DIY Scala Enums (with optional exhaustiveness checking)

ScalaEnum.scala

trait Enum { //DIY enum type
  import java.util.concurrent.atomic.AtomicReference //Concurrency paranoia

  type EnumVal <: Value //This is a type that needs to be found in the implementing class

  private val _values = new AtomicReference(Vector[EnumVal]()) //Stores our enum values

  //Adds an EnumVal to our storage, uses CCAS to make sure it's thread safe, returns the ordinal
  private final def addEnumVal(newVal: EnumVal): Int = { import _values.{get, compareAndSet => CAS}
    val oldVec = get
    val newVec = oldVec :+ newVal
    if((get eq oldVec) && CAS(oldVec, newVec)) newVec.indexWhere(_ eq newVal) else addEnumVal(newVal)
  }

  def values: Vector[EnumVal] = _values.get //Here you can get all the enums that exist for this type

  //This is the trait that we need to extend our EnumVal type with, it does the book-keeping for us
  protected trait Value { self: EnumVal => //Enforce that no one mixes in Value in a non-EnumVal type
    final val ordinal = addEnumVal(this) //Adds the EnumVal and returns the ordinal

    def name: String //All enum values should have a name

    override def toString = name //And that name is used for the toString operation
    override def equals(other: Any) = this eq other.asInstanceOf[AnyRef]
    override def hashCode = 31 * (this.getClass.## + name.## + ordinal)
  }
}

//And here's how to use it, if you want compiler exhaustiveness checking
object Foos extends Enum {
  sealed trait EnumVal extends Value /*{ you can define your own methods etc here }*/

  val F = new EnumVal { val name = "F" }
  val X = new EnumVal { val name = "X" }
}

/**
scala> Foos.values.find(_.name == "F")
res3: Option[Foos.EnumVal] = Some(F)

scala> Foos.X.ordinal
res4: Int = 1

scala> def doSmth(foo: Foos.EnumVal) = foo match {
  case Foos.X => println("pigdog")
}

<console>:10: warning: match is not exhaustive!
missing combination        $anon$1
missing combination        $anon$2

scala> def doSmth(foo: Foos.EnumVal) = foo match {
         case Foos.X => println("pigdog")
         case Foos.F => println("dogpig")
       }
doSmth: (foo: Foos.EnumVal)Unit
**/

//But if you don't care about getting exhaustiveness warnings, you can do:

object Foos extends Enum {
  case class EnumVal private[Foos](name: String) extends Value /* { you can define your own methods and stuff here } */

  val F = EnumVal("F")
  val X = EnumVal("X")
}

/**
Which is a bit less boilerplatey.


Cheers,
√
**/

Summary:
I can’t get Viktor’s code to do pattern matcher exhaustiveness checking. I figured out how to do it using case objects while also eliminating the class/object initialization ordering issues (which cause the ordinals to not consistently be assigned to the same case object instances). Here’s the tinyurl (longer one below): http://goo.gl/zlCs61

Details:
I have so longed for Java’s Enum in Scala (without having to created Scala/Java mixed project and resorting to using Java’s Enum). As I began a large project several months ago, I decided to try and get the Scala enumeration problem solved again. I was very hopeful someone had created a DIY solution...which worked. When I saw Viktor’s come up in my Google search, I was pretty excited, but...

I have tried extensively to get Viktor's code above to work. I have not found a way to get it to provide pattern matching exhaustiveness checking. And I have tried LOTS of variations.

So, I decided to to move to using case objects (as opposed to a series of val-s) which I knew worked with the pattern matcher’s exhaustiveness checker. I got right to the edge of something that worked exactly like I wanted...when I exposed an issue with class/object initialization ordering, which Rex Kerr and I worked through on StackOverflow (a couple of years ago):
http://stackoverflow.com/questions/14947179/using-a-custom-enum-in-the-scala-worksheet-i-am-receiving-an-error-java-lang-ex

ARGH!!!!!!!!

So, I have now invested a considerable amount of time wrestling many Scala and JVM demons to hack my way to runtime victory. Here is a link to org.public_domain.scala.utils.Enumeration which provides the closest analog to all of the benefits I got from Java’s Enum:

Final solution:
https://gist.github.com/chaotic3quilibrium/57add1cd762eb90f6b24#file-org-public_domain-scala-utils-enumeration-scala

Example usages:
Simple: https://gist.github.com/chaotic3quilibrium/57add1cd762eb90f6b24#file-org-public_domain-chess-chesspiecessimplest-scala

Enhanced:
https://gist.github.com/chaotic3quilibrium/57add1cd762eb90f6b24#file-org-public_domain-chess-chesspiecesenhanced-scala

I would appreciate any feedback you might have on this.

Thank you.

@chaotic3quilibrium - Wow. That code is pretty intense! Any thoughts on releasing it as a library?