From: Kai Kai <kai@...>
Date: 2008-12-09T01:28:49+09:00
Subject: Re: filtering blocks

Hm, I think the problem is not to filter a block (that's also possible, 
because in ruby blocks are also objects of the class Proc). You want to 
filter an Enumeration. Therefore you could use the class Enumerator.

require "enumerator"
class Integer
  def odds()
    if block_given? then
      1.upto(self) do |number| yield number if number % 2 == 1 end
    else
      Enumerable::Enumerator.new(self, :odds)
    end
  end
  def evens()
    if block_given? then
      1.upto(self) do |number| yield number if number % 2 == 0 end
    else
      Enumerable::Enumerator.new(self, :even)
    end
  end
end

5.odds.each do |number| puts number end
# => 1, 3, 5
5.odds.any? do |number| number % 3 == 0 end
# => true


And of course you can add more "filters" by extending the Enumerator 
class, or better, a subclass of Enumerator which handles only with 
Integers.

require "enumerator"
class Integer
  def odds()
    if block_given? then
      1.upto(self) do |number| yield number if number % 2 == 1 end
    else
      Enumerable::Enumerator::Integer.new(self, :odds)
    end
  end
  def evens()
    if block_given? then
      1.upto(self) do |number| yield number if number % 2 == 0 end
    else
      Enumerable::Enumerator::Integer.new(self, :even)
    end
  end
end
module Enumerable
  class Enumerator
    class Integer < Enumerator

      def prime
        if block_given? then
          each do |number|
            # primitive prime algorithm
            yield(number) unless (2..Math.sqrt(number)).any? do |i|
              number % i == 0
            end
          end
        else
          Enumerable::Enumerator.new(self, :prime)
        end
      end

      def odds
        if block_given? then
          each do |number| yield number if number % 2 == 1 end
        else
          Enumerable::Enumerator.new(self, :odds)
        end
      end

      def evens
        if block_given? then
          each do |number| yield number if number % 2 == 0 end
        else
          Enumerable::Enumerator.new(self, :evens)
        end
      end
    end
  end
end

12.odds.prime.each {|x| p x}
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