iota_fold_cop.scala

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

  class CopFoldTo[B] {
    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)
  }

  object Finn
  object Cinder
  object Bridger

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

  val dog0 = Finn.inject[Dog]
  val dog1 = Cinder.inject[Dog]

  val res = foldTo[String](dog0)(Prod(
    (finn: Finn.type) => s"we got finn $finn",
    (cinder: Cinder.type) => s"we got cinder $cinder",
    (bridger: Bridger.type) => s"we got bridger $bridger"))

  println(res)