From: Patrick May <patrick@...>
Date: 2004-06-28T05:44:16+09:00
Subject: Re: Is it considered Harmful?

I think that #become and #class= are _theoretically_ wrong.  Such a 
method makes handling object life cycle ugly.

Objects expect to be initialized first, they expect to have various 
pieces of state setup in a predictable manner.  If an object of one 
class #become's another class, then the new class's methods can't count 
on having the proper state.  Core dumps is what you get when this 
happens in c, but you'll have similar problems with 100% *.rb code.  
Edge cases not being handled correctly, missing instance variables, etc.

To get around these errors, you end up needing an extra initialize 
function.  This extra initialize always takes one argument, the object 
that is becoming the class.  Tracking this extra lifecycle per class 
sounds like a recipe for bugs.

To safely support this code:

     a = SomeClass.new
     a.become( AnotherClass )

I end up needing this code in AnotherClass:

     def update_instance_for_changed_class( old_instance )
         case old_instance.class
         when SomeClass
              self.initialize( old_instance.attribute1, 
old_instance.attribute2 )
         when ....
               ....
         end
     end

Other than being uglier, I don't see how #become is different from:

     a = SomeClass.new
     a = AnotherClass.new( a.attribute1, a.attribute2 )

This example gets the mapping of SomeClass to AnotherClass out of 
AnotherClass and into the code that wants to map the two together.  And 
my variable has effectively "become" a new class.

Is the problem that you want to add methods?  Then use Modules.

Is the problem that you want to update all references to the new a?  
For one, #replace_self raises different issues than #become.  Still, I 
have a hunch that for most uses of #replace_self there is a way to use 
of accessor methods instead.

So, to sum up my ivory tower critique of #become and #class=

   * #become creates a duplicate lifecycle for objects, resulting in 
lots of complication
   * It doesn't really add that much functionality

Cheers,

Patrick

p.s. Thanks for raising an interesting issue!  I enjoyed following the 
conversation and thinking about the implications of #become / #class=