[wschroed]

That's a delegation strategy, which is a very limited form of composition.

A class with a trait/role can be queried for that interface:

  smile() if $my_object->does('SomeTrait');

Methods provided by a trait/role have direct access to $self.

Also, Moose gives you the ability to apply traits to classes that are not predefined with those traits:

  use Moose::Util;
  my $instance = Moose::Util::with_traits('MooseClass', 'MyTrait')->new(%params);

You can parameterize roles (as in, pass parameters that affect what the role provides). If you put some method modifiers into your roles, you can enhance existing functionality; for example, you can add logging, memoization, and persistence, all without the class having those features beforehand.

In other words, roles/traits make your OOP functionality much more composable.