在Elm中附加可能列表

Question

我有一个List a和一个Maybe a.我想附加可能的值,如果它是,Just a但如果它是什么都不做Nothing.

这就是我目前使用的:

aList ++ case maybeValue of
           Just value ->
             [ value ]
           Nothing ->
             []

这样做有更好的(更惯用的)方法吗？

请注意,如果有更简洁的方法,那么前置也很好.列表顺序无关紧要.

Answer 1

根据乍得的建议，预先准备更便宜：

prependMaybe : List a -> Maybe a -> List a
prependMaybe list maybe =
   case maybe of
           Just value ->
             value :: list
           Nothing ->
             list

Answer 2

我认为你可以使用Maybe.map List.singleton yourMaybe |> Maybe.withDefault [].

这里有一个完整的例子：

appendMaybe : List a -> Maybe a -> List a
appendMaybe list maybe =
    Maybe.map List.singleton maybe
        |> Maybe.withDefault []
        |> (++) list

你可以在艾莉身上试试