From: William Rutiser <wruyahoo05@...>
Date: 2010-12-06T23:35:45+09:00
Subject: Re: Singleton class, metaclass, eigenclass: what do they mean?

Rick DeNatale wrote:

[snip]
> As an aside, which is probably more apropos the title of this thread.
> I don't personally consider singleton_class, eigenclass, and metaclass
> to be equivalent terms.
>
> In particular I reserved the term metaclass to be the 'singleton'
> class of a class object.  I put singleton in quotes here because,
> although MRI uses the singleton class mechanism to implement
> metaclasses, metaclasses are 'less' singleton when you consider that
> 'singleton' methods which are found in the mtab of the object's
> singleton class, are not shared by any other object, class methods
> which found in the mtab of the metaclass ARE shared in the case where
> the class in question has one or more subclasses.
>
> The etymology of the word metaclass is similar to that of metalanguage
>
> meta- (also met- before a vowel or h)
> combining form
> 1 denoting a change of position or condition : metamorphosis | metathesis.
> 2 denoting position behind, after, or beyond: : metacarpus.
> 3 denoting something of a higher or second-order kind : metalanguage | metonym.
>
> i.e. the third definition of meta here.
>
> and
>
> metalanguage |��met����la ng (g)wij|
> noun
> a form of language or set of terms used for the description or
> analysis of another language. Compare with object language (sense 1).
> ��� Logic a system of propositions about propositions.
>
> So as I define it (and as is normal in most OO languages) a metaclass
> is a form of class used to describe a class.  A singleton class used
> to describe singleton methods of an object is NOT a metaclass but
> rather a singleton CLASS for that object, which inherits from the
> objects original class, and is hidden from Ruby's reflection methods
> because that 'virtual'  bit is set.
>
> IMHO, _why sowed a seed of confusion when he used metaclass in a way
> which conflicts with the common definition, in the poignant guide.
>
>
>   
>> * Note 1: If I understand correctly, for an object that is not
>>  a class, the singleton_class is not prepared a priori, but
>>  is generated at the time when it is called.
>>     
>
> This is true, although it's really an implementation detail.  I don't
> think you can detect this from Ruby without the aid of a C extension
> to let you look for a singleton class without having one created
> automatically as soon as you ask for it with, say class <<
> self;self;end
>
> I suppose I should really turn this into a blog article.
>
>   
Is the word "singleton" used widely or at all in this sense in 
connection with languages other than Ruby? I don't recall encountering 
this use except for Ruby.

Do I understand that the non-existence of a singleton_class until needed 
is just an implementation alternative to always having a possibly empty 
singleton_class?

I am looking forward to a blog article.