Perl 6 and Parrot Essentials, Second Edition

Roles

A role is a reusable unit of class code. Much like a module exports subroutines into your program or another module, a role exports methods and attributes into a class. If your first thought on reading this is “Isn’t that just inheritance?”, then welcome to a whole new world. Inheritance is one way to reuse code, but many relationships other than isa are possible. Various languages pick an alternative and provide syntax for it: Ruby has mixins , Java has interfaces , and some versions of Smalltalk have traits . Perl roles go a bit beyond all of them.

You define a role using the role keyword:

role Hitchhiker { . . . }

You pull a role into a class using the does keyword:

class Arthur does Hitchhiker { . . . }

Roles cannot instantiate objects directly. To create an object that makes use of a role, you make a new object from a class that uses that role:

$person = Arthur.new( . . . );

Composition

Like classes, roles can define both attributes and methods:

role Hitchhiker {
  has $.towel;
  method thumb_ride ($self: $ship) { . . . }
   . . . 
}

Unlike classes, when you pull a role’s methods and attributes into a class they aren’t maintained in an inheritance hierarchy to be searched later. Instead, they are composed into the class almost as if they had been defined in that class. All methods defined in the role are accessible in the composed class, even if they wouldn’t be inherited. All attributes defined in the role are accessible in the composed class by their direct $.name, not just by their ...