expression problem

exprPattern.scala

object exprPattern extends App {
  sealed trait Expr
  case class Add(e1: Expr, e2: Expr) extends Expr
  case class Sub(e1: Expr, e2: Expr) extends Expr
  case class Num(n: Int) extends Expr

  def value(e: Expr): Int = e match {
    case Add(e1, e2) => value(e1) + value(e2)
    case Sub(e1, e2) => value(e1) - value(e2)
    case Num(n) => n
  }

  println(value(Add(Num(1), Sub(Num(2), Num(1)))))

  // Adding new cases -- hard if there are many pattern matches (every match has the full algebra)
}

exprPoly.scala

object exprPoly extends App {

  sealed trait Expr {
    def v: Int
  }

  case class Add(e1: Expr, e2: Expr) extends Expr {
    def v = e1.v + e2.v
  }

  case class Sub(e1: Expr, e2: Expr) extends Expr {
    def v = e1.v - e2.v
  }

  case class Num(n: Int) extends Expr {
    val v = n
  }

  def value(e: Expr): Int = e.v

  println(value(Add(Num(1), Sub(Num(3), Num(1)))))

  // Adding new functions -- hard if there are many functions (every case has all functions)
}