Question

我正在尝试解析JSON文件并将其显示给用户，这是一个简化版本

{
  "posts": [{ 
    // some properties 
    comments: {
      // some more properties
    }
}

这是我的代码，我知道很多我并不确定要删除什么来隔离问题，每次我尝试时都会遇到一个似乎无处可寻的错误。

type Action
  = NoOp
  | GetPosts
  | ShowPosts (Maybe { posts : List Post.Model })


init =
  ( { posts = Nothing }, Effects.none )


update action model =
  case action of
    NoOp ->
      ( model, Effects.none )

    GetPosts ->
      ( { model | posts = Nothing }, getPosts )

    ShowPosts maybePosts ->
      ( { model | posts = maybePosts }, Effects.none )


view address model =
  div
    []
    [ button [ onClick address GetPosts ] [ text "Click to get posts!" ]
    , viewPosts model.posts
    ]


viewPosts maybePosts =
  case maybePosts of
    Nothing ->
      div [] [ text "No posts to display. Try clicking the button" ]

    Just posts ->
      ul [] (List.map Post.view posts)



-- This is the key to map the result of the HTTP GET to an Action
-- Note: Task.toMaybe swallows any HTTP or JSON decoding errors


getPosts : Effects Action
getPosts =
  Http.get decoderColl "./posts.json"
    |> Task.toMaybe
    |> Task.map ShowPosts
    |> Effects.task


type alias PostListContainerModel =
  { posts : List Post.Model }


postDecoder : Decoder Post.Model
postDecoder =
  Decode.object5
    Post.Model
    ("img" := Decode.string)
    ("text" := Decode.string)
    ("source" := Decode.string)
    ("date" := Decode.string)
    ("comments" := Decode.list commentDecoder)


commentDecoder : Decoder Comment.Model
commentDecoder =
  Decode.object2
    Comment.Model
    ("text" := Decode.string)
    ("date" := Decode.string)


decoderColl : Decoder PostListContainerModel
decoderColl =
  Decode.object1
    PostListContainerModel
    ("posts" := Decode.list postDecoder)

我从编译器中收到此错误

函数start期望参数为：

{ ...
, view :
      Signal.Address Action
      -> { posts : Maybe { posts : List Post.Model } }
      -> Html
}

但它是：

{ ...
, view :
      Signal.Address Action -> { posts : Maybe (List Post.Model) } -> Html
}

我无法理解{ posts : Maybe定义中额外view的来源。

上一个问题有一些额外的背景：Parsing nested JSON in Elm

更新：

在elm社区google小组中找到了答案，这里的要点是https://gist.github.com/rundis/23d7ef6ea42842e6f527

Answer 1

我认为ShowPosts的定义正在阻碍。你有这个：

ShowPosts (Maybe { posts : List Post.Model })

但它应该是这样的：

ShowPosts (Maybe (List Post.Model))

进行此更改将导致您必须更新其他一些地方，但请遵循编译器消息，它应该引导您到达正确的位置。

需要更新的地方之一是getPosts，您需要从该包装对象中获取帖子列表。这应该就是这么简单：

|> Task.map (ShowPosts << .posts)

解析JSON

1 个答案: