Pythonic M

m.py

from dataclasses import dataclass
from typing import Callable, Protocol, Self, TypeVar


T = TypeVar("T", covariant=True)


class Result(Protocol[T]):
    def then(self, f: Callable[[T], Self]) -> Self: ...


@dataclass
class Success(Result[T]):
    value: T

    def then(self, f: Callable[[T], Self]) -> Self:
        return f(self.value)


@dataclass
class Failure(Result[T]):
    error: Exception

    def then(self, f: Callable[[T], Self]) -> Self:
        return self


def read_and_add(prev: int = 0) -> Result[int]:
    try:
        return Success(prev + int(input("Enter a number: ")))
    except KeyboardInterrupt:
        return Failure(Exception("Cancelled"))
    except Exception as e:
        return Failure(e)


if __name__ == "__main__":
    result = read_and_add().then(read_and_add).then(read_and_add)

    match (result):
        case Success(value):
            print(f"Sum: {value}")
        case Failure(error):
            print(f"Error: {error}")