From: dblack@...
Date: 2006-11-08T12:36:01+09:00
Subject: Re: Can't bind a singleton method to a subclass?

Hi --

On Wed, 8 Nov 2006, David Walker wrote:

> On 11/7/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
>> The fact that B can call A.class_method is due to the special-casing
>> of singleton classes of Class objects; it's the one case where a given
>> object can execute singleton methods of another object.  But those
>> singleton methods still belong, unambiguously, to the first object, A.
>> The reason B can call them is that A's singleton class serves as the
>> superclass of B's singleton class -- so class_method lies on the
>> method look-up path of B.  But class_method is still defined only in
>> A's singleton class.
>
> Thanks for the info. That's mostly what I thought - I guess I should
> have said, "This seems silly to me," not, "This doesn't make sense to
> me." :)
>
> How, though, is this a special case?

What I mean is: the fact that you can call B.class_method, even though
class_method is a singleton method of an object other than B, is a
special dispensation for class objects.

> For instance methods, you can
> bind an instance method to any object who's class is <= the class the
> method is defined on. Why should it be different for singletons? I.e.
> I have an instance method (instance from the perspective of the
> singleton class A), I should be able to bind it to an object who's
> class is a subclass of the class the instance is defined on. If A's
> singleton class is a superclass of B's singleton class (which it ought
> to be) then this relation holds. I think. :)

I guess it's just a matter of how the logic cascades.  Forbidding a
rebinding of a singleton method on a different object takes precedence
over the special situation where class singleton methods are sort of
not really singleton (i.e., can be called by subclasses).

> In any case, "B.method(:class_method)" gave me a #<Method:
> B(A).class_method>. My original question still stands: is there no way
> to recover this binding once it's been unbound?

Maybe, but not that I can think of.


David

-- 
                   David A. Black | dblack@wobblini.net
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org