From: "David A. Black" <dblack@...>
Date: 2010-08-01T22:28:59+09:00
Subject: Re: what is the correct way to extend native methods?

Hi --

On Sun, 1 Aug 2010, Maurizio De Santis wrote:

> David A. Black wrote:
>>
>> It's one underlying method (C function) with two Ruby names. If you
>> redefine one of the Ruby names you break the binding between that name
>> and the underlying method, and if you add an alias then there's a third
>> name. Each binding is separate; they don't automatically change
>> together.
>>
>
> uhhh I think I've understood: In order to do what I want to do, I should
> implement something like this:
>
>
> class Array
>
>  def slice_with_regexp(*args)
>    return self.select{ |val| val =~ args[0] } if args.size == 1 and
> args[0].is_a?(Regexp)
>    # code that executes C function called by [] and slice
>  end
>
>  alias_method :slice_with_regexp, :slice
>  alias_method :slice_with_regexp, :[]
>
> end
>
>
> that is stupid 'cause breaks Array.slice performances.
>
> Right?
>
> Thanks a lot David!

I should add that my full advice is to avoid overriding slice and/or []
globally. A better way to alter core functionality is on a per-object
basis:

   module ExtendedSlicer
     def slice(*args)
       arg0 = args[0]
       case arg0
       when Regexp
         grep(arg0)
       else
         super
       end
     end
   end

   a = %w{ one two three four }.extend(ExtendedSlicer)
   a.slice(0,2)   # ["one", "two"]
   a.slice(/o/)   # ["one", "two", "four"]

The main advantage here is that it's a better fit: you need an object to
behave a certain way, and you teach that object to behave that way.
Another advantage is that it makes you work a little harder, and
therefore helps you evaluate more carefully whether or not you need to
perform the modification at all.


David

-- 
David A. Black, Senior Developer, Cyrus Innovation Inc.

   The                   Ruby training with Black/Brown/McAnally
   Compleat              Philadelphia, PA, October 1-2, 2010
   Rubyist               http://www.compleatrubyist.com