From: Rick DeNatale <rick.denatale@...>
Date: 2008-03-23T07:28:32+09:00
Subject: Re: #meta

On 3/22/08, Trans <transfire@gmail.com> wrote:

>  > I also side with Jim Weirich about the
>  > distinction between metaclass and singleton class or eigenclass or
>  > whatever you want to call them.
>
>
> That's Smalltalk not Ruby. I understand the distinction people want to
>  draw, but in Ruby they are the same construct.

I respectfully disgree.

Singleton classes in Ruby are an implementation construct which is
used for two distinct purposes:

1) To provide a place to put instance behavior.  For example

    a = "abc"
    def a.foo
         :foo
    end

2) To provide a class for a class, i.e. a metaclass.

One difference is that the first can't have subclasses, while
singleton classes used as metaclasses can and must since the
inheritance chain of metaclasses parallels the inheritance chain of
their instances.  The otehr is that metaclasses is the class of a
class, not an arbitrary object. For all practical purposes, the second
kind acts exactly as metaclasses do in Smalltalk, and Matz was quoted
as such in the 2nd ed of the Pickaxe.  Unfortunately many in the Ruby
community misunderstood that page as equating metaclass with singleton
class rather than seeing that although singleton classes are used to
implement metaclasses in Ruby, all singleton classes are NOT
metaclasses, a term which has a meaning even beyond Smalltalk of being
the class of a class.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/