Question

这是一个我无法解决的简单问题我试图通过以下代码获得maybe_divide作为所需的分配，但终端给我一个错误消息

test2.hs：1：112：模式中的解析错误:(只是x）

maybe_devide :: Maybe Float -> Maybe Float -> Maybe Float
maybe_devide maybeX maybeY = case maybeX maybeY of
  (Just x) (Just y)
    |x/=0 && y/=0 -> Just (div x y)
    |x=0 && y/=0  -> Just 0
    |x/=0 && y=0  -> Nothing
  Nothing (Just y) -> Nothing
  (Just x) Nothing -> Nothing

谁愿意告诉我它有什么问题？感谢

Answer 1

maybeX maybeY在参数maybeX上应用函数maybeY，而您打算对(maybeX,maybeY)对进行大小写分析。您还应该使用==表示相等，而不是=。以下将按您的意图执行：

maybe_devide :: Maybe Float -> Maybe Float -> Maybe Float
maybe_devide maybeX maybeY = case (maybeX,maybeY) of
  (Just x,Just y)
    |x/=0 && y/=0 -> Just (div x y)
    |x==0 && y/=0  -> Just 0
    |x/=0 && y==0  -> Nothing
  (Nothing,Just y) -> Nothing
  (Just x,Nothing) -> Nothing

或者，您可以使用Maybe monad，如

maybe_devide :: Maybe Float -> Maybe Float -> Maybe Float
maybe_devide maybeX maybeY = do
        x <- maybeX
        y <- maybeY
        case (x,y) of
                (0,y) -> Just 0
                (x,0) -> Nothing
                (x,y) -> Just (div x y)

可能除以解析错误（Haskell）

1 个答案: