From: James Coglan <jcoglan@...>
Date: 2008-06-20T07:27:26+09:00
Subject: Re: Why metaclasses?

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> 13:32:55 OPSC_Gold_bas_dev_R1.2.2$ irb
> Ruby version 1.8.6
> irb(main):001:0> x = Class.new
> => #<Class:0x7ff9ece4>
> irb(main):002:0> o = x.new
> => #<#<Class:0x7ff9ece4>:0x7ff9b56c>
> irb(main):003:0> o.class
> => #<Class:0x7ff9ece4>
> irb(main):004:0> o.class == x
> => true
> irb(main):005:0> x === o
> => true
> irb(main):006:0> y = Class.new x
> => #<Class:0x7ff81dc4>
> irb(main):007:0> y.superclass
> => #<Class:0x7ff9ece4>
> irb(main):008:0> y.superclass == x
> => true
> irb(main):009:0> u = y.new
> => #<#<Class:0x7ff81dc4>:0x7ff77518>
> irb(main):010:0> x === u
> => true
> irb(main):011:0> y === u
> => true
>
> irb(main):012:0> m = Module.new
> => #<Module:0x7ff6e058>
> irb(main):013:0> y.class_eval { include m }
> => #<Class:0x7ff81dc4>
> irb(main):014:0> m === u
> => true
> irb(main):015:0> y.ancestors
> => [#<Class:0x7ff81dc4>, #<Module:0x7ff6e058>, #<Class:0x7ff9ece4>,
> Object, Kernel]
> irb(main):016:0>
>

So x is a class, y is a class that inherits from x (which is to say, y
'includes' x as its first inclusion). o is an x, u is a y (and is therefore
an x). Fine. y includes m, so now u is an m as well. y's ancestors are
itself, m, x, Object and Kernel. Kernel is the root module, Object is the
root class (and therefore includes Kernel). A module's (and therefore a
class's) ancestors are itself and its included modules (and *their* included
modules, depth first). For a class, this means its included modules, its
superclass, its superclass's included modules, and so on.

So far I can't see how this is inconsistent with using metamodules instead
of metaclasses, and with class inheritance being a special case of module
inclusion. With apologies for my apparent slow-wittedness, what am I
missing?

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