haskell - Haskell：表达式上下文中的模式语法：_

Question

我是 Haskell 的新人。这是我的程序：

maybe_divide :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybe_divide a b = case (a, b) of  
 (Just a, Just b)
 | (Just a, Just b)            
 | (Nothing, _)    -> Nothing
 | (_, Just 0)     -> Nothing
 | (_, Nothing)    -> Nothing
 | (Just a,Just b) -> Just (a `div` b)

编译器显示：表达式上下文中的模式语法：_。当我只使用没有防护装置的情况时，它可以工作。为什么它在警卫内不起作用？

Answer 1

如帖子中所述，模式匹配不需要守卫。您可以删除一些多余的案例。

maybe_divide :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybe_divide a b = case (a, b) of
                    (Just _, Just 0) -> Nothing
                    (Just n, Just d) -> Just (n `div` d)
                    _ -> Nothing

但是您根本不必编写该函数。因为 Maybe 是 Monad，所以你可以写这个。

maybeDivide :: Integer -> Integer -> Maybe Integer
maybeDivide _ 0 = Nothing
maybeDivide n d = Just (n `div` d)

然后要获得你的 Maybe_divide 版本，你可以写这个。

maybe_divide' :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybe_divide' a b = a >>= (\n -> b >>= (\d -> maybeDivide n d))

这可能更容易阅读。

maybe_divide'' :: Maybe Integer -> Maybe Integer -> Maybe Integer
maybe_divide'' a b = do
    n <- a
    d <- b
    maybeDivide n d