Combine 2 shapeless HLists of the same type using cats Semigroup

semigroup-hlist.scala

import shapeless._
import cats.std.all._

trait ShapelessCatsSemigroup {
  implicit object HNilSemigroup extends Semigroup[HNil] {
    override def combine(h1: HNil, h2: HNil): HNil = HNil
  }

  implicit def HListSemigroup[H : Semigroup, T <: HList : Semigroup]: Semigroup[H :: T] =
    new Semigroup[H :: T] {
      override def combine(hlist1: H :: T, hlist2: H :: T): H :: T = (hlist1, hlist2) match {
        case (h1 :: t1, h2 :: t2) => Semigroup[H].combine(h1, h2) :: Semigroup[T].combine(t1, t2)
      }
    }
}

implicit class ShapelessCatsSemigroupOps[H <: HList](val h1: H) extends AnyVal {
  def combine(h2: H)(implicit semigroup: Semigroup[H]): H = semigroup.combine(h1, h2)
}

object ShapelessCats
  extends ShapelessCatsSemigroup


import ShapelessCats._

val h1: String :: Int :: HNil = "foo" :: 2 :: HNil
val h2: String :: Int :: HNil = "bar" :: 3 :: HNil

println(Semigroup[HNil].combine(HNil, HNil))

println(Semigroup[String :: Int :: HNil].combine(h1, h2))
println(h1.combine(h2))

// Output:
// HNil
// foobar :: 5 :: HNil
// foobar :: 5 :: HNil