https://t.me/progmsk/135818

fibs.py

import itertools


class Cons:
    def __init__(self, head, tail_fn):
        self.head = head
        self._tail_fn = tail_fn
        self._tail = None

    def tail(self):
        if self._tail:
            return self._tail
        self._tail = self._tail_fn()
        return self._tail

    def __iter__(self):
        while True:
            yield self.head
            self = self.tail()


def cons_sum(a, b):
    return Cons(a.head + b.head,
                lambda: cons_sum(a.tail(), b.tail()))


def fibs():
    cons = Cons(0, lambda:
                Cons(1, lambda:
                     cons_sum(cons, cons.tail())))
    return cons


def take(n, it):
    return list(itertools.islice(it, n))


assert [0, 1, 1, 2, 3, 5, 8, 13, 21, 34] == take(10, fibs())