How to decode union types in Elm

DecodeUnionType.elm

import Html exposing (text)
import Json.Decode exposing (..)


type Status 
  = Done
  | Open
  | Failed


type alias Stuff = 
  { f: Status }


statusDecoder =
  let
    statusHelper s =
      case s of
        "Done" -> Done
        "Open" -> Open
        _ -> Failed
        
  in
    map statusHelper string


stuffDecoder = 
  map Stuff
    (field "f" statusDecoder)


main =
  "{\"f\": \"Done\"}" 
  |> decodeString stuffDecoder 
  |> toString 
  |> text