(disjoint) unions, second attempt

AEither.scala

trait AEither[L <: TList] {

  // an `HList` where all elements are functions producing `R`
  type Fold[R] = L#ToKList[({ type λ[α] = α => R })#λ]#Down

  def fold[R](list: Fold[R]): R

}

case class AEitherCons[H, T <: TList](either: Either[H, AEither[T]]) extends AEither[TCons[H, T]] {

  def fold[R](list: Fold[R]): R = either.fold(list.head, (t: AEither[T]) => t.fold[R](list.tail))

}

case class AEitherEnd[T](elem: T) extends AEither[TCons[T, TNil]] {

  def fold[R](list: Fold[R]): R = list.head(elem)

}

// Usage

scala> val list = ((x: Int) => "int") :: ((s: String) => "string") :: HNil

scala> val x: AEither[TCons[Int, TCons[String, TNil]]] = AEitherCons(Left(3))
x: AEither[TCons[Int,TCons[String,TNil]]] = AEitherCons(Left(3))

scala> x.fold(list)
res0: java.lang.String = int


// also, this works:

scala> AEitherCons(Right(AEitherEnd("foo"))).fold[String](list)
res3: String = string

// but this doesn't (which is okay, I'd say):

scala> AEitherCons(Left(3)).fold[String](list)

// what would be cool:

val x: AEither[Int :|: String :|: TNil] = AEither[_1]("foo")