Example of summoning an HList of type class instances with Shapeless

ex.scala

import shapeless._

package object ex {

  implicit class TypeClassHListOps[L <: HList](val self: L) extends AnyVal {
    def summon[TC[_]](implicit op: HListTypeClassSummoner[TC, L]) = op.instance
  }

  trait HListTypeClassSummoner[TC[_], L <: HList] {
    type Out <: HList
    val instance: Out
  }

  object HListTypeClassSummoner {

    type Aux[TC[_], L <: HList, O <: HList] = HListTypeClassSummoner[TC, L] { type Out = O }

    implicit def forHNil[TC[_]]: Aux[TC, HNil, HNil] = new HListTypeClassSummoner[TC, HNil] { type Out = HNil; val instance = HNil }

    implicit def forHCons[TC[_], H, T <: HList, TCT <: HList](implicit
      tch: TC[H],
      rest: HListTypeClassSummoner.Aux[TC, T, TCT]
    ): Aux[TC, H :: T, TC[H] :: TCT] = new HListTypeClassSummoner[TC, H :: T] {
      type Out = TC[H] :: TCT
      val instance = tch :: rest.instance
    }
  }

  // Example
  trait Semigroup[A] { def op(x: A, y: A): A }
  implicit val sgInt = new Semigroup[Int] { def op(x: Int, y: Int) = x + y }
  implicit val sgString = new Semigroup[String] { def op(x: String, y: String) = x + y }
  implicit def sgList[A] = new Semigroup[List[A]] { def op(x: List[A], y: List[A]) = x ++ y }

  val xs = 1 :: "Hello" :: List(1, 2, 3) :: HNil
  val tc = xs.summon[Semigroup]
}