Haskell:表达式上下文中的模式语法:_

Question

我是哈斯克尔的一个新人。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：_。当我只是在没有防护的情况下使用案例时，它可以工作。为什么它在警卫中不起作用？

Answer 1

如帖子中所述，模式匹配不需要防护装置。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

但是您根本不需要编写该函数。因为也许是一个Monad，你可以写这个。

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

然后要获得您的maybe_divide版本，您可以编写此内容。

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

这样可能更容易理解。

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