What was wrong with SML?(blog.plover.com)

    class Thunk:
        def __init__(self, f):
            self.evaluated = False
            self.val = f
        def __call__(self):
            if not self.evaluated:
                self.val = self.val()
                self.evaluated = True
            return self.val

    class Cons:
        def __init__(self, x, xs):
            assert isinstance(xs, Thunk)
            self.head = x
            self._tail = xs
        @property
        def tail(self):
            return self._tail()
        def nth(self, n):
            for _ in range(n):
                self = self.tail
            return self.head

    Thunk(lambda: ones))

    print([ones.nth(i) for i in range(10)])
    # [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

    def lazy_map(xs, f):
        return Cons(f(xs.head), Thunk(lambda: lazy_map(xs.tail, f)))

    nats = Cons(0, Thunk(lambda: lazy_map(nats, lambda x: x + 1)))

    print([nats.nth(i) for i in range(10)])
    # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

    def lazy_zip(xs, ys, f):
        return Cons(f(xs.head, ys.head), Thunk(lambda: lazy_zip(xs.tail, ys.tail, f)))

    fibs = Cons(0, Thunk(lambda: Cons(1, Thunk(lambda: lazy_zip(fibs, fibs.tail, lambda x, y: x + y)))))

    print([fibs.nth(i) for i in range(10)])
    # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]