A sample partial implementation of a lazy enumerable. See http://gist.github.com/441028 for example usage.

lazy.rb

module Lazy
  module LazyEnumerable; end

  class Enumerator
    include LazyEnumerable

    def initialize(enumerable=nil, &block)
      if enumerable
        @enumeration_block = proc {|send| enumerable.each(&send) }
      else
        @enumeration_block = block
      end
    end

    def each(&block)
      @enumeration_block.call(block)
    end
  end

  module LazyEnumerable
    def to_a
      arr = []
      self.each {|element| arr << element }
      arr
    end

    def map(&block)
      Enumerator.new do |send|
        self.each do |element|
          send.call block.call(element)
        end
      end
    end
    alias_method :collect, :map

    def find_all(&block)
      Enumerator.new do |send|
        self.each do |element|
          send.call(element) if block.call(element)
        end
      end
    end
    alias_method :select, :find_all
  end
end