# JOURNAL — parse phase (Builder)

## 2026-06-15 — Initial implementation

**Design decisions:**

Grammar used (standard precedence climbing via two layers):
```
expr    := term (('+' | '-') term)*
term    := factor (('*' | '/') factor)*
factor  := '-' factor | primary
primary := NUMBER | '(' expr ')'
```

Left associativity falls out of the `while` loop pattern in `_expr()` and `_term()` — each new BinOp wraps the accumulated left node.

Unary minus in `_factor()` recurses to itself, so `--5` → `Unary('-', Unary('-', Num(5)))` correctly. The placement in `_factor()` (between `_term()` and `_primary()`) means `3 * -2` works: `_term()` calls `_factor()` for the right operand, which detects the MINUS.

**Verification runs:**

D1:
```
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('1+2*3')))"
BinOp('+', Num(1), BinOp('*', Num(2), Num(3)))
```

D2:
```
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('8-3-2')))"
BinOp('-', BinOp('-', Num(8), Num(3)), Num(2))
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('8/4/2')))"
BinOp('/', BinOp('/', Num(8), Num(4)), Num(2))
```

D3:
```
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('(1+2)*3')))"
BinOp('*', BinOp('+', Num(1), Num(2)), Num(3))
```

D4:
```
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('-5')))"
Unary('-', Num(5))
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('-(1+2)')))"
Unary('-', BinOp('+', Num(1), Num(2)))
$ python -c "from calc.lexer import tokenize; from calc.parser import parse; print(parse(tokenize('3 * -2')))"
BinOp('*', Num(3), Unary('-', Num(2)))
```

D5 (all 5 required error cases + extras):
```
OK: '1 +' -> ParseError: unexpected token 'EOF'
OK: '(1' -> ParseError: unclosed parenthesis, got 'EOF'
OK: '1 2' -> ParseError: unexpected token 'NUMBER'
OK: ')(' -> ParseError: unexpected token 'RPAREN'
OK: '' -> ParseError: empty input
```

D6:
```
$ python -m unittest -q
----------------------------------------------------------------------
Ran 48 tests in 0.002s
OK
```