Roles: Composable Units of Behavior

• Traits are a simple composition mechanism for structuring object-oriented programs. A Trait is essentially a parameterized set of methods; it serves as a behavioral building block for classes and is the primitive unit of code reuse. With Traits, classes are still organized in a single inheritance hierarchy, but they can make use of Traits to specify the incremental difference in behavior with respect to their superclasses.
• http://www.iam.unibe.ch/~scg/Research/Traits/
• Perl 6 and Moose refer to traits as roles