# JOURNAL — phase `lex`

## Implementation

Built `calc/lexer.py` with:
- `Token` dataclass with `kind: str` and `value: Union[int, float, str, None]`
- `LexError(Exception)` for invalid characters
- `tokenize(src: str) -> list[Token]` scanning left-to-right

Number handling: checks `ch.isdigit()` OR `ch == '.' followed by digit` (for `.5` case).
Collects integer digits, then optionally a `.` and fractional digits.
Result is `int` if no `.` seen, `float` otherwise — handles `10.` (trailing dot) correctly.

Operators: simple char-dispatch to the 6 operator/paren token kinds.

Whitespace: space and tab explicitly skipped via `continue`.

Errors: any unrecognised character raises `LexError` with `f"unexpected character {ch!r} at position {i}"`.

EOF appended unconditionally as the final token.

## Test run

```
$ python -m unittest -q
......................
Ran 21 tests in 0.000s

OK
```

## Verification

```
$ python -c "from calc.lexer import tokenize; print([(t.kind,t.value) for t in tokenize('3.5*(1-2)')])"
[('NUMBER', 3.5), ('STAR', '*'), ('LPAREN', '('), ('NUMBER', 1), ('MINUS', '-'), ('NUMBER', 2), ('RPAREN', ')'), ('EOF', None)]

$ python -c "from calc.lexer import tokenize; tokenize('1 @ 2')"
Traceback (most recent call last):
  ...
calc.lexer.LexError: unexpected character '@' at position 2
```