test.scala

import shapeless._

trait F[A] {
  def xmap[B](f: A => B, g: B => A): F[B]
  def as[B] = new AsAux[B]
  class AsAux[B] {
    def apply[C](implicit gen: Generic.Aux[B, C], ev: A =:= C): F[B] =
      xmap(a => gen.from(ev(a)), b => gen.to(b).asInstanceOf[A])
  }
}

case class Bar(x: Int, y: String)

object Example {
  val a: F[Int :: String :: HNil] = null
  val b: F[Bar] = a.as[Bar].apply // TODO Get rid of need to call apply here
}