Perl 6 and Parrot Essentials, Second Edition

Submethods

A submethod is declared like a method, but behaves more like a sub in that it’s not inherited but can be pulled in with a role. Submethods are useful for inheriting interface without inheriting implementation—you can override a method from a parent class without inflicting the changes on any child classes.

The Frogstar::A class defines a set of methods and attributes for the Frogstar Scout robots:

class Frogstar::A {
    has Laser::Beam     $.beam;
    has Antimatter::Ray $.ray;
    has Electron::Ram   $.ram;
     . . . 
    method zap   ($target) { $.beam.fire($target); }
    method shoot ($target) { $.ray.fire($target);  }
    method smash ($target) { $.ram.fire($target);  }
     . . . 
}

In addition to methods for navigation, propulsion, etc., the Frogstar Scouts boast an astounding number of destructive methods (as is to be expected). The Frogstar::B class inherits all of Frogstar::A’s methods and attributes, and also adds its own additional weaponry. Instead of defining a new method for the Zenon Emitter, the Frogstar Scout B overrides the smash method to use the emitter:

class Frogstar::B is Frogstar::A {
    has Zenon::Emitter $.emitter;
    submethod smash ($target) { $.emitter.fire($target); }
}

It still smashes the target, only faster, as you might expect from a newer model. Since the overridden method was declared as a submethod, it has no effect on the Frogstar Scout C, which inherits from Frogstar::B:

class Frogstar::C is Frogstar::B { . . . } . . . $fighter = Frogstar::C.new( . . . ); $fighter.smash($floor); # ...