From: Phil Stone <lists@...>
Date: 2012-04-17T06:54:28+09:00
Subject: Re: A Beginner's Question about Metaprogramming

Thanks for all the help, but there are some more question that I have:

1. Why is it necessary to use a Singleton class in the above example at 
all?

Here's the code where it is used:

    arr.each do |a|
        metaclass.instance_eval do
            define_method( a ) do |val|
               @traits ||= {}
               @traits[a] = val
            end
        end
     end

(I know this code is outdated, I'm just trying to understand the 
concepts.)
Why are the methods defined onto the Singleton class? I would rather 
think that the following is correct, but for some reason it doesn't 
work:

    arr.each do |a|
        class_eval do
            define_method( a ) ...

For what the code is supposed to do, this seems like the right 
function... but it does not work. Why? Why was a metaclass necessary?

2.. I still don't grasp exactly what class << self means. I know that 
class << ClassName would add on to the class of ClassName. Please 
explain.
Thanks.

Lastly, for anybody else who is struggling with Ruby metaprogramming, I 
found this article to be very useful:
http://rubysource.com/ruby-metaprogramming-part-i/

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