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DIY Scala Enums (with optional exhaustiveness checking)
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) = other.asInstanceOf[AnyRef] eq this
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,
**/
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