tp3.scala

trait Shape {
  def area: Double
}

case class Circle(radius: Double)
case class Rectangle(width: Double, length: Double)

//               v  (no constructor parameters)
class CircleShape extends Shape {
  override def area : Double = ??? // radius?
}

//                  v (no constructor parameters)
class RectangleShape extends Shape {
  override def area: Double = ??? // width and height?
}

def areaOf(shape: Shape): Double = shape.area

areaOf(new CircleShape)
areaOf(new RectangleShape)