Elm: Decode Json to a simple record structure

Question

Struggling to find the right way to complete my Decoder. I start with data of the form

[{_id:'interests', [{obj1}, {obj1}]}
,{_id:'countries', [{obj2}, {...}]} 
,{_id:'sections', [{obj3}, {...}]}]

I want to get to a Decoder Summary where

type alias Summary =
    { sections : List Section
    , interests : List Interest
    , countries : List Country
    }

So far, the best I've been able to get to is an output of this sort:

[ Interests (List Interest), Countries (List Country), Sections (List Section)]

but that still requires some rather brittle pattern matching (relying upon a consistent order of the array and is therefore very problematic with 0.16). For that I use

summaryItemDecoder : String -> Decoder SummaryInfo
summaryItemDecoder item =
    let dec =
        case item of
            "sections" -> Json.map Sections sectionsDecoder
            "interests" -> Json.map Interests interestsDecoder
            "countries" -> Json.map Countries countriesDecoder
    in ("data" := dec)

listSummaryDecoder : Decoder (List SummaryInfo)
listSummaryDecoder =
    ("_id" := string) `Json.andThen` summaryItemDecoder
    |> list

Full code here. Grateful for some final tips

Answer 1

I'm not sure you can do much better; you are attempting to parse a format that can express things you cannot represent in your types, so your only option is to fail.

To satisfy the pattern matching gods, perhaps drop an otherwise clause in featuring the fail decoder? (http://package.elm-lang.org/packages/elm-lang/core/3.0.0/Json-Decode#fail)