From: Gary Wright <gwtmp01@...>
Date: 2009-05-16T14:24:33+09:00
Subject: Re: Superclass of eigenclass


On May 15, 2009, at 7:46 AM, Joshua Ballanco wrote:
>> Right. So then the question: is there a compelling reason that they  
>> should be allowed to be subclassed? (I mean, other than the  
>> obvious: "Because that's not the way Ruby was designed" answer.) I  
>> cannot come up with any compelling technical reason why the  
>> artificial limitation of being unable to subclass "virtual  
>> classes"[1] should exist.

The purpose of an eigenclass is to hold per object methods.  As such,  
there is always always a one-to-one relationship between an eigenclass  
and its "eigeninstance" (sorry, but I can't think of a clearer term at  
the moment).

The idea of subclassing an eigenclass contradicts that one-to-one  
relationship.  If you could subclass an eigenclass, there would be no  
way to cause ruby to search the new subclass for instance methods  
since it wouldn't have a matching eigeninstance (sorry, again).  It  
would not be in the method lookup chain of any object.

Just to be complete, I'll point out that there is a way for  
eigenclasses to have a subclass and that is when the eigeninstances  
are themselves classes.  This is most clear in 1.9.  If B is a  
subclass of A, then B.eigenclass will be a subclass of A.eigenclass.   
Even in this case A and B have to exist before their eigenclasses can  
be accessed and not the other way around.

Gary Wright