From: David Masover <ninja@...>
Date: 2009-09-18T03:06:29+09:00
Subject: Re: Mucking about with dynamically adding methods to objects

On Thursday 17 September 2009 10:28:25 am Paul Smith 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.

Break it down into pieces. You already know how to do:

def obj.talk
  ...
end

Or, to define a class method:

Class Foo
  def self.bar
    ...
  end
end
Foo.bar

The next step is to realize that this is equivalent to:

class << obj
  def talk
    ...
  end
end

It might not be immediately obvious, but just take my word for it: that is 
equivalent to 'def obj.talk'.

But it looks like a class definition -- and it sort of is. It's the metaclass 
-- you can think of it as a class that only that object belongs to, which 
inherits from the object's real, official class.

Now, think about this example:

class Foo
  def self.bar

You might've already done it this way:

def Foo.bar

So what is self inside a class? Easy, it's the class itself.

So, what's self inside "class << object"? It's the metaclass I just described.

So, if you do this:

meta_class = class << object
  self
end

you extract the metaclass. (The last statement in a class definition is the 
return value of the class definition.)

Once you understand that much, instance_eval may make sense. In fact, you 
could do this:

class Object
  def metaclass
    class << self; self; end
  end
end

Now you can do object.metaclass.instance_eval. This is so incredibly common 
that it could be shortened, too:

class Object
  def meta_eval &block
    metaclass.instance_eval &block
  end
end

I use this a lot -- even when "class << obj" would work just as well. And I'm 
blatantly stealing this from _why -- he distributed it as a gem called 
'metaid'. His Rubyforge account seems to work, and the gem seems to be alive, 
so I think it's safe -- especially since it's so fundamental and obvious (17 
lines of code) that it's not hard to replace if you have to.

So, do this instead:

gem install metaid

require 'metaid'
def talkify(obj, str)
  obj.meta_eval do
    define_method :talk do
      str
    end
  end
end

I might shorten it like this, if it fits well:

require 'metaid'
def talkify obj, str
  obj.metaclass.send :define_method, :talk do
    str
  end
end

It's probably covered somewhere in that book, but even once it is, I still 
suggest metaid.