lazye.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-
require 'fiber'

module Lazy
  module Enumerable
    def lazy(method = :each)
      Lazy::Enumerator.new do
        if block_given?
          Fiber.yield yield while true
        else
          e = ::Enumerator.new self, method
          Fiber.yield e.next while true
        end
      end
    end
  end

  class Enumerator
    include Lazy::Enumerable

    def initialize
      @context = Fiber.new do
        yield if block_given?
      end
    end

    def next
      @context.resume
    end

    def to_a
      ::Enumerator.new(self).map { |it| it }
    end

    def each
      if block_given?
        loop { yield self.next }
      else
        self
      end
    end

    def map
      lazy do
        it = self.next
        block_given? ? yield(it) : it
      end
    end
  end
end

Array.send :include, Lazy::Enumerable

e = (0...10).to_a.lazy
e = e.map { |x| x + 1 }
e = e.map { |x| x * 2 }
e.each { |x| puts x }