ex8 - https://medium.com/@lu4nm3/use-adt-specific-behaviors-on-a-common-interface-76ae82ee27bc

ex8_adt.scala

import shapeless._

object PriceCompanion extends LabelledTypeClassCompanion[Price] {

  object typeClass extends LabelledTypeClass[Price] {
    def emptyProduct: Price[HNil] = new Price[HNil] {
      def price(t: HNil) = 0.0
    }

    def product[F, T <: HList](name: String, sh: Price[F], st: Price[T]): Price[F :: T] = new Price[F :: T] {
      def price(ft: F :: T): Double = {
        sh.price(ft.head) + st.price(ft.tail)
      }
    }

    def emptyCoproduct: Price[CNil] = new Price[CNil] {
      def price(t: CNil): Double = 0.0
    }

    def coproduct[L, R <: Coproduct](name: String, rl: => Price[L], rr: => Price[R]): Price[L :+: R] = new Price[L :+: R] {
      def price(lr: L :+: R): Double = lr match {
        case Inl(l) => rl.price(l)
        case Inr(r) => rr.price(r)
      }
    }

    def project[F, G](instance: => Price[G], to: F => G, from: G => F): Price[F] = new Price[F] {
      def price(f: F): Double = {
        instance.price(to(f))
      }
    }
  }

}