folding_dogs.scala

  import iota._
  import iota.TList.::
  import iota.TList.Op
  import iota.syntax.inject._

  def foldTo[B]: CopFoldTo[B] = new CopFoldTo[B]

  class CopFoldTo[B] {
    // library code to add to iota
    def apply[L <: TList, LF <: TList](a: Cop[L])(fs: Prod[LF])(
      implicit ev: TList.Compute.Aux[Op.Map[? => B, L], LF]
    ): B =
      fs.values(a.index).asInstanceOf[Any => B].apply(a.value)
  }

  case class Finn()
  case class Cinder()
  case class Bridger()

  type Dog  = Cop[Finn :: Cinder :: Bridger :: TNil]

  val dogs: List[Dog] = List(
    Bridger().inject[Dog],
    Finn().inject[Dog],
    Cinder().inject[Dog],
    Bridger().inject[Dog],
    Finn().inject[Dog])

  val res: List[String] = dogs.map(dog =>
    foldTo[String](dog)(Prod(
      (f: Finn)    => s"we got finn $f",
      (c: Cinder)  => s"we got cinder $c",
      (b: Bridger) => s"we got bridger $b")))

  println(res)
  // List(
  //  we got bridger Bridger(),
  //  we got finn Finn(),
  //  we got cinder Cinder(),
  //  we got bridger Bridger(),
  //  we got finn Finn())