Type Test/Type Cast in Scala --- Matching Object With Patterns

TypeTestAndCast.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 TypeTestCast extends App {
	val numObj = new Num(1)
	val varObj = new Var("var")
	val mulObj = new Mul(numObj, varObj)

	// Simplification rule:
	if (mulObj.isInstanceOf[Mul]) {
		val mul = mulObj.asInstanceOf[Mul]
		val leftOfMul = mul.left
		if (leftOfMul.isInstanceOf[Num]) {
			val num = leftOfMul.asInstanceOf[Num]
			println("The left value of the mul is : " + num.value)
		} else {
			println("The left value of the mul is not a Num object.")
		}
	} else {
		println("The mul is not a Mul object.")
	}
}