promise.rb

require 'deferrable_gratification'

class Promise
  def initialize
    @df = EM::DefaultDeferrable.new
    DG.enhance! EM::DefaultDeferrable
  end

  def done(&block)
    tap { @df.callback(&block) }
  end

  def fail(&block)
    tap { @df.errback(&block) }
  end

  def always(&block)
    tap { @df.bothback(&block) }
  end

  def resolve(*args)
    @df.succeed(*args)
  end

  def reject(*args)
    @df.fail(*args)
  end

  def then(_done, _fail=nil)
    df = Promise.new

    done { |v| result = _done.call(v); df.resolve(result) }.
      fail { |v| result = _fail.call(v); df.reject(result) }

    df
  end

  def transform(_done, _fail=nil)
    tap do
      @df.transform(&_done)
      @df.transform_error(&_fail) if _fail
    end
  end
end