gen_object: the start of an erlang OO behaviour

a_cat.erl

% This is a cat. It behaves like a generic object.

-module(a_cat).
-behaviour(gen_object).
-export([initialise/1, handle_message/3]).

initialise([Name]) ->
  {Name, "Meow"}.

handle_message(say, [Word], {Name, Word}) ->
  {Word, {Name, Word}};
handle_message(say, _, State) ->
  {"", State}.

gen_object.erl

% This is the behaviour definition for a generic object.

-module(gen_object).
-export([new/2, delete/1, send/3]).

-callback initialise(Args :: [any()]) -> State :: any().
-callback handle_message(Msg :: atom(), Args :: [any()], State :: any()) -> {Response :: any(), New_State :: any()}.

send(Obj, Msg, Args) ->
  Obj ! {self(), Msg, Args},
  receive
    Response ->
      Response
  end.

new(CBM, Args) ->
  State = apply(CBM, initialise, [Args]),
  spawn(fun() -> loop(CBM, State) end).

loop(CBM, State) ->
  receive
    gen_object__bif__exit ->
      ok;
    {From, Msg, Args} ->
      {Resp, New_State} = apply(CBM, handle_message, [Msg, Args, State]),
      From ! Resp,
      loop(CBM, New_State)
  end.

delete(Obj) ->
  Obj ! gen_object__bif__exit.

test_a_cat.erl

% This is where we test the cat to make sure that it does not bark.

-module(test_a_cat).
-export([main/0]).

main() ->
  MyCat = gen_object:new(a_cat, ["Fred"]),
  FredSays = gen_object:send(MyCat, say, ["Woof"]),
  gen_object:delete(MyCat),

  io:format("Fred says: ~p~n", [FredSays]).