hlists-and-semigroups.scala

object gist {

  import shapeless._

  trait Semigroup[A] {
    def combine(a0: A, a1: A): A
  }

  implicit final class SemigroupCombineOps[A](a0: A)(implicit S: Semigroup[A]) {
    def combine(a1: A): A = S.combine(a0, a1)
  }

  //The HList here is a meta tag - we never instantiate it.  The order of elements in the hlist isn't important.
  case class Quantity[H <: HList](value: Double)

  object Quantity {
    implicit def quantitySemigroup[H <: HList]: Semigroup[Quantity[H]] = new Semigroup[Quantity[H]] {
      def combine(q0: Quantity[H], q1: Quantity[H]): Quantity[H] = Quantity[H](q0.value + q1.value)
    }
  }


  object ThisCantWork {
    type Foo
    type Bar

    //for all intents and purposes, these represent the same thing.  Physically, we should be able to combine them.
    val q0: Quantity[Foo :: Bar :: HNil] = Quantity(1.0)
    val q1: Quantity[Bar :: Foo :: HNil] = Quantity(2.0)
    //of course, we can't combine them - the types are different
    //q0 combine q1
  }

  object WorkAround {
    import shapeless.ops.hlist.Align
    //we can safely cast, since both hlists have the same elements
    implicit def realignQuantity[A <: HList, B <: HList](q: Quantity[A])(implicit ev: Align[A, B]): Quantity[B] = q.asInstanceOf[Quantity[B]]
    type Foo
    type Bar


    val q0: Quantity[Foo :: Bar :: HNil] = Quantity(1.0)
    val q1: Quantity[Bar :: Foo :: HNil] = Quantity(2.0)
    q0 combine q1
  }
}