Decode JSON string enum into elm union type

A.elm

module A exposing (..)

import Json.Decode as JD
import Json.Encode as JE


type alias User =
    { id : Int
    , theme : Theme
    }


type Theme
    = Light
    | Dark


userDecoder =
    (JD.map2 User
        ( JD.field "id" JD.int )
        ( JD.field "theme" decodeTheme )
    )

-- Can't figure out how to make a custom Json.Decoder.Decoder a
decodeTheme toDecode =
    let
        valueDe =
            JD.decodeString JD.string toDecode
    in
        case valueDe of
            Ok name ->
                if (String.toLower name) == "light" then
                    Light
                else
                    Dark

            Err msg ->
                Dark

Maybe call it unknownTheme

How to do this if custom type contains values that should also be decoded from JSON? If, for example, Dark contains Int-value telling how dark it is.

Can anyone answer LimmaPaulus' question? How would themeDecoder look if Dark had a payload?

type Theme
= Light
| Dark Int

I haven't tried this myself, but this is what I imagine would work

So your JSON might look like

{
    "theme": "Light"
}

or

{
    "theme": ["Dark", 2]
}

So your decoder will need to look like this

type Theme
    = Light
    | Dark Int

type Intermediate =
    Intermediate String (Maybe Int)

decoder : Decoder Theme
decoder =
    Decode.map2 Intermediate (index 0 Decode.string) (index 1 Decode.maybe <| Decode.int)
        |> Decode.andThen fromIntermediate

fromIntermediate : Intermediate -> Decoder Theme
fromIntermediate (Intermediate string maybeInt) =
    case (string, maybeInt) of
        ("light", Nothing) ->
            Decode.succeed Light

        ("dark", Just int) ->
            Decode.succeed <| Dark int

        other ->
            Decode.fail "Invalid json"

Great example!!