Perl 6 and Parrot Essentials, 2nd Edition

Delegation

Delegation is yet another possible relationship between an object and another body of code. Rather than pull methods into a class, you call methods in another object as if they were methods of the class. In Perl 6, delegation can be done in either a class or a role. A delegated object is simply an attribute defined in the class or role. The handles keyword specifies which methods of the delegated object will act as methods of the class. This example declares that any calls to the thumb_ride method on an object with the Hitchhiker role, will actually call the method on its $.thumb attribute:

role Hitchhiker {
     . . . 
    has Electronic::Thumb $.thumb handles 'thumb_ride';
     . . . 
}

The handles keyword accepts many variations in the syntax to delegate methods. You can pass it an array reference of multiple method names:

has Electronic::Thumb $.thumb handles ['thumb_ride', 'sub_etha'];

or a quoted list:

has Electronic::Thumb $.thumb handles «thumb_ride sub_etha»;

A pair in place of a string method name gives the method a different name in the class. This example declares a method named hitch in the class, but any calls to it are delegated to the thumb_ride method on the $.thumb object:

has Electronic::Thumb $.thumb handles :hitch«thumb_ride»;

If the method name is given as a pattern, it’s a wildcard delegation and all methods that match that pattern will be delegated to the attribute. This example delegates all methods that start with “thumb” to $.thumb:

has Electronic::Thumb $.thumb ...