some.scala

abstract class Foo[X](val value : X) { type T = X }
 
abstract class FooFactory[S <: Foo[_]](val P : (S#T) => boolean)
{
    def apply(t : S#T) : Option[S] = if(P(t)) Some(fetch(t)) else None
    
    def unapply(s : S) : Option[S#T] = if(s == null) None else Some(s.value)
 
    def fetch(t : S#T) = mk(t)
 
    protected[this] val mk : (S#T) => S
}
 
class Name private (v : String) extends Foo(v)
object Name extends FooFactory[Name](_.length > 0){ val mk = new Name(_)}