Parameterized enums

Enums can be parameterized.

enum Color(val rgb: Int):
  case Red   extends Color(0xFF0000)
  case Green extends Color(0x00FF00)
  case Blue  extends Color(0x0000FF)

As the example shows, you can define the parameter value by using an explicit extends clause.

Methods defined for enums

The values of an enum correspond to unique integers. The integer associated with an enum value is returned by its ordinal method:

scala> val red = Color.Red
val red: Color = Red
scala> red.ordinal
val res0: Int = 0

The companion object of an enum also defines three utility methods. The valueOf method obtains an enum value by its name. The values method returns all enum values defined in an enumeration in an Array. The fromOrdinal method obtains an enum value from its ordinal (Int) value.

scala> Color.valueOf("Blue")
val res0: Color = Blue
scala> Color.values
val res1: Array[Color] = Array(Red, Green, Blue)
scala> Color.fromOrdinal(0)
val res2: Color = Red

User-defined members of enums

It is possible to add your own definitions to an enum. Example:

enum Planet(mass: Double, radius: Double):
  private final val G = 6.67300E-11
  def surfaceGravity = G * mass / (radius * radius)
  def surfaceWeight(otherMass: Double) = otherMass * surfaceGravity

  case Mercury extends Planet(3.303e+23, 2.4397e6)
  case Venus   extends Planet(4.869e+24, 6.0518e6)
  case Earth   extends Planet(5.976e+24, 6.37814e6)
  case Mars    extends Planet(6.421e+23, 3.3972e6)
  case Jupiter extends Planet(1.9e+27,   7.1492e7)
  case Saturn  extends Planet(5.688e+26, 6.0268e7)
  case Uranus  extends Planet(8.686e+25, 2.5559e7)
  case Neptune extends Planet(1.024e+26, 2.4746e7)
end Planet

User-defined companion object of enums

It is also possible to define an explicit companion object for an enum:

object Planet:
  def main(args: Array[String]) =
    val earthWeight = args(0).toDouble
    val mass = earthWeight / Earth.surfaceGravity
    for p <- values do
      println(s"Your weight on $p is ${p.surfaceWeight(mass)}")
end Planet

Restrictions on Enum Cases

Enum case declarations are similar to secondary constructors: they are scoped outside of the enum template, despite being declared within it. This means that enum case declarations cannot access inner members of the enum class.

Similarly, enum case declarations may not directly reference members of the enum's companion object, even if they are imported (directly, or by renaming). For example:

import Planet.*
enum Planet(mass: Double, radius: Double):
  private final val (mercuryMass, mercuryRadius) = (3.303e+23, 2.4397e6)

  case Mercury extends Planet(mercuryMass, mercuryRadius)             // Not found
  case Venus   extends Planet(venusMass, venusRadius)                 // illegal reference
  case Earth   extends Planet(Planet.earthMass, Planet.earthRadius)   // ok
object Planet:
  private final val (venusMass, venusRadius) = (4.869e+24, 6.0518e6)
  private final val (earthMass, earthRadius) = (5.976e+24, 6.37814e6)
end Planet

The fields referenced by Mercury are not visible, and the fields referenced by Venus may not be referenced directly (using import Planet.*). You must use an indirect reference, such as demonstrated with Earth.

Deprecation of Enum Cases

As a library author, you may want to signal that an enum case is no longer intended for use. However you could still want to gracefully handle the removal of a case from your public API, such as special casing deprecated cases.

To illustrate, say that the Planet enum originally had an additional case:

 enum Planet(mass: Double, radius: Double):
   ...
   case Neptune extends Planet(1.024e+26, 2.4746e7)
+  case Pluto   extends Planet(1.309e+22, 1.1883e3)
 end Planet

We now want to deprecate the Pluto case. First we add the scala.deprecated annotation to Pluto:

 enum Planet(mass: Double, radius: Double):
   ...
   case Neptune extends Planet(1.024e+26, 2.4746e7)
-  case Pluto   extends Planet(1.309e+22, 1.1883e3)
+
+  @deprecated("refer to IAU definition of planet")
+  case Pluto extends Planet(1.309e+22, 1.1883e3)
 end Planet

Outside the lexical scopes of enum Planet or object Planet, references to Planet.Pluto will produce a deprecation warning, but within those scopes we can still reference it to implement introspection over the deprecated cases:

trait Deprecations[T <: reflect.Enum] {
  extension (t: T) def isDeprecatedCase: Boolean
}

object Planet {
  given Deprecations[Planet] with {
    extension (p: Planet)
      def isDeprecatedCase = p == Pluto
  }
}

We could imagine that a library may use type class derivation to automatically provide an instance for Deprecations.

Compatibility with Java Enums

If you want to use the Scala-defined enums as Java enums, you can do so by extending the class java.lang.Enum, which is imported by default, as follows:

enum Color extends Enum[Color] { case Red, Green, Blue }

The type parameter comes from the Java enum definition and should be the same as the type of the enum. There is no need to provide constructor arguments (as defined in the Java API docs) to java.lang.Enum when extending it – the compiler will generate them automatically.

After defining Color like that, you can use it like you would a Java enum:

scala> Color.Red.compareTo(Color.Green)
val res15: Int = -1

For a more in-depth example of using Scala 3 enums from Java, see this test. In the test, the enums are defined in the MainScala.scala file and used from a Java source, Test.java.

Implementation

Enums are represented as sealed classes that extend the scala.reflect.Enum trait. This trait defines a single public method, ordinal:

package scala.reflect

/** A base trait of all Scala enum definitions */
transparent trait Enum extends Any, Product, Serializable:

  /** A number uniquely identifying a case of an enum */
  def ordinal: Int

Enum values with extends clauses get expanded to anonymous class instances. For instance, the Venus value above would be defined like this:

val Venus: Planet = new Planet(4.869E24, 6051800.0):
  def ordinal: Int = 1
  override def productPrefix: String = "Venus"
  override def toString: String = "Venus"

Enum values without extends clauses all share a single implementation that can be instantiated using a private method that takes a tag and a name as arguments. For instance, the first definition of value Color.Red above would expand to:

val Red: Color = $new(0, "Red")

Reference

For more information, see Issue #1970 and PR #4003.