Karen Etheridge > Moose-2.0802 > Moose::Cookbook::Meta::WhyMeta


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Moose::Cookbook::Meta::WhyMeta - Welcome to the meta world (Why Go Meta?)


version 2.0802


You might want to read Moose::Manual::MOP if you haven't done so yet.

If you've ever thought "Moose is great, but I wish it did X differently", then you've gone meta. The meta recipes demonstrate how to change and extend the way Moose works by extending and overriding how the meta classes (Moose::Meta::Class, Moose::Meta::Attribute, etc) work.

The metaclass API is a set of classes that describe classes, roles, attributes, etc. The metaclass API lets you ask questions about a class, like "what attributes does it have?", or "what roles does the class do?"

The metaclass system also lets you make changes to a class, for example by adding new methods or attributes.

The interface presented by Moose.pm (has, with, extends) is just a thin layer of syntactic sugar over the underlying metaclass system.

By extending and changing how this metaclass system works, you can create your own Moose variant.


Let's say that you want to add additional properties to attributes. Specifically, we want to add a "label" property to each attribute, so we can write My::Class->meta()->get_attribute('size')->label(). The first recipe shows how to do this using an attribute trait.

You might also want to add additional properties to your metaclass. For example, if you were writing an ORM based on Moose, you could associate a table name with each class via the class's metaclass object, letting you write My::Class->meta()->table_name().


Many of the MooseX modules on CPAN implement metaclass extensions. A couple good examples include MooseX::Aliases and MooseX::UndefTolerant. For a more complex example see Fey::ORM or Bread::Board::Declare.






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