From: Daniel Waite <rabbitblue@...>
Date: 2007-10-26T04:28:43+09:00
Subject: Re: Need help understanding metaclassing

Rick Denatale wrote:
> class Foo
>       metaclass #=> the singleton class of Foo.
>       metaclass.instance_eval do
>              # Inside this block self is bound to the singleton class of 
> foo
>              define_method(:bar) {"bar"}
>       end
> end
> 
> This has the same effect as
> class Foo
>      def self.bar
>          "bar"
>      end
> end
> 
> I.e. it defines a class method of Foo.
> 
> Now I'd usually use class_eval, or module_eval which do the same thing
> but only work if the receiver is a class/module.

Hold on, now I'm confused. Given this...

class Creature

  def singleton
    class << self
      self
    end
  end

end

c = Creature.new
cs = c.singleton

cs.instance_eval do
  define_method(:eat) do
    puts "You eat."
  end
end

cs.eat # => errors:
NoMethodError: undefined method `eat' for 
#<Class:#<Creature:0xb7f78ae8>>

Why is that? I've tried class_eval, too and I get the same thing. I 
thought I understood this stuff pretty well but I guess not.
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