Extractors in Scala --- Matching Object With Patterns

Extractor.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

// Extractors here:
// apply is the injection
// unapply is the extraction
object Num {
	def apply(value: Int) = new Num(value)
	def unapply(n: Num) = Some(n.value)
}

object Var {
	def apply(name: String) = new Var(name)
	def unapply(v: Var) = Some(v.name)
}

object Mul {
	def apply(left: Expr, right: Expr) = new Mul(left, right)
	def unapply(m: Mul) = Some(m.left, m.right)
	///// --
	// Removing the target type from the unapply method
	// to hide the underlying representation below
	///// --
	// def unapply(x: Expr) = x match {
	// 	case m: Mul => Some(m.left, m.right)
	// 	case _ => None
	// }
}

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

	// Simplification rule:
	mulObj match {
		case Mul(Num(1), Var(x)) => {
			println("The right value of the mul is : " + x)
			Var(x)
		}
		case _ => mulObj
	}
}