Does Ruby have a Metaobject protocol and if not, is it possible to implement one?

Question

Pardon my ignorance, but What is a Metaobject protocol, and does Ruby have one? If not, is it possible to implement one for Ruby? What features might a Metaobject protocol possess if Ruby was to have one?

Answer 1

What is a Metaobject protocol?

The best description I've come across is from the Class::MOP documentation:

A meta object protocol is an API to an object system.

To be more specific, it abstracts the components of an object system (classes, object, methods, object attributes, etc.). These abstractions can then be used to inspect and manipulate the object system which they describe.

It can be said that there are two MOPs for any object system; the implicit MOP and the explicit MOP. The implicit MOP handles things like method dispatch or inheritance, which happen automatically as part of how the object system works. The explicit MOP typically handles the introspection/reflection features of the object system.

All object systems have implicit MOPs. Without one, they would not work. Explicit MOPs are much less common, and depending on the language can vary from restrictive (Reflection in Java or C#) to wide open (CLOS is a perfect example).

Does Ruby have one?

According to this thread on Reopening builtin classes, redefining builtin functions? Perlmonks article I think the answer is no (at least in the strictest sense of what a MOP is).

Clearly there is some wriggle room here so it might be worth posting a question in the Perl side of SO because the Class::MOP / Moose author does answer questions there.

Answer 2

If you look closer to the definition, youll see that Ruby does have a MOP. Is it like the one in CLOS? No, CLOS is a meta-circular MOP which is great (I'd even say genius), but it's not the one true way, take a look at Smalltalk. To implement a (let's say basic) MOP all you need is to provide functions that allow your runtime to:

1. Create or delete a new class
2. Create a new property or method
3. Cause a class to inherit from a different class ("change the class structure")
4. Generate or change the code defining the methods of a class.

And Ruby provides a way to do all that.

On a side note: The author of Class::MOP is right (IMHO) when it claims that some of the things you can do with a meta circular MOP can be hard to do in Ruby (DISCLAIMER: I have zero, zilch, nada Perl knowledge, so I'm thinking Smalltalk like MOP vs CLOS like MOP here) but most of them are very specific (I'm thinking about metaclass instantation here) and there are ways to make things work without them. I think it all depends on your point view, meta circular MOPs are cooler but more on the academic side and non meta circular MOPs are more practical and easier to implement.