From: 7stud -- <bbxx789_05ss@...>
Date: 2009-09-19T11:10:34+09:00
Subject: Re: Mucking about with dynamically adding methods to objects

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.

I just started chapter 14 of the "The Well Grounded Rubyist", and 
instance_eval() and define_method() are explained in regards to a 
similar example.

Something that bothered me when I initially read that code was the use 
of instance_eval().  I was left wondering why the more natural-reading 
class_eval() wasn't used?  After all, the goal is to create a method in 
the singleton *class*.

You can do this with class_eval():

class FairyGodmother

  def get_singleton_class(obj)

    class << obj
      self
    end
    #explained on p.392

  end


  def talkify(obj, str)
    obj_singleton = get_singleton_class(obj)

    obj_singleton.class_eval %Q{

      def talk
        puts "hello"
      end

    }

  end
end


...but if you try this:


  def talkify(obj, str)
    obj_singleton = get_singleton_class(obj)

    obj_singleton.class_eval %Q{

      def talk
        puts str    #<---CHANGE HERE****
      end

    }

  end


...it doesn't work.  The def creates a new scope, and str is not defined 
inside the def.

Using a string as an argument for class_eval() is unwieldy.  Luckily, 
class_eval() will take a block--instead of a string:

  def talkify(obj, str)
    obj_singleton = get_singleton_class(obj)

    obj_singleton.class_eval do

      def talk
        puts 'hello'
      end

    end

But once again, the def creates a new scope, so the code inside the def 
can't reference variables outside the def--like str(which is one of 
talkify()'s parameter variables).

define_method() to the rescue:

  def talkify(obj, str)
    obj_singleton = get_singleton_class(obj)

    obj_singleton.class_eval do

      define_method("talk") do
        puts str
      end

    end

Neither of the blocks used with class_eva() or define_method() creates a 
new scope, so code inside them can make use of variables defined in the 
surrounding scope.

Anyway, after working through that example, I wonder if there is a 
difference between instance_eval() and class_eval() when the receiver is 
a class?







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