foo.elm

type Request 
    = User User.Msg
    | Room Room.Msg

type Endpoint
    = User
    | Room

type User.Msg
    = Create UserPayload
    | Delete User
    | List
    
type Room.Msg
    = Create RoomPayload
    | Expand Room
    | Clean

type alias OutboundRequest
    = { endpoint : String
      , message : String
      , payload : Json.Encode.Value
      }

encodeRequest : Request -> OutboundRequest