# JOURNAL — eval phase

## 2026-06-15

### Implementation approach

Read the existing lexer/parser to understand AST node shapes: `Num(value)`, `BinOp(op, left, right)`, `Unary(op, operand)`.

Implemented `calc/evaluator.py`:
- `EvalError(Exception)` — wraps division-by-zero and unknown nodes; never lets `ZeroDivisionError` escape.
- `evaluate(node)` — recursive AST walk; delegates to `_coerce` after each operation.
- `_coerce(value)` — if `isinstance(value, float) and value == int(value)` → return `int(value)`; else return value as-is. This is the D3 rule applied uniformly at every arithmetic result.

Created `calc/test_evaluator.py` with 20 tests across 3 test classes (TestArithmetic, TestDivision, TestResultType).

Created top-level `calc.py` CLI: parses one arg, catches `LexError | ParseError | EvalError`, prints to stderr + exits 1 on error, prints result + exits 0 on success.

### Test run output

```
$ python -m unittest -q
Ran 68 tests in 0.001s
OK
```

### CLI spot-checks

```
$ python calc.py "2+3*4"
14
$ python calc.py "(2+3)*4"
20
$ python calc.py "7/2"
3.5
$ python calc.py "4/2"
2
$ python calc.py "1/0"
error: division by zero
(exit 1)
$ python calc.py "1 +"
error: unexpected token 'EOF' (None)
(exit 1)
```