Haskell模式匹配中的錯誤

Question

這是我的實驗問題之一，我必須編寫一個maybeDiv函數，這是我的工作。 Just xy存在錯誤，請問是什么原因導致此錯誤？ 謝謝。

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

您的程序應該看起來像這樣：

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PC當前正在忙於制作大型動畫，因此無法檢查。 除非有人糾正我，否則我以后再檢查。

但這將不起作用，因為x/y不是整數。 您寧願要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

編輯

如果不允許您使用2元組(mx,my) ，如您在評論中所述，則可以執行以下操作：

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)

如果要將歐幾里得除數的商作為輸出，請執行以下操作：

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)

您也可以泛化該函數以使其接受Integer和Int類型，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

由於這是在實驗室中進行的，因此您可能不被允許使用Applicative-但是，為清楚起見，以下是您的功能可能/應該是的：

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

這最終將等同於其他答案，但是可能更容易閱讀。