From: John Mair <jrmair@...>
Date: 2010-10-07T08:33:35+09:00
Subject: Why does Module#include exclude the module's metaclass?

When classes are inherited in Ruby the singleton classes are also
inherited:

class A
  def self.hello
    puts "hello"
  end
end

class B < A
end

B.hello #=> "hello"
Yet with modules, this is not the case:

module M
  def self.goodbye
    puts "goodbye"
  end
end

class A
  include M
end

A.goodbye #=> NameError
To get around this limitation many ppl resort to this ugly hack:

module M
  def self.included(c)
    c.extend ClassMethods
  end

  module ClassMethods
    def goodbye
      puts "goodbye"
    end
  end
end
Ok, so my question: is there a theoretical/conceptual reason behind this
limitation on modules? or was it just an implementation difficulty?

After looking at the C source code (YARV/MRI) I can identify there is an
implementation difficulty (not insurmountable, but one all the same),
but is this the sole reason? Are there any other reasons for this
limitation?

thanks
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