A simple Erlang example showing that messages that aren't immediately processed aren't lost, but are held for possible matching later. If you spawn a process P in the wait state initially, msg messages that are sent to it are not processed, but they're not lost, they're just held in the mailbox. When you then switch P to it's run state, then it …

message_queue.erl

%% @author mcphee
%% @doc A simple example showing that messages that
%% aren't immediately processed aren't lost, but are
%% held for possible matching later.

-module(message_queue).

-export([wait/0, run/0]).

% P = spawn(message_queue, wait, [])

% None of these messages are processed because P starts
% out in the wait state, which doesn't deal with msg
% messages.

% P ! {msg, 0}
% P ! {msg, 1}
% P ! {msg, 2}

% When we switch to the run state, however, by sending
% P the start message, then we process all those queued
% up messages.

% P ! start

wait() ->
	receive
		start -> 
			io:format("Got the start message~n"),
			run()
	end.

run() ->
	receive
		{msg, N} -> 
			io:format("Got message ~w~n", [N]),
			run();
		stop ->
			true
	end.