Instead, we do away with the visitor code and create an Algebraic Data Type.

sealed trait OneOrNone[+A]
final case object None extends OneOrNone[Nothing]
final case class One[+A](a: A) extends OneOrNone[A]
            

Then we pattern match when destructuring:

def stringOne[X](o: OneOrNone[X]) = o match {
  case One(o) => "get it! " + o
  case None => "don't got one"                
}
            

Much tidier!