Visitors in Scala --- Matching Object With Patterns

Visitors.scala

// Cited from the essay "Matching Object With Patterns"
// Class hierarchy:
trait Visitor[T] {
	def caseMul(t: Mul): T = otherwise(t)
	def caseNum(t: Num): T = otherwise(t)
	def caseVar(t: Var): T = otherwise(t)
	def otherwise(t: Expr): T = throw new NullPointerException
}

trait Expr {
	def matchWith[T](v: Visitor[T]): T
}

class Num(val value: Int) extends Expr {
	def matchWith[T](v: Visitor[T]): T = v.caseNum(this)
}

class Var(val name: String) extends Expr {
	def matchWith[T](v: Visitor[T]): T = v.caseVar(this)
}

class Mul(val left: Expr, val right: Expr) extends Expr {
	def matchWith[T](v: Visitor[T]): T = v.caseMul(this)
}

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

	// Simplification rule:
	mulObj.matchWith {
		new Visitor[Expr] {
			override def caseMul(m: Mul) = 
				m.left.matchWith {
					new Visitor[Expr] {
						override def caseNum(n: Num) = 
							if (1 == n.value) {
								println("The left Expr of the mul is : " + n.value)
								m.left
							} else {
								mulObj
							}
						override def otherwise(e: Expr) = {
							println("The left Expr of the mul is not a number")
							e
						}
					}
				}
			override def otherwise(e: Expr) = {
				println("The object is not a mul.")
				e
			}
		}
	}
}