sessions/01.md

../neo/neo 
YOU: create math expr eval app
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS assistant false
-- AGENT START WORKING 1
--- STOP_REASON: tool_use
AI: 

I'll start by assessing this task and breaking it into the standard phases. First, let me explore the current project state.
CALLING TOOL: dir_list {"name":"."}
$$$ {"entries":null}

--- STOP_REASON: tool_use
AI: Empty project. Let me create the phase-based TODO structure for this task. Before diving in, I have some clarification questions.
CALLING TOOL: todo_item_add {"items":[{"title":"Research: Gather Requirements","description":"Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.","assignedTo":"assistant","parentId":"1","items":[{"title":"What operators/functions should be supported?","description":"Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.","assignedTo":"user"},{"title":"What number types should be supported?","description":"Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.","assignedTo":"user"},{"title":"Should variables be supported?","description":"E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.","assignedTo":"user"},{"title":"What is the target interface?","description":"CLI app? Library API? REPL? Web service? This determines how we package the evaluator.","assignedTo":"user"},{"title":"What programming language?","description":"Need to know which language to implement in. This affects all implementation decisions.","assignedTo":"user"},{"title":"What error handling is expected?","description":"Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?","assignedTo":"user"}],"parallel":true}]}
$$$ "done"

--- STOP_REASON: end_turn
AI: I've created the Research phase with clarification questions. These are assigned to you and can be answered in parallel since they're independent.

Key questions (sorted by importance):

1. **What operators/functions should be supported?** (basic arithmetic, parentheses, exponentiation, trig functions, etc.)
2. **What number types?** (integers, floats, scientific notation)
3. **Should variables be supported?** (e.g. `x + 2`)
4. **What programming language?**
5. **What is the target interface?** (CLI, library, REPL, web service)
6. **What error handling is expected?**
## AGENT NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	1.1: Research: Gather Requirements - IN PROGRESS assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
		1.1.1: What operators/functions should be supported? - IN PROGRESS user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		1.1.2: What number types should be supported? - IN PROGRESS user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		1.1.3: Should variables be supported? - IN PROGRESS user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		1.1.4: What is the target interface? - IN PROGRESS user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		1.1.5: What programming language? - IN PROGRESS user false
		Need to know which language to implement in. This affects all implementation decisions.
		1.1.6: What error handling is expected? - IN PROGRESS user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
-- YOU START WORKING 6
YOU 1.1.1 What operators/functions should be supported?: basic + parentheses
YOU 1.1.2 What number types should be supported?: float
YOU 1.1.3 Should variables be supported?: no
YOU 1.1.4 What is the target interface?: CLI REPL
YOU 1.1.5 What programming language?: go
YOU 1.1.6 What error handling is expected?: abort on error
-- YOU END WORKING
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	1.1: Research: Gather Requirements - IN PROGRESS assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
		1.1.1: What operators/functions should be supported? - IN PROGRESS assistant false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
			 - user: basic + parentheses
		1.1.2: What number types should be supported? - IN PROGRESS assistant false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
			 - user: float
		1.1.3: Should variables be supported? - IN PROGRESS assistant false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
			 - user: no
		1.1.4: What is the target interface? - IN PROGRESS assistant false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
			 - user: CLI REPL
		1.1.5: What programming language? - IN PROGRESS assistant false
		Need to know which language to implement in. This affects all implementation decisions.
			 - user: go
		1.1.6: What error handling is expected? - IN PROGRESS assistant false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
			 - user: abort on error
-- AGENT START WORKING 1.1.1, 1.1.2, 1.1.3, 1.1.4, 1.1.5, 1.1.6
--- STOP_REASON: tool_use
AI: 

I have all the answers from the user. Let me summarize each item and mark them done, then summarize the overall research findings.
CALLING TOOL: todo_item_add_comment {"id":"1.1.1","comment":"Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions."}
$$$ "done"

CALLING TOOL: todo_item_add_comment {"id":"1.1.2","comment":"Confirmed: Floating point numbers supported."}
$$$ "done"

CALLING TOOL: todo_item_add_comment {"id":"1.1.3","comment":"Confirmed: No variables. Pure numeric expressions only."}
$$$ "done"

CALLING TOOL: todo_item_add_comment {"id":"1.1.4","comment":"Confirmed: CLI REPL (read-eval-print loop)."}
$$$ "done"

CALLING TOOL: todo_item_add_comment {"id":"1.1.5","comment":"Confirmed: Go."}
$$$ "done"

CALLING TOOL: todo_item_add_comment {"id":"1.1.6","comment":"Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues)."}
$$$ "done"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.1.1","summary":"Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions."}
$$$ "done"

CALLING TOOL: todo_item_mark_done {"id":"1.1.2","summary":"Floating point numbers."}
$$$ "done"

CALLING TOOL: todo_item_mark_done {"id":"1.1.3","summary":"No variables. Pure numeric expressions only."}
$$$ "done"

CALLING TOOL: todo_item_mark_done {"id":"1.1.4","summary":"CLI REPL interface."}
$$$ "done"

CALLING TOOL: todo_item_mark_done {"id":"1.1.5","summary":"Go language."}
$$$ "done"

CALLING TOOL: todo_item_mark_done {"id":"1.1.6","summary":"Abort on error — print error, continue REPL loop."}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now let me mark the parent research item as done with a comprehensive summary.
CALLING TOOL: todo_item_mark_done {"id":"1.1","summary":"Research complete. Requirements gathered:\n- Language: Go\n- Operators: +, -, *, / with parentheses\n- Numbers: floating point\n- Variables: none\n- Interface: CLI REPL\n- Error handling: abort on error (print message, continue REPL)\n\nThis is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell."}
$$$ "done"

--- STOP_REASON: end_turn
AI: Research phase is complete. Here's the summary:

**Requirements:**
| Aspect | Decision |
|---|---|
| Language | Go |
| Operators | `+`, `-`, `*`, `/` and `()` |
| Numbers | Floating point |
| Variables | None |
| Interface | CLI REPL |
| Errors | Print error, continue REPL |

This is a well-scoped problem. The expression grammar has only two precedence levels, no variables, and no functions — making it a clean recursive-descent parsing task.

Ready to move to **Design** phase on your confirmation.
## AGENT NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
-- YOU START WORKING 1
YOU 1 create math expr eval app: ASSIGN
-- YOU END WORKING
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS assistant false
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
-- AGENT START WORKING 1
--- STOP_REASON: tool_use
AI: 

Research phase is complete. Now I'll move to the **Design** phase.
CALLING TOOL: todo_item_add {"items":[{"title":"Design: Architecture & Component Design","description":"Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.","assignedTo":"assistant","parentId":"1","items":[{"title":"Analyze approaches and define architecture","description":"Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.","assignedTo":"assistant"}]}]}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now let me work on the design. Let me think through the approaches and produce a design document.

**Approach Analysis:**

**1. Recursive-Descent Parser (with AST)**
- Lexer → Parser → AST → Evaluator → REPL
- Clean separation of concerns, each component is independently testable
- Easy to extend (add operators, functions later)
- Slightly more code due to AST node types

**2. Recursive-Descent Parser (direct evaluation, no AST)**
- Lexer → Parser/Evaluator → REPL
- Parser evaluates as it parses — fewer components
- Harder to extend, test, or add features like pretty-printing
- Couples parsing and evaluation

**3. Shunting-Yard Algorithm**
- Lexer → Shunting-Yard → RPN → Evaluator → REPL
- Good for operator-precedence grammars
- Less intuitive, harder to produce good error messages
- Overkill for 2 precedence levels

**Decision:** Approach 1 — Recursive-Descent with AST. Best separation of concerns, most testable, most extensible.

Let me write the design document:
!!!!! {"command":"mkdir -p docs"}: OK
CALLING TOOL: bash_command {"command":"mkdir -p docs"}
$$$ ""

--- STOP_REASON: tool_use
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- 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"}
$$$ "done"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.2.1","summary":"Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md."}
$$$ "done"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.2","summary":"Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md."}
$$$ "done"

--- STOP_REASON: end_turn
AI: ---

**Design phase complete.** Here's the summary:

**Architecture:** `Lexer → Parser → AST → Evaluator → REPL`

**5 components, each doing one thing:**
| Component | Responsibility | Signature |
|-----------|---------------|-----------|
| **Token** | Data types for tokens | `Type`, `Token` struct |
| **Lexer** | `string → []Token` | `Tokenize(input string) ([]Token, error)` |
| **Parser** | `[]Token → AST` | `Parse(tokens []Token) (Node, error)` |
| **Evaluator** | `AST → float64` | `Eval(node Node) (float64, error)` |
| **REPL** | Interactive loop | `Run(r io.Reader, w io.Writer)` |

**Grammar:**
```
expr   → term (('+' | '-') term)*
term   → factor (('*' | '/') factor)*
factor → NUMBER | '(' expr ')'
```

**3 approaches considered:**
1. ✅ Recursive-descent with AST — best testability and extensibility
2. ❌ Recursive-descent with direct eval — couples parsing and evaluation
3. ❌ Shunting-yard — overkill for 2 precedence levels

Full design document is at `docs/design.md`. 

**Shall I proceed to the Plan phase?**
## AGENT NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
-- YOU START WORKING 1
YOU 1 create math expr eval app: looks good
-- YOU END WORKING
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS assistant false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
-- AGENT START WORKING 1
--- STOP_REASON: tool_use
AI: 

Let me review the current state of the project and the design document before proceeding to the Plan phase.
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: dir_list {"name":"."}
$$$ {"entries":[{"name":"docs","is_dir":true}]}

--- STOP_REASON: tool_use
AI: Good. Research and Design phases are complete. The design is solid. Now I'll move to the **Plan** phase.
CALLING TOOL: todo_item_add {"items":[{"title":"Plan: Detailed Implementation Plan","description":"Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.","assignedTo":"assistant","parentId":"1"}]}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now let me create the detailed plan document and the corresponding TODO items.
CALLING TOOL: file_write {"path":"docs/plan.md","contents":"# 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 & README\n- Write README.md with usage instructions\n- Final commit\n"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now let me create the TODO sub-items for the implementation phase:
CALLING TOOL: todo_item_add {"items":[{"title":"Implement: Execute the plan","description":"Implement all steps from docs/plan.md sequentially using TDD.","assignedTo":"assistant","parentId":"1","items":[{"title":"Step 1: Project skeleton (go mod, dirs, placeholder main.go)","assignedTo":"assistant","description":"Initialize Go module and create directory structure."},{"title":"Step 2: Token package (Type enum, Token struct)","assignedTo":"assistant","description":"Define token types and Token struct with Pos tracking."},{"title":"Step 3: Lexer (Tokenize + tests)","assignedTo":"assistant","description":"Implement tokenizer. TDD: write tests first, then implementation."},{"title":"Step 4: AST package (Node interface, NumberLit, BinaryExpr)","assignedTo":"assistant","description":"Define AST node types with sealed interface."},{"title":"Step 5: Parser (recursive-descent + tests)","assignedTo":"assistant","description":"Implement parser following grammar. TDD: tests first."},{"title":"Step 6: Evaluator (Eval + tests)","assignedTo":"assistant","description":"Implement AST walker. TDD: tests first."},{"title":"Step 7: REPL (Run + tests)","assignedTo":"assistant","description":"Implement REPL loop with io.Reader/Writer. TDD."},{"title":"Step 8: main.go (wire REPL)","assignedTo":"assistant","description":"Entry point calling repl.Run."},{"title":"Step 9: Integration test","assignedTo":"assistant","description":"End-to-end tests through all stages."},{"title":"Step 10: README & final commit","assignedTo":"assistant","description":"Documentation and final commit."}]}]}
$$$ "done"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.3","summary":"Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README."}
$$$ "done"

--- STOP_REASON: end_turn
AI: **Plan phase complete.** Here's the summary:

### Implementation Plan (10 steps)

| Step | What | Key Detail |
|------|------|------------|
| 1 | Project skeleton | `go mod init`, directory structure |
| 2 | Token package | `Type` enum, `Token` struct |
| 3 | Lexer + tests | `Tokenize()` — TDD |
| 4 | AST package | `Node` interface, `NumberLit`, `BinaryExpr` |
| 5 | Parser + tests | Recursive-descent — TDD |
| 6 | Evaluator + tests | `Eval()` — TDD |
| 7 | REPL + tests | `Run(r, w)` — TDD |
| 8 | main.go | Wire REPL to stdin/stdout |
| 9 | Integration test | End-to-end verification |
| 10 | README + commit | Documentation |

Full details in `docs/plan.md`. Shall I proceed to the **Implement** phase?
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - IN PROGRESS assistant false
		Initialize Go module and create directory structure.
		1.4.2: Step 2: Token package (Type enum, Token struct) - IN PROGRESS assistant false
		Define token types and Token struct with Pos tracking.
		1.4.3: Step 3: Lexer (Tokenize + tests) - IN PROGRESS assistant false
		Implement tokenizer. TDD: write tests first, then implementation.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - IN PROGRESS assistant false
		Define AST node types with sealed interface.
		1.4.5: Step 5: Parser (recursive-descent + tests) - IN PROGRESS assistant false
		Implement parser following grammar. TDD: tests first.
		1.4.6: Step 6: Evaluator (Eval + tests) - IN PROGRESS assistant false
		Implement AST walker. TDD: tests first.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.1
--- STOP_REASON: tool_use
AI: 

I'll start by reading the design and plan documents to understand the exact structure needed.
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"

CALLING TOOL: dir_list {"name":"."}
$$$ {"entries":[{"name":"docs","is_dir":true}]}

--- STOP_REASON: tool_use
AI: Clear. Step 1 is simple: initialize Go module, create directory structure, and a placeholder `main.go`. Let me do it.
!!!!! {"command":"go mod init matheval"
OK
--- STOP_REASON: tool_use
!!!!! {"command":"mkdir -p cmd/matheval token lexer ast parser evaluator repl"}: CALLING TOOL: bash_command {"command":"mkdir -p cmd/matheval token lexer ast parser evaluator repl"}
$$$ ""

--- STOP_REASON: tool_use
CALLING TOOL: file_write {"path":"cmd/matheval/main.go","contents":"package main\n\nimport \"fmt\"\n\nfunc main() {\n\tfmt.Println(\"matheval - math expression evaluator\")\n}\n"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"cd /root && go build ./cmd/matheval/ && ./matheval"}: no need to cd /root. you are already in project root dir
--- STOP_REASON: tool_use
!!!!! {"command":"go build ./cmd/matheval/ && ./matheval"}: OK
CALLING TOOL: bash_command {"command":"go build ./cmd/matheval/ && ./matheval"}
ERR: exit status 1
--- STOP_REASON: tool_use
!!!!! {"command":"go build -o matheval ./cmd/matheval/ 2>&1 && ./matheval 2>&1"}: OK
CALLING TOOL: bash_command {"command":"go build -o matheval ./cmd/matheval/ 2>&1 && ./matheval 2>&1"}
ERR: exit status 1
--- STOP_REASON: tool_use
!!!!! {"command":"go build -o matheval ./cmd/matheval/ 2>&1; echo \"EXIT: $?\""}: OK
CALLING TOOL: bash_command {"command":"go build -o matheval ./cmd/matheval/ 2>&1; echo \"EXIT: $?\""}
$$$ "go: go.mod file not found in current directory or any parent directory; see 'go help modules'\nEXIT: 1\n"

--- STOP_REASON: tool_use
!!!!! {"command":"pwd && ls -la"}: OK
CALLING TOOL: bash_command {"command":"pwd && ls -la"}
$$$ "/Users/lekva/dev/src/math\ntotal 0\ndrwxr-xr-x  10 lekva  staff   320 Feb 28 19:06 .\ndrwxr-xr-x@ 41 lekva  staff  1312 Feb 26 10:55 ..\ndrwxr-xr-x   2 lekva  staff    64 Feb 28 19:06 ast\ndrwxr-xr-x   3 lekva  staff    96 Feb 28 19:06 cmd\ndrwxr-xr-x   4 lekva  staff   128 Feb 28 19:05 docs\ndrwxr-xr-x   2 lekva  staff    64 Feb 28 19:06 evaluator\ndrwxr-xr-x   2 lekva  staff    64 Feb 28 19:06 lexer\ndrwxr-xr-x   2 lekva  staff    64 Feb 28 19:06 parser\ndrwxr-xr-x   2 lekva  staff    64 Feb 28 19:06 repl\ndrwxr-xr-x   2 lekva  staff    64 Feb 28 19:06 token\n"

--- STOP_REASON: tool_use
AI: The `go mod init` didn't actually create the file (no error output but no go.mod). Let me check and create it properly.
!!!!! {"command":"ls go.mod 2>&1; echo \"---\"; go version 2>&1"}: OK
CALLING TOOL: bash_command {"command":"ls go.mod 2>&1; echo \"---\"; go version 2>&1"}
$$$ "ls: go.mod: No such file or directory\n---\ngo version go1.23.1 darwin/arm64\n"

--- STOP_REASON: tool_use
!!!!! {"command":"go mod init matheval 2>&1; echo \"EXIT: $?\""}: OK
CALLING TOOL: bash_command {"command":"go mod init matheval 2>&1; echo \"EXIT: $?\""}
$$$ "go: creating new go.mod: module matheval\ngo: to add module requirements and sums:\n\tgo mod tidy\nEXIT: 0\n"

--- STOP_REASON: tool_use
!!!!! {"command":"go build -o matheval ./cmd/matheval/ 2>&1 && ./matheval 2>&1; echo \"EXIT: $?\""}: OK
CALLING TOOL: bash_command {"command":"go build -o matheval ./cmd/matheval/ 2>&1 && ./matheval 2>&1; echo \"EXIT: $?\""}
$$$ "matheval - math expression evaluator\nEXIT: 0\n"

--- STOP_REASON: tool_use
AI: Project skeleton builds and runs. Let me verify the full directory structure and commit.
!!!!! {"command":"find . -not -path './.git/*' -not -name '.git' -not -name '.DS_Store' -not -name 'matheval' | sort"}: OK
CALLING TOOL: bash_command {"command":"find . -not -path './.git/*' -not -name '.git' -not -name '.DS_Store' -not -name 'matheval' | sort"}
$$$ ".\n./ast\n./cmd\n./cmd/matheval/main.go\n./docs\n./docs/design.md\n./docs/plan.md\n./evaluator\n./go.mod\n./lexer\n./parser\n./repl\n./token\n"

--- STOP_REASON: tool_use
!!!!! {"command":"rm -f matheval && git add -A && git commit -m \"Step 1: project skeleton - go mod, directory structure, placeholder main.go\" 2>&1"}: OK
CALLING TOOL: bash_command {"command":"rm -f matheval && git add -A && git commit -m \"Step 1: project skeleton - go mod, directory structure, placeholder main.go\" 2>&1"}
ERR: exit status 128
--- STOP_REASON: tool_use
!!!!! {"command":"git init 2>&1 && git add -A 2>&1 && git commit -m \"Step 1: project skeleton - go mod, directory structure, placeholder main.go\" 2>&1"}: OK
CALLING TOOL: bash_command {"command":"git init 2>&1 && git add -A 2>&1 && git commit -m \"Step 1: project skeleton - go mod, directory structure, placeholder main.go\" 2>&1"}
$$$ "hint: Using 'master' as the name for the initial branch. This default branch name\nhint: is subject to change. To configure the initial branch name to use in all\nhint: of your new repositories, which will suppress this warning, call:\nhint: \nhint: \tgit config --global init.defaultBranch \u003cname\u003e\nhint: \nhint: Names commonly chosen instead of 'master' are 'main', 'trunk' and\nhint: 'development'. The just-created branch can be renamed via this command:\nhint: \nhint: \tgit branch -m \u003cname\u003e\nInitialized empty Git repository in /Users/lekva/dev/src/math/.git/\n[master (root-commit) 719e823] Step 1: project skeleton - go mod, directory structure, placeholder main.go\n 4 files changed, 261 insertions(+)\n create mode 100644 cmd/matheval/main.go\n create mode 100644 docs/design.md\n create mode 100644 docs/plan.md\n create mode 100644 go.mod\n"

--- STOP_REASON: tool_use
AI: Done. Step 1 complete:

- `go.mod` initialized (module `matheval`, Go 1.23.1)
- Directory structure created: `cmd/matheval/`, `token/`, `lexer/`, `ast/`, `parser/`, `evaluator/`, `repl/`
- Placeholder `main.go` — builds and runs successfully
- Committed to git
CALLING TOOL: todo_item_mark_done {"id":"1.4.1","summary":"Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: TODO 1.4.1 is done. Ready for 1.4.2.
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - IN PROGRESS assistant false
		Define token types and Token struct with Pos tracking.
		1.4.3: Step 3: Lexer (Tokenize + tests) - IN PROGRESS assistant false
		Implement tokenizer. TDD: write tests first, then implementation.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - IN PROGRESS assistant false
		Define AST node types with sealed interface.
		1.4.5: Step 5: Parser (recursive-descent + tests) - IN PROGRESS assistant false
		Implement parser following grammar. TDD: tests first.
		1.4.6: Step 6: Evaluator (Eval + tests) - IN PROGRESS assistant false
		Implement AST walker. TDD: tests first.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.2
--- STOP_REASON: tool_use
AI: 

Let me first check the current project state and the design document to understand the token package requirements.
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"

CALLING TOOL: dir_list {"name":"."}
$$$ {"entries":[{"name":".git","is_dir":true},{"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":"lexer","is_dir":true},{"name":"parser","is_dir":true},{"name":"repl","is_dir":true},{"name":"token","is_dir":true}]}

--- STOP_REASON: tool_use
CALLING TOOL: dir_list {"name":"token"}
$$$ {"entries":null}

CALLING TOOL: file_read {"path":"go.mod"}
$$$ "module matheval\n\ngo 1.23.1\n"

--- STOP_REASON: tool_use
AI: Token directory is empty. This is a small, straightforward step — define the `Type` enum, `Token` struct, and a `String()` method. No tests needed for pure data types. Let me implement it directly.
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\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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"cd /root && go build ./token/"}: no need for cd /root
--- STOP_REASON: tool_use
!!!!! {"command":"cd /app && go build ./token/"}:you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"go build ./token/"}: OK
CALLING TOOL: bash_command {"command":"go build ./token/"}
$$$ ""

--- STOP_REASON: tool_use
AI: Builds cleanly. Let me add a quick unit test for the `String()` method to follow TDD practices, then commit.
CALLING TOOL: file_write {"path":"token/token_test.go","contents":"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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"go test ./token/ -v"}: OK
CALLING TOOL: bash_command {"command":"go test ./token/ -v"}
$$$ "=== RUN   TestTypeString\n--- PASS: TestTypeString (0.00s)\nPASS\nok  \tmatheval/token\t0.571s\n"

--- STOP_REASON: tool_use
AI: All tests pass. Let me commit.
!!!!! {"command":"git add -A && git commit -m \"token: add Type enum, Token struct, and String() method\""}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"token: add Type enum, Token struct, and String() method\""}
$$$ "[master d04f362] token: add Type enum, Token struct, and String() method\n 2 files changed, 73 insertions(+)\n create mode 100644 token/token.go\n create mode 100644 token/token_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.2","summary":"Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: **Done.** Token package implemented and committed:

- **`token/token.go`**: `Type` enum with 8 constants (`Number`, `Plus`, `Minus`, `Star`, `Slash`, `LParen`, `RParen`, `EOF`), `Token` struct with `Type`, `Literal`, `Pos` fields, and `String()` method for debugging.
- **`token/token_test.go`**: Table-driven test covering all type strings plus unknown fallback.
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - IN PROGRESS assistant false
		Implement tokenizer. TDD: write tests first, then implementation.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - IN PROGRESS assistant false
		Define AST node types with sealed interface.
		1.4.5: Step 5: Parser (recursive-descent + tests) - IN PROGRESS assistant false
		Implement parser following grammar. TDD: tests first.
		1.4.6: Step 6: Evaluator (Eval + tests) - IN PROGRESS assistant false
		Implement AST walker. TDD: tests first.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.3
--- STOP_REASON: tool_use
AI: 

I'll start by reviewing the existing code to understand the token package and project structure, then implement the lexer using TDD.
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"

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":"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"

--- STOP_REASON: tool_use
AI: Good. I have full context. Now let me check the existing lexer directory:
CALLING TOOL: dir_list {"name":"lexer"}
$$$ {"entries":null}

--- STOP_REASON: tool_use
AI: Empty directory. I'll follow TDD — write tests first, then implement.

**Step 1: Write lexer 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\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: Tests written. Now implement the lexer:
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// 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 >= '0' && ch <= '9'\n}\n"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now run the tests:
!!!!! {"command":"cd /home/user/repos/matheval && go test ./lexer/ -v"}: you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"go test ./lexer/ -v"}: OK
CALLING TOOL: bash_command {"command":"go test ./lexer/ -v"}
$$$ "=== 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)\nPASS\nok  \tmatheval/lexer\t0.580s\n"

--- STOP_REASON: tool_use
AI: All 12 tests pass. Let me also run the full test suite to make sure nothing is broken:
!!!!! {"command":"go test ./..."}: OK
CALLING TOOL: bash_command {"command":"go test ./..."}
$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval/lexer\t0.272s\nok  \tmatheval/token\t0.486s\n"

--- STOP_REASON: tool_use
AI: All tests pass. Now commit:
!!!!! {"command":"git add -A && git commit -m \"Add lexer: Tokenize function with full test coverage\n\nImplements lexer.Tokenize(input string) ([]token.Token, error)\n- Skips whitespace\n- Parses integer and decimal numbers (including leading dot e.g. .5)\n- Handles all operators: + - * /\n- Handles parentheses: ( )\n- Appends EOF token\n- Returns error on invalid characters with position info\n- 12 unit tests covering: empty, whitespace-only, integers, decimals,\n  leading-dot numbers, operators, parens, full expressions, no-space\n  expressions, invalid chars, multiple decimals (1.2.3)\""}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Add lexer: Tokenize function with full test coverage\n\nImplements lexer.Tokenize(input string) ([]token.Token, error)\n- Skips whitespace\n- Parses integer and decimal numbers (including leading dot e.g. .5)\n- Handles all operators: + - * /\n- Handles parentheses: ( )\n- Appends EOF token\n- Returns error on invalid characters with position info\n- 12 unit tests covering: empty, whitespace-only, integers, decimals,\n  leading-dot numbers, operators, parens, full expressions, no-space\n  expressions, invalid chars, multiple decimals (1.2.3)\""}
$$$ "[master cdbb189] Add lexer: Tokenize function with full test coverage\n 2 files changed, 252 insertions(+)\n create mode 100644 lexer/lexer.go\n create mode 100644 lexer/lexer_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.3","summary":"Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: **Step 1.4.3 complete.** 

Summary of what was implemented:

- **`lexer/lexer.go`** — `Tokenize(input string) ([]token.Token, error)`
  - Skips whitespace (space, tab, newline, carriage return)
  - Parses numbers: integers (`42`), decimals (`3.14`), leading-dot (`.5`); stops at second dot (`1.2.3` → `"1.2"` + `".3"`)
  - Single-character operators: `+`, `-`, `*`, `/`
  - Parentheses: `(`, `)`
  - Always appends `EOF` token with position at end of input
  - Returns descriptive error for invalid characters with position

- **`lexer/lexer_test.go`** — 12 tests covering all cases: empty, whitespace-only, single number, decimal, leading dot, operators, parens, full expression, no-space expression, invalid characters, and multi-decimal edge case.

All tests pass. Committed as `cdbb189`.
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - IN PROGRESS assistant false
		Define AST node types with sealed interface.
		1.4.5: Step 5: Parser (recursive-descent + tests) - IN PROGRESS assistant false
		Implement parser following grammar. TDD: tests first.
		1.4.6: Step 6: Evaluator (Eval + tests) - IN PROGRESS assistant false
		Implement AST walker. TDD: tests first.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.4
--- STOP_REASON: tool_use
AI: 

I'll start by reviewing the current project state and the design document to understand the expected AST interfaces.
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"

CALLING TOOL: dir_list {"name":"ast"}
$$$ {"entries":null}

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: This is a small, well-defined step. The `ast` directory is empty. I need to create `ast/ast.go` with the `Node` interface, `NumberLit`, and `BinaryExpr` types, plus `ast/ast_test.go` to verify they satisfy the interface.

Let me implement this directly.
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"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now write a test to verify the types implement `Node` and that the structs hold data correctly.
CALLING TOOL: file_write {"path":"ast/ast_test.go","contents":"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 := &NumberLit{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 := &NumberLit{Value: 1}\n\tright := &NumberLit{Value: 2}\n\texpr := &BinaryExpr{\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 := &BinaryExpr{\n\t\tOp: token.Star,\n\t\tLeft: &BinaryExpr{\n\t\t\tOp:    token.Plus,\n\t\t\tLeft:  &NumberLit{Value: 1},\n\t\t\tRight: &NumberLit{Value: 2},\n\t\t},\n\t\tRight: &NumberLit{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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"cd /home/user/repos/matheval && go test ./ast/ -v"}: you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"go test ./ast/ -v"}: OK
CALLING TOOL: bash_command {"command":"go test ./ast/ -v"}
$$$ "=== RUN   TestNumberLit\n--- PASS: TestNumberLit (0.00s)\n=== RUN   TestBinaryExpr\n--- PASS: TestBinaryExpr (0.00s)\n=== RUN   TestNestedBinaryExpr\n--- PASS: TestNestedBinaryExpr (0.00s)\nPASS\nok  \tmatheval/ast\t0.582s\n"

--- STOP_REASON: tool_use
AI: All 3 tests pass. Let me also run the full test suite to make sure nothing is broken.
!!!!! {"command":"go test ./... -v"}: OK
CALLING TOOL: bash_command {"command":"go test ./... -v"}
$$$ "?   \tmatheval/cmd/matheval\t[no test files]\n=== RUN   TestNumberLit\n--- PASS: TestNumberLit (0.00s)\n=== RUN   TestBinaryExpr\n--- PASS: TestBinaryExpr (0.00s)\n=== RUN   TestNestedBinaryExpr\n--- PASS: TestNestedBinaryExpr (0.00s)\nPASS\nok  \tmatheval/ast\t(cached)\n=== 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)\nPASS\nok  \tmatheval/lexer\t(cached)\n=== RUN   TestTypeString\n--- PASS: TestTypeString (0.00s)\nPASS\nok  \tmatheval/token\t(cached)\n"

--- STOP_REASON: tool_use
AI: All tests pass across all packages. Let me commit.
!!!!! {"command":"git add -A && git commit -m \"Add AST package: Node interface, NumberLit, BinaryExpr\n\n- Sealed Node interface with unexported marker method\n- NumberLit holds float64 value\n- BinaryExpr holds Op (token.Type), Left and Right children\n- Tests verify interface satisfaction, data integrity, and nesting\""}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Add AST package: Node interface, NumberLit, BinaryExpr\n\n- Sealed Node interface with unexported marker method\n- NumberLit holds float64 value\n- BinaryExpr holds Op (token.Type), Left and Right children\n- Tests verify interface satisfaction, data integrity, and nesting\""}
$$$ "[master ad74392] Add AST package: Node interface, NumberLit, BinaryExpr\n 2 files changed, 90 insertions(+)\n create mode 100644 ast/ast.go\n create mode 100644 ast/ast_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.4","summary":"AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: **Step 4 complete.** The `ast` package is implemented with:

- **`Node`** — sealed interface (unexported `node()` marker method)
- **`NumberLit`** — holds `Value float64`
- **`BinaryExpr`** — holds `Op token.Type`, `Left Node`, `Right Node`
- **3 tests** — compile-time interface checks, struct field access, nested tree construction

All 15 tests across the project pass. Committed as `ad74392`.

Ready for **1.4.5** (Parser) when you give the go-ahead.
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - DONE user false
		Define AST node types with sealed interface.
		SUMMARY: AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed.
		1.4.5: Step 5: Parser (recursive-descent + tests) - IN PROGRESS assistant false
		Implement parser following grammar. TDD: tests first.
		1.4.6: Step 6: Evaluator (Eval + tests) - IN PROGRESS assistant false
		Implement AST walker. TDD: tests first.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.5
--- STOP_REASON: tool_use
AI: 

I'll start by reviewing the current codebase to understand what's already implemented before writing the parser.
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"

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"

--- STOP_REASON: tool_use
AI: Now let me check existing tests for context:
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":"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: dir_list {"name":"parser"}
$$$ {"entries":null}

--- STOP_REASON: tool_use
AI: Good. I have full context. The parser directory is empty. I'll implement the parser using TDD: write tests first, then the implementation.

**Grammar:**
```
expr   → term (('+' | '-') term)*
term   → factor (('*' | '/') factor)*
factor → NUMBER | '(' expr ')'
```

Let me write the test file first:
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// 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"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now implement the parser:
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// 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 | '(' 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.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: Run the tests:
!!!!! {"command":"cd /home/user/repos/matheval && go test ./parser/ -v 2>&1"}: you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"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\t0.611s\n"

--- STOP_REASON: tool_use
AI: All 19 tests pass. Let me also run the full test suite to make sure nothing is broken:
!!!!! {"command":"go test ./... 2>&1"}: OK
CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval/ast\t0.269s\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t0.496s\nok  \tmatheval/token\t(cached)\n"

--- STOP_REASON: tool_use
AI: All tests pass across all packages. Now commit:
!!!!! {"command":"git add -A && git commit -m \"parser: implement recursive-descent parser with tests\n\n- Parse([]token.Token) (ast.Node, error) converts tokens to AST\n- Grammar: expr → term ((+|-) term)*, term → factor ((*|/) factor)*, factor → NUMBER | '(' expr ')'\n- Correct operator precedence (* / before + -)\n- Left-associative operators\n- Error handling: empty input, missing/unexpected parens, trailing tokens, consecutive operators\n- 19 unit tests covering success and error cases\" 2>&1"}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"parser: implement recursive-descent parser with tests\n\n- Parse([]token.Token) (ast.Node, error) converts tokens to AST\n- Grammar: expr → term ((+|-) term)*, term → factor ((*|/) factor)*, factor → NUMBER | '(' expr ')'\n- Correct operator precedence (* / before + -)\n- Left-associative operators\n- Error handling: empty input, missing/unexpected parens, trailing tokens, consecutive operators\n- 19 unit tests covering success and error cases\" 2>&1"}
$$$ "[master b05c53f] parser: implement recursive-descent parser with tests\n 2 files changed, 598 insertions(+)\n create mode 100644 parser/parser.go\n create mode 100644 parser/parser_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.5","summary":"Parser implemented and tested. Recursive-descent parser with 3 grammar rules (expr, term, factor). Internal `parser` struct tracks position in token slice. `Parse([]token.Token) (ast.Node, error)` is the public API. Handles: single numbers, all 4 operators, correct precedence (*/before+-), left associativity, parentheses (including nested), and all error cases (empty input, missing/unexpected parens, trailing tokens, consecutive operators, empty parens). 19 unit tests all passing. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: **Step 1.4.5 complete.** Parser implemented with:

- **`parser.go`**: `Parse([]token.Token) (ast.Node, error)` — recursive-descent parser with internal `parser` struct tracking position
  - `expr()` — handles `+`, `-` (lowest precedence)
  - `term()` — handles `*`, `/` (higher precedence)
  - `factor()` — handles `NUMBER` and `(expr)`
  - Validates no trailing tokens after parsing
- **`parser_test.go`**: 19 tests covering:
  - Single/decimal numbers
  - All 4 operators individually
  - Precedence (`1+2*3`, `2*3+1`)
  - Left associativity (`1-2-3`)
  - Parentheses and nested parentheses
  - Complex expression (`1+2*3-4/2`)
  - Error cases: empty input, missing `)`, unexpected `)`, trailing operator, trailing tokens, consecutive operators, empty parens

All passing, committed.
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - DONE user false
		Define AST node types with sealed interface.
		SUMMARY: AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed.
		1.4.5: Step 5: Parser (recursive-descent + tests) - DONE user false
		Implement parser following grammar. TDD: tests first.
		SUMMARY: Parser implemented and tested. Recursive-descent parser with 3 grammar rules (expr, term, factor). Internal `parser` struct tracks position in token slice. `Parse([]token.Token) (ast.Node, error)` is the public API. Handles: single numbers, all 4 operators, correct precedence (*/before+-), left associativity, parentheses (including nested), and all error cases (empty input, missing/unexpected parens, trailing tokens, consecutive operators, empty parens). 19 unit tests all passing. Committed.
		1.4.6: Step 6: Evaluator (Eval + tests) - IN PROGRESS assistant false
		Implement AST walker. TDD: tests first.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.6
--- STOP_REASON: tool_use
AI: 

I'll start by reading the existing code to understand the current state before implementing the evaluator.
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":"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":"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: Good. Now let me check the existing evaluator directory:
CALLING TOOL: dir_list {"name":"evaluator"}
$$$ {"entries":null}

--- STOP_REASON: tool_use
AI: Empty directory. Let me implement the evaluator using TDD — tests first, then implementation.

**Writing the test file first:**
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\"testing\"\n)\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"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now the implementation:
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// 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"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Run the tests:
!!!!! {"command":"cd /home/user && go test ./evaluator/ -v"}: you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"go test ./evaluator/ -v"}: OK
CALLING TOOL: bash_command {"command":"go test ./evaluator/ -v"}
$$$ "=== 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\t0.573s\n"

--- STOP_REASON: tool_use
AI: All 10 tests pass. Let me also verify the full project still builds and all existing tests pass:
!!!!! {"command":"go test ./... -v 2>&1 | tail -30"}: OK
CALLING TOOL: bash_command {"command":"go test ./... -v 2>&1 | tail -30"}
$$$ "=== 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\t(cached)\n=== RUN   TestTypeString\n--- PASS: TestTypeString (0.00s)\nPASS\nok  \tmatheval/token\t(cached)\n"

--- STOP_REASON: tool_use
AI: All tests across all packages pass. Now commit:
!!!!! {"command":"git add -A && git commit -m \"Step 6: Implement evaluator with TDD\n\n- Eval(ast.Node) (float64, error) - recursive AST walker\n- Handles all 4 operators: +, -, *, /\n- Division by zero returns error\n- Error propagation from sub-expressions\n- 10 unit tests covering: single number, all operators, nested/deep\n  expressions, division by zero (direct and in sub-expr), floats\""}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Step 6: Implement evaluator with TDD\n\n- Eval(ast.Node) (float64, error) - recursive AST walker\n- Handles all 4 operators: +, -, *, /\n- Division by zero returns error\n- Error propagation from sub-expressions\n- 10 unit tests covering: single number, all operators, nested/deep\n  expressions, division by zero (direct and in sub-expr), floats\""}
$$$ "[master 586f28e] Step 6: Implement evaluator with TDD\n 2 files changed, 220 insertions(+)\n create mode 100644 evaluator/evaluator.go\n create mode 100644 evaluator/evaluator_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.6","summary":"Evaluator implemented and tested. `Eval(ast.Node) (float64, error)` recursively walks the AST via type switch on NumberLit and BinaryExpr. Handles all 4 operators (+, -, *, /). Returns error on division by zero. Errors propagate from sub-expressions. 10 unit tests all passing: single number, each operator, nested/deeply-nested expressions, division by zero (direct and in sub-expression), floating point precision. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: Step 6 complete. The evaluator is a clean, simple recursive AST walker:

- **`Eval(ast.Node) (float64, error)`** — type-switches on `*ast.NumberLit` (returns value) and `*ast.BinaryExpr` (recurses left/right, applies operator)
- Division by zero returns an error
- Errors propagate upward from sub-expressions
- 10 unit tests all passing, full project green

Ready to proceed to **1.4.7** (REPL) when directed.
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - DONE user false
		Define AST node types with sealed interface.
		SUMMARY: AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed.
		1.4.5: Step 5: Parser (recursive-descent + tests) - DONE user false
		Implement parser following grammar. TDD: tests first.
		SUMMARY: Parser implemented and tested. Recursive-descent parser with 3 grammar rules (expr, term, factor). Internal `parser` struct tracks position in token slice. `Parse([]token.Token) (ast.Node, error)` is the public API. Handles: single numbers, all 4 operators, correct precedence (*/before+-), left associativity, parentheses (including nested), and all error cases (empty input, missing/unexpected parens, trailing tokens, consecutive operators, empty parens). 19 unit tests all passing. Committed.
		1.4.6: Step 6: Evaluator (Eval + tests) - DONE user false
		Implement AST walker. TDD: tests first.
		SUMMARY: Evaluator implemented and tested. `Eval(ast.Node) (float64, error)` recursively walks the AST via type switch on NumberLit and BinaryExpr. Handles all 4 operators (+, -, *, /). Returns error on division by zero. Errors propagate from sub-expressions. 10 unit tests all passing: single number, each operator, nested/deeply-nested expressions, division by zero (direct and in sub-expression), floating point precision. Committed.
		1.4.7: Step 7: REPL (Run + tests) - IN PROGRESS assistant false
		Implement REPL loop with io.Reader/Writer. TDD.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.7
--- STOP_REASON: tool_use
AI: 

I'll start by reading the existing codebase to understand the current state, then implement the REPL.
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"

CALLING TOOL: dir_list {"name":"repl"}
$$$ {"entries":null}

--- STOP_REASON: tool_use
AI: Let me look at the existing packages to understand the current API:
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"

--- STOP_REASON: tool_use
AI: Good. I have full context. Now I'll implement the REPL with TDD — write the test first, then the implementation.
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}\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// Check it contains \"5\" but not \"5.0\"\n\tlines := strings.Split(got, \"\\n\")\n\tfound := false\n\tfor _, line := range lines {\n\t\ttrimmed := strings.TrimSpace(line)\n\t\tif trimmed == \"5\" {\n\t\t\tfound = true\n\t\t\tbreak\n\t\t}\n\t}\n\tif !found {\n\t\tt.Errorf(\"expected clean '5' result line, got %q\", got)\n\t}\n}\n"}
$$$ "done"

--- STOP_REASON: tool_use
AI: Now run the tests to confirm they fail (TDD red phase):
!!!!! {"command":"cd /app && go test ./repl/ 2>&1 | head -30"}: you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"cd matheval && go test ./repl/ 2>&1 | head -30"}: you are already in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"pwd && ls"}: OK
CALLING TOOL: bash_command {"command":"pwd && ls"}
$$$ "/Users/lekva/dev/src/math\nast\ncmd\ndocs\nevaluator\ngo.mod\nlexer\nparser\nrepl\ntoken\n"

--- STOP_REASON: tool_use
!!!!! {"command":"go test ./repl/ 2>&1 | head -30"}: OK
CALLING TOOL: bash_command {"command":"go test ./repl/ 2>&1 | head -30"}
$$$ "# matheval/repl [matheval/repl.test]\nrepl/repl_test.go:13:2: undefined: Run\nrepl/repl_test.go:25:2: undefined: Run\nrepl/repl_test.go:41:2: undefined: Run\nrepl/repl_test.go:56:2: undefined: Run\nrepl/repl_test.go:72:2: undefined: Run\nrepl/repl_test.go:84:2: undefined: Run\nrepl/repl_test.go:96:2: undefined: Run\nrepl/repl_test.go:108:2: undefined: Run\nrepl/repl_test.go:121:2: undefined: Run\nrepl/repl_test.go:135:2: undefined: Run\nrepl/repl_test.go:135:2: too many errors\nFAIL\tmatheval/repl [build failed]\nFAIL\n"

--- STOP_REASON: tool_use
AI: Good — tests fail because `Run` doesn't exist yet. Now implement it:
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/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\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) && !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
!!!!! {"command":"go test ./repl/ -v 2>&1"}: OK
CALLING TOOL: bash_command {"command":"go test ./repl/ -v 2>&1"}
ERR: exit status 1
--- 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    repl_test.go:149: expected clean '5' result line, got \"\u003e\u003e 5\\n\u003e\u003e \"\n--- FAIL: TestRun_WholeIntegerNoTrailingZeros (0.00s)\nFAIL\nFAIL\tmatheval/repl\t0.274s\nFAIL\nEXIT: 1\n"

--- STOP_REASON: tool_use
AI: Almost — the test is checking for an exact `"5"` line but the prompt `">> "` appears on the same line prefix of the next prompt. The issue is that the result line is `"5"` but the output is `>> 5\n>> `. Let me check the actual output more carefully. The output is `>> 5\n>> `. So the result `5` appears on a line that starts right after the prompt. The lines after splitting on `\n` would be: `[">> 5", ">> "]`. After trimming: `[">> 5", ">>"]`. Neither is exactly `"5"`.

The result is printed after the prompt of the previous line. Let me fix the test — the prompt is on the same output stream. The actual line content is `>> 5` (prompt then result). Let me adjust the test to account for the prompt:
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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"go test ./repl/ -v 2>&1"}: Ok
--- STOP_REASON: tool_use
!!!!! {"command":"go test ./repl/ -v 2>&1; echo \"---EXIT:$?---\""}: Ok
--- STOP_REASON: tool_use
!!!!! {"command":"go test -v ./repl/"}: OK
CALLING TOOL: bash_command {"command":"go test -v ./repl/"}
$$$ "=== 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\t0.587s\n"

--- STOP_REASON: tool_use
AI: All 10 tests pass. Let me also run the full test suite to make sure nothing is broken:
!!!!! {"command":"go test ./..."}: OK
CALLING TOOL: bash_command {"command":"go test ./..."}
$$$ "?   \tmatheval/cmd/matheval\t[no test files]\nok  \tmatheval/ast\t(cached)\nok  \tmatheval/evaluator\t0.495s\nok  \tmatheval/lexer\t(cached)\nok  \tmatheval/parser\t(cached)\nok  \tmatheval/repl\t0.273s\nok  \tmatheval/token\t(cached)\n"

--- STOP_REASON: tool_use
AI: All tests pass across all packages. Now commit:
!!!!! {"command":"git add -A && git commit -m \"Implement REPL package with Run(io.Reader, io.Writer)\n\n- Read-eval-print loop: reads lines, tokenizes, parses, evaluates, prints result\n- Prints prompt '>> ' before each input\n- Error handling: prints 'error: <msg>' and continues loop\n- Skips empty lines\n- Formats results with %g (clean integers, no trailing zeros)\n- 10 unit tests covering: single/multiple expressions, error recovery,\n  division by zero, empty lines, prompt display, float results,\n  invalid chars, empty input, integer formatting\""}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Implement REPL package with Run(io.Reader, io.Writer)\n\n- Read-eval-print loop: reads lines, tokenizes, parses, evaluates, prints result\n- Prints prompt '>> ' before each input\n- Error handling: prints 'error: <msg>' and continues loop\n- Skips empty lines\n- Formats results with %g (clean integers, no trailing zeros)\n- 10 unit tests covering: single/multiple expressions, error recovery,\n  division by zero, empty lines, prompt display, float results,\n  invalid chars, empty input, integer formatting\""}
$$$ "[master cd59d22] Implement REPL package with Run(io.Reader, io.Writer)\n 2 files changed, 212 insertions(+)\n create mode 100644 repl/repl.go\n create mode 100644 repl/repl_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.7","summary":"REPL implemented and tested. `Run(io.Reader, io.Writer)` reads lines via bufio.Scanner, pipes each through lexer→parser→evaluator pipeline, prints results or errors. Prompt \">> \" displayed before each input. Empty lines skipped. Results formatted with %g. 10 unit tests all passing. Full test suite green. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: **Step 7 complete.** REPL package implemented with:

- **`repl.go`**: `Run(r io.Reader, w io.Writer)` — reads lines, runs them through `lexer.Tokenize → parser.Parse → evaluator.Eval`, prints result or `error: <msg>`, continues on error, skips empty lines, shows `>> ` prompt.
- **`repl_test.go`**: 10 tests covering single/multiple expressions, error recovery, division by zero, empty lines, prompt, float results, invalid characters, empty input, and clean integer formatting. All pass.

Ready to proceed to **1.4.8** (main.go wiring).
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - DONE user false
		Define AST node types with sealed interface.
		SUMMARY: AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed.
		1.4.5: Step 5: Parser (recursive-descent + tests) - DONE user false
		Implement parser following grammar. TDD: tests first.
		SUMMARY: Parser implemented and tested. Recursive-descent parser with 3 grammar rules (expr, term, factor). Internal `parser` struct tracks position in token slice. `Parse([]token.Token) (ast.Node, error)` is the public API. Handles: single numbers, all 4 operators, correct precedence (*/before+-), left associativity, parentheses (including nested), and all error cases (empty input, missing/unexpected parens, trailing tokens, consecutive operators, empty parens). 19 unit tests all passing. Committed.
		1.4.6: Step 6: Evaluator (Eval + tests) - DONE user false
		Implement AST walker. TDD: tests first.
		SUMMARY: Evaluator implemented and tested. `Eval(ast.Node) (float64, error)` recursively walks the AST via type switch on NumberLit and BinaryExpr. Handles all 4 operators (+, -, *, /). Returns error on division by zero. Errors propagate from sub-expressions. 10 unit tests all passing: single number, each operator, nested/deeply-nested expressions, division by zero (direct and in sub-expression), floating point precision. Committed.
		1.4.7: Step 7: REPL (Run + tests) - DONE user false
		Implement REPL loop with io.Reader/Writer. TDD.
		SUMMARY: REPL implemented and tested. `Run(io.Reader, io.Writer)` reads lines via bufio.Scanner, pipes each through lexer→parser→evaluator pipeline, prints results or errors. Prompt ">> " displayed before each input. Empty lines skipped. Results formatted with %g. 10 unit tests all passing. Full test suite green. Committed.
		1.4.8: Step 8: main.go (wire REPL) - IN PROGRESS assistant false
		Entry point calling repl.Run.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.8
--- STOP_REASON: tool_use
AI: 

Let me first check the current state of the project and the existing main.go.
CALLING TOOL: file_read {"path":"cmd/matheval/main.go"}
$$$ "package main\n\nimport \"fmt\"\n\nfunc main() {\n\tfmt.Println(\"matheval - math expression evaluator\")\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: Simple step — just wire `repl.Run` with `os.Stdin` and `os.Stdout`.
CALLING TOOL: file_write {"path":"cmd/matheval/main.go","contents":"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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"cd /home/user/repos/matheval && go build ./cmd/matheval/"}: you are in project dir already
--- STOP_REASON: tool_use
!!!!! {"command":"go build ./cmd/matheval/"}: OK
CALLING TOOL: bash_command {"command":"go build ./cmd/matheval/"}
$$$ ""

--- STOP_REASON: tool_use
AI: Build succeeds. Let me run a quick smoke test via piped input:
!!!!! {"command":"echo -e \"2 + 3\\n(4 + 5) * 2\\n10 / 0\" | go run ./cmd/matheval/"}: --- STOP_REASON: tool_use
!!!!! {"command":"printf \"2 + 3\\n(4 + 5) * 2\\n10 / 0\\n\" | go run ./cmd/matheval/ 2>&1"}: OK
--- STOP_REASON: tool_use
!!!!! {"command":"printf '2 + 3\\n(4 + 5) * 2\\n10 / 0\\n' | go run ./cmd/matheval/"}: --- STOP_REASON: tool_use
AI: Output seems empty — let me capture both stdout and stderr explicitly:
!!!!! {"command":"printf '2 + 3\\n(4 + 5) * 2\\n10 / 0\\n' | go run ./cmd/matheval/ > /tmp/out.txt 2>&1; cat /tmp/out.txt"}: --- STOP_REASON: tool_use
!!!!! {"command":"printf '2 + 3\\n(4 + 5) * 2\\n10 / 0\\n' | go run ./cmd/matheval/ > /tmp/out.txt 2>/tmp/err.txt; echo \"STDOUT:\"; cat /tmp/out.txt; echo \"STDERR:\"; cat /tmp/err.txt"}: --- STOP_REASON: tool_use
!!!!! {"command":"echo \"2 + 3\" | go run ./cmd/matheval/ 2>&1 || true"}: CALLING TOOL: bash_command {"command":"echo \"2 + 3\" | go run ./cmd/matheval/ 2>&1 || true"}
$$$ "\u003e\u003e 5\n\u003e\u003e "

--- STOP_REASON: tool_use
AI: Works. The REPL outputs `>> 5` as expected. Let me test more:
!!!!! {"command":"printf '%s\\n' \"2 + 3\" \"(4 + 5) * 2\" \"10 / 0\" \"3.14 * 2\" | go run ./cmd/matheval/ 2>&1"}: OK
CALLING TOOL: bash_command {"command":"printf '%s\\n' \"2 + 3\" \"(4 + 5) * 2\" \"10 / 0\" \"3.14 * 2\" | go run ./cmd/matheval/ 2>&1"}
$$$ "\u003e\u003e 5\n\u003e\u003e 18\n\u003e\u003e error: division by zero\n\u003e\u003e 6.28\n\u003e\u003e "

--- STOP_REASON: tool_use
AI: All working correctly:
- `2 + 3` → `5`
- `(4 + 5) * 2` → `18`
- `10 / 0` → `error: division by zero`
- `3.14 * 2` → `6.28`

Let me also make sure the full test suite still passes:
!!!!! {"command":"go test ./... 2>&1"}: OK
CALLING TOOL: bash_command {"command":"go test ./... 2>&1"}
$$$ "ok  \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\t(cached)\nok  \tmatheval/token\t(cached)\n"

--- STOP_REASON: tool_use
AI: All tests pass. Let me commit:
!!!!! {"command":"git add -A && git commit -m \"Step 8: Wire REPL to main.go entry point\""}: OK
CALLING TOOL: bash_command {"command":"git add -A && git commit -m \"Step 8: Wire REPL to main.go entry point\""}
$$$ "[master ca85f4e] Step 8: Wire REPL to main.go entry point\n 2 files changed, 6 insertions(+), 2 deletions(-)\n create mode 100755 matheval\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.8","summary":"main.go wired to call repl.Run(os.Stdin, os.Stdout). Builds, runs, and all tests pass. Smoke-tested with piped input: arithmetic, parentheses, division-by-zero, and float expressions all produce correct output. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: Step 8 done. `main.go` now simply calls `repl.Run(os.Stdin, os.Stdout)` — the thinnest possible entry point, delegating all logic to the REPL package. Build, run, and all tests confirmed green. Committed.

Ready to proceed to **1.4.9** (Integration test) when given the go-ahead.
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - DONE user false
		Define AST node types with sealed interface.
		SUMMARY: AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed.
		1.4.5: Step 5: Parser (recursive-descent + tests) - DONE user false
		Implement parser following grammar. TDD: tests first.
		SUMMARY: Parser implemented and tested. Recursive-descent parser with 3 grammar rules (expr, term, factor). Internal `parser` struct tracks position in token slice. `Parse([]token.Token) (ast.Node, error)` is the public API. Handles: single numbers, all 4 operators, correct precedence (*/before+-), left associativity, parentheses (including nested), and all error cases (empty input, missing/unexpected parens, trailing tokens, consecutive operators, empty parens). 19 unit tests all passing. Committed.
		1.4.6: Step 6: Evaluator (Eval + tests) - DONE user false
		Implement AST walker. TDD: tests first.
		SUMMARY: Evaluator implemented and tested. `Eval(ast.Node) (float64, error)` recursively walks the AST via type switch on NumberLit and BinaryExpr. Handles all 4 operators (+, -, *, /). Returns error on division by zero. Errors propagate from sub-expressions. 10 unit tests all passing: single number, each operator, nested/deeply-nested expressions, division by zero (direct and in sub-expression), floating point precision. Committed.
		1.4.7: Step 7: REPL (Run + tests) - DONE user false
		Implement REPL loop with io.Reader/Writer. TDD.
		SUMMARY: REPL implemented and tested. `Run(io.Reader, io.Writer)` reads lines via bufio.Scanner, pipes each through lexer→parser→evaluator pipeline, prints results or errors. Prompt ">> " displayed before each input. Empty lines skipped. Results formatted with %g. 10 unit tests all passing. Full test suite green. Committed.
		1.4.8: Step 8: main.go (wire REPL) - DONE user false
		Entry point calling repl.Run.
		SUMMARY: main.go wired to call repl.Run(os.Stdin, os.Stdout). Builds, runs, and all tests pass. Smoke-tested with piped input: arithmetic, parentheses, division-by-zero, and float expressions all produce correct output. Committed.
		1.4.9: Step 9: Integration test - IN PROGRESS assistant false
		End-to-end tests through all stages.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.9
--- STOP_REASON: tool_use
AI: 

I'll start by examining the current project structure and existing tests to understand what's already covered before writing integration tests.
CALLING TOOL: dir_list {"name":"."}
$$$ {"entries":[{"name":".git","is_dir":true},{"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":"lexer","is_dir":true},{"name":"matheval","is_dir":false},{"name":"parser","is_dir":true},{"name":"repl","is_dir":true},{"name":"token","is_dir":true}]}

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":"repl"}
$$$ {"entries":[{"name":"repl.go","is_dir":false},{"name":"repl_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":"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}]}

--- STOP_REASON: tool_use
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":"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":"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"

--- STOP_REASON: tool_use
AI: Now I have full context. The REPL tests already cover some end-to-end scenarios, but they're testing through the REPL's I/O layer. The integration test should test the full pipeline (lexer → parser → evaluator) directly, covering edge cases like deeply nested parentheses, long expressions, and more complex scenarios. Let me check what the existing unit tests already cover:
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":"go.mod"}
$$$ "module matheval\n\ngo 1.23.1\n"

--- STOP_REASON: tool_use
AI: Now I have full context. The existing unit tests test each component in isolation. The REPL tests test through the I/O layer. What's missing is a dedicated integration test that feeds raw expression strings through the full pipeline (lexer → parser → evaluator) directly, testing the components' interaction without the REPL's I/O concerns. This is the right place for edge cases like deeply nested parentheses, long chained expressions, floating point precision, etc.

Let me create the integration test file:
CALLING TOOL: dir_list {"name":"cmd/matheval"}
$$$ {"entries":[{"name":"main.go","is_dir":false}]}

--- STOP_REASON: tool_use
AI: I'll create the integration test as a separate package at the top level to ensure it only uses public APIs:
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/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) > 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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"cd /tmp && [ -d matheval ] || echo \"need to find project dir\"; find / -name \"go.mod\" -path \"*/matheval/*\" 2>/dev/null | head -5"}: you are 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\t0.601s\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 33 integration tests pass (along with all existing unit tests). Let me verify the integration test count:
!!!!! {"command":"go test -v . 2>&1 | grep -c \"=== RUN\""}: OK
CALLING TOOL: bash_command {"command":"go test -v . 2>&1 | grep -c \"=== RUN\""}
$$$ "36\n"

--- STOP_REASON: tool_use
!!!!! {"command":"go test -v . 2>&1"}: OK
CALLING TOOL: bash_command {"command":"go test -v . 2>&1"}
$$$ "=== 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)\nPASS\nok  \tmatheval\t(cached)\n"

--- STOP_REASON: tool_use
AI: All 36 integration tests pass. Now commit:
!!!!! {"command":"git add integration_test.go && git commit -m \"Add end-to-end integration tests\n\n36 tests covering the full lexer → parser → evaluator pipeline:\n- Basic arithmetic (single numbers, +, -, *, /)\n- Operator precedence (* / before + -)\n- Left associativity for subtraction and division\n- Parentheses (simple, nested, deeply nested, both sides, complex)\n- Complex multi-operator expressions\n- Long chained expressions\n- Floating point (arithmetic, division, precision)\n- Whitespace variations (none, extra, tabs)\n- Error cases (div by zero, invalid chars, mismatched parens,\n  empty parens, trailing/leading/consecutive operators, empty input,\n  consecutive numbers)\" 2>&1"}: OK
CALLING TOOL: bash_command {"command":"git add integration_test.go && git commit -m \"Add end-to-end integration tests\n\n36 tests covering the full lexer → parser → evaluator pipeline:\n- Basic arithmetic (single numbers, +, -, *, /)\n- Operator precedence (* / before + -)\n- Left associativity for subtraction and division\n- Parentheses (simple, nested, deeply nested, both sides, complex)\n- Complex multi-operator expressions\n- Long chained expressions\n- Floating point (arithmetic, division, precision)\n- Whitespace variations (none, extra, tabs)\n- Error cases (div by zero, invalid chars, mismatched parens,\n  empty parens, trailing/leading/consecutive operators, empty input,\n  consecutive numbers)\" 2>&1"}
$$$ "[master afb61d0] Add end-to-end integration tests\n 1 file changed, 261 insertions(+)\n create mode 100644 integration_test.go\n"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.9","summary":"Created integration_test.go with 36 end-to-end tests covering the full lexer → parser → evaluator pipeline. Tests organized into 6 categories: basic arithmetic (7), precedence/associativity (4), parentheses (5), complex expressions (3), floating point (3), whitespace (3), and error cases (11). All tests pass. Committed."}
$$$ "done"

--- STOP_REASON: end_turn
AI: Step 1.4.9 is done. Created `integration_test.go` in the root package (`matheval_test`) with 36 end-to-end tests that exercise the full pipeline directly (lexer → parser → evaluator) without going through the REPL I/O layer.

**Test coverage:**
| Category | Count | Examples |
|---|---|---|
| Basic arithmetic | 7 | Single number, decimal, leading dot, +, -, *, / |
| Precedence & associativity | 4 | `*`/`/` before `+`/`-`, left-associative `-` and `/` |
| Parentheses | 5 | Override precedence, nested, deeply nested (6 levels), both sides, complex |
| Complex expressions | 3 | All 4 operators combined, 10-term chain, mixed chain |
| Floating point | 3 | Float arithmetic, float division, 0.1+0.2 precision |
| Whitespace | 3 | No spaces, extra spaces, tabs |
| Error cases | 11 | Division by zero, invalid char, mismatched parens, empty parens, trailing/leading/consecutive operators, empty input, consecutive numbers |
## USER NO ITEMS
1: create math expr eval app - IN PROGRESS user false
	 - user: looks good
	1.1: Research: Gather Requirements - DONE assistant true
	Before designing anything, we need to understand the full scope of the math expression evaluator. What operations, data types, error handling, and interface are expected.
	SUMMARY: Research complete. Requirements gathered:
	- Language: Go
	- Operators: +, -, *, / with parentheses
	- Numbers: floating point
	- Variables: none
	- Interface: CLI REPL
	- Error handling: abort on error (print message, continue REPL)
	
	This is a straightforward recursive-descent parser problem with two precedence levels (additive and multiplicative) plus parenthesized sub-expressions. No symbol table needed. The architecture will consist of: Lexer → Parser → Evaluator → REPL shell.
		1.1.1: What operators/functions should be supported? - DONE user false
		Need to know the scope: basic arithmetic (+, -, *, /)? Parentheses? Exponentiation? Unary minus? Math functions (sin, cos, sqrt)? This fundamentally affects parser complexity.
		SUMMARY: Basic arithmetic (+, -, *, /) and parentheses. No exponentiation, unary operators, or math functions.
			 - user: basic + parentheses
			 - assistant: Confirmed: Basic arithmetic (+, -, *, /) plus parentheses. No exponentiation, unary minus, or math functions.
		1.1.2: What number types should be supported? - DONE user false
		Integers only? Floating point? Negative numbers? Scientific notation (e.g. 1.5e10)? This affects the tokenizer/lexer design.
		SUMMARY: Floating point numbers.
			 - user: float
			 - assistant: Confirmed: Floating point numbers supported.
		1.1.3: Should variables be supported? - DONE user false
		E.g. 'x + 2' where x is provided externally. This affects whether we need a symbol table / environment concept.
		SUMMARY: No variables. Pure numeric expressions only.
			 - user: no
			 - assistant: Confirmed: No variables. Pure numeric expressions only.
		1.1.4: What is the target interface? - DONE user false
		CLI app? Library API? REPL? Web service? This determines how we package the evaluator.
		SUMMARY: CLI REPL interface.
			 - user: CLI REPL
			 - assistant: Confirmed: CLI REPL (read-eval-print loop).
		1.1.5: What programming language? - DONE user false
		Need to know which language to implement in. This affects all implementation decisions.
		SUMMARY: Go language.
			 - user: go
			 - assistant: Confirmed: Go.
		1.1.6: What error handling is expected? - DONE user false
		Division by zero? Malformed expressions? Mismatched parentheses? How should errors be reported?
		SUMMARY: Abort on error — print error, continue REPL loop.
			 - user: abort on error
			 - assistant: Confirmed: Abort on error (print error message and stop evaluating that expression, but REPL continues).
	1.2: Design: Architecture & Component Design - DONE assistant false
	Define the high-level architecture, component interfaces, and their relationships for the math expression evaluator. Consider multiple approaches and pick the best one.
	SUMMARY: Design complete. Architecture: Lexer → Parser → AST → Evaluator → REPL. Recursive-descent parser with explicit AST chosen over direct-evaluation and shunting-yard approaches. All component interfaces defined. Design document written to docs/design.md.
		1.2.1: Analyze approaches and define architecture - DONE assistant false
		Consider recursive-descent vs Pratt parsing vs shunting-yard. Define component boundaries: Lexer, Parser, AST, Evaluator, REPL. Produce design document.
		SUMMARY: Analyzed 3 approaches (recursive-descent with AST, recursive-descent with direct eval, shunting-yard). Chose recursive-descent with AST for clean separation of concerns. Defined interfaces for all 5 components: Token, Lexer, Parser (with grammar), Evaluator, REPL. Produced design document at docs/design.md.
	1.3: Plan: Detailed Implementation Plan - DONE assistant false
	Create a detailed, step-by-step implementation plan following the design document. Each step should be small and independently completable. Write the plan to docs/plan.md.
	SUMMARY: Plan created at docs/plan.md. 10 sequential implementation steps defined covering: project skeleton, token package, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README.
	1.4: Implement: Execute the plan - IN PROGRESS assistant false
	Implement all steps from docs/plan.md sequentially using TDD.
		1.4.1: Step 1: Project skeleton (go mod, dirs, placeholder main.go) - DONE user false
		Initialize Go module and create directory structure.
		SUMMARY: Project skeleton created: go.mod (module matheval), directories (cmd/matheval, token, lexer, ast, parser, evaluator, repl), placeholder main.go. Builds and runs. Committed.
		1.4.2: Step 2: Token package (Type enum, Token struct) - DONE user false
		Define token types and Token struct with Pos tracking.
		SUMMARY: Token package implemented: Type enum (Number, Plus, Minus, Star, Slash, LParen, RParen, EOF), Token struct (Type, Literal, Pos), String() method on Type for debugging. Unit test covers all type strings including unknown fallback. Committed.
		1.4.3: Step 3: Lexer (Tokenize + tests) - DONE user false
		Implement tokenizer. TDD: write tests first, then implementation.
		SUMMARY: Lexer implemented and tested. `Tokenize(input string) ([]token.Token, error)` handles: whitespace skipping, integer/decimal/leading-dot numbers, all 4 operators, parentheses, EOF, and invalid character errors with position reporting. 12 unit tests all passing. Committed.
		1.4.4: Step 4: AST package (Node interface, NumberLit, BinaryExpr) - DONE user false
		Define AST node types with sealed interface.
		SUMMARY: AST package implemented: sealed Node interface with unexported marker method, NumberLit (float64 value), BinaryExpr (Op token.Type, Left/Right Node). 3 unit tests covering interface satisfaction, data integrity, and nested tree construction. All tests pass. Committed.
		1.4.5: Step 5: Parser (recursive-descent + tests) - DONE user false
		Implement parser following grammar. TDD: tests first.
		SUMMARY: Parser implemented and tested. Recursive-descent parser with 3 grammar rules (expr, term, factor). Internal `parser` struct tracks position in token slice. `Parse([]token.Token) (ast.Node, error)` is the public API. Handles: single numbers, all 4 operators, correct precedence (*/before+-), left associativity, parentheses (including nested), and all error cases (empty input, missing/unexpected parens, trailing tokens, consecutive operators, empty parens). 19 unit tests all passing. Committed.
		1.4.6: Step 6: Evaluator (Eval + tests) - DONE user false
		Implement AST walker. TDD: tests first.
		SUMMARY: Evaluator implemented and tested. `Eval(ast.Node) (float64, error)` recursively walks the AST via type switch on NumberLit and BinaryExpr. Handles all 4 operators (+, -, *, /). Returns error on division by zero. Errors propagate from sub-expressions. 10 unit tests all passing: single number, each operator, nested/deeply-nested expressions, division by zero (direct and in sub-expression), floating point precision. Committed.
		1.4.7: Step 7: REPL (Run + tests) - DONE user false
		Implement REPL loop with io.Reader/Writer. TDD.
		SUMMARY: REPL implemented and tested. `Run(io.Reader, io.Writer)` reads lines via bufio.Scanner, pipes each through lexer→parser→evaluator pipeline, prints results or errors. Prompt ">> " displayed before each input. Empty lines skipped. Results formatted with %g. 10 unit tests all passing. Full test suite green. Committed.
		1.4.8: Step 8: main.go (wire REPL) - DONE user false
		Entry point calling repl.Run.
		SUMMARY: main.go wired to call repl.Run(os.Stdin, os.Stdout). Builds, runs, and all tests pass. Smoke-tested with piped input: arithmetic, parentheses, division-by-zero, and float expressions all produce correct output. Committed.
		1.4.9: Step 9: Integration test - DONE user false
		End-to-end tests through all stages.
		SUMMARY: Created integration_test.go with 36 end-to-end tests covering the full lexer → parser → evaluator pipeline. Tests organized into 6 categories: basic arithmetic (7), precedence/associativity (4), parentheses (5), complex expressions (3), floating point (3), whitespace (3), and error cases (11). All tests pass. Committed.
		1.4.10: Step 10: README & final commit - IN PROGRESS assistant false
		Documentation and final commit.
-- AGENT START WORKING 1.4.10
--- STOP_REASON: tool_use
AI: 

Let me first review the current state of the project and existing documentation.
CALLING TOOL: dir_list {"name":"."}
$$$ {"entries":[{"name":".git","is_dir":true},{"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":"token","is_dir":true}]}

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: Let me also check the actual source files to understand the implementation:
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"

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 I have full context. Let me write the README and make the final commit.
CALLING TOOL: file_write {"path":"README.md","contents":"# 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>> 2 + 3 * 4\n14\n>> (2 + 3) * 4\n20\n>> 7 / 2\n3.5\n>> 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"}
$$$ "done"

--- STOP_REASON: tool_use
!!!!! {"command":"cd /root && go test ./... 2>&1"}: you are in project dir
--- STOP_REASON: tool_use
!!!!! {"command":"go test ./..."}: OK
CALLING TOOL: bash_command {"command":"go test ./..."}
$$$ "?   \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 commit.
!!!!! {"command":"git add -A && git commit -m \"Add README and finalize project\""}: Ok
--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4.10","summary":"README.md written with: build instructions, usage examples (REPL and piped), architecture overview table, grammar specification, and test instructions. All tests pass. Final commit made."}
$$$ "done"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1.4","summary":"All 10 implementation steps complete. Full pipeline implemented with TDD: token, lexer, AST, parser, evaluator, REPL, main.go, integration tests, and README. All tests pass across all packages."}
$$$ "done"

--- STOP_REASON: tool_use
CALLING TOOL: todo_item_mark_done {"id":"1","summary":"Math expression evaluator app complete. Go CLI REPL supporting +, -, *, / with parentheses and floating point numbers. Clean architecture: Lexer → Parser → AST → Evaluator → REPL. Full test coverage including 36 integration tests. All phases (Research, Design, Plan, Implement) completed successfully."}
$$$ "done"

--- STOP_REASON: end_turn
AI: Project complete. All phases done:

1. **Research** — Requirements gathered
2. **Design** — Recursive-descent with AST chosen, interfaces defined
3. **Plan** — 10-step implementation plan
4. **Implement** — All steps executed with TDD, all tests green, README written, committed
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interrupt: 2
~/dev/src/math $