gistfile1.scala

sealed abstract class Expression
case class X() extends Expression
case class Const(value : Int) extends Expression
case class Add(left : Expression, right : Expression) extends Expression
case class Mult(left : Expression, right : Expression) extends Expression
case class Neg(expr : Expression) extends Expression

def eval(e : Expression, x : Int) : Int = e match {
  case X() => x
  case Const(c) => c
  case Add(l, r) => eval(l, x) + eval(r, x)
  case Mult(l, r) => eval(l, x) * eval(r, x)
  case Neg(expr) => - eval(expr, x)
}