Decoding nested JSON structures

Question

I'm trying playing with the source code of elmplayground and I'm trying to create a configuration json file for the blog. The issue I'm having at this point is that I don't know how I can decode a post/page author as a nested structure. What I want is that the author field in posts and pages do a reference to an author in the config.json.

config.json:

{
 "posts": [{
 "slug": "/hello-world",
 "title": "Hello World",
 "name": "hello-world",
 "publishedDate": "2016-10-30",
 "author": "Gabriel",
 "intro": ""
 }],
 "pages": [{
 "slug": "/hello",
 "name": "index",
 "title": "Elm Playground",
 "publishedDate": "2016-09-01",
 "author": "Gabriel",
 "intro": ""
 }],
 "authors": {
 "Gabriel": {
 "name": "Gabriel Perales",
 "avatar": "..."
 }
 }
}

Type Content for pages and posts:

type alias Content =
 { title : String
 , name : String
 , slug : String
 , publishedDate : Date
 , author : Author
 , markdown : WebData String
 , contentType : ContentType
 , intro : String
 }

Type Author:

type alias Author =
 { name : String
 , avatar : String
 }

Currently this is my config decoder:

configDecoder : Decoder Config
configDecoder =
 Decode.map2 Config
 (field "posts" <| Decode.list <| decodeContent Post)
 (field "pages" <| Decode.list <| decodeContent Page)
decodeContent : ContentType -> Decoder Content
decodeContent contentType =
 Decode.map8 Content
 (field "slug" string)
 (field "name" string)
 (field "title" string)
 (field "publishedDate" decodeDate)
 (field "author"
 (string
 -- I want to decode the author from "authors"
 -- I have tried with 
 -- (\name -> at ["authors", name] decodeCofigAuthor) but it doesn\'t work
 |> andThen (\name -> Decode.succeed <| Author name "...")
 )
 )
 (Decode.succeed RemoteData.NotAsked)
 (Decode.succeed contentType)
 (field "intro" string)
decodeConfigAuthor : Decoder Author
decodeConfigAuthor =
 Decode.map2 Author
 (field "name" string)
 (field "avatar" string)

Answer 1

I would start by decoding the authors, and then use andThen to pass the authors Dict into decodeContent. You can then use Decode.map to convert to author name into a lookup in the authors Dict.

decoder =
 (field "authors" <| Decode.dict <| authorDecoder)
 |> Decode.andThen configDecoder
configDecoder authors =
 Decode.map2 Config
 (field "posts" <| Decode.list <| decodeContent Post authors)
 (field "pages" <| Decode.list <| decodeContent Page authors)
decodeContent contentType authors =
 Decode.map8 Content
 -- ...
 (field "author" (string |> Decode.map (\name -> Dict.get name authors)))
 -- ...

This would change your model to use a Maybe Author, but you could also use Decode.andThen and return a Decode.fail if Dict.get returns Nothing.

• gabrielperales: I'm going to try it, but it looks like what I was looking for