future.rb

require 'thread'

class Future
  def initialize &block
    @cv = ConditionVariable.new
    @mutex = Mutex.new

    Thread.start do
      @result = block.call
      @cv.signal
    end
  end
  def get
    @mutex.synchronize do
      unless @result
        @cv.wait(@mutex)
      end
    end
    @result
  end
  def done?
    @result ? true : false
  end
end

if __FILE__ == $0

  def fib(n)
    n < 2 ? 1 : fib(n-1) + fib(n-2)
  end

  task = Future.new do
    fib 30
  end
  puts "task.done? #=> %s" % task.done?
  puts "task.get   #=> %d" % task.get
  puts "task.done? #=> %s" % task.done?
end