Scala: Something like Option (Some, None) but with three states: Some, None, Unknown

Question

I need to return values, and when someone asks for a value, tell them one of three things:

1. Here is the value
2. There is no value
3. We have no information on this value (unknown)

case 2 is subtly different than case 3. Example:

val radio = car.radioType

1. we know the value: return the radio type, say "pioneer"
2. b. there is no value: return None
3. c. we are missing data about this car, we don't know if it has a radio or not

I thought I might extend scala's None and create an Unknown, but that doesn't seem possible.

suggestions?

thanks!

Update:

Ideally I'd like to be able to write code like this:

car.radioType match { 
   case Unknown => 
   case None => 
   case Some(radioType : RadioType) => 
}

Answer 1

Here's a barebones implementation. You probably want to look at the source for the Option class for some of the bells and whistles:

package example

object App extends Application {
  val x: TriOption[String] = TriUnknown

  x match {
    case TriSome(s) => println("found: " + s)
    case TriNone => println("none")
    case TriUnknown => println("unknown")
  }
}

sealed abstract class TriOption[+A]
final case class TriSome[+A](x: A) extends TriOption[A]
final case object TriNone extends TriOption[Nothing]
final case object TriUnknown extends TriOption[Nothing]

Answer 2

Don't tell anyone I suggested this, but you could always use null for Unknown rather than writing a new class.

car.radioType match { 
   case null => 
   case None => 
   case Some(radioType : RadioType) => 
}

Answer 3

You can grab some stuff from Lift: the Box. It has three states, Full, Failure and Empty. Also, Empty and Failure both inherit from EmptyBox.