trait Shape[A] {
  def area(a: A): Double
}

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

// Here >
implicit object CircleShape extends Shape[Circle] {
  override def area(circle: Circle) : Double = math.Pi * math.pow(circle.radius, 2)
}

// And here >
implicit object RectangleShape extends Shape[Rectangle] {
  override def area(rectangle: Rectangle): Double = rectangle.width * rectangle.length
}

def areaOf[A](shape: A)(implicit shapeImpl: Shape[A]): Double = shapeImpl.area(shape)

areaOf(Circle(10))
areaOf(Rectangle(5,5))