JOURNAL — eval phase

2026-06-15 — Implementation

Built evaluator, CLI, and tests in one go.

evaluator.py

evaluate(node) walks the AST recursively:

Num → return node.value directly (already int or float from lexer)
Unary('-', operand) → negate result
BinOp(op, left, right) → evaluate both sides, apply op

Division: uses Python left / right (always returns float). If right == 0, raises EvalError("Division by zero").

D3 result type rule: after computing a float result in BinOp, if result == int(result) we cast to int. This ensures 4/2 → 2 (int) and 7/2 → 3.5 (float). Integer arithmetic returns int naturally.

calc.py CLI

main() accepts exactly one argv argument (the expression string). Catches LexError, ParseError, EvalError → prints Error: <msg> to stderr, exits 1. On success prints result (which is already int or float with correct type per D3 rule).

Test run output

$ python -m unittest -q
Ran 62 tests in 0.001s
OK

(45 prior tests from lex+parse phases, 17 new evaluator tests)

CLI 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 end of input (exit 1)

1.4 KiB Raw Blame History