Dotty: Allowed modifiers for enums and enum cases

[allanrenucci]

What should be the allowed modifiers for enums and enum cases? What about:

• enums: access modifiers (private, protected) and annotations
• enum cases: annotations

[allanrenucci]

Related: #5034

[ctongfei]

Can we do implicit case in enums? This would make Shapeless-style recursive implicits much easier.

[milessabin]

@ctongfei could you elaborate on that a little? What shapeless idiom are you referring to and how do you see implicit cases mapping on to that?

[ctongfei]

@milessabin I once created this Shapless-style thing:

trait IndexOf[A, U] extends DepFn0 {
  type Out <: Nat
  def toInt: Int
}

object IndexOf {

  def apply[A, U](implicit o: IndexOf[A, U]): Aux[A, U, o.Out] = o
  type Aux[A, U, I <: Nat] = IndexOf[A, U] { type Out = I }

  implicit def case0[At <: HList, U]: Aux[U :: At, U, _0] =
    new IndexOf[U :: At, U] {
      type Out = _0
      def apply() = Nat._0
      def toInt = 0
    }

  implicit def caseN[At <: HList, Ah, U, I <: Nat]
  (implicit p: IndexOf.Aux[At, U, I]): Aux[Ah :: At, U, Succ[I]] =
    new IndexOf[Ah :: At, U] {
      type Out = Succ[I]
      def apply() = Succ[I]()
      def toInt = p.toInt + 1
    }
}

Using implicit cases, I hope that this could be simplified to something like this:

enum IndexOf[A, U] extends DepFn0 {
  type Out <: Nat
  def toInt: Int

  implicit case Case0[At <: HList, U] extends IndexOf[U :: At, U] {
      type Out = _0
      def apply() = Nat._0
      def toInt = 0
    }

  implicit case CaseN[At <: HList, Ah, U, I <: Nat]
  (implicit p: IndexOf.Aux[At, U, I]) extends IndexOf[Ah :: At, U] {
      type Out = Succ[I]
      def apply() = Succ[I]()
      def toInt = p.toInt + 1
    }

}

[milessabin]

I see, so the idea would be to use an enum as a shapeless Poly with implicit cases corresponding to the Case's of the Poly ... right?

[ctongfei]

@milessabin Yes that's my idea. You summarized it much better than I could.

[allanrenucci]

@ctongfei Please open a separate issue if you think this is a use case that enums should support. This is out of scope of the current proposal for several reasons:

• enum cases cannot have a body
• enum cases don't desugar to def. Simple cases will desugar to vals. More complex ones with type parameters and/or value parameters will desugar to classes. See http://dotty.epfl.ch/docs/reference/enums/desugarEnums.html