TDA, no mutation, no (used) return values between objects

translate.rb

class Printer
  def initialize(options)
    @out = options.fetch(:out)
    @translator = options.fetch(:translator)
  end

  def say_hello
    @translator.translate("Hello!", self)
  end

  def receive_translation(translated_text)
    @out.puts(translated_text)
  end
end

class Translator
  def translate(text, observer)
    observer.receive_translation(translations[text])
  end

  private

  def translations
    {'Hello!' => 'Guten tag!', 'Foo' => 'Bar'}
  end
end

translator = Translator.new
printer = Printer.new(out: STDOUT, translator: translator)
printer.say_hello