Scala Language Enumerations Using sealed trait and case objects and allValues-macro
Example
This is just an extension on the sealed trait variant where a macro generates a set with all instances at compile time. This nicely omits the drawback that a developer can add a value to the enumeration but forget to add it to the allElements set.
This variant especially becomes handy for large enums.
import EnumerationMacros._
sealed trait WeekDay
object WeekDay {
case object Mon extends WeekDay
case object Tue extends WeekDay
case object Wed extends WeekDay
case object Thu extends WeekDay
case object Fri extends WeekDay
case object Sun extends WeekDay
case object Sat extends WeekDay
val allWeekDays: Set[WeekDay] = sealedInstancesOf[WeekDay]
}
For this to work you need this macro:
import scala.collection.immutable.TreeSet
import scala.language.experimental.macros
import scala.reflect.macros.blackbox
/**
A macro to produce a TreeSet of all instances of a sealed trait.
Based on Travis Brown's work:
http://stackoverflow.com/questions/13671734/iteration-over-a-sealed-trait-in-scala
CAREFUL: !!! MUST be used at END OF code block containing the instances !!!
*/
object EnumerationMacros {
def sealedInstancesOf[A]: TreeSet[A] = macro sealedInstancesOf_impl[A]
def sealedInstancesOf_impl[A: c.WeakTypeTag](c: blackbox.Context) = {
import c.universe._
val symbol = weakTypeOf[A].typeSymbol.asClass
if (!symbol.isClass || !symbol.isSealed)
c.abort(c.enclosingPosition, "Can only enumerate values of a sealed trait or class.")
else {
val children = symbol.knownDirectSubclasses.toList
if (!children.forall(_.isModuleClass)) c.abort(c.enclosingPosition, "All children must be objects.")
else c.Expr[TreeSet[A]] {
def sourceModuleRef(sym: Symbol) = Ident(sym.asInstanceOf[scala.reflect.internal.Symbols#Symbol
].sourceModule.asInstanceOf[Symbol]
)
Apply(
Select(
reify(TreeSet).tree,
TermName("apply")
),
children.map(sourceModuleRef(_))
)
}
}
}
}
