Object Oriented Decomposition in Scala --- Matching Object With Patterns

OODecomposition.scala

// Cited from the essay "Matching Object With Patterns"
// Class hierarchy:
trait Expr {
	def isVar: Boolean = false
	def isNum: Boolean = false
	def isMul: Boolean = false
	def value: Int = throw new NullPointerException
	def name: String = throw new NullPointerException
	def left: Expr = throw new NullPointerException
	def right: Expr = throw new NullPointerException
}

class Num(override val value: Int) extends Expr {
	override def isNum = true
}

class Var(override val name: String) extends Expr {
	override def isVar = true
}

class Mul(override val left: Expr, override val right: Expr) extends Expr {
	override def isMul = true
}

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

	// Simplification rule:
	if (mulObj.isMul) {
		val leftOfMul = mulObj.left
		if (leftOfMul.isNum && 1 == leftOfMul.value) {
			println("The left Expr of the mul is : " + leftOfMul.value)
			// leftOfMul
		} else {
			println("Thee left Expr of the mul is not a number object.")
			// mulObj
		}
	}
}