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Michael G Schwern
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mixin::with - declaring a mix-in class


    package Dog::Retriever;
    use mixin::with 'Dog';


mixin::with is used to declare mix-in classes.

When to use a mixin?

Mixin classes useful for those that add new functionality to an existing class. If you find yourself doing:

    package Foo::ExtraStuff;
    use base 'Foo';
    sub new_method { ... }

    package Bar;
    use base qw(Foo Foo::ExtraStuff);

it's a good indication that Foo::ExtraStuff might do better as a mixin.

Instead of mixins, please consider using traits. See Class::Trait for an implementaiton.


Basic usage is simple:

    package Foo::Extra;
    use mixin::with 'Foo';

    sub new_thing {
        my($self) = shift;
        ...normal method...

use mixin::with 'Foo' is similar to subclassing from 'Foo'.

All public methods of Foo::Extra will be mixed in. mixin::with considers all methods that don't start with an '_' as public.

Limitations of mixins

There's one critical difference between a normal subclass and one intended to be mixin. It can have no private methods. Instead, use lexical methods.

  my $private = sub { ... };

instead of

  sub _private { ... }

Don't worry, it's the same thing.


What if I want to mixin with anything?

Sometimes a mixin does not care what it mixes in with. Consider a logging or error handling mixin. For these, simply mixin with UNIVERSAL.

    package My::Errors;
    use mixin::with qw(UNIVERSAL);
Why do I have to declare what I mixin with?

Two reasons. One is technical, it allows SUPER to work.

The other is organizational. It is rare that a mixin is intended to be mixed with any old class. It often uses methods as if it were a subclass. For this reason it is good that it declares this relationship explicitly else the mixee won't be aware of the mixin's expectations.

Why use mixins instead of traits?

Good question. Traits are definately a better idea then mixins, but mixins have two advantages. They're simpler to explain, acting like a gateway drug to traits by introducing the concept of OO reuse by class composition rather than inheritance.

The other is mixins work more like a drop-in replacement for multiple inheritance. In a large, hairy hierarchy mixins can often be used to trim the inheritance bush and make sense of things with a minimum of modification to the code. Once this basic repair is done, the work of converting to traits can begin.

If these advantages don't apply, proceed directly to traits.


Michael G Schwern <schwern@pobox.com>


Copyright 2002-2010 by Michael G Schwern

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



mixin, ruby from which I stole this idea.