maybe example

~/dev/erl $ erl -pa .
Eshell V6.3  (abort with ^G)
1> maybe:start_link().
{ok,<0.34.0>}
2> maybe:unlift(5).
{error,badarg}
3> N = maybe:nothing().
#Ref<0.0.0.32>
4> maybe:unlift(N).
nothing
5> V = maybe:just(5).
#Ref<0.0.0.53>
6> maybe:unlift(V).
{just,5}
7> 

maybe.erl

-module(maybe).

-behaviour(gen_server).

%% API
-export([start_link/0, just/1, nothing/0, unlift/1]).

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

-define(SERVER, ?MODULE).

-record(state, {storage=ets:new(storage, []), nothing=make_ref()}).

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

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

just(Val) ->
  gen_server:call(?SERVER, {just, Val}).

nothing() ->
  gen_server:call(?SERVER, nothing).

unlift(Id) ->
  gen_server:call(?SERVER, {get, Id}).

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

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

handle_call({just, Value}, _From, State) ->
  Id = make_ref(),
  ets:insert(State#state.storage, {Id, Value}),
  {reply, Id, State};

handle_call(nothing, _From, State) ->
  {reply, State#state.nothing, State};

handle_call({get, Id}, _From, #state{nothing=Id}=State) ->
  {reply, nothing, State};

handle_call({get, Id}, _From, State) ->
  case ets:lookup(State#state.storage, Id) of
    [{Id, Value}] ->
      {reply, {just, Value}, State};
    _ ->
      {reply, {error, badarg}, State}
  end.

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

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

terminate(_Reason, _State) ->
  ok.

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

%%%===================================================================
%%% Internal functions
%%%===================================================================