Naive promise implementation

naive_promise.rb

class Promise
  def initialize(&blk)
    @functions = [blk]
  end

  def then(&blk)
    @functions << blk
  end

  def run
    Thread.new(@functions) do |fs|
      last_result = nil
      until fs.empty?
        last_result = fs.shift.(last_result)
      end
    end
  end
end