elm decoder error

Main.elm

import Html exposing (..)
import Json.Decode exposing (Decoder, list, string)
import Json.Decode

type Elem
    = Para (List Elem)
    | Chunk String

elemListDecoder : Decoder (List Elem)
elemListDecoder =
    Json.Decode.list elemDecoder

elemDecoder : Decoder Elem
elemDecoder =
    Json.Decode.oneOf [ elemDecoderChunk, elemDecoderType ]

elemDecoderChunk : Decoder Elem
elemDecoderChunk =
    Json.Decode.map Chunk string

elemDecoderType : Decoder Elem
elemDecoderType =
    Json.Decode.field "type" string
        |> Json.Decode.andThen elemDecoderTypeHelp

elemDecoderTypeHelp : String -> Decoder Elem
elemDecoderTypeHelp t =
    case t of
        "p" ->
            -- meta: using elemDecoder directly doesn't work somehow
            Json.Decode.map Para (Json.Decode.field "content" (Json.Decode.list elemDecoder))
            -- Json.Decode.map Para (Json.Decode.field "content" (Json.Decode.list (Json.Decode.oneOf [ elemDecoderChunk, elemDecoderType])))
        _ -> Json.Decode.fail <| "Can't do a " ++ t ++ " elem"



--- skeleton from here on


main =
  Html.beginnerProgram { model = model, view = view, update = update }

model : Model
model =
    let js = "[\"foo\", {\"type\": \"p\", \"content\": []}]"
    in 
      case Json.Decode.decodeString elemListDecoder js of
        Ok d -> { doc = d }
        Err err -> Debug.crash err


-- MODEL

type alias Model = { doc : List Elem }


-- UPDATE

type Msg = Reset 

update : Msg -> Model -> Model
update msg model =
  case msg of
    Reset -> model


-- VIEW

view : Model -> Html Msg
view model =
  Html.text "all fine"