promise.rb

require 'monitor'

class Promise
  include MonitorMixin
  NOTHING = Object.new.freeze

  def initialize(&blk)
    @blk = blk
    @value = NOTHING
  end

  def value
    unless value?
      synchronize { work }
    end
    @value
  end

  def value?
    @value !== NOTHING
  end

  private def work
    return if value?
    @value = @blk.call
    @blk = nil
  end
end