From: Gene Tani <gene.tani@...>
Date: 2005-10-03T22:26:47+09:00
Subject: Re: Cannot subclass Class?

Alex martelli's talk on python metaclasses, if you're curious, they're
really not too complicated:

http://www.python.org/pycon/2005/papers/36/pyc05_bla_dp.pdf

Good reminder about meta-slippage.  I've been trying to wrap my brain
around all the meta's in Jim Weirich's picture:

http://onestepback.org/images/rubyobjects.png

David A. Black wrote:
> Hi --
>
> On Mon, 3 Oct 2005, Rob Rypka wrote:
>
> > On 10/2/05, itsme213 <itsme213@hotmail.com> wrote:
> >> irb(main):006:0> class MC < Class
> >> irb(main):007:1> end
> >> TypeError: can't make subclass of Class
> >>
> >> Why is this? Is it a language restriction or a 1.8.2 limitation?
> >>
> >> Are there other ways to define new meta-classes?
> >
> > I was trying to figure out exactly what metaclasses allow you to do,
> > and I came across the following quote:
> >
> > Metaclasses are deeper magic than 99% of users should ever worry
> > about. If you wonder whether you need them, you don't (the people who
> > actually need them know with certainty that they need them, and don't
> > need an explanation about why).
> > -- Python Guru Tim Peters
> >
> > If you've got an example of something you want to do with metaclasses
> > in Ruby, post it here.  I'll bet you get a lot of suggestions that are
> > less "brain-melting," as the Python folks like to say.
>
> I don't know what the term refers to in Python, but I do know that in
> Ruby there's been a lot of terminological slippage over the years.
> Some people use "metaclass" to refer to all singleton classes.  Others
> use it to refer to the singleton classes of class objects.  Other have
> argued that the only real metaclass in Ruby is the class Class (if you
> define a metaclass as a class that produces classes).
>
> Singleton classes in general are definitely not deep magic.  They're
> just the way Ruby handles requests for per-object method definitions:
> stick them in a per-object class.  They're not particularly meta;
> they're just anonymous classes.
>
> Singleton classes of class objects are probably the most frequently
> made use of, since that's where class methods live.  I think you see
> this:
>
>    class C
>      class << self
>
> more often than class << obj where obj is not a class.
>
> And class Class is just class Class :-)
>
> So the question of what "metaclasses" allow you to do depends a bit on
> what you mean exactly, but I don't think there's much mystery or
> reason for people not to try to understand them.
> 
> 
> David
> 
> -- 
> David A. Black
> dblack@wobblini.net