lazy.rb

require "thread"

class Delay
  def initialize(&thunk)
    @semaphore = Mutex.new
    @thunk = thunk
    @evaluated = false
  end

  def  value
    synchronize {
      if @evaluated
        @value
      else
        @value = @thunk.call and @thunk = nil
        @evaluated = true
        freeze and @value
      end
    }
  end

  private
    def synchronize
      if @evaluated
        yield
      else
        @semaphore.synchronize { yield }
      end
    end
end

sum = Delay.new { sleep(1); 2 + 5 }
(0...1000).map { Thread.new { puts sum.value } }
  .map(&:join)

Awesome job!