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| """ | |
| "algebraic data types" in python | |
| """ | |
| from __future__ import annotations | |
| from dataclasses import dataclass | |
| @dataclass | |
| class Value: | |
| value: int | |
| @dataclass | |
| class Add: | |
| left: Expr | |
| right: Expr | |
| @dataclass | |
| class Mul: | |
| left: Expr | |
| right: Expr | |
| Expr = Value | Add | Mul | |
| def eval(expr: Expr) -> int: | |
| match expr: | |
| case Value(value): | |
| return value | |
| case Add(left, right): | |
| return eval(left) + eval(right) | |
| case Mul(left, right): | |
| return eval(left) * eval(right) | |
| def rpn(s: str) -> Expr: | |
| """parse reverse polish notation""" | |
| stack: list[Expr] = [] | |
| for token in s.split(): | |
| match token: | |
| case "+": | |
| right = stack.pop() | |
| left = stack.pop() | |
| stack.append(Add(left, right)) | |
| case "*": | |
| right = stack.pop() | |
| left = stack.pop() | |
| stack.append(Mul(left, right)) | |
| case _: | |
| stack.append(Value(int(token))) | |
| return stack.pop() | |
| if __name__ == "__main__": | |
| expr = rpn("1 2 + 3 4 + *") | |
| print(expr, "=", eval(expr)) |
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