What is the purpose of the emptyCoproduct and coproduct methods of the TypeClass trait in Shapeless

Question

It is not fully clear to me what is the purpose of the emptyCoProduct and coproduct methods of the TypeClass trait in Shapeless.

W

Answer 1

As of shapeless 2.3.2, the above example doesn't seem to compile. here's the updated one for future reference:

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 ProductTC {
  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]]
}