elm - 如何编写解码器来映射自定义数据类型的值列表？

Question

我正在努力为链接列表编写解码器:

listOfLinksDecoder : Decoder (List JsonLink)
listOfLinksDecoder =
    Decode.map (List JsonLink)
        (field "Links" <| Decode.list linkDecoder)

Error:

Decode.map (List JsonLink)

Cannot find variable List

请注意，我已经成功地为单个链接编写了解码器:

linkDecoder : Decoder JsonLink
linkDecoder =
    Decode.map6 JsonLink
        (field "Profile" profileDecoder)
        (field "Title" Decode.string)
        (field "Url" Decode.string)
        (field "ContentType" Decode.string)
        (field "Topics" <| Decode.list topicDecoder)
        (field "IsFeatured" Decode.bool)

请注意，我尝试搜索 this documentation 。但是，我仍然无法找到适合我的案例的示例。

附录:

topicLinks : Id -> Topic -> ContentType -> (Result Http.Error (List JsonLink) -> msg) -> Cmd msg
topicLinks providerId topic contentType msg =
    let
        url =
            baseUrl ++ (getId providerId) ++ "/" ++ "topiclinks"

        body =
            encodeId providerId |> Http.jsonBody

        request =
            Http.post url body linksDecoder
    in
        Http.send msg request

Answer 1

您不需要映射，您可以这样做:

listOfLinksDecoder : Decoder (List JsonLink)
listOfLinksDecoder =
    field "Links" <| Decode.list linkDecoder

另请注意，无法找到变量列表错误是因为List是一种类型，而不是构造函数/函数。