From: Brian Candler <b.candler@...>
Date: 2008-11-04T22:23:53+09:00
Subject: Re: Iterator objects and lazy evaluation

Actually, it turns out you don't even need ruby-1.9 for this. The 
following runs fine in ruby-1.8.6:

  module Enumerable
    class Gen
      include Enumerable
      def initialize(&body)
        @body = body
      end
      def each(&receiver)
        @body.call(receiver)
      end
    end

    def filter(&blk)
      Gen.new do |y|
        each do |*input|
          blk.call(y, *input)
        end
      end
    end
  end

  (1..10).
    filter { |y,e| puts "Sending #{e.inspect}"; y[e]; puts "Sent" }.
    filter { |y,e| y[e] if e % 2 == 0; sleep 0.5 }.
    filter { |y,e| y[e + 100]; sleep 0.5 }.
    each { |e| puts e }

The puts and sleep calls are in there to show this is proper 
'horizontal' evaluation, with no intermediate arrays. But if you want an 
array of the finished product, just replace the last line with "to_a"

I wouldn't be surprised if this pattern exists in a library somewhere, 
but I don't know where.
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