From: Greg Weeks <greg.weeks@...>
Date: 2007-11-16T18:20:16+09:00
Subject: meta-class subclass relationships

Ruby exposes its singleton meta-classes, eg:

    class <<B ; $Meta_B = self ; end

But their relationships are not quite what I expected.  For example:

Suppose that A inherits from B (and B inherits from Object).  Of course,
this means that A instances respond to all the B instance messages.  So:

    A.new.is_a? B  ->  true

At the same time, the A class object responds to all the B class object
messages.  So:

    class <<Object ; $Meta_Object = self ; end
    class <<B ; $Meta_B = self ; end
    class <<A ; $Meta_A = self ; end

    A.is_a? $Meta_A  ->  true
    B.is_a? $Meta_A  ->  false
    A.is_a? $Meta_B  ->  true
    B.is_a? $Meta_B  ->  true

Since everything that is_a $Meta_A also is_a $Meta_B, I expected a
subclass/superclass relationship.  Indeed, I expected the following to
be true:

    $Meta_A.superclass == $Meta_B
    $Meta_B.superclass == $Meta_Object
    $Meta_Object.superclass == Class
    Class.superclass == Module
    Module.superclass == Object
    Object.superclass == nil

The above superclass chain reflects how a message to the class A object
is looked up.  What surprised me is that the first two equalities above
are false.

Should they have been true?

FWIW: The superclass of both $Meta_A and $Meta_B is something called
#<Class:Class>, which is its own superclass and is a subclass of
$Meta_Object:

    $X = $Meta_A.superclass  ->  #<Class:Class>
    $X == $Meta_B.superclass  ->  true
    $X == $X.superclass  ->  true
    $X < $Meta_Object  ->  true
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