tp5.scala

// Shape is now a parametrized trait
// 'A' is placeholder for `Circle`, `Rectangle` or other shape representation
trait Shape[A] {
  def area(a: A): Double
}

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

// We have to extend with Shape[Circle] because we have to pass 'Circle' to `area`
class CircleShape extends Shape[Circle] {
  override def area(circle: Circle) : Double = math.Pi * math.pow(circle.radius, 2)
}

// Same here, 'area' takes 'Rectangle'
class RectangleShape extends Shape[Rectangle] {
  override def area(rectangle: Rectangle): Double = rectangle.width * rectangle.length
}

//                                                  vvv (hmmm)              
def areaOf[A](shape: Shape[A]): Double = shape.area(???)

areaOf(new CircleShape)