On 11/12/06, me@[...].net <me@[...].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 []