Lazy Proxy implementation in ruby 2.1

lazy.rb

class LazyProxy < BasicObject
  def self.promise(&callback)
    new(&callback)
  end

  def initialize(&callback)
    @callback = callback
  end

  def method_missing(method, *args, &block)
    __target__.send(method, *args, &block)
  end

  def __target__
    @target ||= @callback.call
  end
end

module Kernel
  def promise(&callback)
    LazyProxy.promise(&callback)
  end
end