matching case classes with type parameters? | The Scala Programming Language

I have a newbie question that perhaps someone can help me with. I'm trying to have a base class with a type parameter and a pattern match: abstract class X[T](var a : T) { // some type independent code (omitted)... // type dependent def field2val(field : String) { this match { case floatX(_) => a = field.toFloat; case intX(_) => a = field.toInt; } } } case class floatX(a : Float) extends X[Float](