Scala: Inherited enum object doesn't satisfy parent trait - scala

Question

I am trying to use an Enumeration object in a method. The enum object extends from a trait, and the method takes the trait as a parameter. Here is the sample code.
sealed trait canAdd
object DAdder extends Enumeration with canAdd
{
type DAdder = Value
val P, Q = Value
}
class ClassTypeTest extends AnyFlatSpec with Matchers
{
class AClass
{
def add(v: canAdd) = if(v.isInstanceOf[DAdder]) println("Got DAdder") else println("Got IAdder")
def note(v: canAdd) = println("Got some canAdd trait object")
}
val aobj = new AClass
val aseq: Seq[DAdder] = Seq(DAdder.P, DAdder.Q, DAdder.P)
//*** Error in below line *****
aseq.foreach(aobj.add(_))
}
The compiler gives the following error:
Error:(23, 23) type mismatch;
found : x.x.DAdder.DAdder
(which expands to) x.x.DAdder.Value
required: x.x.canAdd
aseq.map(aobj.add(_))
Shouldn't I be able to pass an object that inherits the trait in a method that takes the trait as an argument? How do I fix this?

Answer 1

Enumeration class doesn't allow you to extends its values functionality. It is basically:
abstract class Enumeratum {
sealed trait Value { /* utilities */ }
object Value { /* factory of Values */ }
// utilities
}
Since you have the control only on Enumeratum class you cannot extend Values which is what you ask for.
You can easily have this functionality with sealed traits and case objects
sealed trait DAdder extends canadd
object DAdder {
case object P extends DAdder
case object Q extends DAdder
}
This however misses some utilities like finding value by its name, listing all values, etc.
This problem is solved by Enumeratum library which require that you mix in some traits and cope paste one line (val values = findValues) to have all functionalities of Enumeration and more
import enumeratum._
sealed trait DAdder extends canadd
with EnumEntry // mixin here
object DAdder extends Enum[DAdder] { // and mixin here
case object P extends DAdder
case object Q extends DAdder
val values = findValues // and one line calling macro
}
(There are also specializations for enums that should store some primitives/strings as their values in enumeratum.values._).
In Scala 3 this will probably look slightly different as it introduced enum keyword
sealed trait DAdder extends canadd
object DAdder {
case object P extends DAdder
case object Q extends DAdder
}
will become
enum DAdder extends canadd {
case P, Q
}
Scala 3's enum will ordinal method defined, so libraries like Enumeratum will have to provide a little fewer functionalities. (I guess by mixing in Java's Enum trait you will have access to values so everything else is a matter of extension methods).