gistfile1.scala

// An ADT+shapeless as a drop-in replacement for a standard Scala Enumeration.
//
// First the unsafe standard Scala Enumeration ...
//
object ScalaEnumDemo extends App {
  // Example from scala.Enumeration scaladoc. Terse ...
  object WeekDay extends Enumeration {
    type WeekDay = Value
    val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value
  }

  import WeekDay._

  def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)

  assert((WeekDay.values filter isWorkingDay) == Set(Mon, Tue, Wed, Thu, Fri))

  // However ...

  def isWeekend(d: WeekDay) = d match {
    case Sat | Sun => true
    // Oops! Missing case ... still compiles
  }

  assert(!isWeekend(Mon)) // MatchError at run time
}

// A safer ADT+shapeless alternative ...
//
object ShapelessEnumDemo extends App {
  // ADT as an enumeration. A little more boilerplate ...
  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 Sat extends WeekDay
    case object Sun extends WeekDay

    val values: Set[WeekDay] = Values
  }

  import WeekDay._

  def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)

  assert((WeekDay.values filter isWorkingDay) == Set(Mon, Tue, Wed, Thu, Fri))

  // ... the payoff ...

  def isWeekend(d: WeekDay) = d match {
    case Sat | Sun => true
    case _ => false // compile time non-exhaustive match warning/error without this case
  }

  assert(!isWeekend(Mon)) // 
}

// Infrastructure for the above. Original version due to Travis Brown,
//
//   http://stackoverflow.com/questions/25838411
//
object Values {
  implicit def conv[T](self: this.type)(implicit v: MkValues[T]): Set[T] = Values[T]

  def apply[T](implicit v: MkValues[T]): Set[T] = v.values.toSet

  trait MkValues[T] {
    def values: List[T]
  }

  object MkValues {
    import shapeless._

    implicit def values[T, Repr <: Coproduct]
      (implicit gen: Generic.Aux[T, Repr], v: Aux[T, Repr]): MkValues[T] =
        new MkValues[T] { def values = v.values }

    trait Aux[T, Repr] {
      def values: List[T]
    }

    object Aux {
      implicit def cnilAux[A]: Aux[A, CNil] =
        new Aux[A, CNil] { def values = Nil }

      implicit def cconsAux[T, L <: T, R <: Coproduct]
        (implicit l: Witness.Aux[L], r: Aux[T, R]): Aux[T, L :+: R] =
        new Aux[T, L :+: R] { def values = l.value :: r.values }
    }
  }
}