From: Jan Svitok <jan.svitok@...>
Date: 2006-11-13T02:37:18+09:00
Subject: Re: redefining a method using Module#include

On 11/12/06, me@mikehogan.net <me@mikehogan.net> wrote:
> Hi,
>
> How can I redefine Array#[] using an included Module?  Here is my
> attempt:
>
> module AbsenceHandling
>     def if_absent_call(p)
>         @if_absent = p
>         self
>     end
>
>     def [](index)
>         result = old_accessor(index)
>         if(result.nil? && !@if_absent.nil?)
>             result = @if_absent.call(index)
>             self[index]=result
>         end
>         result
>     end
> end
>
> class Array
>     alias :old_accessor :[]
>     include AbsenceHandling
> end
>
>
> x = [1,2,3].if_absent_call(proc{|index|
>     "#{index}"
> })
> puts x[5]

Hi,

If a class defines a method, it has higher precedence than methods of
any of its included modules. So the "override" never happens in this
case. You can work around it by defining the [] operator directly in
Array method, via Module#included and Module#define_method. You can
alias the old [] in the included as well.

module AbsenceHandling
  def if_absent_call(p)
     @if_absent = p
     self
   end

  def self.included(base)
    # base = Array in this case
    # alias_method and define_method are private, so we must
    # use the send trick
    base.send(:alias_method, :old_accessor, :[])
    base.send(:define_method, :[]) do |index|
      result = old_accessor(index)
      if(result.nil? && !@if_absent.nil?)
        result = @if_absent.call(index)
        self[index]=result
      end
      result
    end
  end
end

class Array
  include AbsenceHandling
end

Note: [] in Array is a special case (as are some other methods in base
C classes) as sometimes their C versions are called directly for the
sake of speed. So e.g. Array#to_s will use the old []