如何在更高階中使用 Maybe function haskell？

Question

我想解決以下 f 返回 Nothing 的問題，然后刪除第一個元素。 如果 f 返回 Just x，則第一個元素將更改為 x。 我下面的答案適用於 Nothing 但我不明白如何區分 Nothing 和 Just 在 function

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

改變（\x -> 無）[1..5] == [2..5]

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

Answer 1

對於一個簡單的解決方案，您可以像這樣在fx上進行模式匹配（使用case ）：

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

那應該做你描述的

---

對於較短的，您可以使用maybeToList並像這樣連接部分：

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