adt6.scala

sealed trait Shape

case class Circle(radius: Int) extends Shape

case class Rectangle(width: Int, height: Int) extends Shape

// 'match' keyword allows us to pattern match on a 
// specific option of the sum type
def area(shape: Shape): Double = shape match {
  case Circle(r) => math.Pi * r * r
  case Rectangle(w, h) => w * h // Note how rectangle's width and height 
                                // are captured in 'w' and 'h'. It's possible
                                // because Rectange is a product type
}