Promises

promise.rb

class Promise
  class << self
    alias :make :new
  end

  def initialize(block)
    @result = []
    @thread = Thread.new(@result) do |result|
      result[0] = block.call
    end
  end

  def result
    @thread.join
    @result[0]
  end
end

def very_long_calculation
  sleep 2
  42
end

p = Promise.make(->{ very_long_calculation })
puts "Doing something"
sleep 3
puts "Done doing something"
puts p.result