From: Florian Frank <flori@...>
Date: 2009-11-20T01:46:50+09:00
Subject: Re: Any official name for Ruby's class which makes "class me

Rick DeNatale wrote:
>> Actually, I think it was Jim Weirich who pointed out that, given the
>> usual sense of the term "metaclass" as a class which produces other
>> classes, Class itself is the only metaclass in Ruby. The term is
>> sometimes used to refer to singleton classes in general, and sometimes
>> to refer only to singleton classes of Class objects:
>>     
And I dislike this usage. The whole problem seems to stem from a general 
confusion of the meaning of the term meta programming (where 
eigen/singleton/virtual classes ofen play a role in Ruby) with the fact, 
that Matz couldn't yet decide how to call these thingies. But some day, 
eventually...
> I usually don't disagree with my younger friend Jim, but...
>
> In general, although in some languages the implementation of
> metaclasses is involved with creating other classes, the more general
> meaning is that a metaclass is a class whose instance(s) are classes.
>   
Do you have found any other examples of these classes whose instances 
are classes besides Class? I haven't. If you try to create instances 
like this

 >> MyClass = Class.new(Class)
 # => MyClass
 >> Foo = MyClass.new do def foo() end end
TypeError: wrong instance allocation
    from (irb):2:in `new'
    from (irb):2

or even this

 >> MyClass = Class.dup
 # => MyClass
 >> Foo = MyClass.new do def foo() end end
TypeError: wrong instance allocation
    from (irb):4:in `new'
    from (irb):4

Ruby does a hard type check and throws an exception. This is where you 
have broken through Ruby's thin layer of abstraction around its 
C-internals, while Smalltalk is much more turtles all the way down.

This is different from Module, BTW:

 >> MyModule = Class.new(Module)
 # => MyModule
 >> Foo = MyModule.new do def bar() end end
 # => Foo
 >> class A; include Foo ; end
 # => A
 >> A.new.bar
 # => nil

So you could say, Ruby has one metaclass, but can have lots of metamodules.

-- 
Florian Frank