Erlang Process Ring!

I though I might just post this. This is a couple of weeks old. It's the exercise from chapter four, regarding the erlang rings.

-module(ring).
-import(erlang, [error/1]).
-export([start/2]).
-export([init/0]).

-ifdef(debug).
-define(DBG(Str, Args), io:format(Str, Args).
-else.
-define(DBG(Str, Args), ok).
-endif.

%% Process that makes the ring
loop(Pid, Id) ->
    receive
 {stop, FromPid} ->
     ?DBG("~p received msg: stop from ~p\n", [self(), FromPid]);
 {Msg, FromPid} ->
     ?DBG("~p received msg: ~p from ~p\n", [self(), Msg, FromPid]),
     case Id of 
  first ->
      case Msg of 
   {_, 0} ->
       Pid ! {stop, self()};
   {Str, Num} ->
       Pid ! {{Str, Num - 1}, self()},
       loop(Pid, Id)     
      end;

  middle ->
      Pid ! {Msg, self()},
      loop(Pid, Id)
     end;

 Msg -> 
     io:format("wtf: ~p\n", [Msg]),
     error("bad message")
    end.


init() ->
    receive
 {next, Pid, _} -> 
     ?DBG("~p sends to ~p\n", [self(), Pid]),
     loop(Pid, middle);
 {first, Pid, Num} ->
            ?DBG("[first:~p] sends to ~p\n", [self(), Pid]),
            Pid ! {{"hello", Num}, self()},
     loop(Pid, first);
 Msg -> 
     io:format("wtf: ~p\n", [Msg]),
     error("bad init message")
    end,
    ok.

%% To set up the ring
create_procs(0, PidList) ->
    PidList;
create_procs(N, PidList) ->
    Pid = spawn(?MODULE, init, []),
    NewPidList = [Pid|PidList],
    create_procs(N - 1, NewPidList).

set_up_ring_(Num, Last, [Pid|[First|[]]]) -> %% Num is the number of messages
    Last ! {next, First, Num},
    ?DBG("~p <- ~p\n", [Last, First]),
    First ! {first, Pid, Num},
    ?DBG("~p <- ~p\n", [First, Pid]);
set_up_ring_(Num, Last, [Pid1|[Pid2|PidList]]) ->
    Pid2 ! {next, Pid1, Num},
    ?DBG("~p <- ~p\n", [Pid2, Pid1]),
    set_up_ring_(Num, Last, [Pid2|PidList]).
    
set_up_ring(Num, [X|[Y|XS]]) ->
    set_up_ring_(Num, X, [X|[Y|XS]]);
set_up_ring(_, _) ->
    error(badarg).

start(NumProcs, NumMessages) when (NumProcs >= 2) and (NumMessages >= 1) ->
    PidList = create_procs(NumProcs, []),
    set_up_ring(NumMessages, PidList).