http://stackoverflow.com/a/25559471/4933928

ShapelessTypeClass.scala

import shapeless._
import shapeless.test._

trait Weight[A] { def apply(a: A): Int }

object Weight {
  def apply[A](f: A => Int) = new Weight[A] { def apply(a: A) = f(a) }
}

case class Foo(i: Int, s: String)

sealed trait Root
case class Bar(i: Int) extends Root
case class Baz(s: String) extends Root

object Base {
  implicit val stringWeight: Weight[String] = Weight(_.size)
  implicit def intWeight: Weight[Int] = Weight(identity)
}

object Product {
  object WeightHelper extends ProductTypeClassCompanion[Weight] {
    object typeClass extends ProductTypeClass[Weight] {
      def emptyProduct: Weight[HNil] = Weight(_ => 0)
      def product[H, T <: HList](hw: Weight[H], tw: Weight[T]): Weight[H :: T] =
        Weight { case (h :: t) => hw(h) + tw(t) }
      def project[F, G](w: => Weight[G], to: F => G,from: G => F): Weight[F] =
        Weight(f => w(to(f)))
    }
  }

  import Base._
  import WeightHelper._

  implicitly[Weight[Foo]]
  implicitly[Weight[Bar]]
  implicitly[Weight[Baz]]

  illTyped("implicitly[Weight[Root]]")
}

object TC {
  object WeightTypeClass extends TypeClassCompanion[Weight] {
    object typeClass extends TypeClass[Weight] {
      def emptyProduct: Weight[HNil] = Weight(_ => 0)
      def product[H, T <: HList](hw: Weight[H], tw: Weight[T]): Weight[H :: T] =
        Weight { case (h :: t) => hw(h) + tw(t) }
      def project[F, G](w: => Weight[G], to: F => G, from: G => F): Weight[F] =
        Weight(f => w(to(f)))
      def emptyCoproduct: Weight[CNil] = Weight(_ => 0)
      def coproduct[L, R <: Coproduct]
        (lw: => Weight[L], rw: => Weight[R]): Weight[L :+: R] = Weight {
        case Inl(h) => lw(h)
        case Inr(t) => rw(t)
      }
    }
  }

  import Base._
  import WeightTypeClass._

  implicitly[Weight[Root]]
}

object Main extends App {
}