nyan.scala

sealed trait T
class A extends T {
  def f:String = "A"
}
class B extends T {
  def g:String = "B"
}

val a = new A
val b = new B

def f(x: T): String = x match {
  case a: A => a.f
  case b: B => b.g
}

println(f(a)) // => A
println(f(b)) // => B