A MOP for Perl 5.16

In the discussion of A Design for Class Attributes in Perl 5.16 or Perl 5.18 (along with other discussions such as Modern Perl: the book), a meta-object protocol is, effectively, a design for an object system that it itself object oriented.

In other words, you have an object which represents the class of an object. To create a new class, you might write something like:

my $class = MetaClass->new( name => 'My::Class' );

You can add methods by calling add_method() and add attributes by calling add_attribute(). This is, in fact, what Class::MOP does in Perl 5, and what Parrot provides (not well, at the moment) to Rakudo Perl 6 and other hosted languages, and what a new MOP built for Rakudo Perl 6 does very well.

A good MOP has at least two important properties. First, it's a consistent and coherent underlying mechanism on which to build language features. Second, it's flexible and extensible.

The latter is even more important than it sounds at first.

Moose encourages the use of well-encapsulated, opaque objects by default, while blessed hash references might as well just be hash references, as anyone can poke and prod in them anytime. (Certainly you can do the same with Moose objects, but the documentation and design of Moose discourage you from even knowing that such a thing is possible.)

If the language syntax (and I've not mentioned them before now) allow you to specify a different metaclass—something other than Class—you can create objects with different behaviors and different storage of methods and attributes and other data trivially.

If the rest of the language, from the internals to all other extension mechanisms, interoperate in terms of the protocol of the MOP, these objects and classes and metaclasses will all behave and interoperate effectively.

In other words, if I were to propose a blue sky feature for Perl 5.16 without worrying about development resources or testing or feasibility of implementation or likelihood of merging into bleadperl, I would say this:

Beg, borrow, and steal a good MOP—perhaps the MOP from Rakudo Perl 6—and reimplement default Perl 5 OO in terms of that MOP.

Then let us discuss adding a class keyword and a method keyword and syntax for declaring attributes. Let's solve the biggest problem first: there's no reason to have multiple object systems in the core, not if they're slightly incompatible. As a bonus, we get much better introspection, dynamic programming, and reflection for free.

(If you're not sure how reimplementing blessed hashes in terms of a MOP makes things easier, consider this: anything which has the same duck-typed or interface-checked or, as is my wont, role-based design nature as the DefaultClass metaclass will be substitutable in any place you'd normally use DefaultClass, whether you use a MooseClass metaclass or a XSFriendlyClass metaclass or a SerializableClass metaclass or a ThreadSafeClass metaclass or what have you. The possibilities are... powerful.)