From: Pit Capitain <pit@...>
Date: 2003-08-07T16:13:30+09:00
Subject: Re: What's New and Shiny in Ruby 1.8.0?

On 7 Aug 2003 at 8:30, Harry Ohlsen wrote:

> Martin DeMello wrote:
> 
> > Still hacky, but general:
> > 
> > class Test
> >   attr_accessor :foo, :bar
> >   def initialize(f, b)
> >     @foo, @bar = f, b
> >   end
> > 
> >   def inspect
> >     "[#{foo}, #{bar}]"
> >   end
> > end
> > 
> > def rev(obj)
> >   a = obj.dup
> >   class << a
> >     alias old_cmp <=>
> >     def <=>(other)
> >       -old_cmp(other)
> >     end
> >   end
> >   a
> > end
> > 
> > a = [
> >   Test.new("Hello", 1),
> >   Test.new("World", -1),
> >   Test.new("Foo", 5),
> >   Test.new("bar", 4)
> > ]
> > 
> > p a
> > p a.sort_by {|x| x.foo}
> > p a.sort_by {|x| rev(x.foo)}

Very nice idea. I like it.

> However, if we try 
> 
>    p a.sort_by { |x| rev(x.bar) }
> 
> we get ...
> 
>    sort_by.rb:20:in `dup': can't dup Fixnum (TypeError)

This could be fixed with something like

  def rev(obj)
    -obj
  rescue NoMethodError
    a = obj.dup
    class << a
      alias old_cmp <=>
      def <=>(other)
        -old_cmp(other)
      end
    end
    a
  end

Not beautiful, but working.

> I tried sticking in an
> 
>    if obj.respond_to? :dup
> 
> but that didn't help.  It would appear that, while Fixnum responds to
> #dup, its response is "I'm sorry, Dave, I'm afraid I can't do that."
> :-(. 
> 
> That doesn't seem to make a lot of sense to me.  I would have thought
> it made more sense for Fixnum#dup to just return self.  Maybe there's
> some good reason for not doing that.

But then you couldn't do things like Martin's code, where you want to 
modify a duplicate and not the original object.

> Alternatively, maybe we need a way for classes to disown methods they
> inherit that they don't want to (or logically shouldn't) implement? 
> Then, Fixnum could disown dup and the respond_to? test would work the
> way I was expecting.

Since dup is defined in Object (Kernel), you'd expect to find it in 
every object, wouldn't you? But if you really want you can undefine 
dup for Fixnums:

  p 5.respond_to?( :dup )  # => true
  p 5.dup rescue puts $!   # => can't dup Fixnum

  Fixnum.send :undef_method, :dup

  p 5.respond_to?( :dup )  # => false
  p 5.dup rescue puts $!   # => undefined method `dup' for 5:Fixnum

Regards,
Pit