This is a demonstration of a gen_server which receives a message, does not reply but puts it into its own mailbox again so it will reply later. This way the execution is deferred and other pending messages are processed first.

msg_looper.erl

% This is a demonstration of a gen_server
% which receives a message, does not reply but
% puts it into its own mailbox again so it will reply later.
% This way the execution is deferred and other pending messages are processed first.

-module(msg_looper).
-behaviour(gen_server).

%% API
-export([start_link/0, stop/1]).
-export([
    msg/1
    ]).

%% gen_server callbacks
-export([
    init/1
    , handle_call/3
    , handle_cast/2
    , handle_info/2
    , terminate/2
    , code_change/3]).

-define(SERVER, ?MODULE).

%%%===================================================================
%%% API
%%%===================================================================

start_link() ->
    gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).

stop(Pid) ->
    gen_server:call(Pid, stop).

msg(Msg) ->
    gen_server:call(?SERVER, {msg, Msg}).

%%%===================================================================
%%% gen_server callbacks
%%%===================================================================

init([]) ->
    {ok, {}}.

handle_call({msg, Msg}, From, _State) ->
    error_logger:info_msg("got message: ~p from ~p\n", [Msg, From]),
    self() ! {'$gen_call', From, {msg_internal, Msg}},
    {noreply, {}};

handle_call({msg_internal, Msg}, From, _State) ->
    error_logger:info_msg("got internal message: ~p from ~p\n", [Msg, From]),
    {reply, Msg, {}}.

handle_cast(_Msg, State) ->
    {noreply, State}.

handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.