From: "David A. Black" <dblack@...>
Date: 2005-04-20T20:13:41+09:00
Subject: Re: [ANN] Article: Seeing Metaclasses Clearly

Hi --

On Wed, 20 Apr 2005, Jim Weirich wrote:

> It also makes the name "Singleton" class a little odd.  I thought it was
> because a singleton class could only have a single instance.  Now I find out
> it can't even have that.  Perhaps they should be Zeroton classes :)

I always thought it was singleton in the sense that, unlike the "birth
class", it was not shared with other objects.  There is of course a
long history of discussion about the term "singleton", and the fact
that it's overloaded....  I've suggested "own class", and others have
suggested other terms, though I don't know that even a pronouncement
from Matz will change usage completely.

>>> Indeed, Class is the Metaclass from which all the singleton metaclasses
>>> derive.
>>
>> And all other Class objects too.
>
> Only things (that would be called metaclasses if they could only have
> instances) are derived (i.e. inherit) from Class.  All other class objects
> are merely instances of Class.

But being a (mere) instance of Class is, I think, by far the closest
thing in Ruby to matching the proto-cookie-cutter thing.  (I know
that's just an analogy, but Class actually fits it rather well.)  Not
that every instance of Class is a metaclass, but rather that Class is
a metaclass, on account of the fact that it can create new classes
(meta or otherwise).

>> But then "[Mm]etaclass" becomes
>> really kind of overloaded -- in the sense that on the one hand it is a
>> suitable term for Class (which is truly the thing that stamps out the
>> cookie cutters themselves), while on the other hand it gets used as a
>> term for something which is unequivocally *not* in the cookie-cutter,
>> meta-cookie-cutter, or factory family at all.
>
> The formal definition does not require metaclasses (or classes) to create
> instances ... only that they /have/ instances (which is you so clearly
> demonstrated singleton classes have no instances).  The factory aspect was
> entirely part of the analogy and not part of the formal definition.

Like "singleton", though, this may be a case where Matz isn't using it
in a way that fits precisely with any previous formal definitions.  I
guess that's one of the hazards of writing a language with a cool,
uncommon design :-)


David

-- 
David A. Black
dblack@wobblini.net