From: "Peña, Botp" <botp@...>
Date: 2008-09-01T11:48:24+09:00
Subject: Re: instance_eval/class_eval including/extending modules

From: Patrick Li [mailto:patrickli_2001@hotmail.com] 
# I must say that I also find the difference between 
# instance_eval/class_eval and include/extend extremely confusing.
# If someone clearly knows the fundamental rules, please please share.
# Here's my understanding:
# def myMethod; end
#   -creates a method "myMethod" callable by instances of self.
# Here's a list of situations that I'm having trouble with:
# 1) class Klass
#      def myMethod; end
#    end
#   -self is equal to Klass
#   -so myMethod() is callable by Klass.new

generally, callable by instances of Klass

# 2) Klass.instance_eval {def myMethod; end}
#  -self is equal to Klass
#  -so myMethod() is callable by Klass.new 

no.
the instance here is Klass, not those created/instanciated w Klass.new. So you are creating a class method for Klass. Remember, classes are objects too, ergo there are class methods too. 

# What's wrong with my understanding? I think I'm not understanding def 
# correctly.

observe eg,

class Klass
  def mymethod; end
end
#=> nil

k=Klass.new
#=> #<Klass:0x28e1ff0>

k.mymethod
#=> nil

ok. fine.
let's try the other way, ie instance_eval.

k.instance_eval{def mymethod3; end}
#=> nil

k.mymethod3
#=> nil

fine still.
now let us try it for Klass itself.

Klass.mymethod
NoMethodError: undefined method `mymethod' for Klass:Class

oops ;)
maybe, we can do it w mymethod3

Klass.mymethod3
NoMethodError: undefined method `mymethod3' for Klass:Class

oops still ;)


Ok. let us define a method that works only for Klass.

Klass.instance_eval{def mymethod2; end}
#=> nil

k.mymethod2
NoMethodError: undefined method `mymethod2' for #<Klass:0x28e1ff0>

heheh, it does not work for instances/creations of Klass.
now, let us test it for Klass itself.

Klass.mymethod2
#=> nil

ah, we see.

hth.
kind regards -botp