如何在高阶函数haskell中使用Maybe？

Question

我想解决以下问题，其中 f 不返回任何内容，然后删除第一个元素。如果 f 返回 Just x，则第一个元素随 x 改变。我下面的答案对 Nothing 正确，但我无法理解如何在函数内部区分 Nothing 和 Just

change:: (a -> Maybe a) -> [a] -> [a]
change f [] = []
change f (x:xs) = xs

改变(\x -> 没有) [1..5] == [2..5]

change(\x -> Just 10) [1,2,3] == [10,2,3]

Answer 1

对于一个简单的解决方案，您可以像这样进行模式匹配（与case）f x：

change:: (a -> Maybe a) -> [a] -> [a]
change _ [] = []
change f (x:xs) =
   case f x of
       Just x' -> x':xs
       Nothing -> xs

那应该做你所描述的

对于较短的，您可以使用maybeToList并连接这样的部分：

change:: (a -> Maybe a) -> [a] -> [a]
change _ [] = []
change f (x:xs) = (maybeToList $ f x) ++ xs