Hello everyone,
I’m a new in elexir and functional language. I’m trying to implement Websocket interraction with server.
On first layer I implement module based on GenServer with functionality like WebSockex and use ninenines/gun as Websocket client. This module name egun, below are the code snippets:

defmodule Egun do
  use GenServer

  @callback handle_connect(state :: term) :: {:ok, new_state :: term}
  @callback handle_message(msg :: term, state :: term) :: {:ok, new_state} | ...
  @callback handle_send(msg :: term, state :: term) :: {:ok, new_state} | ...
...

  defmacro __using__(_opts) do
    quote location: :keep do
      @behaviour Egun
      @doc false
      def handle_connect(state) do
        {:ok, state}
      end

      @doc false
      def handle_message(message, _state) do
        raise "No handle_message/2 clause in #{__MODULE__} provided for #{inspect(message)}"
      end

      @doc false
      def handle_send(message, state) do
        {:reply, {:text, message}, state}
      end

      defoverridable  handle_connect: 1,
                      handle_message: 2,
                      handle_send: 2
   end

  def start_link(conn = %Conn{}) do
    GenServer.start_link(__MODULE__, %__MODULE__{
      conn: conn,
    })
  end

  def init(state) do
     ...
     send(self(), :ws_connected)
     {:ok, state}
  end

  def send_message(client, message) do
    GenServer.call(client, {:ws_syncsend, message})
  end

  def handle_call({:ws_syncsend, message}, sender, state) do
    message = compose_msg(message, state.conn)
   ...
    {:reply, message, state}
  end

  def handle_info(:ws_connected, state) do
    common_handle(:handle_connect, state)
  end

  def handle_info(
    {:gun_ws, _gun_pid, _ws_stream, {:text, message}}, state
  ) do
    case Jason.decode(message, keys: :atoms) do
      {:ok, msg} ->
        common_handle({:handle_message, msg}, state)

      error ->
        {:stop, error, state}
    end
  end

  defp common_handle({function, msg}, state) when is_atom(function)  do
    result = apply(state.module, function, [msg, state])
    common_handle_ret(result, {function, msg}, state)
  end

  defp common_handle(function, state) when is_atom(function)  do
    result = apply(state.module, function, [state])
    common_handle_ret(result, {function, nil}, state)
  end

  defp common_handle_ret(result, {function, msg}, state) do
    case result do
      {:ok, new_state} ->
        {:noreply, new_state}

      {:reply, frame, new_state} ->
        :gun.ws_send(new_state.gun_pid, frame_encode(frame))
        {:noreply, new_state}
      
      ....
    end
  end

This implementation provides messaging through an open Websocket. Next I need extend functionality egun. And create module session which adds business logic methods:

defmodule Session do
  @moduledoc nil

  use Egun

  def create(client) when is_pid(client) do
    tr = Egun.send_message(client, %{
      session: "create",
      transaction: "yes"
      })
    GenServer.call(client, {:push_transaction, tr, client})
  end

  @impl true
  def handle_call({:push_transaction, transaction, callback_pid}, _from, state) do
    trs = Map.put(state.transactions, transaction, callback_pid)
    {:noreply, %{state | transactions: trs}}
  end

  @impl Egun
  def handle_message(message, state) do
    IO.puts("Handle message: #{inspect(message)}")
    IO.puts("State: #{inspect(state)}")
    {:ok, state}
  end
end

After compile I get warning message:

warning: got "@impl Egun" for function handle_call/3 but this behaviour does not specify such callback. The known callbacks are:

  * Egun.handle_connect/1 (function)
  * Egun.handle_message/2 (function)
  * Egun.handle_send/2 (function)

How can I modify my modules so that the Session module can implement the behavior of the GenServer and Egan without using an additional GenServer. On object oriented programming language Session can get access to all methods from GenServer and Egan?