A rudimentary implementation of promises for Ruby.

promise.rb

class Promise
  def initialize(&callback)
    @callback = callback
    @next_promise = nil
  end

  def resolve(callback_result = nil)
    callback.call(callback_result).tap do |result|
      next_promise.resolve(result) if next_promise
    end
  end

  def then(&callback)
    self.next_promise = self.class.new(&callback)
  end

  private

  attr_accessor :callback, :next_promise
end


promise = Promise.new do
  puts 'first'
end

puts 'procedural'

other_promise = promise.then do
  puts 'second'
end

other_promise.then do
  puts 'third'
end

promise.resolve

# prints
# => procedural
# => first
# => second
# => third