Concurrent frequency and ping server

server.erl

-module(server).
-export([ping/1, init/0]).

% Usage in shell:
%register(server, spawn(server, init, [])).
%server ! {request, self(), allocate}.
%server ! {request, self(), {deallocate, Freq}}.
%register(client, spawn(server, init, [])).
%server ! {ping, client}. % WARNING! This ping-pong server will never end!

ping(Io) ->
    receive
        {ping, Pid} ->
            Io ! io:format("~p sent a ping~n", [self()]),
            Pid ! {pong, self()},
            timer:sleep(100);
        {pong, Pid} ->
            Io ! io:format("~p replied with a pong~n", [self()]),
            Pid ! {ping, self()},
            timer:sleep(100)
    end,
    ping(Io).

allocate({[], Allocated}, _Pid) ->
    {{[], Allocated}, {error, no_frequency}};
allocate({[Freq|Free], Allocated}, Pid) ->
    {{Free, [{Freq, Pid}|Allocated]},
     {ok, Freq}}.

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

init() ->
    Frequencies = {get_frequencies(), []},
    loop(Frequencies).

get_frequencies() -> [10, 14, 15, 17, 20].

loop({_Free, Allocated}=Frequencies) ->
    receive
        {request, Pid, allocate} ->
            IsAllocated = lists:keymember(Pid, 2, Allocated),
            case IsAllocated of
                true ->
                    Pid ! {reply, {error, already_allocated}},
                    loop(Frequencies);
                false ->
                    {NewFrequencies, Reply} = allocate(Frequencies, Pid),
                    Pid ! {reply, Reply},
                    loop(NewFrequencies)
            end;
        {request, Pid, {deallocate, Freq}} ->
            case lists:keyfind(Freq, 1, Allocated) of
                {Freq, Pid} ->
                    NewFrequencies = deallocate(Frequencies, Freq),
                    Pid ! {reply, ok},
                    loop(NewFrequencies);
                false ->
                    Pid ! {reply, {error, no_matching_freq_pid}},
                    loop(Frequencies)
            end;
        {request, Pid, stop} ->
            Pid ! {reply, stop}
    end.