EnumCoproduct.scala

import shapeless._
import scala.annotation.implicitNotFound

/*
 * Used type names:
 * - A = type of the sealed trait
 * - C = type of the coproduct
 * - H = type of the iterated singleton (case object) instance
 * - T = type of the iterated coproduct
 */

trait AllSingletons[A, C <: Coproduct] {
  def values: List[A]
}

object AllSingletons {
  implicit def cnilSingletons[A]: AllSingletons[A, CNil] = new AllSingletons[A, CNil] {
    def values: Nil.type = Nil
  }

  implicit def coproductSingletons[A, H <: A, T <: Coproduct](
      implicit tail: AllSingletons[A, T],
      witness: Witness.Aux[H]): AllSingletons[A, H :+: T] = new AllSingletons[A, H :+: T] {

    def values: List[A] = witness.value :: tail.values
  }
}

@implicitNotFound(
  "Cannot enumerate the ${A} sealed trait. Make sure only case objects extends this trait.")
trait EnumCoproduct[A] {

  /**
    * Returns all ''case object'' instances for the given ''sealed trait''. If any ''case class''
    * extends this sealed trait, this call will end up with compile error.
    *
    * @return set of case objects extending sealed trait
    */
  def values: Set[A]
}

/**
  * Utilities allowing to enumerate the case objects of the sealed trait.
  */
object EnumCoproduct {
  implicit def fromAllSingletons[A, C <: Coproduct](
      implicit gen: Generic.Aux[A, C],
      singletons: AllSingletons[A, C]): EnumCoproduct[A] = new EnumCoproduct[A] {

    def values: Set[A] = singletons.values.toSet
  }

  def apply[A](implicit instance: EnumCoproduct[A]): EnumCoproduct[A] = instance
}