http://spin.atomicobject.com/2012/06/04/bringing-fp-to-oo/ Methods without side effects.

driving.rb

class Driving
 
  def start(car)
    car.clone.merge(running: true)
  end
 
  def accelerate(car)
    car.clone.merge(speed: car[:speed] + 1)
  end
 
  def get_in(car, passenger)
    car.clone.merge(passengers: car[:passengers] + [passenger])
  end
end