Promises are Futures.

promise.rb

require 'celluloid'

class Politician
  include Celluloid

  def lower_taxes
    # Try really hard to do this
    result_of_complicated_code = [true, false].sample

    yield result_of_complicated_code
  end
end

some_dude = Politician.new

promise = some_dude.future.lower_taxes do |result|
  result ? 'this was successful' : 'this was unsuccessful'
end

puts promise.value