From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2009-09-18T02:01:32+09:00
Subject: Re: Mucking about with dynamically adding methods to objects

On Thu, Sep 17, 2009 at 5:28 PM, Paul Smith <paul@pollyandpaul.co.uk> wrote:
> 2009/9/17 Jesús Gabriel y Galán <jgabrielygalan@gmail.com>:
>> On Thu, Sep 17, 2009 at 11:44 AM, Paul Smith <paul@pollyandpaul.co.uk> wrote:

> I think I have a lot more reading to do before
>
> irb(main):014:2> (class << obj;self;end).instance_eval do
> irb(main):015:3* define_method :talk do
>
> makes any kind of sense.

David's article is a good starting point to understand singleton
classes and methods. Anyway,
(class << obj; self; end) is a very common idiom to access the
singleton class of an object.
Some people even do:

class Object
  def singleton_class
    class << self; self; end;
  end
end

Maybe with this addition, what I wrote is a little bit more clear:

obj.singleton_class.instance_eval do
  define_method :talk do
    puts str
  end
end

As we need to refer to str, we use define_method, which doesn't define
a new scope, so str is available in the block passed to define_method,
which is a closure. In order to call define_method, which is private,
we need an environment in which self is the object that we wan't to
call define_method on*. In our case this object is the singleton class
of "obj". The method instance_eval does exactly that: evaluate the
block in a context in which self is set to the receiver of
instance_eval.

Hope this clarifies a little bit more.

* Another way is to use send

obj.singleton_class.send(:define_method, :talk) do
  puts str
end

but I like instance_eval better.

Jesus.