recur3.py

from __future__ import annotations
import functools
from typing import Callable, ParamSpec, TypeVar, Generic

P = ParamSpec("P")
T = TypeVar("T")


class Recur(BaseException, Generic[P, T]):
    f: Callable[P, T] | None
    args: P.args
    kwargs: P.kwargs

    def __init__(self, f: Recurrent[P, T] | Callable[P, T] | None, *args: P.args, **kwargs: P.kwargs):
        super().__init__(*args)
        self.f = f.f if isinstance(f, Recurrent) else f
        self.kwargs = kwargs

    def __call__(self) -> T:
        assert self.f is not None
        return self.f(*self.args, *self.kwargs)

class Recurrent(Generic[P, T]):
    f: Callable[P, T]

    def __init__(self, f: Callable[P, T]) -> None:
        self.f = f

    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> T:
        r = Recur(self.f, *args, **kwargs)
        while True:
            try:
                return r()
            except Recur as exc:
                r.f = r.f if exc.f is None else exc.f
                r.args = exc.args
                r.kwargs = exc.kwargs

@Recurrent
def gcd(a: int, b: int) -> int:
    print(f"gcd({a}, {b})")
    if b == 0:
        return a
    raise Recur(gcd, b, a % b)

@Recurrent
def even(a: int, whatever: str) -> bool:
    assert a >= 0
    if a == 0: return True
    raise Recur(odd, a-1)

@Recurrent
def odd(a: int) -> bool:
    assert a >= 0
    if a == 0: return False
    raise Recur(even, a-1, "boo")

print(gcd(1071, 462))
print(odd(37))