Simple evaluator in Scala

GADT.scala

abstract class Term[A]

case class Val[A](v : A) extends Term[A]
case class Fn[A, B](f : A => B) extends Term[A => B]
case class Apply[A, B](fn : Term[A => B], input : Term[A]) extends Term[B]

def eval[A](input : Term[A]) : A = input match {
  case Val(v) => v
  case Fn(f) => f
  case Apply(m,d) => eval(m)(eval(d))
}

def badEval[A](input : Term[A]) : A = input match {
  case Val(v) => v
  case Fn(f) => f

  // here is where we do not eval(v) first, which should be a type error!
  case Apply(m,v) => eval(m)(v)
}

val five = Val(5)
val timesFive = Fn({x : Int => x * 5})
val fiveTimesFive = Apply(timesFive, five)
println("eval gives:" + eval(fiveTimesFive))
println("badEval gives:" + badEval(fiveTimesFive))