Added the tail recursion utility.

"""

This utility makes it easy to write recursion with tail call

optimization. To make a function utilize tail call optimization,

decorate a function with a "tail_call" decorator, and wrap the

return value with "Res", recursive descent with "Recursive".

TODO: support for tail call optimization when a different

function is called in return

"""

from __future__ import annotations

class Res:

    __slots__='res'

    def __init__(self, res):

        self.res = res

class Recursive:

    __slots__='args', 'kwargs'

    def __init__(self, *args, **kwargs):

        self.args = args

        self.kwargs = kwargs

def tail_recursion(f):

    def _wrap(*args, **kwargs):

        res: Res | Recursive = Recursive(*args, **kwargs)

        while isinstance(res, Recursive):

            res = f(*res.args, **res.kwargs)

        return res.res

    return _wrap

@tail_recursion

def factorial(x: int, accum=1):

    if x == 0:

        return Res(accum)

    return Recursive(x-1, accum*x)

print(factorial(10))

print(factorial(100))

print(factorial(10000))