elm - 解码列表时忽略无效项

Question

解码列表时是否可以忽略无效项？示例:我有一个模型

type Type
    = A
    | B

type alias Section =
    { sectionType : Type
    , index : Int
    }


getTypeFromString : String -> Maybe Type
getTypeFromString input =
    case input of
        “a” ->
            Just A

        “b” ->
            Just B

        _ ->
            Nothing

decodeType : Decoder Type
decodeType =
    Decode.string
        |> Decode.andThen
            (\str ->
                case getTypeFromString str of
                    Just sectionType ->
                        Decode.succeed sectionType

                    Nothing ->
                        Decode.fail <| ("Unknown type" ++ str)
            )


decodeSection : Decoder Section
decodeSection =
    Decode.map2 Section
        (Decode.field "type" decodeType)
        (Decode.field "index" Decode.int)

如果我解码 JSON

{
  "sections": [{type: "A", index: 1}, {type: "invalid-type", index: 2}]
}

我希望我的部分 = [ {type = A, index= 1} ]

Answer 1

通常，处理这些问题的方法是将其解码为表示选项的 Elm 类型，然后使用 map 进行后处理。

所以例如在你的例子中，我会选择这样的东西:

decodeMaybeType : Decoder (Maybe Type)
decodeMaybeType =
    Decode.string
        |> Decode.map getTypeFromString 


decodeMaybeSection : Decoder (Maybe Section)
decodeMaybeSection =
    Decode.map2 (\maybeType index -> Maybe.map (\t -> Section t index) maybeType)
        (Decode.field "type" decodeMaybeType)
        (Decode.field "index" Decode.int)


decodeSections : Decoder (List Section)
decodeSections =
    Decode.list decodeMaybeSection
       |> Decode.map (List.filterMap identity)

注意:List.filterMap identity 是一个 List (Maybe a) -> List a，它过滤掉 Nothing 并删除Maybe 一气呵成。