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| 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()) |
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