From: Pit Capitain <pit@...>
Date: 2003-03-12T06:39:18+09:00
Subject: Re: extend / instance_eval interactions

On 12 Mar 2003 at 6:12, James Davis wrote:

> Why does instance_eval interfere with extend?  Should it?  If I remove
> instance_eval, or place extend before it, meth1 in Klass is overriden
> with meth1 in ExtMod.  Ruby 1.6.8 Windows.  Thanks,

James, I'm not sure but I think it has to do with chaining of modules 
and singleton classes. See below:

> module ExtMod
> 	def meth1;	puts 'C - ExtMod meth1';	end
> 	def meth2;	puts 'D - ExtMod meth2';	end
> end
> 
> class Klass
> 	def meth1;	puts 'A - Klass meth1';	end
> end
> 
> s = "def meth1; puts 'B - instance_eval meth1'; end"
> a=Klass.new

Here a is of class Klass:

  a
    Klass (meth1)

> a.meth1                 #-> A - Klass meth1
> a.instance_eval(s)

This creates the singleton class of a, a class that holds a's own 
methods. This class comes before all other classes a inherits from:

  a
    Singleton class (meth1)
    Klass (meth1)

so whenever you call a.meth1, you get meth1 of the singleton class.

> a.meth1                 #-> B - instance_eval meth1
> a.extend(ExtMod)

Now you add ExtMod to the list of a's ancestors, again AFTER the 
singleton class:

  a
    Singleton class (meth1)
    ExtMod (meth1, meth2)
    Klass (meth1)

When you call a.meth1, you still get meth1 of the singleton class, 
although you included ExtMod after creating the singleton class.

> a.meth1                 #-> B - instance_eval meth1
> a.meth2                 #-> D - ExtMod meth2

HTH

Regards,
Pit