countrefs.scala

import shapeless._

final class AnyRefCounter[L <: HList, N <: Nat]
object AnyRefCounter {
  implicit def empty = new AnyRefCounter[HNil, Nat._0]
  implicit def anyRef[H <: AnyRef, T <: HList, N <: Nat](implicit c: AnyRefCounter[T, N]) = new AnyRefCounter[H :: T, Succ[N]]
  implicit def anyVal[H <: AnyVal, T <: HList, N <: Nat](implicit c: AnyRefCounter[T, N]) = new AnyRefCounter[H :: T, N]

  def countRefs[L <: HList, N <: Nat](xs: L)(implicit c: AnyRefCounter[L, N], n: ToInt[N]): Int = n()
}