foo.erl:

-module(foo).

-export([doit/0, doit/1, doit/2]).

doit() ->
    doit.

doit(A) ->
    [doit, A].

doit(A, B) ->
    [doit, A, B].

Module bar.erl which 'mixes in' foo:

-module(bar).
-include_lib("mixer/include/mixer.hrl").
-mixin([foo]).

or all except specific functions from foo:

-module(bar).
-include_lib("mixer/include/mixer.hrl").
-mixin([{foo, except, [doit/0, doit/2]}]).

or only specific functions from foo:

-module(bar).
-include_lib("mixer/include/mixer.hrl").
-mixin([{foo, [doit/0, doit/2]}]).

Another version of bar.erl which mixes in all functions from foo and select functions from baz:

-module(bar).
-include_lib("mixer/include/mixer.hrl").
-mixin([foo, {baz, [doit/0, doit/1]}]).

One more version of bar.erl which mixes in foo:doit/0 and renames it to do_it_now/0:

-module(bar).
-include_lib("mixer/include/mixer.hrl").
-mixin([{foo, [{doit/0, do_it_now}]}]).

The original motivation for this parse transform was to permit reuse of functions implementing common logic for tasks such as signature verification and authorization across multiple webmachine resources.