Structure mismatch while parsing JSON

Question

I am trying to parse a JSON file and show it to the user, here a simplified version

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

This is my code, I know it's a lot I'm not really sure what to remove to isolate the problem, every time I try I get a different error that seems to lead me nowhere.

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)

I am getting this error from the compiler

Function start is expecting the argument to be:

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

But it is:

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

I can't understand where that extra { posts : Maybe is coming from in the definition of view.

Previous question with some extra background: Parsing nested JSON in Elm

UPDATE:

Got an answer in the elm community google group, here's the gist https://gist.github.com/rundis/23d7ef6ea42842e6f527

Answer 1

I think the definition of ShowPosts is getting in the way. You have this:

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

But it should probably be this:

ShowPosts (Maybe (List Post.Model))

Making that change will cause you to have to update a few other places, but follow the compiler messages and it should lead you to the right place.

One of the places that will need updating is getPosts, where you'll need to take the list of posts out of that wrapper object. That should be as simple as this:

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

• Alberto Zaccagni: Hey! Thanks :) Yeah, that's one of the things I've tried but I got an error I didn't really understand. I'll try harder.
• Alberto Zaccagni: I tried changing this definition type alias PostListContainerModel = List Post.Model because it makes this way for me, but no, I get Cannot find variable PostListContainerModel` which puzzles me even more.
• Chad Gilbert: I've updated the answer to include an example of eliminating the need to pass around PostListContainerModel. That is useful for decoding but really, you can avoid passing it around by using List Post.Model everywhere else by updating getPosts.
• Alberto Zaccagni: Thank you, I am trying to understand where to go after that, I've changed decoderColl = Decode.object1 (List Post.Model) ("posts" := Decode.list postDecoder) but that errors, I thought that was correct but I probably misunderstood the docs.
• Chad Gilbert: Sorry for not being clear in that last comment - PostListContainerModel is still needed for json decoding in the example you posted, but you can just pass around a List Post.Model for all other functions rather than a record that looks like PostListContainerModel