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

You could create your own with the three possibilities. Or as of one your car.radioType types you could have unknown, and then use guards on your case's to handle it. If you roll your own, you should include the Product trait as well. liftweb has the Box type, which is an option close that allows for full, empty and erorr to happen.

Answer 2

You can use scala.Either. Use Left for the exceptional value, and Right for the expected value which can be an Option in this case:

scala> type Result = Either[String, Option[String]]
defined type alias Result

scala> val hasValue: Result = Right(Some("pioneer"))
hasValue: Result = Right(Some(pioneer))

scala> val noValue: Result = Right(None)
noValue: Result = Right(None)

scala> val unknownValue = Left("unknown")
unknownValue: Left[java.lang.String,Nothing] = Left(unknown)

Answer 3

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 4

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.

Answer 5

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) => 
}