haskell - 如何在高阶函数haskell中使用Maybe？

Question

我想解决以下问题，其中 f 返回 Nothing，然后删除第一个元素。如果 f 返回 Just x，则第一个元素将更改为 x。我在下面的回答对 Nothing 有效，但我不明白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

更改(\x -> 无)[1..5] == [2..5]

改变(\x -> 仅 10) [1,2,3] == [10,2,3]

Answer 1

对于一个简单的解决方案，您可以像这样在 f x 上进行模式匹配(使用 case):

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

那应该按照你描述的去做

---

对于较短的，您可以使用 maybeToList并像这样连接部分:

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