Decode different format of json in elm

decode.elm

import Html exposing (text)
import Json.Decode as Decode exposing ( Decoder, (:=), at )

jsonString1 =
  "{ \"enclosure\" : { \"url\": \"aaa\" } }"
  
jsonString2 =
  "{ \"enclosure\" : [ {\"url\": \"ccc\"}, {\"url\" : \"bbb\"} ] }"

type alias Enclosure =
  { url : String}

main =
  let
    a = Decode.decodeString decoder jsonString1
    url1 = getUrl a
    b = Decode.decodeString decoder jsonString2
    url2 = getUrl b
  in
    text <| "Hello, " ++ url1 ++ "," ++ url2
    
    

decoder : Decoder Enclosure
decoder =
  Decode.oneOf 
    [ "enclosure" := enclosureDecoder
    , ("enclosure" := Decode.list enclosureDecoder ) 
          `Decode.andThen` 
          (\list ->
            let 
              first = List.head list
            in
              case first of
                Just f -> Decode.succeed f
                Nothing -> Decode.fail "failed" 
          )
    ]


enclosureDecoder : Decoder Enclosure
enclosureDecoder =
  Decode.object1 Enclosure
    ( "url" :=  Decode.string )
    
    
getUrl : Result String Enclosure -> String
getUrl a =
  case a of
    Ok e -> 
      e.url
      
    Err err -> 
      "error"