Seamless tuple deconstruction in type parameter declaration

Question

I am using RightFolder, which returns Tuple2 and would like to return a part _1. The first version rightFoldUntupled1works fine, but uses an extra class IsComposite.

In the second version, rightFoldUntupled2I try to achieve the same without IsCompositedestructuring Pas Product2[_, Values]. It doesn't work anymore - the compiler happily testified that P is Product2[_,_], but as soon as I use the named type parameter for _1, it no longer compiles. Any idea why?

The full source (with sbt ready to run with implicit debugging output) is here: https://github.com/mpollmeier/shapeless-playground/tree/f3cf049

case class Label[A](name: String)

val label1 = Label[Int]("a")
val labels = label1 :: HNil

object getValue extends Poly2 {
  implicit def atLabel[A, B <: HList] = at[Label[A], (B, Map[String, Any])] {
    case (label, (acc, values)) ⇒
      (values(label.name).asInstanceOf[A] :: acc, values)
  }
}

// compiles fine
val untupled1: Int :: HNil = rightFoldUntupled1(labels)

// [error] could not find implicit value for parameter folder:
// shapeless.ops.hlist.RightFolder.Aux[shapeless.::[Main.DestructureTupleTest.Label[Int],shapeless.HNil],
//(shapeless.HNil, Map[String,Any]),Main.DestructureTupleTest.getValue.type,P]
val untupled2: Int :: HNil = rightFoldUntupled2(labels)

def rightFoldUntupled1[L <: HList, P <: Product2[_, _], Values <: HList](labels: L)(
  implicit folder: RightFolder.Aux[L, (HNil, Map[String, Any]), getValue.type, P],
  ic: IsComposite.Aux[P, Values, _]
): Values = {
  val state = Map("a" -> 5, "b" -> "five")
  val resultTuple = labels.foldRight((HNil: HNil, state))(getValue)
  ic.head(resultTuple)
}

def rightFoldUntupled2[L <: HList, Values, P <: Product2[_, Values]](labels: L)(
  implicit folder: RightFolder.Aux[L, (HNil, Map[String, Any]), getValue.type, P]
): Values = {
  val state = Map("a" -> 5, "b" -> "five")
  val resultTuple = labels.foldRight((HNil: HNil, state))(getValue)
  resultTuple._1
}

Answer 1

, rightFoldUntupled1:

def rightFoldUntupled2[L <: HList, Values, M](labels: L)(
  implicit folder: RightFolder.Aux[L, (HNil, Map[String, Any]), getValue.type, (Values, M)]
  ): Values = {
  val state = Map("a" -> 5, "b" -> "five")
  val resultTuple = labels.foldRight((HNil: HNil, state))(getValue)
  resultTuple._1
}