From: Hugh Sasse Staff Elec Eng <hgs@...>
Date: 2002-02-05T02:53:21+09:00
Subject: Re: Mixins and accessing earlier definitions.

On Tue, 5 Feb 2002, ts wrote:

> >>>>> "H" == Hugh Sasse Staff Elec Eng <hgs@dmu.ac.uk> writes:
>
> H> Yes, that is what I tried -- it makes no difference, really, because it
> H> still ends up as a superclass...
>
>  Try this
>
> pigeon% cat b.rb
> #!/usr/bin/ruby
> module B
>    def self.append_features(kl)

        # OK this gets called when included in another module/class..

>       klass = self
>       kl.class_eval do

           # In that class...

>          if method_defined?(:aa) && !method_defined?("_aa_#{klass}_")
>             alias_method "_aa_#{klass}_", :aa
              # rename the method to reflect where it now is...

>             remove_method :aa
              # and remove the original so the new aa will be found...

>          end
>       end
>       super
>    end

     # The new aa ia this:
>    def aa
>       puts "B::aa"
        # call the old one...
>       _aa_B_ if respond_to? :_aa_B_
>    end
> end
>
> def aa
>    puts "object"
> end
>
> class A
>    def aa
>       puts "A::aa"
> #      super
>    end
>
>    include B
> end
>
> A.new.aa
> pigeon%
>
> H> I don't think it will, but I'll bear that in mind.
>
>  To see the problem with #super, remove the comment in A#aa

It loops forever, but Object.aa never gets called... because the module
is interposed btween A and Object...  I can't see where the loop is
occurring, though.

I am beginning to get the feeling that my whole approach to this problem
is counter to the design of Ruby.  From the exposure to Ruby's design
that I have had, this suggests that my approach is wrong.  Being able to
create new classes by plugging code into them, redefining methods at the
time of plugging in, without having to create a whole tree of subclasses
would seem like a fairly structured approach.....

I wondered if I needed some kind of Factory pattern for this, but
looking at RubyGarden I suspect that DecoratorPattern

 http://www.rubygarden.org/ruby?DecoratorPattern

might be more appropriate.  Now, if I put the code using singleton
methods and method aliases (at the end of the page) inside a module,
maybe using class << self, would this work...  I suspect not, because
the module would still be the partent of my class...  But if I put it
inside:
    module B
        def self.append_features(kl)
            klass = self
            kl.class_eval do
                ...
            end
        end
    end
can I get away with having a def inside a def?  This still feels too
much like an uphill struggle for idiomatic Ruby

> Guy Decoux
>
>
	Hugh