shapeless wizardry

gistfile1.scala

object foo extends App {
  case class Elem(a: Int, b: String, c: Int)

  val l = Elem(1, "asf", 2)

  trait R[T] {
    def r(x: T): T
  }

  object defaultConvs extends Poly1 {
    implicit def default[T: R] = at[T](implicitly[R[T]].r)
  }

  def keep[T]: R[T] = new R[T] {
    def r(t: T) = t
  }

  implicit val stringR: R[String] = new R[String] {
    def r(x: String): String = {
      val y = x.reverse
      println(y)
      y
    }
  }

  implicit val intR: R[Int] = new R[Int] {
    def r(x: Int): Int = {
      val y = x * 2
      println(y)
      y
    }
  }

  // ayyyyyyyyy lmao it works
  class R2[T, L <: HList](g: Generic[T] { type Repr = L })(implicit m: Mapper[defaultConvs.type, L]) extends R[T] {
    def r(x: T): T = {
      g.from((g.to(x) map defaultConvs).asInstanceOf[L])
    }
  }
  def scrub[T: R](x: T): T = implicitly[R[T]].r(x)

  implicit val elemR = new R2(Generic[Elem])
  val m = scrub(l)
  println(m) //Elem(2,fsa,4)
  println(l) //Elem(1,asf,2)

}