From: Rick DeNatale <rick.denatale@...>
Date: 2007-10-26T03:32:43+09:00
Subject: Re: Need help understanding metaclassing

On 10/25/07, Chris Czub <chris.czub@gmail.com> wrote:
> I've been reading lots of Ruby tutorials trying to figure out
> metaclassing - the one that got me closest was Why?s poignant guide to
> Ruby, but I'm still struggling over some of the Ruby-specific syntax.
>
> I attached a class I've been writing that needs to be highly extensible,
> and it's loaded with comments I wrote to try to understand each part of
> it. If any part of the comments are wrong, it would be nice to have my
> misconceptions cleared up. I've only been using Ruby for a day and, wow.
>
> Here's the part I'm having the hardest time figuring out, and Why?s
> guide kind of skipped over it:
>
>       metaclass.instance_eval do
>         define_method( a ) do |val|
>           @attribs ||= {}
>           @attribs[a] = val
>         end
>       end
>
> So, I understand that the block inside the instance_eval will be
> executed in the instance of the object(meaning when I create a new
> object, right?)

Wrong!

the block argument to instance_eval is evaluated right away, in the
context of metaclass.

The context is missing but I'm assuming that this is inside a class
definition, and metaclass has been defined as a method to return the
singleton class.  So lets say we had:

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.

> However, I don't understand the next part. define_method is a method
> call to the method that is defined further down, right? But what is the
> " do |val|" after the method call, and what is the point of the two
> lines after that?

David A. Black has already answered this.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/