NAME

mixin - Mix-in inheritance, an alternative to multiple inheritance

SYNOPSIS

  package Dog;
  sub speak { print "Bark!\n" }
  sub new { my $class = shift;  bless {}, $class }

  package Dog::Small;
  use base 'Dog';
  sub speak { print "Yip!\n"; }

  package Dog::Retriever;
  use mixin::with 'Dog';
  sub fetch { print "Get your own stinking $_[1]\n" }

  package Dog::Small::Retriever;
  use base 'Dog::Small';
  use mixin 'Dog::Retriever';

  my $small_retriever = Dog::Small::Retriever->new;
  $small_retriever->speak;          # Yip!
  $small_retriever->fetch('ball');  # Get your own stinking ball

DESCRIPTION

NOTE You probably want to look into the similar but superior concept of traits/roles instead. See "SEE ALSO" for suggested modules.

Mixin inheritance is an alternative to the usual multiple-inheritance and solves the problem of knowing which parent will be called. It also solves a number of tricky problems like diamond inheritence.

The idea is to solve the same sets of problems which MI solves without the problems of MI. For all practical purposes you can think of a mixin as multiple inheritance without the actual inheritance.

Mixins are a band-aid for the problems of MI. A better solution is to use traits (called "Roles" in Perl 6), which are like mixins on steroids. Class::Trait implements this.

Using a mixin class

There are two steps to using a mixin-class.

First, make sure you are inherited from the class with which the mixin-class is to be mixed.

  package Dog::Small::Retriever;
  use base 'Dog::Small';

Since Dog::Small isa Dog, that does it. Then simply mixin the new functionality

  use mixin 'Dog::Retriever';

and now you can use fetch().

Writing a mixin class

See mixin::with.

Mixins, Inheritance and SUPER

A class which uses a mixin does not inherit from it. However, through some clever trickery, "SUPER" continues to work. Here's an example.

    {
        package Parent;
        sub foo { "Parent" }
    }

    {
        package Middle;
        use mixin::with "Parent";

        sub foo {
            my $self = shift;
            return $self->SUPER::foo(), "Middle";
        }
    }

    {
        package Child;
        use base "Parent";
        use mixin "Middle";

        sub foo {
            my $self = shift;
            return $self->SUPER::foo(), "Child";
        }
    }

    print join " ", Child->foo;  # Parent Middle Child

This will print "Parent Middle Child". You'll note that this is the same result if Child inherited from Middle and Middle from Parent. Its also the same result if Child multiply inherited from Middle and Parent but NOT if it inherited from Parent then Middle. The advantage of mixins vs multiple inheritance is such ambiguities do not exist.

Note that even though both the Child and Middle define foo() the Middle mixin does not overwrite Child's foo(). A mixin does not simply export its methods into the mixer and thus does not blow over existing methods.

NOTES

A mixin will not warn if the mixin and the user define the same method.

AUTHOR

Michael G Schwern <schwern@pobox.com>

LICENSE

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

<http://dev.perl.org/licenses/>