Type Case in Scala --- Matching Object With Patterns

TypeCase.scala

// Cited from the essay "Matching Object With Patterns"
// Class hierarchy:
trait Expr
class Num(val value: Int) extends Expr
class Var(val name: String) extends Expr
class Mul(val left: Expr, val right: Expr) extends Expr

object Typecase extends App {
	val numObj = new Num(1)
	val varObj = new Var("var")
	val mulObj = new Mul(numObj, varObj)

	// Simplification rule:
	mulObj match {
		case m: Mul => 
			m.left match {
				case n: Num => 
					if (1 == n.value) {
						println("The left value of the mul is : " + n.value)
						m.left
					} else {
						println("The left value of the mul is not 1.")
						mulObj
					}
				case _ => {
					println("The left value of the mul is not a Num object.")
					mulObj
				}
			}
		case _ => {
			println("The mul is not a Mul object.")
			mulObj
		}
	}
}