Concise value types

object ValueTypes6
    {

        abstract class Type[T](val p : (T) => boolean)
        {
            sealed protected[Type] case class X[T] protected[Type] (val value : T)

            type Type = X[T]

            def apply(t : T) : Option[Type] = if(p(t)) Some(mk(t)) else None
            def unapply(v : Type) : Option[T] = if(v == null) None else Some(v.value)

            private def mk(t : T) : Type = X(t)
        }

        object Zipcode extends Type[String](_.length > 0)
        object City extends Type[String](_.length > 4)

        case class Foo(val zipcode : Zipcode.Type, val city : City.Type)

        def main(a : Array[String]) : Unit = {
            val x = for(
                z <- Zipcode("foo");
                c <- City("bartown")) yield Foo(z,c);

            x.map( _ match { case Foo(Zipcode(x),City(y)) => println("blipp: " + x + " " + y);
                             case x => println(x)} )
        }
    }