Question

在榆木中将List (Maybe a)转换为Maybe (List a)的好方法是什么？

逻辑很简单：

如果所有项目均为Just (List a)，请返回Just a
否则，返回Nothing。

Example 1:

input:  [ Just 1, Just 2, Just 3 ]
output: Just [ 1, 2, 3 ]

Example 2:

input:  [ Just 1, Nothing, Just 3 ]
output: Nothing

可以使用某些内置功能轻松完成吗？

我想出的最好的样子是这样的：

listOfMaybesToMaybeList : List (Maybe a) -> Maybe (List a)
listOfMaybesToMaybeList listOfMaybes =
    List.foldl
        (\maybeItem ->
            \maybeResultList ->
                case ( maybeItem, maybeResultList ) of
                    ( Just item, Just resultList ) ->
                        Just (List.append resultList [ item ])

                    ( _, _ ) ->
                        Nothing
        )
        (Just [])
        listOfMaybes

这种函数的合适名称是什么？在寻找答案时，我发现Haskell中有一个名为sequence的函数，似乎在做类似的事情。

Answer 1

您可以使用Elm Fancy Search工具并搜索功能签名：List (Maybe a) -> Maybe (List a)

第一个结果出现Maybe.Extra.combine

Answer 2

@Chad Gilbert的答案肯定是正确的，但是如果您正在寻找这种功能的更简单的实现，那么下面的代码就可以解决问题：

listOfMaybesToMaybeList : List (Maybe a) -> Maybe (List a)
listOfMaybesToMaybeList listOfMaybes =
    List.foldr (Maybe.map2 (::)) (Just []) listOfMaybes

或者只是：

listOfMaybesToMaybeList : List (Maybe a) -> Maybe (List a)
listOfMaybesToMaybeList = List.foldr (Maybe.map2 (::)) (Just [])

Maybe.map2仅接受一个函数和两个Maybe值并将该函数应用于这些值：

> Maybe.map2 (+) (Just 2) (Just 3)
Just 5 : Maybe.Maybe number
> Maybe.map2 (::) (Just 2) (Just [1])
Just [2,1] : Maybe.Maybe (List number)

请注意，使用(::)函数（在列表之前）代替了(++)或List.append，因为它在列表中表现更好。然后必须使用foldr而不是foldl来保持顺序。

在榆木中将清单（也许是a）转换为也许（清单a）

2 个答案: