Question

这是我的实验室问题之一，我必须写一个maybeDiv函数，这是我的工作。 Just x y中有错误，请问导致此错误的原因是什么？感谢。

maybeDiv :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybeDiv mx my = case mx my of
    Just x y
      | y == 0 -> Just Nothing
      | otherwise -> Just (x / y)
    Nothing -> Nothing

Answer 1

你的程序应该看起来像：

maybeDiv :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybeDiv mx my = case (mx, my) of
    (Just x, Just y)
      | y == 0 -> Nothing
      | otherwise -> Just (x / y)
    _ -> Nothing

我的电脑目前正在忙着做一个巨大的动画，所以我无法检查。我会稍后检查，除非有人在以前纠正我。

但这不起作用，因为x/y不是整数。你应该想要一个Double。所以：

maybeDiv :: Maybe Integer -> Maybe Integer -> Maybe Double
maybeDiv mx my = case (mx, my) of
    (Just x, Just y)
      | y == 0 -> Nothing
      | otherwise -> Just (fromInteger x / fromInteger y)
    _ -> Nothing

修改

如果您不允许使用2-tuple (mx,my)，正如您在评论中所说，您可以这样做：

maybeDiv :: Maybe Integer -> Maybe Integer -> Maybe Double
maybeDiv mx my =
  case my of
    Nothing -> Nothing
    Just 0  -> Nothing
    Just y  -> case mx of
      Nothing -> Nothing
      Just x  -> Just (fromInteger x / fromInteger y)

如果您想要欧几里德分区的商作为输出，请执行：

maybeDiv :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybeDiv mx my =
  case my of
    Nothing -> Nothing
    Just 0  -> Nothing
    Just y  -> case mx of
      Nothing -> Nothing
      Just x  -> Just (x `quot` y)

您还可以对该功能进行概括，以便它接受Integer和Int类型，如下所示：

maybeDiv :: Integral a => Maybe a -> Maybe a -> Maybe a
maybeDiv mx my =
  case my of
    Nothing -> Nothing
    Just 0  -> Nothing
    Just y  -> case mx of
      Nothing -> Nothing
      Just x  -> Just (x `quot` y)

Answer 2

由于这是针对实验室的，因此可能不允许使用Applicative - 但是，为了清楚起见，这是您的功能可能/应该实际的内容：

maybeDiv :: Maybe Int -> Maybe Int -> Maybe Int
maybeDiv ma mb = quot <$> ma <*> mb

这最终会相当于其他答案，但阅读起来会更清晰。

Haskell模式匹配中的错误

2 个答案:

修改