\n- Add `FuncDef` struct: `Name string`, `Params []string`, `Body Node`; implements `Statement`\n- No tests needed — pure data types\n\n### Step 3: Lexer (`lexer/lexer.go`)\n- Add `isLetter(ch byte) bool` helper\n- Before the single-char switch, add branch: if `isLetter(ch)`, scan identifier (letter then letters/digits), emit `Ident` token\n- Add `','` → `Comma` and `'='` → `Equals` to single-char switch\n- **Tests:** identifiers (`x`, `foo`, `f1`), comma, equals, full definition `f(x) = x + 1`, call `f(1, 2)`, mixed with numbers\n\n### Step 4: Parser (`parser/parser.go`)\n- Extend `factor()`:\n  - `Ident` followed by `LParen` → parse `FuncCall`: consume `(`, parse args as comma-separated exprs, consume `)`\n  - `Ident` not followed by `LParen` → return `\u0026ast.Ident{Name}`\n- Add `parseFuncDef()`: expects `Ident(` params `) = expr`\n- Add `ParseLine(tokens) (Statement, error)`:\n  - Scan for `Equals` token (not inside parens)\n  - If found → `parseFuncDef()` → `*ast.FuncDef`\n  - If not → `expr()` → `*ast.ExprStmt{Expr}`\n- Keep `Parse()` unchanged for backward compat\n- **Tests:** ParseLine for defs and exprs, factor for ident and func call, error cases\n\n### Step 5: Evaluator (`evaluator/evaluator.go`)\n- Add `Evaluator` struct with `funcs map[string]*ast.FuncDef`\n- `New() *Evaluator`\n- `Define(def *ast.FuncDef) error` — error on redefinition\n- `Eval(node ast.Node, env map[string]float64) (float64, error)`:\n  - `*ast.NumberLit` → return value\n  - `*ast.BinaryExpr` → recurse left/right with same env\n  - `*ast.Ident` → lookup in env, error if not found\n  - `*ast.FuncCall` → lookup func, eval args in caller env, bind params, eval body in new env\n- Keep package-level `Eval(node) (float64, error)` as backward-compat wrapper\n- **Tests:** all existing tests still pass, new tests for Ident, FuncCall, Define, errors\n\n### Step 6: REPL (`repl/repl.go`)\n- In `Run()`: create `evaluator.New()` before loop\n- Replace `evalLine()` with inline logic using `ParseLine()`\n- `*ast.FuncDef` → `ev.Define(def)`, print `\"defined \u003cname\u003e\"`\n- `*ast.ExprStmt` → `ev.Eval(stmt.Expr, nil)`, print result\n- **Tests:** define + call across lines, redefine error, undefined func error\n\n### Step 7: Integration tests (`integration_test.go`)\n- Update `eval()`/`evalErr()` helpers to use `Evaluator` struct\n- Add tests:\n  - Define and call single-param function\n  - Define and call multi-param function\n  - Cross-function calls\n  - Nested function calls in expressions\n  - Error: undefined function\n  - Error: wrong argument count\n  - Error: function redefinition\n  - Error: undefined variable\n\n## Backward Compatibility\n- `Parse()` remains unchanged — returns `ast.Node`\n- Package-level `Eval()` remains — wraps `New().Eval(node, nil)`\n- Existing tests must continue to pass at every step\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me examine the current state of the codebase:
+CALLING TOOL: dir_list {"name":"."}
+$$$ {"entries":[{"name":".git","is_dir":true},{"name":"README.md","is_dir":false},{"name":"ast","is_dir":true},{"name":"cmd","is_dir":true},{"name":"docs","is_dir":true},{"name":"evaluator","is_dir":true},{"name":"go.mod","is_dir":false},{"name":"integration_test.go","is_dir":false},{"name":"lexer","is_dir":true},{"name":"matheval","is_dir":false},{"name":"parser","is_dir":true},{"name":"repl","is_dir":true},{"name":"sessions","is_dir":true},{"name":"token","is_dir":true}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"token/token.go"}
+$$$ "package token\n\nimport \"fmt\"\n\n// Type represents the type of a lexical token.\ntype Type int\n\nconst (\n\tNumber Type = iota // numeric literal\n\tPlus               // +\n\tMinus              // -\n\tStar               // *\n\tSlash              // /\n\tLParen             // (\n\tRParen             // )\n\tIdent              // identifier (e.g. \"f\", \"x\")\n\tComma              // ,\n\tEquals             // =\n\tEOF                // end of input\n)\n\n// String returns a human-readable name for the token type.\nfunc (t Type) String() string {\n\tswitch t {\n\tcase Number:\n\t\treturn \"Number\"\n\tcase Plus:\n\t\treturn \"+\"\n\tcase Minus:\n\t\treturn \"-\"\n\tcase Star:\n\t\treturn \"*\"\n\tcase Slash:\n\t\treturn \"/\"\n\tcase LParen:\n\t\treturn \"(\"\n\tcase RParen:\n\t\treturn \")\"\n\tcase Ident:\n\t\treturn \"Ident\"\n\tcase Comma:\n\t\treturn \",\"\n\tcase Equals:\n\t\treturn \"=\"\n\tcase EOF:\n\t\treturn \"EOF\"\n\tdefault:\n\t\treturn fmt.Sprintf(\"Unknown(%d)\", int(t))\n\t}\n}\n\n// Token represents a single lexical token.\ntype Token struct {\n\tType    Type   // the kind of token\n\tLiteral string // raw text (e.g. \"3.14\", \"+\")\n\tPos     int    // byte offset in input string\n}\n"
+
+CALLING TOOL: file_read {"path":"ast/ast.go"}
+$$$ "package ast\n\nimport \"matheval/token\"\n\n// Node is the interface all AST nodes implement.\n// The unexported marker method seals the interface — only types\n// in this package can implement it.\ntype Node interface {\n\tnode() // sealed marker\n}\n\n// NumberLit represents a numeric literal (e.g. 3.14).\ntype NumberLit struct {\n\tValue float64\n}\n\nfunc (*NumberLit) node() {}\n\n// BinaryExpr represents a binary operation (e.g. 1 + 2).\ntype BinaryExpr struct {\n\tOp    token.Type // Plus, Minus, Star, Slash\n\tLeft  Node\n\tRight Node\n}\n\nfunc (*BinaryExpr) node() {}\n\n// Ident represents a variable reference (e.g. x, y).\ntype Ident struct {\n\tName string\n}\n\nfunc (*Ident) node() {}\n\n// FuncCall represents a function call (e.g. f(1, 2+3)).\ntype FuncCall struct {\n\tName string\n\tArgs []Node\n}\n\nfunc (*FuncCall) node() {}\n\n// Statement is the interface for top-level REPL input.\n// A line is either an expression to evaluate or a function definition.\n// The unexported marker method seals the interface.\ntype Statement interface {\n\tstmt() // sealed marker\n}\n\n// ExprStmt wraps an expression as a top-level statement.\ntype ExprStmt struct {\n\tExpr Node\n}\n\nfunc (*ExprStmt) stmt() {}\n\n// FuncDef represents a function definition (e.g. f(x, y) = x + y).\ntype FuncDef struct {\n\tName   string\n\tParams []string\n\tBody   Node\n}\n\nfunc (*FuncDef) stmt() {}\n"
+
+CALLING TOOL: file_read {"path":"lexer/lexer.go"}
+$$$ "package lexer\n\nimport (\n\t\"fmt\"\n\t\"matheval/token\"\n)\n\n// Tokenize converts an input string into a slice of tokens.\n// Returns an error if the input contains invalid characters.\nfunc Tokenize(input string) ([]token.Token, error) {\n\tvar tokens []token.Token\n\ti := 0\n\n\tfor i \u003c len(input) {\n\t\tch := input[i]\n\n\t\t// Skip whitespace.\n\t\tif ch == ' ' || ch == '\\t' || ch == '\\n' || ch == '\\r' {\n\t\t\ti++\n\t\t\tcontinue\n\t\t}\n\n\t\t// Number: digits and dots.\n\t\tif isDigit(ch) || ch == '.' {\n\t\t\tstart := i\n\t\t\thasDot := false\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isDigit(input[i]) || input[i] == '.') {\n\t\t\t\tif input[i] == '.' {\n\t\t\t\t\tif hasDot {\n\t\t\t\t\t\tbreak // second dot ends this number\n\t\t\t\t\t}\n\t\t\t\t\thasDot = true\n\t\t\t\t}\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Number,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Identifier: starts with letter, continues with letters/digits.\n\t\tif isLetter(ch) {\n\t\t\tstart := i\n\t\t\tfor i \u003c len(input) \u0026\u0026 (isLetter(input[i]) || isDigit(input[i])) {\n\t\t\t\ti++\n\t\t\t}\n\t\t\ttokens = append(tokens, token.Token{\n\t\t\t\tType:    token.Ident,\n\t\t\t\tLiteral: input[start:i],\n\t\t\t\tPos:     start,\n\t\t\t})\n\t\t\tcontinue\n\t\t}\n\n\t\t// Single-character tokens.\n\t\tvar typ token.Type\n\t\tswitch ch {\n\t\tcase '+':\n\t\t\ttyp = token.Plus\n\t\tcase '-':\n\t\t\ttyp = token.Minus\n\t\tcase '*':\n\t\t\ttyp = token.Star\n\t\tcase '/':\n\t\t\ttyp = token.Slash\n\t\tcase '(':\n\t\t\ttyp = token.LParen\n\t\tcase ')':\n\t\t\ttyp = token.RParen\n\t\tcase ',':\n\t\t\ttyp = token.Comma\n\t\tcase '=':\n\t\t\ttyp = token.Equals\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"unexpected character %q at position %d\", string(ch), i)\n\t\t}\n\n\t\ttokens = append(tokens, token.Token{\n\t\t\tType:    typ,\n\t\t\tLiteral: string(ch),\n\t\t\tPos:     i,\n\t\t})\n\t\ti++\n\t}\n\n\t// Always append EOF.\n\ttokens = append(tokens, token.Token{\n\t\tType:    token.EOF,\n\t\tLiteral: \"\",\n\t\tPos:     i,\n\t})\n\n\treturn tokens, nil\n}\n\nfunc isDigit(ch byte) bool {\n\treturn ch \u003e= '0' \u0026\u0026 ch \u003c= '9'\n}\n\nfunc isLetter(ch byte) bool {\n\treturn (ch \u003e= 'a' \u0026\u0026 ch \u003c= 'z') || (ch \u003e= 'A' \u0026\u0026 ch \u003c= 'Z') || ch == '_'\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser.go"}
+$$$ "package parser\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strconv\"\n)\n\n// Parse converts a slice of tokens into an AST.\n// Returns an error for malformed expressions.\nfunc Parse(tokens []token.Token) (ast.Node, error) {\n\tp := \u0026parser{tokens: tokens}\n\tnode, err := p.expr()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\t// Ensure all tokens have been consumed (only EOF remains).\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n\treturn node, nil\n}\n\n// ParseLine parses a full REPL line, returning either a function definition\n// or an expression statement.\nfunc ParseLine(tokens []token.Token) (ast.Statement, error) {\n\tif len(tokens) == 0 || tokens[0].Type == token.EOF {\n\t\treturn nil, fmt.Errorf(\"empty input\")\n\t}\n\n\t// Detect function definition: look for Equals token.\n\t// A function definition has the form: Ident LParen params RParen Equals body\n\tif isFuncDef(tokens) {\n\t\treturn parseFuncDef(tokens)\n\t}\n\n\t// Otherwise, parse as expression.\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\treturn \u0026ast.ExprStmt{Expr: node}, nil\n}\n\n// isFuncDef checks if the token stream looks like a function definition.\n// Pattern: Ident LParen ... RParen Equals ...\nfunc isFuncDef(tokens []token.Token) bool {\n\tif len(tokens) \u003c 5 {\n\t\treturn false\n\t}\n\tif tokens[0].Type != token.Ident {\n\t\treturn false\n\t}\n\tif tokens[1].Type != token.LParen {\n\t\treturn false\n\t}\n\t// Find matching RParen, then check for Equals.\n\tdepth := 0\n\tfor i := 1; i \u003c len(tokens); i++ {\n\t\tswitch tokens[i].Type {\n\t\tcase token.LParen:\n\t\t\tdepth++\n\t\tcase token.RParen:\n\t\t\tdepth--\n\t\t\tif depth == 0 {\n\t\t\t\t// Next token must be Equals for this to be a func def.\n\t\t\t\tif i+1 \u003c len(tokens) \u0026\u0026 tokens[i+1].Type == token.Equals {\n\t\t\t\t\treturn true\n\t\t\t\t}\n\t\t\t\treturn false\n\t\t\t}\n\t\tcase token.EOF:\n\t\t\treturn false\n\t\t}\n\t}\n\treturn false\n}\n\n// parseFuncDef parses: Ident LParen param1, param2, ... RParen Equals body\nfunc parseFuncDef(tokens []token.Token) (*ast.FuncDef, error) {\n\tp := \u0026parser{tokens: tokens}\n\n\t// Function name.\n\tnameTok, err := p.expect(token.Ident)\n\tif err != nil {\n\t\treturn nil, fmt.Errorf(\"expected function name: %w\", err)\n\t}\n\tname := nameTok.Literal\n\n\t// Opening paren.\n\tif _, err := p.expect(token.LParen); err != nil {\n\t\treturn nil, fmt.Errorf(\"expected '(' after function name: %w\", err)\n\t}\n\n\t// Parameters: comma-separated identifiers.\n\tvar params []string\n\tif p.current().Type != token.RParen {\n\t\tparamTok, err := p.expect(token.Ident)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"expected parameter name: %w\", err)\n\t\t}\n\t\tparams = append(params, paramTok.Literal)\n\n\t\tfor p.current().Type == token.Comma {\n\t\t\tp.advance() // consume comma\n\t\t\tparamTok, err := p.expect(token.Ident)\n\t\t\tif err != nil {\n\t\t\t\treturn nil, fmt.Errorf(\"expected parameter name after ',': %w\", err)\n\t\t\t}\n\t\t\tparams = append(params, paramTok.Literal)\n\t\t}\n\t}\n\n\t// Closing paren.\n\tif _, err := p.expect(token.RParen); err != nil {\n\t\treturn nil, fmt.Errorf(\"expected ')' after parameters: %w\", err)\n\t}\n\n\t// Equals sign.\n\tif _, err := p.expect(token.Equals); err != nil {\n\t\treturn nil, fmt.Errorf(\"expected '=' in function definition: %w\", err)\n\t}\n\n\t// Body expression.\n\tbody, err := p.expr()\n\tif err != nil {\n\t\treturn nil, fmt.Errorf(\"error in function body: %w\", err)\n\t}\n\n\t// Ensure all tokens consumed.\n\tif p.current().Type != token.EOF {\n\t\ttok := p.current()\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d after function body\", tok.Type, tok.Pos)\n\t}\n\n\treturn \u0026ast.FuncDef{\n\t\tName:   name,\n\t\tParams: params,\n\t\tBody:   body,\n\t}, nil\n}\n\n// parser holds the state for a single parse operation.\ntype parser struct {\n\ttokens []token.Token\n\tpos    int\n}\n\n// current returns the token at the current position.\nfunc (p *parser) current() token.Token {\n\tif p.pos \u003e= len(p.tokens) {\n\t\treturn token.Token{Type: token.EOF}\n\t}\n\treturn p.tokens[p.pos]\n}\n\n// advance moves to the next token and returns the previous one.\nfunc (p *parser) advance() token.Token {\n\ttok := p.current()\n\tp.pos++\n\treturn tok\n}\n\n// expect consumes a token of the given type or returns an error.\nfunc (p *parser) expect(typ token.Type) (token.Token, error) {\n\ttok := p.current()\n\tif tok.Type != typ {\n\t\treturn tok, fmt.Errorf(\"expected %v but got %v at position %d\", typ, tok.Type, tok.Pos)\n\t}\n\tp.advance()\n\treturn tok, nil\n}\n\n// expr → term (('+' | '-') term)*\nfunc (p *parser) expr() (ast.Node, error) {\n\tleft, err := p.term()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Plus || p.current().Type == token.Minus {\n\t\top := p.advance()\n\t\tright, err := p.term()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// term → factor (('*' | '/') factor)*\nfunc (p *parser) term() (ast.Node, error) {\n\tleft, err := p.factor()\n\tif err != nil {\n\t\treturn nil, err\n\t}\n\tfor p.current().Type == token.Star || p.current().Type == token.Slash {\n\t\top := p.advance()\n\t\tright, err := p.factor()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tleft = \u0026ast.BinaryExpr{\n\t\t\tOp:    op.Type,\n\t\t\tLeft:  left,\n\t\t\tRight: right,\n\t\t}\n\t}\n\treturn left, nil\n}\n\n// factor → NUMBER | IDENT | IDENT '(' args ')' | '(' expr ')'\nfunc (p *parser) factor() (ast.Node, error) {\n\ttok := p.current()\n\tswitch tok.Type {\n\tcase token.Number:\n\t\tp.advance()\n\t\tval, err := strconv.ParseFloat(tok.Literal, 64)\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"invalid number %q at position %d: %w\", tok.Literal, tok.Pos, err)\n\t\t}\n\t\treturn \u0026ast.NumberLit{Value: val}, nil\n\n\tcase token.Ident:\n\t\tp.advance()\n\t\t// If followed by '(', this is a function call.\n\t\tif p.current().Type == token.LParen {\n\t\t\tp.advance() // consume '('\n\t\t\tvar args []ast.Node\n\t\t\tif p.current().Type != token.RParen {\n\t\t\t\targ, err := p.expr()\n\t\t\t\tif err != nil {\n\t\t\t\t\treturn nil, err\n\t\t\t\t}\n\t\t\t\targs = append(args, arg)\n\t\t\t\tfor p.current().Type == token.Comma {\n\t\t\t\t\tp.advance() // consume ','\n\t\t\t\t\targ, err := p.expr()\n\t\t\t\t\tif err != nil {\n\t\t\t\t\t\treturn nil, err\n\t\t\t\t\t}\n\t\t\t\t\targs = append(args, arg)\n\t\t\t\t}\n\t\t\t}\n\t\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\t\treturn nil, fmt.Errorf(\"expected ')' after function arguments at position %d\", p.current().Pos)\n\t\t\t}\n\t\t\treturn \u0026ast.FuncCall{Name: tok.Literal, Args: args}, nil\n\t\t}\n\t\t// Otherwise, it's a variable reference.\n\t\treturn \u0026ast.Ident{Name: tok.Literal}, nil\n\n\tcase token.LParen:\n\t\tp.advance() // consume '('\n\t\tnode, err := p.expr()\n\t\tif err != nil {\n\t\t\treturn nil, err\n\t\t}\n\t\tif _, err := p.expect(token.RParen); err != nil {\n\t\t\treturn nil, fmt.Errorf(\"missing closing parenthesis at position %d\", p.current().Pos)\n\t\t}\n\t\treturn node, nil\n\n\tdefault:\n\t\treturn nil, fmt.Errorf(\"unexpected token %v at position %d\", tok.Type, tok.Pos)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator.go"}
+$$$ "package evaluator\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n)\n\n// Eval evaluates an AST node and returns the result.\n// Returns an error on division by zero.\nfunc Eval(node ast.Node) (float64, error) {\n\tswitch n := node.(type) {\n\tcase *ast.NumberLit:\n\t\treturn n.Value, nil\n\n\tcase *ast.BinaryExpr:\n\t\tleft, err := Eval(n.Left)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\t\tright, err := Eval(n.Right)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\n\t\tswitch n.Op {\n\t\tcase token.Plus:\n\t\t\treturn left + right, nil\n\t\tcase token.Minus:\n\t\t\treturn left - right, nil\n\t\tcase token.Star:\n\t\t\treturn left * right, nil\n\t\tcase token.Slash:\n\t\t\tif right == 0 {\n\t\t\t\treturn 0, fmt.Errorf(\"division by zero\")\n\t\t\t}\n\t\t\treturn left / right, nil\n\t\tdefault:\n\t\t\treturn 0, fmt.Errorf(\"unknown operator: %v\", n.Op)\n\t\t}\n\n\tdefault:\n\t\treturn 0, fmt.Errorf(\"unknown node type: %T\", node)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl.go"}
+$$$ "package repl\n\nimport (\n\t\"bufio\"\n\t\"fmt\"\n\t\"io\"\n\t\"math\"\n\t\"strings\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\nconst prompt = \"\u003e\u003e \"\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer) {\n\tscanner := bufio.NewScanner(r)\n\tfmt.Fprint(w, prompt)\n\n\tfor scanner.Scan() {\n\t\tline := strings.TrimSpace(scanner.Text())\n\t\tif line == \"\" {\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\tresult, err := evalLine(line)\n\t\tif err != nil {\n\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t} else {\n\t\t\tfmt.Fprintln(w, formatResult(result))\n\t\t}\n\n\t\tfmt.Fprint(w, prompt)\n\t}\n}\n\n// evalLine tokenizes, parses, and evaluates a single expression string.\nfunc evalLine(line string) (float64, error) {\n\ttokens, err := lexer.Tokenize(line)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn 0, err\n\t}\n\n\treturn evaluator.Eval(tree)\n}\n\n// formatResult formats a float64 for display.\n// Whole numbers are printed without decimal points.\nfunc formatResult(val float64) string {\n\tif val == math.Trunc(val) \u0026\u0026 !math.IsInf(val, 0) \u0026\u0026 !math.IsNaN(val) {\n\t\treturn fmt.Sprintf(\"%g\", val)\n\t}\n\treturn fmt.Sprintf(\"%g\", val)\n}\n"
+
+CALLING TOOL: file_read {"path":"integration_test.go"}
+$$$ "package matheval_test\n\nimport (\n\t\"math\"\n\t\"strings\"\n\t\"testing\"\n\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\n// eval is a helper that runs the full pipeline: lexer → parser → evaluator.\nfunc eval(t *testing.T, input string) float64 {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\tt.Fatalf(\"lexer error for %q: %v\", input, err)\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"parser error for %q: %v\", input, err)\n\t}\n\tresult, err := evaluator.Eval(tree)\n\tif err != nil {\n\t\tt.Fatalf(\"evaluator error for %q: %v\", input, err)\n\t}\n\treturn result\n}\n\n// evalErr is a helper that expects the full pipeline to return an error.\nfunc evalErr(t *testing.T, input string) error {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\treturn err\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn err\n\t}\n\t_, err = evaluator.Eval(tree)\n\treturn err\n}\n\nfunc assertApprox(t *testing.T, input string, expected, got float64) {\n\tt.Helper()\n\tif math.Abs(expected-got) \u003e 1e-9 {\n\t\tt.Errorf(\"%q: expected %v, got %v\", input, expected, got)\n\t}\n}\n\n// --- Basic arithmetic ---\n\nfunc TestIntegration_SingleNumber(t *testing.T) {\n\tassertApprox(t, \"42\", 42, eval(t, \"42\"))\n}\n\nfunc TestIntegration_DecimalNumber(t *testing.T) {\n\tassertApprox(t, \"3.14\", 3.14, eval(t, \"3.14\"))\n}\n\nfunc TestIntegration_LeadingDot(t *testing.T) {\n\tassertApprox(t, \".5\", 0.5, eval(t, \".5\"))\n}\n\nfunc TestIntegration_Addition(t *testing.T) {\n\tassertApprox(t, \"1 + 2\", 3, eval(t, \"1 + 2\"))\n}\n\nfunc TestIntegration_Subtraction(t *testing.T) {\n\tassertApprox(t, \"10 - 4\", 6, eval(t, \"10 - 4\"))\n}\n\nfunc TestIntegration_Multiplication(t *testing.T) {\n\tassertApprox(t, \"3 * 7\", 21, eval(t, \"3 * 7\"))\n}\n\nfunc TestIntegration_Division(t *testing.T) {\n\tassertApprox(t, \"10 / 4\", 2.5, eval(t, \"10 / 4\"))\n}\n\n// --- Precedence and associativity ---\n\nfunc TestIntegration_PrecedenceMulOverAdd(t *testing.T) {\n\t// 2 + 3 * 4 = 2 + 12 = 14\n\tassertApprox(t, \"2 + 3 * 4\", 14, eval(t, \"2 + 3 * 4\"))\n}\n\nfunc TestIntegration_PrecedenceDivOverSub(t *testing.T) {\n\t// 10 - 6 / 3 = 10 - 2 = 8\n\tassertApprox(t, \"10 - 6 / 3\", 8, eval(t, \"10 - 6 / 3\"))\n}\n\nfunc TestIntegration_LeftAssociativitySub(t *testing.T) {\n\t// 10 - 3 - 2 = (10 - 3) - 2 = 5\n\tassertApprox(t, \"10 - 3 - 2\", 5, eval(t, \"10 - 3 - 2\"))\n}\n\nfunc TestIntegration_LeftAssociativityDiv(t *testing.T) {\n\t// 24 / 4 / 3 = (24 / 4) / 3 = 2\n\tassertApprox(t, \"24 / 4 / 3\", 2, eval(t, \"24 / 4 / 3\"))\n}\n\n// --- Parentheses ---\n\nfunc TestIntegration_ParensOverridePrecedence(t *testing.T) {\n\t// (2 + 3) * 4 = 20\n\tassertApprox(t, \"(2 + 3) * 4\", 20, eval(t, \"(2 + 3) * 4\"))\n}\n\nfunc TestIntegration_NestedParens(t *testing.T) {\n\t// ((1 + 2)) = 3\n\tassertApprox(t, \"((1 + 2))\", 3, eval(t, \"((1 + 2))\"))\n}\n\nfunc TestIntegration_DeeplyNestedParens(t *testing.T) {\n\t// ((((((1 + 2)))))) = 3\n\tassertApprox(t, \"((((((1 + 2))))))\", 3, eval(t, \"((((((1 + 2))))))\"))\n}\n\nfunc TestIntegration_ParensOnBothSides(t *testing.T) {\n\t// (1 + 2) * (3 + 4) = 3 * 7 = 21\n\tassertApprox(t, \"(1 + 2) * (3 + 4)\", 21, eval(t, \"(1 + 2) * (3 + 4)\"))\n}\n\nfunc TestIntegration_ParensNestedComplex(t *testing.T) {\n\t// ((2 + 3) * (4 - 1)) / 5 = (5 * 3) / 5 = 3\n\tassertApprox(t, \"((2 + 3) * (4 - 1)) / 5\", 3, eval(t, \"((2 + 3) * (4 - 1)) / 5\"))\n}\n\n// --- Complex expressions ---\n\nfunc TestIntegration_AllOperators(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2 = 1 + 6 - 2 = 5\n\tassertApprox(t, \"1 + 2 * 3 - 4 / 2\", 5, eval(t, \"1 + 2 * 3 - 4 / 2\"))\n}\n\nfunc TestIntegration_LongChainedAddition(t *testing.T) {\n\t// 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55\n\tassertApprox(t, \"1+2+3+4+5+6+7+8+9+10\", 55, eval(t, \"1+2+3+4+5+6+7+8+9+10\"))\n}\n\nfunc TestIntegration_LongChainedMixed(t *testing.T) {\n\t// 2 * 3 + 4 * 5 - 6 / 2 + 1 = 6 + 20 - 3 + 1 = 24\n\tassertApprox(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\", 24, eval(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\"))\n}\n\n// --- Floating point ---\n\nfunc TestIntegration_FloatArithmetic(t *testing.T) {\n\t// 1.5 + 2.5 = 4.0\n\tassertApprox(t, \"1.5 + 2.5\", 4.0, eval(t, \"1.5 + 2.5\"))\n}\n\nfunc TestIntegration_FloatDivision(t *testing.T) {\n\t// 7 / 2 = 3.5\n\tassertApprox(t, \"7 / 2\", 3.5, eval(t, \"7 / 2\"))\n}\n\nfunc TestIntegration_FloatPrecision(t *testing.T) {\n\t// 0.1 + 0.2 ≈ 0.3 (within tolerance)\n\tassertApprox(t, \"0.1 + 0.2\", 0.3, eval(t, \"0.1 + 0.2\"))\n}\n\n// --- Whitespace variations ---\n\nfunc TestIntegration_NoSpaces(t *testing.T) {\n\tassertApprox(t, \"1+2*3\", 7, eval(t, \"1+2*3\"))\n}\n\nfunc TestIntegration_ExtraSpaces(t *testing.T) {\n\tassertApprox(t, \"  1  +  2  \", 3, eval(t, \"  1  +  2  \"))\n}\n\nfunc TestIntegration_TabsAndSpaces(t *testing.T) {\n\tassertApprox(t, \"1\\t+\\t2\", 3, eval(t, \"1\\t+\\t2\"))\n}\n\n// --- Error cases ---\n\nfunc TestIntegration_DivisionByZero(t *testing.T) {\n\terr := evalErr(t, \"1 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n\tif !strings.Contains(err.Error(), \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_DivisionByZeroInSubExpr(t *testing.T) {\n\terr := evalErr(t, \"1 + 2 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestIntegration_InvalidCharacter(t *testing.T) {\n\terr := evalErr(t, \"1 @ 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenLeft(t *testing.T) {\n\terr := evalErr(t, \"(1 + 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing closing paren\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenRight(t *testing.T) {\n\terr := evalErr(t, \"1 + 2)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected closing paren\")\n\t}\n}\n\nfunc TestIntegration_EmptyParens(t *testing.T) {\n\terr := evalErr(t, \"()\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\nfunc TestIntegration_TrailingOperator(t *testing.T) {\n\terr := evalErr(t, \"1 +\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestIntegration_LeadingOperator(t *testing.T) {\n\terr := evalErr(t, \"* 1\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for leading operator\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveOperators(t *testing.T) {\n\terr := evalErr(t, \"1 + * 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestIntegration_EmptyInput(t *testing.T) {\n\t// Empty string should produce only EOF, parser should error\n\terr := evalErr(t, \"\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveNumbers(t *testing.T) {\n\terr := evalErr(t, \"1 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive numbers without operator\")\n\t}\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me check if there are any existing tests for the lexer, parser, and evaluator:
+CALLING TOOL: dir_list {"name":"lexer"}
+$$$ {"entries":[{"name":"lexer.go","is_dir":false},{"name":"lexer_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"parser"}
+$$$ {"entries":[{"name":"parser.go","is_dir":false},{"name":"parser_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"evaluator"}
+$$$ {"entries":[{"name":"evaluator.go","is_dir":false},{"name":"evaluator_test.go","is_dir":false}]}
+
+CALLING TOOL: dir_list {"name":"repl"}
+$$$ {"entries":[{"name":"repl.go","is_dir":false},{"name":"repl_test.go","is_dir":false}]}
+
+--- STOP_REASON: tool_use
+CALLING TOOL: file_read {"path":"lexer/lexer_test.go"}
+$$$ "package lexer\n\nimport (\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestTokenizeEmpty(t *testing.T) {\n\ttokens, err := Tokenize(\"\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeWhitespaceOnly(t *testing.T) {\n\ttokens, err := Tokenize(\"   \\t\\n  \")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif len(tokens) != 1 || tokens[0].Type != token.EOF {\n\t\tt.Fatalf(\"expected single EOF token, got %v\", tokens)\n\t}\n}\n\nfunc TestTokenizeSingleNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"42\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeDecimalNumber(t *testing.T) {\n\ttokens, err := Tokenize(\"3.14\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"3.14\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeLeadingDotNumber(t *testing.T) {\n\ttokens, err := Tokenize(\".5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \".5\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeOperators(t *testing.T) {\n\ttokens, err := Tokenize(\"+-*/\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 0},\n\t\t{Type: token.Minus, Literal: \"-\", Pos: 1},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 2},\n\t\t{Type: token.Slash, Literal: \"/\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeParens(t *testing.T) {\n\ttokens, err := Tokenize(\"()\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 1},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFullExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"(1 + 2.5) * 3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 0},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 1},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2.5\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 10},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 12},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 13},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeNoSpaces(t *testing.T) {\n\ttokens, err := Tokenize(\"1+2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeInvalidCharacter(t *testing.T) {\n\t_, err := Tokenize(\"1 + @\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleInvalidCharacters(t *testing.T) {\n\t_, err := Tokenize(\"1 \u0026 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestTokenizeMultipleDecimals(t *testing.T) {\n\t// \"1.2.3\" — the lexer should read \"1.2\" as a number, then \".3\" as another number\n\ttokens, err := Tokenize(\"1.2.3\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Number, Literal: \"1.2\", Pos: 0},\n\t\t{Type: token.Number, Literal: \".3\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeSingleIdent(t *testing.T) {\n\ttokens, err := Tokenize(\"x\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeMultiCharIdent(t *testing.T) {\n\ttokens, err := Tokenize(\"foo\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"foo\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeIdentWithDigits(t *testing.T) {\n\ttokens, err := Tokenize(\"x2\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"x2\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeIdentWithUnderscore(t *testing.T) {\n\ttokens, err := Tokenize(\"my_func\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"my_func\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 7},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeComma(t *testing.T) {\n\ttokens, err := Tokenize(\",\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Comma, Literal: \",\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeEquals(t *testing.T) {\n\ttokens, err := Tokenize(\"=\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFuncDefinition(t *testing.T) {\n\ttokens, err := Tokenize(\"f(x) = x + 1\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 7},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 9},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 11},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 12},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFuncCallWithArgs(t *testing.T) {\n\ttokens, err := Tokenize(\"f(1, 2)\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 5},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 6},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 7},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeMultiParamFuncDef(t *testing.T) {\n\ttokens, err := Tokenize(\"add(x, y) = x + y\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"add\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 3},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 4},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"y\", Pos: 7},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 10},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 12},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 14},\n\t\t{Type: token.Ident, Literal: \"y\", Pos: 16},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 17},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\nfunc TestTokenizeFuncCallInExpression(t *testing.T) {\n\ttokens, err := Tokenize(\"f(1+2, 3*4) + 5\")\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\texpect := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 4},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 7},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 8},\n\t\t{Type: token.Number, Literal: \"4\", Pos: 9},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 10},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 12},\n\t\t{Type: token.Number, Literal: \"5\", Pos: 14},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 15},\n\t}\n\tassertTokens(t, expect, tokens)\n}\n\n// assertTokens is a test helper that compares two token slices.\nfunc assertTokens(t *testing.T, want, got []token.Token) {\n\tt.Helper()\n\tif len(want) != len(got) {\n\t\tt.Fatalf(\"token count: want %d, got %d\\nwant: %v\\ngot:  %v\", len(want), len(got), want, got)\n\t}\n\tfor i := range want {\n\t\tif want[i].Type != got[i].Type {\n\t\t\tt.Errorf(\"token[%d].Type: want %v, got %v\", i, want[i].Type, got[i].Type)\n\t\t}\n\t\tif want[i].Literal != got[i].Literal {\n\t\t\tt.Errorf(\"token[%d].Literal: want %q, got %q\", i, want[i].Literal, got[i].Literal)\n\t\t}\n\t\tif want[i].Pos != got[i].Pos {\n\t\t\tt.Errorf(\"token[%d].Pos: want %d, got %d\", i, want[i].Pos, got[i].Pos)\n\t\t}\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"parser/parser_test.go"}
+$$$ ""
+
+CALLING TOOL: file_read {"path":"evaluator/evaluator_test.go"}
+$$$ "package evaluator\n\nimport (\n\t\"math\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\nfunc TestEvalNumberLit(t *testing.T) {\n\tresult, err := Eval(\u0026ast.NumberLit{Value: 42.5})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 42.5 {\n\t\tt.Fatalf(\"expected 42.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalAddition(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 3 {\n\t\tt.Fatalf(\"expected 3, got %v\", result)\n\t}\n}\n\nfunc TestEvalSubtraction(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Minus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalMultiplication(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Star,\n\t\tLeft:  \u0026ast.NumberLit{Value: 3},\n\t\tRight: \u0026ast.NumberLit{Value: 7},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 21 {\n\t\tt.Fatalf(\"expected 21, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivision(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 2.5 {\n\t\tt.Fatalf(\"expected 2.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZero(t *testing.T) {\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  \u0026ast.NumberLit{Value: 5},\n\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestEvalNestedExpr(t *testing.T) {\n\t// (1 + 2) * (8 / 4) = 3 * 2 = 6\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 1},\n\t\t\tRight: \u0026ast.NumberLit{Value: 2},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 8},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalDeeplyNested(t *testing.T) {\n\t// ((2 + 3) * 4) - (10 / 5) = 20 - 2 = 18\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp: token.Minus,\n\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\tOp: token.Star,\n\t\t\tLeft: \u0026ast.BinaryExpr{\n\t\t\t\tOp:    token.Plus,\n\t\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\t\tRight: \u0026ast.NumberLit{Value: 3},\n\t\t\t},\n\t\t\tRight: \u0026ast.NumberLit{Value: 4},\n\t\t},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 10},\n\t\t\tRight: \u0026ast.NumberLit{Value: 5},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 18 {\n\t\tt.Fatalf(\"expected 18, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZeroInSubExpr(t *testing.T) {\n\t// 1 + (2 / 0) — error should propagate\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:   token.Plus,\n\t\tLeft: \u0026ast.NumberLit{Value: 1},\n\t\tRight: \u0026ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  \u0026ast.NumberLit{Value: 2},\n\t\t\tRight: \u0026ast.NumberLit{Value: 0},\n\t\t},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error from sub-expression\")\n\t}\n}\n\nfunc TestEvalFloatingPoint(t *testing.T) {\n\t// 1.5 + 2.3 = 3.8\n\tnode := \u0026ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  \u0026ast.NumberLit{Value: 1.5},\n\t\tRight: \u0026ast.NumberLit{Value: 2.3},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif math.Abs(result-3.8) \u003e 1e-12 {\n\t\tt.Fatalf(\"expected 3.8, got %v\", result)\n\t}\n}\n"
+
+CALLING TOOL: file_read {"path":"repl/repl_test.go"}
+$$$ "package repl\n\nimport (\n\t\"bytes\"\n\t\"strings\"\n\t\"testing\"\n)\n\nfunc TestRun_SingleExpression(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n}\n\nfunc TestRun_MultipleExpressions(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n3 * 4\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"12\") {\n\t\tt.Errorf(\"expected output to contain '12', got %q\", got)\n\t}\n}\n\nfunc TestRun_ErrorContinues(t *testing.T) {\n\t// First line has error, second is valid.\n\tin := strings.NewReader(\"1 +\\n2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"5\") {\n\t\tt.Errorf(\"expected output to contain '5' after error recovery, got %q\", got)\n\t}\n}\n\nfunc TestRun_DivisionByZero(t *testing.T) {\n\tin := strings.NewReader(\"1 / 0\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyLine(t *testing.T) {\n\t// Empty lines should be skipped, not cause errors.\n\tin := strings.NewReader(\"\\n1 + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"2\") {\n\t\tt.Errorf(\"expected output to contain '2', got %q\", got)\n\t}\n\t// Should not contain any error.\n\tif strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"empty line should not produce error, got %q\", got)\n\t}\n}\n\nfunc TestRun_Prompt(t *testing.T) {\n\tin := strings.NewReader(\"42\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected prompt '\u003e\u003e ' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_FloatResult(t *testing.T) {\n\tin := strings.NewReader(\"7 / 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3.5\") {\n\t\tt.Errorf(\"expected output to contain '3.5', got %q\", got)\n\t}\n}\n\nfunc TestRun_InvalidCharacter(t *testing.T) {\n\tin := strings.NewReader(\"1 @ 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyInput(t *testing.T) {\n\t// No input at all — just EOF.\n\tin := strings.NewReader(\"\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Should just show the prompt and exit gracefully.\n\tif !strings.Contains(got, \"\u003e\u003e \") {\n\t\tt.Errorf(\"expected at least one prompt, got %q\", got)\n\t}\n}\n\nfunc TestRun_WholeIntegerNoTrailingZeros(t *testing.T) {\n\t// 2 + 3 = 5, should print \"5\" not \"5.000000\".\n\tin := strings.NewReader(\"2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, \u0026out)\n\n\tgot := out.String()\n\t// Result line is \"5\\n\" (between prompts).\n\tif !strings.Contains(got, \"5\\n\") {\n\t\tt.Errorf(\"expected '5\\\\n' in output, got %q\", got)\n\t}\n\t// Should not contain \"5.0\"\n\tif strings.Contains(got, \"5.0\") {\n\t\tt.Errorf(\"expected no trailing zeros, got %q\", got)\n\t}\n}\n"
+
+--- STOP_REASON: tool_use
+AI: Now let me check if existing tests pass:
+!!!!! {"command":"cd /root && go test ./... 2>&1"}: you are already in project dir
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
+ERR: exit status 1
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... 2>&1; echo \"EXIT: $?\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1; echo \"EXIT: $?\""}
+$$$ "parser/parser_test.go:1:1: expected 'package', found 'EOF'\nEXIT: 1\n"
+
+--- STOP_REASON: tool_use
+AI: The parser_test.go is empty. Let me check:
+!!!!! {"command":"cat parser/parser_test.go"}: OK
+CALLING TOOL: bash_command {"command":"cat parser/parser_test.go"}
+$$$ ""
+
+--- STOP_REASON: tool_use
+AI: Empty file. Let me check the go.mod and also understand what we need. The parser test file is empty, which means step 4 tests haven't been written yet. Let me understand the full picture:
+
+- **Step 1 (Token)**: ✅ Done - `Ident`, `Comma`, `Equals` are in token.go
+- **Step 2 (AST)**: ✅ Done - `Ident`, `FuncCall`, `Statement`, `ExprStmt`, `FuncDef` are in ast.go
+- **Step 3 (Lexer)**: ✅ Done - Identifier scanning, comma, equals support with tests
+- **Step 4 (Parser)**: ✅ Code done - `ParseLine`, `isFuncDef`, `parseFuncDef`, factor with Ident/FuncCall. **Tests empty.**
+- **Step 5 (Evaluator)**: ❌ Not done - Still only has the old `Eval(node)` function, no `Evaluator` struct, no env support
+- **Step 6 (REPL)**: ❌ Not done - Still uses old `evalLine` with `Parse` + `Eval`
+- **Step 7 (Integration tests)**: ❌ Not done
+CALLING TOOL: todo_item_add {"items":[{"title":"Implement function definition support (remaining steps)","description":"Steps 1-3 and step 4 code are done. Need: parser tests (step 4), evaluator with env/funcs (step 5), REPL update (step 6), integration tests (step 7).","assignedTo":"assistant","parentId":"1","parallel":false,"items":[{"title":"Step 4: Write parser tests","description":"Parser code is done but parser_test.go is empty. Write tests for: ParseLine for defs and exprs, factor for ident and func call, error cases.","assignedTo":"assistant"},{"title":"Step 5: Evaluator with Evaluator struct, env, and function support","description":"Add Evaluator struct with funcs map, New(), Define(), Eval(node, env). Handle Ident, FuncCall, NumberLit, BinaryExpr. Keep backward-compat package-level Eval(). Write tests.","assignedTo":"assistant"},{"title":"Step 6: Update REPL to use ParseLine and Evaluator","description":"Replace evalLine with ParseLine + Evaluator. Handle FuncDef (define + print 'defined <name>') and ExprStmt (eval + print result). Update REPL tests.","assignedTo":"assistant"},{"title":"Step 7: Integration tests for function definitions","description":"Update eval/evalErr helpers to use Evaluator struct. Add tests for define+call, multi-param, cross-function calls, nested calls, errors (undefined func, wrong arg count, redefinition, undefined var).","assignedTo":"assistant"}]}]}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Let me start with Step 4: Writing parser tests. First, let me fix the empty parser_test.go so tests can run:
+CALLING TOOL: file_write {"path":"parser/parser_test.go","contents":"package parser\n\nimport (\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"testing\"\n)\n\n// --- Parse (backward compatibility) ---\n\nfunc TestParse_SingleNumber(t *testing.T) {\n\ttokens := []token.Token{\n\t\t{Type: token.Number, Literal: \"42\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 2},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tnum, ok := node.(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.NumberLit, got %T\", node)\n\t}\n\tif num.Value != 42 {\n\t\tt.Fatalf(\"expected 42, got %v\", num.Value)\n\t}\n}\n\nfunc TestParse_BinaryExpr(t *testing.T) {\n\ttokens := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 2},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 4},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tbin, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tif bin.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus, got %v\", bin.Op)\n\t}\n}\n\n// --- factor: Ident ---\n\nfunc TestParse_Ident(t *testing.T) {\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 0},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 1},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tident, ok := node.(*ast.Ident)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.Ident, got %T\", node)\n\t}\n\tif ident.Name != \"x\" {\n\t\tt.Fatalf(\"expected 'x', got %q\", ident.Name)\n\t}\n}\n\nfunc TestParse_IdentInExpr(t *testing.T) {\n\t// x + 1\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 2},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 4},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tbin, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\tleft, ok := bin.Left.(*ast.Ident)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.Ident, got %T\", bin.Left)\n\t}\n\tif left.Name != \"x\" {\n\t\tt.Fatalf(\"expected 'x', got %q\", left.Name)\n\t}\n}\n\n// --- factor: FuncCall ---\n\nfunc TestParse_FuncCallNoArgs(t *testing.T) {\n\t// f()\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfc, ok := node.(*ast.FuncCall)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncCall, got %T\", node)\n\t}\n\tif fc.Name != \"f\" {\n\t\tt.Fatalf(\"expected 'f', got %q\", fc.Name)\n\t}\n\tif len(fc.Args) != 0 {\n\t\tt.Fatalf(\"expected 0 args, got %d\", len(fc.Args))\n\t}\n}\n\nfunc TestParse_FuncCallOneArg(t *testing.T) {\n\t// f(42)\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"42\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 4},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfc, ok := node.(*ast.FuncCall)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncCall, got %T\", node)\n\t}\n\tif fc.Name != \"f\" {\n\t\tt.Fatalf(\"expected 'f', got %q\", fc.Name)\n\t}\n\tif len(fc.Args) != 1 {\n\t\tt.Fatalf(\"expected 1 arg, got %d\", len(fc.Args))\n\t}\n\targ, ok := fc.Args[0].(*ast.NumberLit)\n\tif !ok {\n\t\tt.Fatalf(\"expected arg to be *ast.NumberLit, got %T\", fc.Args[0])\n\t}\n\tif arg.Value != 42 {\n\t\tt.Fatalf(\"expected 42, got %v\", arg.Value)\n\t}\n}\n\nfunc TestParse_FuncCallMultiArgs(t *testing.T) {\n\t// f(1, 2, 3)\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 5},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 6},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 8},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 9},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 10},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfc, ok := node.(*ast.FuncCall)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncCall, got %T\", node)\n\t}\n\tif len(fc.Args) != 3 {\n\t\tt.Fatalf(\"expected 3 args, got %d\", len(fc.Args))\n\t}\n}\n\nfunc TestParse_FuncCallExprArgs(t *testing.T) {\n\t// f(1+2, 3*4)\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 3},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 4},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n\t\t{Type: token.Number, Literal: \"3\", Pos: 7},\n\t\t{Type: token.Star, Literal: \"*\", Pos: 8},\n\t\t{Type: token.Number, Literal: \"4\", Pos: 9},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 10},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 11},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfc, ok := node.(*ast.FuncCall)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncCall, got %T\", node)\n\t}\n\tif len(fc.Args) != 2 {\n\t\tt.Fatalf(\"expected 2 args, got %d\", len(fc.Args))\n\t}\n\t// First arg: 1+2\n\t_, ok = fc.Args[0].(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected first arg to be *ast.BinaryExpr, got %T\", fc.Args[0])\n\t}\n}\n\nfunc TestParse_FuncCallInExpr(t *testing.T) {\n\t// f(1) + 2\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 5},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 7},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 8},\n\t}\n\tnode, err := Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tbin, ok := node.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.BinaryExpr, got %T\", node)\n\t}\n\t_, ok = bin.Left.(*ast.FuncCall)\n\tif !ok {\n\t\tt.Fatalf(\"expected left to be *ast.FuncCall, got %T\", bin.Left)\n\t}\n}\n\nfunc TestParse_FuncCallMissingRParen(t *testing.T) {\n\t// f(1\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 3},\n\t}\n\t_, err := Parse(tokens)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing closing paren in func call\")\n\t}\n}\n\n// --- ParseLine: expression statement ---\n\nfunc TestParseLine_ExprStmt(t *testing.T) {\n\t// \"1 + 2\"\n\ttokens := []token.Token{\n\t\t{Type: token.Number, Literal: \"1\", Pos: 0},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 2},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 4},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 5},\n\t}\n\tstmt, err := ParseLine(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tes, ok := stmt.(*ast.ExprStmt)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.ExprStmt, got %T\", stmt)\n\t}\n\t_, ok = es.Expr.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected expr to be *ast.BinaryExpr, got %T\", es.Expr)\n\t}\n}\n\nfunc TestParseLine_ExprStmtFuncCall(t *testing.T) {\n\t// \"f(1)\"\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 4},\n\t}\n\tstmt, err := ParseLine(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tes, ok := stmt.(*ast.ExprStmt)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.ExprStmt, got %T\", stmt)\n\t}\n\t_, ok = es.Expr.(*ast.FuncCall)\n\tif !ok {\n\t\tt.Fatalf(\"expected expr to be *ast.FuncCall, got %T\", es.Expr)\n\t}\n}\n\n// --- ParseLine: function definition ---\n\nfunc TestParseLine_FuncDefSingleParam(t *testing.T) {\n\t// \"f(x) = x + 1\"\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 7},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 9},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 11},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 12},\n\t}\n\tstmt, err := ParseLine(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfd, ok := stmt.(*ast.FuncDef)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncDef, got %T\", stmt)\n\t}\n\tif fd.Name != \"f\" {\n\t\tt.Fatalf(\"expected name 'f', got %q\", fd.Name)\n\t}\n\tif len(fd.Params) != 1 || fd.Params[0] != \"x\" {\n\t\tt.Fatalf(\"expected params [x], got %v\", fd.Params)\n\t}\n\t// Body should be x + 1\n\tbin, ok := fd.Body.(*ast.BinaryExpr)\n\tif !ok {\n\t\tt.Fatalf(\"expected body to be *ast.BinaryExpr, got %T\", fd.Body)\n\t}\n\tif bin.Op != token.Plus {\n\t\tt.Fatalf(\"expected Plus in body, got %v\", bin.Op)\n\t}\n}\n\nfunc TestParseLine_FuncDefMultiParam(t *testing.T) {\n\t// \"add(x, y) = x + y\"\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"add\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 3},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 4},\n\t\t{Type: token.Comma, Literal: \",\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"y\", Pos: 7},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 8},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 10},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 12},\n\t\t{Type: token.Plus, Literal: \"+\", Pos: 14},\n\t\t{Type: token.Ident, Literal: \"y\", Pos: 16},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 17},\n\t}\n\tstmt, err := ParseLine(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfd, ok := stmt.(*ast.FuncDef)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncDef, got %T\", stmt)\n\t}\n\tif fd.Name != \"add\" {\n\t\tt.Fatalf(\"expected name 'add', got %q\", fd.Name)\n\t}\n\tif len(fd.Params) != 2 || fd.Params[0] != \"x\" || fd.Params[1] != \"y\" {\n\t\tt.Fatalf(\"expected params [x y], got %v\", fd.Params)\n\t}\n}\n\nfunc TestParseLine_FuncDefNoParams(t *testing.T) {\n\t// \"c() = 42\"\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"c\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 2},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 4},\n\t\t{Type: token.Number, Literal: \"42\", Pos: 6},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 8},\n\t}\n\tstmt, err := ParseLine(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tfd, ok := stmt.(*ast.FuncDef)\n\tif !ok {\n\t\tt.Fatalf(\"expected *ast.FuncDef, got %T\", stmt)\n\t}\n\tif fd.Name != \"c\" {\n\t\tt.Fatalf(\"expected name 'c', got %q\", fd.Name)\n\t}\n\tif len(fd.Params) != 0 {\n\t\tt.Fatalf(\"expected 0 params, got %d\", len(fd.Params))\n\t}\n}\n\n// --- ParseLine: error cases ---\n\nfunc TestParseLine_Empty(t *testing.T) {\n\ttokens := []token.Token{\n\t\t{Type: token.EOF, Literal: \"\", Pos: 0},\n\t}\n\t_, err := ParseLine(tokens)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestParseLine_FuncDefMissingBody(t *testing.T) {\n\t// \"f(x) =\"\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 5},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 6},\n\t}\n\t_, err := ParseLine(tokens)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing function body\")\n\t}\n}\n\nfunc TestParseLine_FuncDefBadParams(t *testing.T) {\n\t// \"f(1) = 2\" — params must be identifiers\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 5},\n\t\t{Type: token.Number, Literal: \"2\", Pos: 7},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 8},\n\t}\n\t_, err := ParseLine(tokens)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for numeric parameter in func def\")\n\t}\n}\n\nfunc TestParseLine_FuncDefTrailingTokens(t *testing.T) {\n\t// \"f(x) = x 1\" — extra token after body\n\ttokens := []token.Token{\n\t\t{Type: token.Ident, Literal: \"f\", Pos: 0},\n\t\t{Type: token.LParen, Literal: \"(\", Pos: 1},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 2},\n\t\t{Type: token.RParen, Literal: \")\", Pos: 3},\n\t\t{Type: token.Equals, Literal: \"=\", Pos: 5},\n\t\t{Type: token.Ident, Literal: \"x\", Pos: 7},\n\t\t{Type: token.Number, Literal: \"1\", Pos: 9},\n\t\t{Type: token.EOF, Literal: \"\", Pos: 10},\n\t}\n\t_, err := ParseLine(tokens)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing tokens after function body\")\n\t}\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./parser/ -v 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./parser/ -v 2>&1"}
+$$$ "=== RUN   TestParse_SingleNumber\n--- PASS: TestParse_SingleNumber (0.00s)\n=== RUN   TestParse_BinaryExpr\n--- PASS: TestParse_BinaryExpr (0.00s)\n=== RUN   TestParse_Ident\n--- PASS: TestParse_Ident (0.00s)\n=== RUN   TestParse_IdentInExpr\n--- PASS: TestParse_IdentInExpr (0.00s)\n=== RUN   TestParse_FuncCallNoArgs\n--- PASS: TestParse_FuncCallNoArgs (0.00s)\n=== RUN   TestParse_FuncCallOneArg\n--- PASS: TestParse_FuncCallOneArg (0.00s)\n=== RUN   TestParse_FuncCallMultiArgs\n--- PASS: TestParse_FuncCallMultiArgs (0.00s)\n=== RUN   TestParse_FuncCallExprArgs\n--- PASS: TestParse_FuncCallExprArgs (0.00s)\n=== RUN   TestParse_FuncCallInExpr\n--- PASS: TestParse_FuncCallInExpr (0.00s)\n=== RUN   TestParse_FuncCallMissingRParen\n--- PASS: TestParse_FuncCallMissingRParen (0.00s)\n=== RUN   TestParseLine_ExprStmt\n--- PASS: TestParseLine_ExprStmt (0.00s)\n=== RUN   TestParseLine_ExprStmtFuncCall\n--- PASS: TestParseLine_ExprStmtFuncCall (0.00s)\n=== RUN   TestParseLine_FuncDefSingleParam\n--- PASS: TestParseLine_FuncDefSingleParam (0.00s)\n=== RUN   TestParseLine_FuncDefMultiParam\n--- PASS: TestParseLine_FuncDefMultiParam (0.00s)\n=== RUN   TestParseLine_FuncDefNoParams\n--- PASS: TestParseLine_FuncDefNoParams (0.00s)\n=== RUN   TestParseLine_Empty\n--- PASS: TestParseLine_Empty (0.00s)\n=== RUN   TestParseLine_FuncDefMissingBody\n--- PASS: TestParseLine_FuncDefMissingBody (0.00s)\n=== RUN   TestParseLine_FuncDefBadParams\n--- PASS: TestParseLine_FuncDefBadParams (0.00s)\n=== RUN   TestParseLine_FuncDefTrailingTokens\n--- PASS: TestParseLine_FuncDefTrailingTokens (0.00s)\nPASS\nok  \tmatheval/parser\t0.587s\n"
+
+--- STOP_REASON: tool_use
+AI: All parser tests pass. Let me verify all existing tests still pass:
+!!!!! {"command":"go test ./... 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t0.694s\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t(cached)\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t0.300s\nok  \tmatheval/repl\t1.073s\nok  \tmatheval/token\t(cached)\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass. Let me commit and move to Step 5.
+!!!!! {"command":"git add -A && git commit -m \"Step 4: Add parser tests for Ident, FuncCall, ParseLine, and error cases\" 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Step 4: Add parser tests for Ident, FuncCall, ParseLine, and error cases\" 2>&1"}
+$$$ "[master 547feda] Step 4: Add parser tests for Ident, FuncCall, ParseLine, and error cases\n 2 files changed, 461 insertions(+), 332 deletions(-)\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.1.1","summary":"Parser tests written and passing. 19 tests covering: Parse backward compat (number, binary expr), Ident, FuncCall (no args, one arg, multi args, expr args, in expressions, missing rparen), ParseLine (expr stmt, func call expr, func def single/multi/no params), error cases (empty, missing body, bad params, trailing tokens)."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now Step 5: Evaluator with `Evaluator` struct, env, and function support.
+CALLING TOOL: file_write {"path":"evaluator/evaluator.go","contents":"package evaluator\n\nimport (\n\t\"fmt\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n)\n\n// Evaluator holds function definitions and evaluates AST nodes.\ntype Evaluator struct {\n\tfuncs map[string]*ast.FuncDef\n}\n\n// New creates a new Evaluator with no defined functions.\nfunc New() *Evaluator {\n\treturn &Evaluator{funcs: make(map[string]*ast.FuncDef)}\n}\n\n// Define registers a function definition.\n// Returns an error if a function with the same name is already defined.\nfunc (ev *Evaluator) Define(def *ast.FuncDef) error {\n\tif _, exists := ev.funcs[def.Name]; exists {\n\t\treturn fmt.Errorf(\"function %q already defined\", def.Name)\n\t}\n\tev.funcs[def.Name] = def\n\treturn nil\n}\n\n// Eval evaluates an AST node with the given variable environment.\n// env maps variable names to their values; nil is treated as empty.\nfunc (ev *Evaluator) Eval(node ast.Node, env map[string]float64) (float64, error) {\n\tswitch n := node.(type) {\n\tcase *ast.NumberLit:\n\t\treturn n.Value, nil\n\n\tcase *ast.Ident:\n\t\tif env != nil {\n\t\t\tif val, ok := env[n.Name]; ok {\n\t\t\t\treturn val, nil\n\t\t\t}\n\t\t}\n\t\treturn 0, fmt.Errorf(\"undefined variable %q\", n.Name)\n\n\tcase *ast.BinaryExpr:\n\t\tleft, err := ev.Eval(n.Left, env)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\t\tright, err := ev.Eval(n.Right, env)\n\t\tif err != nil {\n\t\t\treturn 0, err\n\t\t}\n\n\t\tswitch n.Op {\n\t\tcase token.Plus:\n\t\t\treturn left + right, nil\n\t\tcase token.Minus:\n\t\t\treturn left - right, nil\n\t\tcase token.Star:\n\t\t\treturn left * right, nil\n\t\tcase token.Slash:\n\t\t\tif right == 0 {\n\t\t\t\treturn 0, fmt.Errorf(\"division by zero\")\n\t\t\t}\n\t\t\treturn left / right, nil\n\t\tdefault:\n\t\t\treturn 0, fmt.Errorf(\"unknown operator: %v\", n.Op)\n\t\t}\n\n\tcase *ast.FuncCall:\n\t\tdef, ok := ev.funcs[n.Name]\n\t\tif !ok {\n\t\t\treturn 0, fmt.Errorf(\"undefined function %q\", n.Name)\n\t\t}\n\t\tif len(n.Args) != len(def.Params) {\n\t\t\treturn 0, fmt.Errorf(\"function %q expects %d arguments, got %d\", n.Name, len(def.Params), len(n.Args))\n\t\t}\n\t\t// Evaluate arguments in caller's environment.\n\t\tnewEnv := make(map[string]float64, len(def.Params))\n\t\tfor i, param := range def.Params {\n\t\t\tval, err := ev.Eval(n.Args[i], env)\n\t\t\tif err != nil {\n\t\t\t\treturn 0, err\n\t\t\t}\n\t\t\tnewEnv[param] = val\n\t\t}\n\t\t// Evaluate function body in new environment.\n\t\treturn ev.Eval(def.Body, newEnv)\n\n\tdefault:\n\t\treturn 0, fmt.Errorf(\"unknown node type: %T\", node)\n\t}\n}\n\n// Eval is a backward-compatible package-level function.\n// It evaluates an AST node without any variable/function context.\nfunc Eval(node ast.Node) (float64, error) {\n\treturn New().Eval(node, nil)\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me verify existing evaluator tests still pass:
+!!!!! {"command":"go test ./evaluator/ -v 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"go test ./evaluator/ -v 2>&1"}
+$$$ "=== RUN   TestEvalNumberLit\n--- PASS: TestEvalNumberLit (0.00s)\n=== RUN   TestEvalAddition\n--- PASS: TestEvalAddition (0.00s)\n=== RUN   TestEvalSubtraction\n--- PASS: TestEvalSubtraction (0.00s)\n=== RUN   TestEvalMultiplication\n--- PASS: TestEvalMultiplication (0.00s)\n=== RUN   TestEvalDivision\n--- PASS: TestEvalDivision (0.00s)\n=== RUN   TestEvalDivisionByZero\n--- PASS: TestEvalDivisionByZero (0.00s)\n=== RUN   TestEvalNestedExpr\n--- PASS: TestEvalNestedExpr (0.00s)\n=== RUN   TestEvalDeeplyNested\n--- PASS: TestEvalDeeplyNested (0.00s)\n=== RUN   TestEvalDivisionByZeroInSubExpr\n--- PASS: TestEvalDivisionByZeroInSubExpr (0.00s)\n=== RUN   TestEvalFloatingPoint\n--- PASS: TestEvalFloatingPoint (0.00s)\nPASS\nok  \tmatheval/evaluator\t0.592s\n"
+
+--- STOP_REASON: tool_use
+AI: All backward-compatible tests pass. Now add new tests for the Evaluator struct features:
+CALLING TOOL: file_write {"path":"evaluator/evaluator_test.go","contents":"package evaluator\n\nimport (\n\t\"math\"\n\t\"matheval/ast\"\n\t\"matheval/token\"\n\t\"strings\"\n\t\"testing\"\n)\n\n// --- Backward-compatible package-level Eval ---\n\nfunc TestEvalNumberLit(t *testing.T) {\n\tresult, err := Eval(&ast.NumberLit{Value: 42.5})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 42.5 {\n\t\tt.Fatalf(\"expected 42.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalAddition(t *testing.T) {\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  &ast.NumberLit{Value: 1},\n\t\tRight: &ast.NumberLit{Value: 2},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 3 {\n\t\tt.Fatalf(\"expected 3, got %v\", result)\n\t}\n}\n\nfunc TestEvalSubtraction(t *testing.T) {\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Minus,\n\t\tLeft:  &ast.NumberLit{Value: 10},\n\t\tRight: &ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalMultiplication(t *testing.T) {\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Star,\n\t\tLeft:  &ast.NumberLit{Value: 3},\n\t\tRight: &ast.NumberLit{Value: 7},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 21 {\n\t\tt.Fatalf(\"expected 21, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivision(t *testing.T) {\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  &ast.NumberLit{Value: 10},\n\t\tRight: &ast.NumberLit{Value: 4},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 2.5 {\n\t\tt.Fatalf(\"expected 2.5, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZero(t *testing.T) {\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Slash,\n\t\tLeft:  &ast.NumberLit{Value: 5},\n\t\tRight: &ast.NumberLit{Value: 0},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestEvalNestedExpr(t *testing.T) {\n\t// (1 + 2) * (8 / 4) = 3 * 2 = 6\n\tnode := &ast.BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: &ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  &ast.NumberLit{Value: 1},\n\t\t\tRight: &ast.NumberLit{Value: 2},\n\t\t},\n\t\tRight: &ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  &ast.NumberLit{Value: 8},\n\t\t\tRight: &ast.NumberLit{Value: 4},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvalDeeplyNested(t *testing.T) {\n\t// ((2 + 3) * 4) - (10 / 5) = 20 - 2 = 18\n\tnode := &ast.BinaryExpr{\n\t\tOp: token.Minus,\n\t\tLeft: &ast.BinaryExpr{\n\t\t\tOp: token.Star,\n\t\t\tLeft: &ast.BinaryExpr{\n\t\t\t\tOp:    token.Plus,\n\t\t\t\tLeft:  &ast.NumberLit{Value: 2},\n\t\t\t\tRight: &ast.NumberLit{Value: 3},\n\t\t\t},\n\t\t\tRight: &ast.NumberLit{Value: 4},\n\t\t},\n\t\tRight: &ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  &ast.NumberLit{Value: 10},\n\t\t\tRight: &ast.NumberLit{Value: 5},\n\t\t},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 18 {\n\t\tt.Fatalf(\"expected 18, got %v\", result)\n\t}\n}\n\nfunc TestEvalDivisionByZeroInSubExpr(t *testing.T) {\n\t// 1 + (2 / 0) — error should propagate\n\tnode := &ast.BinaryExpr{\n\t\tOp:   token.Plus,\n\t\tLeft: &ast.NumberLit{Value: 1},\n\t\tRight: &ast.BinaryExpr{\n\t\t\tOp:    token.Slash,\n\t\t\tLeft:  &ast.NumberLit{Value: 2},\n\t\t\tRight: &ast.NumberLit{Value: 0},\n\t\t},\n\t}\n\t_, err := Eval(node)\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error from sub-expression\")\n\t}\n}\n\nfunc TestEvalFloatingPoint(t *testing.T) {\n\t// 1.5 + 2.3 = 3.8\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  &ast.NumberLit{Value: 1.5},\n\t\tRight: &ast.NumberLit{Value: 2.3},\n\t}\n\tresult, err := Eval(node)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif math.Abs(result-3.8) > 1e-12 {\n\t\tt.Fatalf(\"expected 3.8, got %v\", result)\n\t}\n}\n\n// --- Evaluator struct: Ident ---\n\nfunc TestEvaluator_Ident(t *testing.T) {\n\tev := New()\n\tenv := map[string]float64{\"x\": 7}\n\tresult, err := ev.Eval(&ast.Ident{Name: \"x\"}, env)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 7 {\n\t\tt.Fatalf(\"expected 7, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_IdentUndefined(t *testing.T) {\n\tev := New()\n\t_, err := ev.Eval(&ast.Ident{Name: \"x\"}, nil)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for undefined variable\")\n\t}\n\tif !strings.Contains(err.Error(), \"undefined variable\") {\n\t\tt.Errorf(\"expected 'undefined variable' in error, got: %v\", err)\n\t}\n}\n\nfunc TestEvaluator_IdentInExpr(t *testing.T) {\n\tev := New()\n\tenv := map[string]float64{\"x\": 3, \"y\": 4}\n\t// x + y\n\tnode := &ast.BinaryExpr{\n\t\tOp:    token.Plus,\n\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\tRight: &ast.Ident{Name: \"y\"},\n\t}\n\tresult, err := ev.Eval(node, env)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 7 {\n\t\tt.Fatalf(\"expected 7, got %v\", result)\n\t}\n}\n\n// --- Evaluator struct: Define + FuncCall ---\n\nfunc TestEvaluator_DefineAndCall(t *testing.T) {\n\tev := New()\n\t// f(x) = x + 1\n\terr := ev.Define(&ast.FuncDef{\n\t\tName:   \"f\",\n\t\tParams: []string{\"x\"},\n\t\tBody: &ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\t\tRight: &ast.NumberLit{Value: 1},\n\t\t},\n\t})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\n\t// f(5) = 6\n\tresult, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"f\",\n\t\tArgs: []ast.Node{&ast.NumberLit{Value: 5}},\n\t}, nil)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 6 {\n\t\tt.Fatalf(\"expected 6, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_DefineMultiParam(t *testing.T) {\n\tev := New()\n\t// add(x, y) = x + y\n\terr := ev.Define(&ast.FuncDef{\n\t\tName:   \"add\",\n\t\tParams: []string{\"x\", \"y\"},\n\t\tBody: &ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\t\tRight: &ast.Ident{Name: \"y\"},\n\t\t},\n\t})\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\n\t// add(3, 4) = 7\n\tresult, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"add\",\n\t\tArgs: []ast.Node{\n\t\t\t&ast.NumberLit{Value: 3},\n\t\t\t&ast.NumberLit{Value: 4},\n\t\t},\n\t}, nil)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 7 {\n\t\tt.Fatalf(\"expected 7, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_DefineRedefinitionError(t *testing.T) {\n\tev := New()\n\tdef := &ast.FuncDef{\n\t\tName:   \"f\",\n\t\tParams: []string{\"x\"},\n\t\tBody:   &ast.Ident{Name: \"x\"},\n\t}\n\tif err := ev.Define(def); err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\terr := ev.Define(def)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for redefining function\")\n\t}\n\tif !strings.Contains(err.Error(), \"already defined\") {\n\t\tt.Errorf(\"expected 'already defined' in error, got: %v\", err)\n\t}\n}\n\nfunc TestEvaluator_UndefinedFunction(t *testing.T) {\n\tev := New()\n\t_, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"f\",\n\t\tArgs: []ast.Node{&ast.NumberLit{Value: 1}},\n\t}, nil)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for undefined function\")\n\t}\n\tif !strings.Contains(err.Error(), \"undefined function\") {\n\t\tt.Errorf(\"expected 'undefined function' in error, got: %v\", err)\n\t}\n}\n\nfunc TestEvaluator_WrongArgCount(t *testing.T) {\n\tev := New()\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"f\",\n\t\tParams: []string{\"x\"},\n\t\tBody:   &ast.Ident{Name: \"x\"},\n\t})\n\t_, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"f\",\n\t\tArgs: []ast.Node{\n\t\t\t&ast.NumberLit{Value: 1},\n\t\t\t&ast.NumberLit{Value: 2},\n\t\t},\n\t}, nil)\n\tif err == nil {\n\t\tt.Fatal(\"expected error for wrong argument count\")\n\t}\n\tif !strings.Contains(err.Error(), \"expects 1 arguments, got 2\") {\n\t\tt.Errorf(\"expected arg count error, got: %v\", err)\n\t}\n}\n\nfunc TestEvaluator_FuncCallInExpr(t *testing.T) {\n\tev := New()\n\t// f(x) = x * 2\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"f\",\n\t\tParams: []string{\"x\"},\n\t\tBody: &ast.BinaryExpr{\n\t\t\tOp:    token.Star,\n\t\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\t\tRight: &ast.NumberLit{Value: 2},\n\t\t},\n\t})\n\t// f(3) + 1 = 7\n\tnode := &ast.BinaryExpr{\n\t\tOp: token.Plus,\n\t\tLeft: &ast.FuncCall{\n\t\t\tName: \"f\",\n\t\t\tArgs: []ast.Node{&ast.NumberLit{Value: 3}},\n\t\t},\n\t\tRight: &ast.NumberLit{Value: 1},\n\t}\n\tresult, err := ev.Eval(node, nil)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 7 {\n\t\tt.Fatalf(\"expected 7, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_NestedFuncCall(t *testing.T) {\n\tev := New()\n\t// f(x) = x + 1\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"f\",\n\t\tParams: []string{\"x\"},\n\t\tBody: &ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\t\tRight: &ast.NumberLit{Value: 1},\n\t\t},\n\t})\n\t// f(f(1)) = f(2) = 3\n\tnode := &ast.FuncCall{\n\t\tName: \"f\",\n\t\tArgs: []ast.Node{\n\t\t\t&ast.FuncCall{\n\t\t\t\tName: \"f\",\n\t\t\t\tArgs: []ast.Node{&ast.NumberLit{Value: 1}},\n\t\t\t},\n\t\t},\n\t}\n\tresult, err := ev.Eval(node, nil)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 3 {\n\t\tt.Fatalf(\"expected 3, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_CrossFunctionCall(t *testing.T) {\n\tev := New()\n\t// double(x) = x * 2\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"double\",\n\t\tParams: []string{\"x\"},\n\t\tBody: &ast.BinaryExpr{\n\t\t\tOp:    token.Star,\n\t\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\t\tRight: &ast.NumberLit{Value: 2},\n\t\t},\n\t})\n\t// quad(x) = double(double(x))\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"quad\",\n\t\tParams: []string{\"x\"},\n\t\tBody: &ast.FuncCall{\n\t\t\tName: \"double\",\n\t\t\tArgs: []ast.Node{\n\t\t\t\t&ast.FuncCall{\n\t\t\t\t\tName:   \"double\",\n\t\t\t\t\tArgs:   []ast.Node{&ast.Ident{Name: \"x\"}},\n\t\t\t\t},\n\t\t\t},\n\t\t},\n\t})\n\t// quad(3) = double(double(3)) = double(6) = 12\n\tresult, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"quad\",\n\t\tArgs: []ast.Node{&ast.NumberLit{Value: 3}},\n\t}, nil)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 12 {\n\t\tt.Fatalf(\"expected 12, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_FuncNoParams(t *testing.T) {\n\tev := New()\n\t// c() = 42\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"c\",\n\t\tParams: []string{},\n\t\tBody:   &ast.NumberLit{Value: 42},\n\t})\n\tresult, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"c\",\n\t\tArgs: []ast.Node{},\n\t}, nil)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\tif result != 42 {\n\t\tt.Fatalf(\"expected 42, got %v\", result)\n\t}\n}\n\nfunc TestEvaluator_ArgEvaluatedInCallerEnv(t *testing.T) {\n\tev := New()\n\t// f(x) = x + 1\n\tev.Define(&ast.FuncDef{\n\t\tName:   \"f\",\n\t\tParams: []string{\"x\"},\n\t\tBody: &ast.BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  &ast.Ident{Name: \"x\"},\n\t\t\tRight: &ast.NumberLit{Value: 1},\n\t\t},\n\t})\n\t// Evaluate f(y) with y=10 in caller env\n\tcallerEnv := map[string]float64{\"y\": 10}\n\tresult, err := ev.Eval(&ast.FuncCall{\n\t\tName: \"f\",\n\t\tArgs: []ast.Node{&ast.Ident{Name: \"y\"}},\n\t}, callerEnv)\n\tif err != nil {\n\t\tt.Fatalf(\"unexpected error: %v\", err)\n\t}\n\t// f(10) = 11\n\tif result != 11 {\n\t\tt.Fatalf(\"expected 11, got %v\", result)\n\t}\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./evaluator/ -v 2>&1"}: Ok
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./evaluator/ -v 2>&1; echo \"---EXIT:$?---\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./evaluator/ -v 2>&1; echo \"---EXIT:$?---\""}
+$$$ "=== RUN   TestEvalNumberLit\n--- PASS: TestEvalNumberLit (0.00s)\n=== RUN   TestEvalAddition\n--- PASS: TestEvalAddition (0.00s)\n=== RUN   TestEvalSubtraction\n--- PASS: TestEvalSubtraction (0.00s)\n=== RUN   TestEvalMultiplication\n--- PASS: TestEvalMultiplication (0.00s)\n=== RUN   TestEvalDivision\n--- PASS: TestEvalDivision (0.00s)\n=== RUN   TestEvalDivisionByZero\n--- PASS: TestEvalDivisionByZero (0.00s)\n=== RUN   TestEvalNestedExpr\n--- PASS: TestEvalNestedExpr (0.00s)\n=== RUN   TestEvalDeeplyNested\n--- PASS: TestEvalDeeplyNested (0.00s)\n=== RUN   TestEvalDivisionByZeroInSubExpr\n--- PASS: TestEvalDivisionByZeroInSubExpr (0.00s)\n=== RUN   TestEvalFloatingPoint\n--- PASS: TestEvalFloatingPoint (0.00s)\n=== RUN   TestEvaluator_Ident\n--- PASS: TestEvaluator_Ident (0.00s)\n=== RUN   TestEvaluator_IdentUndefined\n--- PASS: TestEvaluator_IdentUndefined (0.00s)\n=== RUN   TestEvaluator_IdentInExpr\n--- PASS: TestEvaluator_IdentInExpr (0.00s)\n=== RUN   TestEvaluator_DefineAndCall\n--- PASS: TestEvaluator_DefineAndCall (0.00s)\n=== RUN   TestEvaluator_DefineMultiParam\n--- PASS: TestEvaluator_DefineMultiParam (0.00s)\n=== RUN   TestEvaluator_DefineRedefinitionError\n--- PASS: TestEvaluator_DefineRedefinitionError (0.00s)\n=== RUN   TestEvaluator_UndefinedFunction\n--- PASS: TestEvaluator_UndefinedFunction (0.00s)\n=== RUN   TestEvaluator_WrongArgCount\n--- PASS: TestEvaluator_WrongArgCount (0.00s)\n=== RUN   TestEvaluator_FuncCallInExpr\n--- PASS: TestEvaluator_FuncCallInExpr (0.00s)\n=== RUN   TestEvaluator_NestedFuncCall\n--- PASS: TestEvaluator_NestedFuncCall (0.00s)\n=== RUN   TestEvaluator_CrossFunctionCall\n--- PASS: TestEvaluator_CrossFunctionCall (0.00s)\n=== RUN   TestEvaluator_FuncNoParams\n--- PASS: TestEvaluator_FuncNoParams (0.00s)\n=== RUN   TestEvaluator_ArgEvaluatedInCallerEnv\n--- PASS: TestEvaluator_ArgEvaluatedInCallerEnv (0.00s)\nPASS\nok  \tmatheval/evaluator\t0.576s\n---EXIT:0---\n"
+
+--- STOP_REASON: tool_use
+AI: All 23 evaluator tests pass. Let me verify all tests:
+!!!!! {"command":"go test ./... 2>&1; echo \"---EXIT:$?---\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1; echo \"---EXIT:$?---\""}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t0.422s\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t0.642s\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t1.001s\nok  \tmatheval/token\t(cached)\n---EXIT:0---\n"
+
+--- STOP_REASON: tool_use
+AI: All pass. Commit.
+!!!!! {"command":"git add -A && git commit -m \"Step 5: Add Evaluator struct with env, Define, FuncCall support; backward-compat Eval preserved\" 2>&1"}: Ok
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.1.2","summary":"Evaluator struct implemented with New(), Define(), Eval(node, env). Handles NumberLit, Ident, BinaryExpr, FuncCall. Package-level Eval() preserved for backward compat. 23 tests all passing."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now Step 6: Update REPL to use ParseLine and Evaluator.
+CALLING TOOL: file_write {"path":"repl/repl.go","contents":"package repl\n\nimport (\n\t\"bufio\"\n\t\"fmt\"\n\t\"io\"\n\t\"math\"\n\t\"strings\"\n\n\t\"matheval/ast\"\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\nconst prompt = \">> \"\n\n// Run starts the read-eval-print loop, reading from r and writing to w.\nfunc Run(r io.Reader, w io.Writer) {\n\tscanner := bufio.NewScanner(r)\n\tev := evaluator.New()\n\tfmt.Fprint(w, prompt)\n\n\tfor scanner.Scan() {\n\t\tline := strings.TrimSpace(scanner.Text())\n\t\tif line == \"\" {\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\ttokens, err := lexer.Tokenize(line)\n\t\tif err != nil {\n\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\tstmt, err := parser.ParseLine(tokens)\n\t\tif err != nil {\n\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t\tfmt.Fprint(w, prompt)\n\t\t\tcontinue\n\t\t}\n\n\t\tswitch s := stmt.(type) {\n\t\tcase *ast.FuncDef:\n\t\t\tif err := ev.Define(s); err != nil {\n\t\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t\t} else {\n\t\t\t\tfmt.Fprintf(w, \"defined %s\\n\", s.Name)\n\t\t\t}\n\t\tcase *ast.ExprStmt:\n\t\t\tresult, err := ev.Eval(s.Expr, nil)\n\t\t\tif err != nil {\n\t\t\t\tfmt.Fprintf(w, \"error: %s\\n\", err)\n\t\t\t} else {\n\t\t\t\tfmt.Fprintln(w, formatResult(result))\n\t\t\t}\n\t\t}\n\n\t\tfmt.Fprint(w, prompt)\n\t}\n}\n\n// formatResult formats a float64 for display.\n// Whole numbers are printed without decimal points.\nfunc formatResult(val float64) string {\n\tif val == math.Trunc(val) && !math.IsInf(val, 0) && !math.IsNaN(val) {\n\t\treturn fmt.Sprintf(\"%g\", val)\n\t}\n\treturn fmt.Sprintf(\"%g\", val)\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now let me run the existing REPL tests to make sure backward compat is preserved:
+!!!!! {"command":"go test ./repl/ -v 2>&1; echo \"---EXIT:$?---\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./repl/ -v 2>&1; echo \"---EXIT:$?---\""}
+$$$ "=== RUN   TestRun_SingleExpression\n--- PASS: TestRun_SingleExpression (0.00s)\n=== RUN   TestRun_MultipleExpressions\n--- PASS: TestRun_MultipleExpressions (0.00s)\n=== RUN   TestRun_ErrorContinues\n--- PASS: TestRun_ErrorContinues (0.00s)\n=== RUN   TestRun_DivisionByZero\n--- PASS: TestRun_DivisionByZero (0.00s)\n=== RUN   TestRun_EmptyLine\n--- PASS: TestRun_EmptyLine (0.00s)\n=== RUN   TestRun_Prompt\n--- PASS: TestRun_Prompt (0.00s)\n=== RUN   TestRun_FloatResult\n--- PASS: TestRun_FloatResult (0.00s)\n=== RUN   TestRun_InvalidCharacter\n--- PASS: TestRun_InvalidCharacter (0.00s)\n=== RUN   TestRun_EmptyInput\n--- PASS: TestRun_EmptyInput (0.00s)\n=== RUN   TestRun_WholeIntegerNoTrailingZeros\n--- PASS: TestRun_WholeIntegerNoTrailingZeros (0.00s)\nPASS\nok  \tmatheval/repl\t0.605s\n---EXIT:0---\n"
+
+--- STOP_REASON: tool_use
+AI: All existing REPL tests pass. Now add new REPL tests for function definitions:
+CALLING TOOL: file_write {"path":"repl/repl_test.go","contents":"package repl\n\nimport (\n\t\"bytes\"\n\t\"strings\"\n\t\"testing\"\n)\n\nfunc TestRun_SingleExpression(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n}\n\nfunc TestRun_MultipleExpressions(t *testing.T) {\n\tin := strings.NewReader(\"1 + 2\\n3 * 4\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3\") {\n\t\tt.Errorf(\"expected output to contain '3', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"12\") {\n\t\tt.Errorf(\"expected output to contain '12', got %q\", got)\n\t}\n}\n\nfunc TestRun_ErrorContinues(t *testing.T) {\n\t// First line has error, second is valid.\n\tin := strings.NewReader(\"1 +\\n2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"5\") {\n\t\tt.Errorf(\"expected output to contain '5' after error recovery, got %q\", got)\n\t}\n}\n\nfunc TestRun_DivisionByZero(t *testing.T) {\n\tin := strings.NewReader(\"1 / 0\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n\tif !strings.Contains(got, \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyLine(t *testing.T) {\n\t// Empty lines should be skipped, not cause errors.\n\tin := strings.NewReader(\"\\n1 + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"2\") {\n\t\tt.Errorf(\"expected output to contain '2', got %q\", got)\n\t}\n\t// Should not contain any error.\n\tif strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"empty line should not produce error, got %q\", got)\n\t}\n}\n\nfunc TestRun_Prompt(t *testing.T) {\n\tin := strings.NewReader(\"42\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \">> \") {\n\t\tt.Errorf(\"expected prompt '>> ' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_FloatResult(t *testing.T) {\n\tin := strings.NewReader(\"7 / 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"3.5\") {\n\t\tt.Errorf(\"expected output to contain '3.5', got %q\", got)\n\t}\n}\n\nfunc TestRun_InvalidCharacter(t *testing.T) {\n\tin := strings.NewReader(\"1 @ 2\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected output to contain 'error:', got %q\", got)\n\t}\n}\n\nfunc TestRun_EmptyInput(t *testing.T) {\n\t// No input at all — just EOF.\n\tin := strings.NewReader(\"\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\t// Should just show the prompt and exit gracefully.\n\tif !strings.Contains(got, \">> \") {\n\t\tt.Errorf(\"expected at least one prompt, got %q\", got)\n\t}\n}\n\nfunc TestRun_WholeIntegerNoTrailingZeros(t *testing.T) {\n\t// 2 + 3 = 5, should print \"5\" not \"5.000000\".\n\tin := strings.NewReader(\"2 + 3\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\t// Result line is \"5\\n\" (between prompts).\n\tif !strings.Contains(got, \"5\\n\") {\n\t\tt.Errorf(\"expected '5\\\\n' in output, got %q\", got)\n\t}\n\t// Should not contain \"5.0\"\n\tif strings.Contains(got, \"5.0\") {\n\t\tt.Errorf(\"expected no trailing zeros, got %q\", got)\n\t}\n}\n\n// --- Function definition tests ---\n\nfunc TestRun_DefineFunction(t *testing.T) {\n\tin := strings.NewReader(\"f(x) = x + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"defined f\") {\n\t\tt.Errorf(\"expected 'defined f' in output, got %q\", got)\n\t}\n}\n\nfunc TestRun_DefineAndCallFunction(t *testing.T) {\n\tin := strings.NewReader(\"f(x) = x + 1\\nf(5)\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"defined f\") {\n\t\tt.Errorf(\"expected 'defined f' in output, got %q\", got)\n\t}\n\tif !strings.Contains(got, \"6\") {\n\t\tt.Errorf(\"expected output to contain '6', got %q\", got)\n\t}\n}\n\nfunc TestRun_DefineMultiParamAndCall(t *testing.T) {\n\tin := strings.NewReader(\"add(x, y) = x + y\\nadd(3, 4)\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"defined add\") {\n\t\tt.Errorf(\"expected 'defined add' in output, got %q\", got)\n\t}\n\tif !strings.Contains(got, \"7\") {\n\t\tt.Errorf(\"expected output to contain '7', got %q\", got)\n\t}\n}\n\nfunc TestRun_RedefineError(t *testing.T) {\n\tin := strings.NewReader(\"f(x) = x\\nf(x) = x + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"defined f\") {\n\t\tt.Errorf(\"expected 'defined f' in output, got %q\", got)\n\t}\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected 'error:' for redefinition, got %q\", got)\n\t}\n\tif !strings.Contains(got, \"already defined\") {\n\t\tt.Errorf(\"expected 'already defined' in error, got %q\", got)\n\t}\n}\n\nfunc TestRun_UndefinedFunctionError(t *testing.T) {\n\tin := strings.NewReader(\"f(1)\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"error:\") {\n\t\tt.Errorf(\"expected 'error:' for undefined function, got %q\", got)\n\t}\n\tif !strings.Contains(got, \"undefined function\") {\n\t\tt.Errorf(\"expected 'undefined function' in error, got %q\", got)\n\t}\n}\n\nfunc TestRun_FuncCallInExpression(t *testing.T) {\n\tin := strings.NewReader(\"f(x) = x * 2\\nf(3) + 1\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"7\") {\n\t\tt.Errorf(\"expected output to contain '7', got %q\", got)\n\t}\n}\n\nfunc TestRun_CrossFunctionCalls(t *testing.T) {\n\tin := strings.NewReader(\"double(x) = x * 2\\nquad(x) = double(double(x))\\nquad(3)\\n\")\n\tvar out bytes.Buffer\n\n\tRun(in, &out)\n\n\tgot := out.String()\n\tif !strings.Contains(got, \"12\") {\n\t\tt.Errorf(\"expected output to contain '12', got %q\", got)\n\t}\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./repl/ -v 2>&1; echo \"---EXIT:$?---\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./repl/ -v 2>&1; echo \"---EXIT:$?---\""}
+$$$ "=== RUN   TestRun_SingleExpression\n--- PASS: TestRun_SingleExpression (0.00s)\n=== RUN   TestRun_MultipleExpressions\n--- PASS: TestRun_MultipleExpressions (0.00s)\n=== RUN   TestRun_ErrorContinues\n--- PASS: TestRun_ErrorContinues (0.00s)\n=== RUN   TestRun_DivisionByZero\n--- PASS: TestRun_DivisionByZero (0.00s)\n=== RUN   TestRun_EmptyLine\n--- PASS: TestRun_EmptyLine (0.00s)\n=== RUN   TestRun_Prompt\n--- PASS: TestRun_Prompt (0.00s)\n=== RUN   TestRun_FloatResult\n--- PASS: TestRun_FloatResult (0.00s)\n=== RUN   TestRun_InvalidCharacter\n--- PASS: TestRun_InvalidCharacter (0.00s)\n=== RUN   TestRun_EmptyInput\n--- PASS: TestRun_EmptyInput (0.00s)\n=== RUN   TestRun_WholeIntegerNoTrailingZeros\n--- PASS: TestRun_WholeIntegerNoTrailingZeros (0.00s)\n=== RUN   TestRun_DefineFunction\n--- PASS: TestRun_DefineFunction (0.00s)\n=== RUN   TestRun_DefineAndCallFunction\n--- PASS: TestRun_DefineAndCallFunction (0.00s)\n=== RUN   TestRun_DefineMultiParamAndCall\n--- PASS: TestRun_DefineMultiParamAndCall (0.00s)\n=== RUN   TestRun_RedefineError\n--- PASS: TestRun_RedefineError (0.00s)\n=== RUN   TestRun_UndefinedFunctionError\n--- PASS: TestRun_UndefinedFunctionError (0.00s)\n=== RUN   TestRun_FuncCallInExpression\n--- PASS: TestRun_FuncCallInExpression (0.00s)\n=== RUN   TestRun_CrossFunctionCalls\n--- PASS: TestRun_CrossFunctionCalls (0.00s)\nPASS\nok  \tmatheval/repl\t0.604s\n---EXIT:0---\n"
+
+--- STOP_REASON: tool_use
+AI: All 17 REPL tests pass. Let me verify everything still works together:
+!!!!! {"command":"go test ./... 2>&1; echo \"---EXIT:$?---\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./... 2>&1; echo \"---EXIT:$?---\""}
+$$$ "ok  \tmatheval\t(cached)\nok  \tmatheval/ast\t(cached)\n?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval/evaluator\t(cached)\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t0.273s\nok  \tmatheval/token\t(cached)\n---EXIT:0---\n"
+
+--- STOP_REASON: tool_use
+AI: Commit.
+!!!!! {"command":"git add -A && git commit -m \"Step 6: Update REPL to use ParseLine + Evaluator; add function definition REPL tests\" 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Step 6: Update REPL to use ParseLine + Evaluator; add function definition REPL tests\" 2>&1"}
+$$$ "[master 17f2439] Step 6: Update REPL to use ParseLine + Evaluator; add function definition REPL tests\n 1 file changed, 101 insertions(+)\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.1.3","summary":"REPL updated to use ParseLine + Evaluator. Handles FuncDef (prints \"defined <name>\") and ExprStmt (evaluates and prints result). All 17 REPL tests passing including 7 new function-related tests."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+AI: Now Step 7: Integration tests for function definitions.
+CALLING TOOL: file_write {"path":"integration_test.go","contents":"package matheval_test\n\nimport (\n\t\"math\"\n\t\"strings\"\n\t\"testing\"\n\n\t\"matheval/ast\"\n\t\"matheval/evaluator\"\n\t\"matheval/lexer\"\n\t\"matheval/parser\"\n)\n\n// eval is a helper that runs the full pipeline: lexer → parser → evaluator.\nfunc eval(t *testing.T, input string) float64 {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\tt.Fatalf(\"lexer error for %q: %v\", input, err)\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\tt.Fatalf(\"parser error for %q: %v\", input, err)\n\t}\n\tresult, err := evaluator.Eval(tree)\n\tif err != nil {\n\t\tt.Fatalf(\"evaluator error for %q: %v\", input, err)\n\t}\n\treturn result\n}\n\n// evalErr is a helper that expects the full pipeline to return an error.\nfunc evalErr(t *testing.T, input string) error {\n\tt.Helper()\n\ttokens, err := lexer.Tokenize(input)\n\tif err != nil {\n\t\treturn err\n\t}\n\ttree, err := parser.Parse(tokens)\n\tif err != nil {\n\t\treturn err\n\t}\n\t_, err = evaluator.Eval(tree)\n\treturn err\n}\n\n// evalLine is a helper that uses ParseLine + Evaluator for a multi-line session.\n// It processes lines sequentially on a shared evaluator and returns the result\n// of the last expression statement.\nfunc evalLines(t *testing.T, ev *evaluator.Evaluator, lines ...string) float64 {\n\tt.Helper()\n\tvar lastResult float64\n\tfor _, line := range lines {\n\t\ttokens, err := lexer.Tokenize(line)\n\t\tif err != nil {\n\t\t\tt.Fatalf(\"lexer error for %q: %v\", line, err)\n\t\t}\n\t\tstmt, err := parser.ParseLine(tokens)\n\t\tif err != nil {\n\t\t\tt.Fatalf(\"parser error for %q: %v\", line, err)\n\t\t}\n\t\tswitch s := stmt.(type) {\n\t\tcase *ast.FuncDef:\n\t\t\tif err := ev.Define(s); err != nil {\n\t\t\t\tt.Fatalf(\"define error for %q: %v\", line, err)\n\t\t\t}\n\t\tcase *ast.ExprStmt:\n\t\t\tresult, err := ev.Eval(s.Expr, nil)\n\t\t\tif err != nil {\n\t\t\t\tt.Fatalf(\"eval error for %q: %v\", line, err)\n\t\t\t}\n\t\t\tlastResult = result\n\t\t}\n\t}\n\treturn lastResult\n}\n\n// evalLinesErr processes lines and expects the last one to produce an error.\nfunc evalLinesErr(t *testing.T, ev *evaluator.Evaluator, lines ...string) error {\n\tt.Helper()\n\tfor i, line := range lines {\n\t\ttokens, err := lexer.Tokenize(line)\n\t\tif err != nil {\n\t\t\tif i == len(lines)-1 {\n\t\t\t\treturn err\n\t\t\t}\n\t\t\tt.Fatalf(\"lexer error for %q: %v\", line, err)\n\t\t}\n\t\tstmt, err := parser.ParseLine(tokens)\n\t\tif err != nil {\n\t\t\tif i == len(lines)-1 {\n\t\t\t\treturn err\n\t\t\t}\n\t\t\tt.Fatalf(\"parser error for %q: %v\", line, err)\n\t\t}\n\t\tswitch s := stmt.(type) {\n\t\tcase *ast.FuncDef:\n\t\t\tif err := ev.Define(s); err != nil {\n\t\t\t\tif i == len(lines)-1 {\n\t\t\t\t\treturn err\n\t\t\t\t}\n\t\t\t\tt.Fatalf(\"define error for %q: %v\", line, err)\n\t\t\t}\n\t\tcase *ast.ExprStmt:\n\t\t\t_, err := ev.Eval(s.Expr, nil)\n\t\t\tif err != nil {\n\t\t\t\tif i == len(lines)-1 {\n\t\t\t\t\treturn err\n\t\t\t\t}\n\t\t\t\tt.Fatalf(\"eval error for %q: %v\", line, err)\n\t\t\t}\n\t\t}\n\t}\n\treturn nil\n}\n\nfunc assertApprox(t *testing.T, input string, expected, got float64) {\n\tt.Helper()\n\tif math.Abs(expected-got) > 1e-9 {\n\t\tt.Errorf(\"%q: expected %v, got %v\", input, expected, got)\n\t}\n}\n\n// --- Basic arithmetic ---\n\nfunc TestIntegration_SingleNumber(t *testing.T) {\n\tassertApprox(t, \"42\", 42, eval(t, \"42\"))\n}\n\nfunc TestIntegration_DecimalNumber(t *testing.T) {\n\tassertApprox(t, \"3.14\", 3.14, eval(t, \"3.14\"))\n}\n\nfunc TestIntegration_LeadingDot(t *testing.T) {\n\tassertApprox(t, \".5\", 0.5, eval(t, \".5\"))\n}\n\nfunc TestIntegration_Addition(t *testing.T) {\n\tassertApprox(t, \"1 + 2\", 3, eval(t, \"1 + 2\"))\n}\n\nfunc TestIntegration_Subtraction(t *testing.T) {\n\tassertApprox(t, \"10 - 4\", 6, eval(t, \"10 - 4\"))\n}\n\nfunc TestIntegration_Multiplication(t *testing.T) {\n\tassertApprox(t, \"3 * 7\", 21, eval(t, \"3 * 7\"))\n}\n\nfunc TestIntegration_Division(t *testing.T) {\n\tassertApprox(t, \"10 / 4\", 2.5, eval(t, \"10 / 4\"))\n}\n\n// --- Precedence and associativity ---\n\nfunc TestIntegration_PrecedenceMulOverAdd(t *testing.T) {\n\t// 2 + 3 * 4 = 2 + 12 = 14\n\tassertApprox(t, \"2 + 3 * 4\", 14, eval(t, \"2 + 3 * 4\"))\n}\n\nfunc TestIntegration_PrecedenceDivOverSub(t *testing.T) {\n\t// 10 - 6 / 3 = 10 - 2 = 8\n\tassertApprox(t, \"10 - 6 / 3\", 8, eval(t, \"10 - 6 / 3\"))\n}\n\nfunc TestIntegration_LeftAssociativitySub(t *testing.T) {\n\t// 10 - 3 - 2 = (10 - 3) - 2 = 5\n\tassertApprox(t, \"10 - 3 - 2\", 5, eval(t, \"10 - 3 - 2\"))\n}\n\nfunc TestIntegration_LeftAssociativityDiv(t *testing.T) {\n\t// 24 / 4 / 3 = (24 / 4) / 3 = 2\n\tassertApprox(t, \"24 / 4 / 3\", 2, eval(t, \"24 / 4 / 3\"))\n}\n\n// --- Parentheses ---\n\nfunc TestIntegration_ParensOverridePrecedence(t *testing.T) {\n\t// (2 + 3) * 4 = 20\n\tassertApprox(t, \"(2 + 3) * 4\", 20, eval(t, \"(2 + 3) * 4\"))\n}\n\nfunc TestIntegration_NestedParens(t *testing.T) {\n\t// ((1 + 2)) = 3\n\tassertApprox(t, \"((1 + 2))\", 3, eval(t, \"((1 + 2))\"))\n}\n\nfunc TestIntegration_DeeplyNestedParens(t *testing.T) {\n\t// ((((((1 + 2)))))) = 3\n\tassertApprox(t, \"((((((1 + 2))))))\", 3, eval(t, \"((((((1 + 2))))))\"))\n}\n\nfunc TestIntegration_ParensOnBothSides(t *testing.T) {\n\t// (1 + 2) * (3 + 4) = 3 * 7 = 21\n\tassertApprox(t, \"(1 + 2) * (3 + 4)\", 21, eval(t, \"(1 + 2) * (3 + 4)\"))\n}\n\nfunc TestIntegration_ParensNestedComplex(t *testing.T) {\n\t// ((2 + 3) * (4 - 1)) / 5 = (5 * 3) / 5 = 3\n\tassertApprox(t, \"((2 + 3) * (4 - 1)) / 5\", 3, eval(t, \"((2 + 3) * (4 - 1)) / 5\"))\n}\n\n// --- Complex expressions ---\n\nfunc TestIntegration_AllOperators(t *testing.T) {\n\t// 1 + 2 * 3 - 4 / 2 = 1 + 6 - 2 = 5\n\tassertApprox(t, \"1 + 2 * 3 - 4 / 2\", 5, eval(t, \"1 + 2 * 3 - 4 / 2\"))\n}\n\nfunc TestIntegration_LongChainedAddition(t *testing.T) {\n\t// 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55\n\tassertApprox(t, \"1+2+3+4+5+6+7+8+9+10\", 55, eval(t, \"1+2+3+4+5+6+7+8+9+10\"))\n}\n\nfunc TestIntegration_LongChainedMixed(t *testing.T) {\n\t// 2 * 3 + 4 * 5 - 6 / 2 + 1 = 6 + 20 - 3 + 1 = 24\n\tassertApprox(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\", 24, eval(t, \"2 * 3 + 4 * 5 - 6 / 2 + 1\"))\n}\n\n// --- Floating point ---\n\nfunc TestIntegration_FloatArithmetic(t *testing.T) {\n\t// 1.5 + 2.5 = 4.0\n\tassertApprox(t, \"1.5 + 2.5\", 4.0, eval(t, \"1.5 + 2.5\"))\n}\n\nfunc TestIntegration_FloatDivision(t *testing.T) {\n\t// 7 / 2 = 3.5\n\tassertApprox(t, \"7 / 2\", 3.5, eval(t, \"7 / 2\"))\n}\n\nfunc TestIntegration_FloatPrecision(t *testing.T) {\n\t// 0.1 + 0.2 ≈ 0.3 (within tolerance)\n\tassertApprox(t, \"0.1 + 0.2\", 0.3, eval(t, \"0.1 + 0.2\"))\n}\n\n// --- Whitespace variations ---\n\nfunc TestIntegration_NoSpaces(t *testing.T) {\n\tassertApprox(t, \"1+2*3\", 7, eval(t, \"1+2*3\"))\n}\n\nfunc TestIntegration_ExtraSpaces(t *testing.T) {\n\tassertApprox(t, \"  1  +  2  \", 3, eval(t, \"  1  +  2  \"))\n}\n\nfunc TestIntegration_TabsAndSpaces(t *testing.T) {\n\tassertApprox(t, \"1\\t+\\t2\", 3, eval(t, \"1\\t+\\t2\"))\n}\n\n// --- Error cases ---\n\nfunc TestIntegration_DivisionByZero(t *testing.T) {\n\terr := evalErr(t, \"1 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n\tif !strings.Contains(err.Error(), \"division by zero\") {\n\t\tt.Errorf(\"expected 'division by zero' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_DivisionByZeroInSubExpr(t *testing.T) {\n\terr := evalErr(t, \"1 + 2 / 0\")\n\tif err == nil {\n\t\tt.Fatal(\"expected division by zero error\")\n\t}\n}\n\nfunc TestIntegration_InvalidCharacter(t *testing.T) {\n\terr := evalErr(t, \"1 @ 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for invalid character\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenLeft(t *testing.T) {\n\terr := evalErr(t, \"(1 + 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for missing closing paren\")\n\t}\n}\n\nfunc TestIntegration_MismatchedParenRight(t *testing.T) {\n\terr := evalErr(t, \"1 + 2)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for unexpected closing paren\")\n\t}\n}\n\nfunc TestIntegration_EmptyParens(t *testing.T) {\n\terr := evalErr(t, \"()\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty parentheses\")\n\t}\n}\n\nfunc TestIntegration_TrailingOperator(t *testing.T) {\n\terr := evalErr(t, \"1 +\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for trailing operator\")\n\t}\n}\n\nfunc TestIntegration_LeadingOperator(t *testing.T) {\n\terr := evalErr(t, \"* 1\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for leading operator\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveOperators(t *testing.T) {\n\terr := evalErr(t, \"1 + * 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive operators\")\n\t}\n}\n\nfunc TestIntegration_EmptyInput(t *testing.T) {\n\t// Empty string should produce only EOF, parser should error\n\terr := evalErr(t, \"\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for empty input\")\n\t}\n}\n\nfunc TestIntegration_ConsecutiveNumbers(t *testing.T) {\n\terr := evalErr(t, \"1 2\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for consecutive numbers without operator\")\n\t}\n}\n\n// --- Function definitions (full pipeline) ---\n\nfunc TestIntegration_DefineAndCallSingleParam(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev, \"f(x) = x + 1\", \"f(5)\")\n\tassertApprox(t, \"f(5)\", 6, result)\n}\n\nfunc TestIntegration_DefineAndCallMultiParam(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev, \"add(x, y) = x + y\", \"add(3, 4)\")\n\tassertApprox(t, \"add(3, 4)\", 7, result)\n}\n\nfunc TestIntegration_CrossFunctionCalls(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev,\n\t\t\"double(x) = x * 2\",\n\t\t\"quad(x) = double(double(x))\",\n\t\t\"quad(3)\",\n\t)\n\tassertApprox(t, \"quad(3)\", 12, result)\n}\n\nfunc TestIntegration_NestedFuncCallsInExpr(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev,\n\t\t\"f(x) = x + 1\",\n\t\t\"f(f(f(1)))\",\n\t)\n\t// f(1)=2, f(2)=3, f(3)=4\n\tassertApprox(t, \"f(f(f(1)))\", 4, result)\n}\n\nfunc TestIntegration_FuncCallInBinaryExpr(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev,\n\t\t\"f(x) = x * 2\",\n\t\t\"f(3) + f(4)\",\n\t)\n\t// f(3)=6, f(4)=8, 6+8=14\n\tassertApprox(t, \"f(3)+f(4)\", 14, result)\n}\n\nfunc TestIntegration_FuncWithExprBody(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev,\n\t\t\"area(w, h) = w * h\",\n\t\t\"area(3, 5) + 1\",\n\t)\n\tassertApprox(t, \"area(3,5)+1\", 16, result)\n}\n\nfunc TestIntegration_FuncNoParams(t *testing.T) {\n\tev := evaluator.New()\n\tresult := evalLines(t, ev, \"pi() = 3\", \"pi() + 1\")\n\tassertApprox(t, \"pi()+1\", 4, result)\n}\n\n// --- Function error cases (full pipeline) ---\n\nfunc TestIntegration_UndefinedFunction(t *testing.T) {\n\tev := evaluator.New()\n\terr := evalLinesErr(t, ev, \"f(1)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for undefined function\")\n\t}\n\tif !strings.Contains(err.Error(), \"undefined function\") {\n\t\tt.Errorf(\"expected 'undefined function' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_WrongArgCount(t *testing.T) {\n\tev := evaluator.New()\n\terr := evalLinesErr(t, ev, \"f(x) = x\", \"f(1, 2)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for wrong argument count\")\n\t}\n\tif !strings.Contains(err.Error(), \"expects 1 arguments, got 2\") {\n\t\tt.Errorf(\"expected arg count error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_FunctionRedefinition(t *testing.T) {\n\tev := evaluator.New()\n\terr := evalLinesErr(t, ev, \"f(x) = x\", \"f(x) = x + 1\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for function redefinition\")\n\t}\n\tif !strings.Contains(err.Error(), \"already defined\") {\n\t\tt.Errorf(\"expected 'already defined' in error, got: %v\", err)\n\t}\n}\n\nfunc TestIntegration_UndefinedVariable(t *testing.T) {\n\tev := evaluator.New()\n\terr := evalLinesErr(t, ev, \"f(x) = x + y\", \"f(1)\")\n\tif err == nil {\n\t\tt.Fatal(\"expected error for undefined variable\")\n\t}\n\tif !strings.Contains(err.Error(), \"undefined variable\") {\n\t\tt.Errorf(\"expected 'undefined variable' in error, got: %v\", err)\n\t}\n}\n"}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test -v -run TestIntegration 2>&1; echo \"---EXIT:$?---\""}: OK
+CALLING TOOL: bash_command {"command":"go test -v -run TestIntegration 2>&1; echo \"---EXIT:$?---\""}
+$$$ "=== RUN   TestIntegration_SingleNumber\n--- PASS: TestIntegration_SingleNumber (0.00s)\n=== RUN   TestIntegration_DecimalNumber\n--- PASS: TestIntegration_DecimalNumber (0.00s)\n=== RUN   TestIntegration_LeadingDot\n--- PASS: TestIntegration_LeadingDot (0.00s)\n=== RUN   TestIntegration_Addition\n--- PASS: TestIntegration_Addition (0.00s)\n=== RUN   TestIntegration_Subtraction\n--- PASS: TestIntegration_Subtraction (0.00s)\n=== RUN   TestIntegration_Multiplication\n--- PASS: TestIntegration_Multiplication (0.00s)\n=== RUN   TestIntegration_Division\n--- PASS: TestIntegration_Division (0.00s)\n=== RUN   TestIntegration_PrecedenceMulOverAdd\n--- PASS: TestIntegration_PrecedenceMulOverAdd (0.00s)\n=== RUN   TestIntegration_PrecedenceDivOverSub\n--- PASS: TestIntegration_PrecedenceDivOverSub (0.00s)\n=== RUN   TestIntegration_LeftAssociativitySub\n--- PASS: TestIntegration_LeftAssociativitySub (0.00s)\n=== RUN   TestIntegration_LeftAssociativityDiv\n--- PASS: TestIntegration_LeftAssociativityDiv (0.00s)\n=== RUN   TestIntegration_ParensOverridePrecedence\n--- PASS: TestIntegration_ParensOverridePrecedence (0.00s)\n=== RUN   TestIntegration_NestedParens\n--- PASS: TestIntegration_NestedParens (0.00s)\n=== RUN   TestIntegration_DeeplyNestedParens\n--- PASS: TestIntegration_DeeplyNestedParens (0.00s)\n=== RUN   TestIntegration_ParensOnBothSides\n--- PASS: TestIntegration_ParensOnBothSides (0.00s)\n=== RUN   TestIntegration_ParensNestedComplex\n--- PASS: TestIntegration_ParensNestedComplex (0.00s)\n=== RUN   TestIntegration_AllOperators\n--- PASS: TestIntegration_AllOperators (0.00s)\n=== RUN   TestIntegration_LongChainedAddition\n--- PASS: TestIntegration_LongChainedAddition (0.00s)\n=== RUN   TestIntegration_LongChainedMixed\n--- PASS: TestIntegration_LongChainedMixed (0.00s)\n=== RUN   TestIntegration_FloatArithmetic\n--- PASS: TestIntegration_FloatArithmetic (0.00s)\n=== RUN   TestIntegration_FloatDivision\n--- PASS: TestIntegration_FloatDivision (0.00s)\n=== RUN   TestIntegration_FloatPrecision\n--- PASS: TestIntegration_FloatPrecision (0.00s)\n=== RUN   TestIntegration_NoSpaces\n--- PASS: TestIntegration_NoSpaces (0.00s)\n=== RUN   TestIntegration_ExtraSpaces\n--- PASS: TestIntegration_ExtraSpaces (0.00s)\n=== RUN   TestIntegration_TabsAndSpaces\n--- PASS: TestIntegration_TabsAndSpaces (0.00s)\n=== RUN   TestIntegration_DivisionByZero\n--- PASS: TestIntegration_DivisionByZero (0.00s)\n=== RUN   TestIntegration_DivisionByZeroInSubExpr\n--- PASS: TestIntegration_DivisionByZeroInSubExpr (0.00s)\n=== RUN   TestIntegration_InvalidCharacter\n--- PASS: TestIntegration_InvalidCharacter (0.00s)\n=== RUN   TestIntegration_MismatchedParenLeft\n--- PASS: TestIntegration_MismatchedParenLeft (0.00s)\n=== RUN   TestIntegration_MismatchedParenRight\n--- PASS: TestIntegration_MismatchedParenRight (0.00s)\n=== RUN   TestIntegration_EmptyParens\n--- PASS: TestIntegration_EmptyParens (0.00s)\n=== RUN   TestIntegration_TrailingOperator\n--- PASS: TestIntegration_TrailingOperator (0.00s)\n=== RUN   TestIntegration_LeadingOperator\n--- PASS: TestIntegration_LeadingOperator (0.00s)\n=== RUN   TestIntegration_ConsecutiveOperators\n--- PASS: TestIntegration_ConsecutiveOperators (0.00s)\n=== RUN   TestIntegration_EmptyInput\n--- PASS: TestIntegration_EmptyInput (0.00s)\n=== RUN   TestIntegration_ConsecutiveNumbers\n--- PASS: TestIntegration_ConsecutiveNumbers (0.00s)\n=== RUN   TestIntegration_DefineAndCallSingleParam\n--- PASS: TestIntegration_DefineAndCallSingleParam (0.00s)\n=== RUN   TestIntegration_DefineAndCallMultiParam\n--- PASS: TestIntegration_DefineAndCallMultiParam (0.00s)\n=== RUN   TestIntegration_CrossFunctionCalls\n--- PASS: TestIntegration_CrossFunctionCalls (0.00s)\n=== RUN   TestIntegration_NestedFuncCallsInExpr\n--- PASS: TestIntegration_NestedFuncCallsInExpr (0.00s)\n=== RUN   TestIntegration_FuncCallInBinaryExpr\n--- PASS: TestIntegration_FuncCallInBinaryExpr (0.00s)\n=== RUN   TestIntegration_FuncWithExprBody\n--- PASS: TestIntegration_FuncWithExprBody (0.00s)\n=== RUN   TestIntegration_FuncNoParams\n--- PASS: TestIntegration_FuncNoParams (0.00s)\n=== RUN   TestIntegration_UndefinedFunction\n--- PASS: TestIntegration_UndefinedFunction (0.00s)\n=== RUN   TestIntegration_WrongArgCount\n--- PASS: TestIntegration_WrongArgCount (0.00s)\n=== RUN   TestIntegration_FunctionRedefinition\n--- PASS: TestIntegration_FunctionRedefinition (0.00s)\n=== RUN   TestIntegration_UndefinedVariable\n--- PASS: TestIntegration_UndefinedVariable (0.00s)\nPASS\nok  \tmatheval\t0.630s\n---EXIT:0---\n"
+
+--- STOP_REASON: tool_use
+AI: All 46 integration tests pass. Final full test run:
+!!!!! {"command":"go test ./... -count=1 2>&1; echo \"---EXIT:$?---\""}: Ok
+--- STOP_REASON: tool_use
+!!!!! {"command":"go test ./... -count=1 2>&1 && echo \"ALL PASS\" || echo \"FAIL\""}: OK
+CALLING TOOL: bash_command {"command":"go test ./... -count=1 2>&1 && echo \"ALL PASS\" || echo \"FAIL\""}
+$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval\t0.449s\nok  \tmatheval/ast\t0.820s\nok  \tmatheval/evaluator\t1.039s\nok  \tmatheval/lexer\t2.176s\nok  \tmatheval/parser\t1.406s\nok  \tmatheval/repl\t2.397s\nok  \tmatheval/token\t1.769s\nALL PASS\n"
+
+--- STOP_REASON: tool_use
+AI: All tests pass across all packages. Commit.
+!!!!! {"command":"git add -A && git commit -m \"Step 7: Add integration tests for function definitions - define+call, multi-param, cross-function, nested, errors\" 2>&1"}: OK
+CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Step 7: Add integration tests for function definitions - define+call, multi-param, cross-function, nested, errors\" 2>&1"}
+$$$ "[master 0279013] Step 7: Add integration tests for function definitions - define+call, multi-param, cross-function, nested, errors\n 1 file changed, 176 insertions(+)\n"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.1.4","summary":"Integration tests added: 11 new function-related tests (define+call single/multi-param, cross-function calls, nested calls, func in binary expr, func with expr body, no-params func, errors for undefined function/wrong arg count/redefinition/undefined variable). All 46 integration tests passing."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1.1","summary":"All 4 remaining steps implemented: parser tests (step 4), evaluator with Evaluator struct/env/functions (step 5), REPL updated to use ParseLine+Evaluator (step 6), integration tests (step 7). Full backward compatibility maintained."}
+$$$ "done"
+
+--- STOP_REASON: tool_use
+CALLING TOOL: todo_item_mark_done {"id":"1","summary":"Function definition support fully implemented. All 7 steps from plan.md completed: tokens (Ident/Comma/Equals), AST nodes (Ident/FuncCall/Statement/ExprStmt/FuncDef), lexer (identifier scanning), parser (ParseLine/isFuncDef/parseFuncDef/factor with Ident+FuncCall), evaluator (Evaluator struct with Define/Eval/env/funcs), REPL (ParseLine+Evaluator integration), integration tests. Total test count: 19 parser, 23 evaluator, 17 REPL, 46 integration — all passing."}
+$$$ "done"
+
+--- STOP_REASON: end_turn
+AI: Implementation complete. Here's a summary of what was done:
+
+**Steps 1-3 & 4 (code)** were already done. I completed:
+
+1. **Step 4 — Parser tests** (19 tests): Backward compat (`Parse`), `Ident`, `FuncCall` (no args, one arg, multi args, expr args, in expressions, missing rparen), `ParseLine` (expr stmt, func call, func def single/multi/no params), error cases (empty, missing body, bad params, trailing tokens).
+
+2. **Step 5 — Evaluator struct** (23 tests): `Evaluator` struct with `funcs map[string]*ast.FuncDef`, `New()`, `Define()` (with redefinition error), `Eval(node, env)` handling `NumberLit`, `Ident` (env lookup), `BinaryExpr`, `FuncCall` (arg count check, caller-env arg evaluation, new-env body evaluation). Package-level `Eval()` preserved as backward-compat wrapper.
+
+3. **Step 6 — REPL update** (17 tests): Replaced `evalLine` with `ParseLine` + `Evaluator`. `FuncDef` → `ev.Define()` + prints "defined \<name\>". `ExprStmt` → `ev.Eval()` + prints result. Added 7 new REPL tests for define, call, multi-param, redefinition error, undefined func, func in expr, cross-function calls.
+
+4. **Step 7 — Integration tests** (46 tests): Added `evalLines`/`evalLinesErr` helpers using shared `Evaluator`. 11 new tests: single/multi-param define+call, cross-function calls, nested calls, func in binary expr, func with expr body, no-params func, undefined function, wrong arg count, redefinition, undefined variable.
+## AGENT NO ITEMS
+## USER NO ITEMS
+interrupt: 2
+								
+~/dev/src/math $