The Perl Toolchain Summit needs more sponsors. If your company depends on Perl, please support this very important event.
 / 
Zydeco-0.511
River stage one • 3 direct dependents • 4 total dependents
21 ++
/ Zydeco::Manual::01_ClassesRolesEtc

NAME

Zydeco::Manual::01_ClassesRolesEtc - Classes, roles, and abstract classes

SYNOPSIS

  package Farming {
    use Zydeco;
    
    class Animal {
      has name;
    }
    
    role Milkable {
      method milk {
        say "giving milk...";
      }
    }
    
    class Cow extends Animal with Milkable;
  }
  
  my $daisy = Farming->new_cow(name => "Daisy");
  $daisy->milk();

MANUAL

The object-oriented programming paradigm, is based on the concept of "objects" which are structures representing real-world things or abstract ideas, containing data (a.k.a. attributes) and the code (a.k.a. methods) needed to operate on this data.

Classes

Most object-oriented programming languages, including Perl, are class-based. Each object is a member of a class, and it is the class where these attributes and methods are defined.

In Zydeco, you can create a class with the class keyword. The simplest class definition consists of class followed by a name for the class:

  class Bucket;

It is possible to create a class without specifying a name, in which case Zydeco will think of a name and return it. However, as the class keyword is always a complete statement, you cannot do:

  my $Bucket = class;

Instead you must wrap it in a do block:

  my $Bucket = do { class; };

This is called an anonymous class.

Naming

Zydeco prefixes your class names with the package name.

  package MyApp {
    use Zydeco;
    class Foo;          # class name is "MyApp::Foo"
    class Foo::Bar;     # class name is "MyApp::Foo::Bar"
  }

Prefixing a class name with "::" avoids this.

  package MyApp {
    use Zydeco;
    class ::Foo;        # class name is "Foo"
    class ::Foo::Bar;   # class name is "Foo::Bar"
  }

Objects

When you've created a class, the next thing to do is create objects of that class. The class "Bucket" represents the concept of buckets, but objects of that class represent individual buckets.

For named classes in Perl, the typical way to create an object of class "Bucket" would be:

  my $red_bucket = Bucket->new;

Zydeco does things a little differently though. Zydeco defines classes within "factory packages". Rather than asking the "Bucket" class for a new bucket object, you ask your factory package for a new bucket object.

  package Farming {
    use Zydeco;
    class Bucket;
  }
  
  my $red_bucket = Farming->new_bucket;

For anonymous classes, the factory does not know how to make them, so to instantiate them ("instantiate" means "create an instance of"), you call new on the class directly.

  my $Bucket      = do { class; };
  my $red_bucket  = $Bucket->new;

You can check to see if an object is a Bucket using the isa method.

  if ( $object->isa("Farming::Bucket") ) {
    ...;
  }

But notice this hard-codes the name of your package ("Farming"). We don't like hard-coding stuff, so a better way is to make use of the "Farming::Types" type library. While you define the "Farming" factory package, Zydeco will be at work in the background, defining "Farming::Types".

  use Farming;
  use Farming::Types -is;
  
  if ( is_Bucket $object ) {
    ...;
  }

With anonymous classes, you need to stick with isa though.

  if ( $object->isa($Bucket) ) {
    ...;
  }

Roles

A role is a way to bundle up a collection of behaviours that can then be imported (a.k.a. consumed) by a class. Goats, sheep, and cows are all milkable, so in our farming example, we might want to define the behaviour for a milkable animal in a "Milkable" role which the "Cow", "Goat", and "Sheep" classes can all consume.

Roles are defined with the role keyword which mostly uses the same syntax as the class keyword.

  package Farming {
    use Zydeco;
    role Milkable;
    
    class Cow with Milkable;
    class Goat with Milkable;
    class Sheep with Milkable;
  }

Roles cannot be instantiated like classes can. You can create a new "Cow" object which will be "Milkable", but you can't directly create a "Milkable" object.

Anonymous roles are possible.

  my $Milkable = do { role; };

Consuming anonymous roles is a little tricky.

  class Cow {
    with {"::$Milkable"};
  }

The braces mean that the role should be evaluated as a block. The leading "::" is required because you don't want the prefix to be added to the role name.

The equivalent of isa for roles is does.

  if ( $daisy->does("Farming::Milkable") ) {
    ...;
  }

But like with checking isa, it's desirable to avoid hard-coding that "Farming" everywhere. Better to use a type check.

  use Farming;
  use Farming::Types -is;
  
  if ( is_Milkable $daisy ) {
    ...;
  }

The type check functions look so much more elegant anyway.

If you have a role name as a string, you can consume it by putting it in braces.

Tag Roles

For some roles, there may not be any real "behaviour" associated with them. For example, piglets and rabbits are adorable.

  package Farming {
    class Piglet with Adorable?;
    class Rabbit with Adorable?;
  }

This just tags the "Rabbit" and "Piglet" classes with the "Adorable" role which can be checked using does or is_Adorable, without us having to declare the "Adorable" role explicitly. It's just a shortcut for:

  package Farming {
    role Adorable;
    class Piglet with Adorable;
    class Rabbit with Adorable;
  }

Inheritance

Inheritance allows you make one class into a subclass of another.

For example, all piglets are pigs, so if we have an existing "Pig" class, we can say that "Piglet" is a subclass of that.

  package Farming {
    class Pig;
    class Piglet extends Pig;
  }

Objects of class Piglet are also objects of class Pig.

  use Farming;
  use Farming::Types -is;
  
  my $oinker = Farming->new_piglet;
  
  is_Piglet($oinker);     # ==> true
  is_Pig($oinker);        # ==> true

Any behaviours defined by the "Pig" class will be inherited by the Piglet class, but the "Piglet" class may define its own unique behaviours too, or override behaviour from the parent class.

It is possible to both inherit and consume at the same time:

  class Piglet extends Pig with Adorable?;

Extending anonymous classes is much like consuming anonymous roles.

  class Piglet {
    extends { "::" . "Pig" };
  }

Abstract Classes

It it possible to mark a class as abstract. An abstract class is one that, like a role, cannot be instantiated, but can be inherited from.

For example, it may be desirable to have an abstract class "Animal" for our farmyward, which all other animals inherit from, but disallow creating "Animal" objects directly. We do this using the abstract keyword, which is just a prefix for class.

  package Farming {
    use Zydeco;
    role Milkable;
    abstract class Animal;
    class Horse extends Animal;
    class Cow extends Animal with Milkable;
    class Pig extends Animal;
    class Piglet extends Pig with Adorable?;
    class Sheep extends Animal with Milkable;
    class Goat extends Animal with Milkable;
    class Rabbit extends Animal with Adorable?
  }

Nesting Classes

You might notice this getting a little repetitive. Zydeco allows you to nest subclass definitions within their parent class as a shortcut.

  package Farming {
    use Zydeco;
    
    role Milkable;
    
    abstract class Animal {
      class Horse;
      class Cow with Milkable;
      class Pig {
        class Piglet with Adorable?;
      }
      class Sheep with Milkable;
      class Goat with Milkable;
      class Rabbit with Adorable?
    }
  }

Plus-Sign Prefix

If you prefix a class name with a plus sign, the class name will be prefixed with its parent class.

  package MyApp {
    class Foo {          # class name is "MyApp::Foo"
      class +Bar {       # class name is "MyApp::Foo::Bar"
        class +Baz::Bat; # class name is "MyApp::Foo::Bar::Baz::Bat"
      }
    }
  }

This even works with extends.

  package MyApp {
    class Foo;               # class name is "MyApp::Foo"
    class +Bar extends Foo;  # class name is "MyApp::Foo::Bar"
  }

KEYWORDS

In this chapter, we looked at the following keywords:

class
role
with
extends
abstract class

TODO

This page should probably detail version, authority, and toolkit.

NEXT STEPS

The structure and behaviour of objects is defined by classes. In the next chapter we will see how classes can define the structure of their objects.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2020 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.