module Base =
  struct
    type server_msg = .. [@@deriving yojson]
    type server_msg +=
      | SOk of string
      [@@deriving yojson]

    type client_msg = .. [@@deriving yojson]
    type client_msg +=
      | Push
      [@@deriving yojson]
  end

module Make_base() = struct include Base end

module type P =
  sig
    type app_server_msg = .. [@@deriving yojson]
    type app_client_msg = .. [@@deriving yojson]

    include (module type of Base)

    val pack_server_msg : string -> server_msg -> app_server_msg
    val unpack_server_msg : app_server_msg -> (string * server_msg) option

    val pack_client_msg : string -> client_msg -> app_client_msg
    val unpack_client_msg : app_client_msg -> (string * client_msg) option
  end