arity.scala

package fail.sauce.example123

import cats.Monad

import shapeless.{HList, HNil, ::}
import shapeless.Poly2
import shapeless.UnaryTCConstraint
import shapeless.UnaryTCConstraint.*->*
import shapeless.ops.hlist.RightFolder

object Example extends App {
  println("example time")

  object monadFolder extends Poly2 {
    implicit def caseApplicative[A, B <: HList, F[_]: Monad] = at[F[A], F[B]] {
      (a, b) => Monad[F].ap(Monad[F].map(b)(bb => (_: A) :: bb))(a)
    }
  }

  def hlistMagic1[F[_]: Monad, L <: HList: *->*[F]#λ, M <: HList](l: L)(
    implicit
    folder: RightFolder.Aux[L, F[HNil], monadFolder.type, F[M]]
  ): F[M] = l.foldRight(Monad[F].pure(HNil: HNil))(monadFolder)

  // using context bounds and kind projector plugin (instead of type lambdas)
  def hlistMagic2[
    F[_]: Monad,
    L <: HList : UnaryTCConstraint[?, F] : RightFolder.Aux[?, F[HNil], monadFolder.type, F[M]],
    M <: HList
  ] (l: L): F[M] =
    l.foldRight(Monad[F].pure(HNil: HNil))(monadFolder)


  import cats.std.option._

  val input = Option("hello") :: Option(1) :: Option("world") :: HNil
  val output1: Option[String :: Int :: String :: HNil] = hlistMagic1(input)
  val output2: Option[String :: Int :: String :: HNil] = hlistMagic2(input)

  println("output1 " + output1)
  println("output2 " + output2)

}