Last element of a product

last.scala

  import shapeless._
  import scala.annotation.implicitNotFound

  trait DeepFn1[T] {
    type Out
    def apply(t: T): Out
  }

  @implicitNotFound("Implicit not found: Last[${L}]. ${L} is empty, so there is no last element.")
  trait Last[L] extends DeepFn1[L]

  object Last extends LastSyntax {
    def apply[L](implicit last: Last[L]): Aux[L, last.Out] = last

    type Aux[L, Out0] = Last[L] {type Out = Out0}

    implicit def hsingleLast[H]: Aux[H :: HNil, H] =
      new Last[H :: HNil] {
        type Out = H
        def apply(h: H :: HNil): Out = h.head
      }

    implicit def hlistLast[H, T <: HList, OutT]
    (implicit last: Last.Aux[T, OutT]): Aux[H :: T, OutT] =
      new Last[H :: T] {
        type Out = OutT
        def apply(hlist: H :: T): Out = last(hlist.tail)
      }

    implicit def genericLast[In, Gen, OutT]
    (implicit gen: Generic.Aux[In, Gen],
     last: Last.Aux[Gen, OutT]): Last.Aux[In, OutT] =
      new Last[In] {
        type Out = OutT
        def apply(in: In): OutT =
          last(gen.to(in))
      }
  }

  trait LastSyntax {
    implicit def toLastOps[A](a: A)(implicit ev: Last[A]): LastOps[A, ev.Out] =
      new LastOps[A, ev.Out] {
        def last: ev.Out = ev(a)
      }

    sealed trait LastOps[A, Out] {
      def last: Out
    }
  }

  import Last._

  case class Person(name: String, age: Int)
  val john = Person("John", 25)
  val age: Int = john.last // 25