Concurrent Programming in Erlang - The University of Kent - chapter 4.5

frequency_server.erl

-module(frequency_server).
-behaviour(gen_server).
-export([init/1, handle_call/3, handle_cast/2]).
-export([start/0, stop/0, allocate/0, deallocate/1]).

get_frequencies() -> [10,11,12,13,14,15].

%% gen_sever implementation

init(_Args) ->
  Freqs = {get_frequencies(), []},
  {ok, Freqs}.

handle_call(allocate, _Pid, {[], Allocated}) ->
  {reply, {error, no_frequency}, {[], Allocated}};

handle_call(allocate, Pid, {[Freq | Free], Allocated}) ->
  NewFreqs = {Free, [{Freq, Pid} | Allocated]},
  {reply, {ok, Freq}, NewFreqs}.

handle_cast({deallocate, Freq}, {Free, Allocated}) ->
  NewAllocated = lists:keydelete(Freq, 1, Allocated),
  NewFreqs = {[Freq | Free], NewAllocated},
  {noreply, NewFreqs}.

%% API

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

stop() ->
  gen_server:stop(?MODULE).

allocate() ->
  gen_server:call(?MODULE, allocate).

deallocate(Freq) ->
  gen_server:cast(?MODULE, {deallocate, Freq}).

Nice!