Make use of pmod_pt parse_transform

In order to use it, the monad transformers have new behaviour:
monad_t. That's beause the transform alters the arity of exported
functions and they are no longer the same as those of monad.

rebar.config

@@ -1,3 +1,4 @@
 %% -*- erlang -*-
 {erl_opts, [debug_info, warnings_as_errors]}.
 {cover_enabled, true}.
+{deps, [{pmod_transform, ".*", {git, "git://github.com/erlang/pmod_transform.git", "master"}}]}.

src/error_t.erl

@@ -14,10 +14,11 @@
 %% Copyright (c) 2011-2013 VMware, Inc.  All rights reserved.
 %%
 
+-compile({parse_transform, pmod_pt}).
 -module(error_t, [InnerMonad]).
 -compile({parse_transform, do}).
 
--behaviour(monad).
+-behaviour(monad_t).
 -export(['>>='/2, return/1, fail/1, run/1, lift/1]).
 
 -ifdef(use_specs).

src/monad_t.erl

@@ -0,0 +1,10 @@
+-module(monad_t).
+
+-export([behaviour_info/1]).
+
+behaviour_info(callbacks) ->
+    [{'>>=',  3},
+     {return, 2},
+     {fail,   2}];
+behaviour_info(_Other) ->
+    undefined.

src/state_t.erl

@@ -14,10 +14,11 @@
 %% Copyright (c) 2011-2013 VMware, Inc.  All rights reserved.
 %%
 
+-compile({parse_transform, pmod_pt}).
 -module(state_t, [InnerMonad]).
 -compile({parse_transform, do}).
 
--behaviour(monad).
+-behaviour(monad_t).
 -export(['>>='/2, return/1, fail/1]).
 -export([get/0, put/1, eval/2, exec/2, run/2,
          modify/1, modify_and_return/1, lift/1]).