python - 为什么我翻译成 Python 的 Haskell 不能正常工作？

Question

haskell :

average x y = (x + y) / 2

sqrt' :: (Ord a, Fractional a) => a -> Int -> a
sqrt' 0 _ = 0.0
sqrt' 1 _ = 1.0
sqrt' s approximations = (infsqr' s) !! approximations

infsqr' n = unfoldr acc 1 where
    acc guess | guess < 0 = Nothing
              | otherwise = Just (newguess', newguess') where
                newguess' = average guess (n / guess)

python :

def unfold(f, x):
    while True:
        w, x = f(x)
        yield w

def average(x, y):
    return float((x + y) / 2)

def acc(guess):
    if guess < 1:
        return None
    else:
        newguess = average(guess, (float(n/guess)))
        return (newguess, newguess) 
n = 9
print unfold(acc, 1).next()
print unfold(acc, 1).next()

它应该输出列表的下两个值，例如5.0, 3.4

但是它输出了两次 5.0，为什么？

Answer 1

如果你再次调用 unfold ，你的生成器将重新生成，所以你需要将它赋值给变量。

>>> res = unfold(acc, 1)
>>> print res.next()
5.0
>>> print res.next()
3.4
>>>