From: "David A. Black" <dblack@...>
Date: 2009-11-17T20:30:36+09:00
Subject: Re: Any official name for Ruby's class which makes "class me

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Hi --

On Mon, 16 Nov 2009, Marnen Laibow-Koser wrote:

> Hunt Jon wrote:
>> We all know Ruby really doesn't have class methods.
>>
>> But how do you call such classes that have "class methods"?
>> I hear Japanese say 特異 class, but is there any English word for that?
>
> You mean instances of class Class?  The official docs call them
> metaclasses.

I don't think that's right. String and Array and

   class MyClass

and Class.new are all instances of Class, but they're not metaclasses.

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:

   s = ""
   class << s; self; end   # s's singleton class
   class << String; self; end  # String's singleton class or metaclass

This usage (using it for classes' singleton classes) has some
justification because classes' s. classes aren't quite the same as
those of other objects (they allow for method-sharing by subclasses,
so they're not strictly "per-object" definitions). I myself usually
stick to "singleton class" all around, partly just to do my bit to
stop the name-proliferation madness :-) I don't see anything "meta"
about singleton classes in general, so I don't call them metaclasses.

>> Is eigenclass an official name for that?

No (at least, I hope not :-) Matz had expressed uncertainty about the
naming scheme, and a bunch of more or less whimsical names were
suggested, some of which seem to have a certain following. The
unfortunate effect is that it's all a bit unclear.

I recommend "singleton class", plus "metaclass" for classes' singleton
classes if you feel strongly about their special status. I'll add that
in nine years of using Ruby, I have yet to see more than perhaps one
or two cases of confusion between singleton classes and
classes that include the Singleton module (totalling about four
seconds and cleared up with: "No, that's different"), though that
coincidence of names is often adduced as evidence that "singleton
class" must be a confusing name.


David

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