From: dblack@...
Date: 2006-11-08T13:43:39+09:00
Subject: Re: Can't bind a singleton method to a subclass?

Hi --

On Wed, 8 Nov 2006, ara.t.howard@noaa.gov wrote:

> On Wed, 8 Nov 2006 dblack@wobblini.net wrote:
>
>> I think it's not so much that you're unbinding it *from* B, as that 
>> you're
>> grabbing it *through* B.  So, by the time you unbind it, it doesn't 
>> "know"
>> that you got it that way; it's just a Method object, and it allows itself 
>> to
>> be unbound.
>
> yeah.  at first i thought that, but:
>
>    harp:~ > cat a.rb
>    class A
>      def self.class_method() 42 end
>    end
>    class B < A; end
>
>    p A.method(:class_method)
>    p B.method(:class_method)
>
>
>    harp:~ > ruby a.rb
>    #<Method: A.class_method>
>    #<Method: B(A).class_method>
>
> so it seems like the method is aware that it's receiver is B.  but i take 
> your point.

Interesting.  The dot is the singleton-method indicator in that
notation:

irb(main):013:0> obj = Object.new
=> #<Object:0xb7f04aa8>
irb(main):014:0> def obj.x; end
=> nil
irb(main):015:0> obj.method(:x)
=> #<Method: #<Object:0xb7f04aa8>.x>

In your example, it's A.class_method and not
#<Class:0xabcdef>.class_method.  But it still really means the
singleton class of A.  I guess it's all in the service of allowing for
that special case singleton-of-Class-object thing.

>> I'd say that's a side-effect of the fact that A's singleton
>> methods can be got at via B at all.
>
> yes.  put another way, and something i was be-moaning several years ago, is
> this:
>
>    harp:~ > cat a.rb
>    class A
>      SINGLETON_CLASS = class << self
>        self
>      end
>    end
>    class B < A
>      SINGLETON_CLASS = class << self
>        self
>      end
>    end
>
>    p B::SINGLETON_CLASS.ancestors
>
>    harp:~ > ruby a.rb
>    [Class, Module, Object, Kernel]
>
> singleton classes should respect inheritence.

I'm not sure what you mean here.  Is it that ancestors doesn't show
singleton classes?


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