README.md

# matheval

A math expression evaluator with an interactive REPL, written in Go.

## Features

- Arithmetic operators: `+`, `-`, `*`, `/`
- Parentheses for grouping
- Floating point numbers (including `.5` syntax)
- Correct operator precedence (`*` and `/` bind tighter than `+` and `-`)
- Left-to-right associativity
- Clear error messages with position reporting

## Build

```sh
go build -o matheval ./cmd/matheval
```

## Usage

Run the REPL:

```sh
./matheval
```

Then type expressions:

```
>> 2 + 3 * 4
14
>> (2 + 3) * 4
20
>> 7 / 2
3.5
>> 1 / 0
error: division by zero
```

Press `Ctrl+D` (EOF) to exit.

You can also pipe input:

```sh
echo "2 + 3" | ./matheval
```

## Architecture

```
Input string → Lexer → Parser → AST → Evaluator → Result
```

| Package     | Responsibility                        |
|-------------|---------------------------------------|
| `token`     | Token types and data structures       |
| `lexer`     | Tokenizes input string                |
| `ast`       | AST node types (`NumberLit`, `BinaryExpr`) |
| `parser`    | Recursive-descent parser              |
| `evaluator` | Walks AST and computes result         |
| `repl`      | Read-eval-print loop                  |

## Grammar

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

## Tests

```sh
go test ./...
```

This runs unit tests for each package plus integration tests covering the full pipeline.