sequencing-hlists.scala

import scalaz._, Scalaz._, shapeless._, ops.hlist.{ RightFolder, Tupler }

// Might as well stay generic in `F` for this part.
object applicativeFolder extends Poly2 {
  implicit def caseApplicative[A, B <: HList, F[_]](implicit
    app: Applicative[F]
  ) = at[F[A], F[B]] {
    (a, b) => app.ap(a)(app.map(b)(bb => (_: A) :: bb))
  }
}

// It should be possible to make this part generic in `F` as well,
// but type inference makes it tricky, so we specialize to `Option`.
def sequence[T, EL <: HList, L <: HList, OL <: HList, OT](t: T)(implicit
  gen: Generic.Aux[T, EL],
  eq: EL =:= L,
  folder: RightFolder.Aux[L, Option[HNil], applicativeFolder.type, Option[OL]],
  tupler: Tupler.Aux[OL, OT]
): Option[OT] =
  eq(gen.to(t)).foldRight(some(HNil: HNil))(applicativeFolder).map(tupler(_))

If you provide the zero element you can:

def sequence[OL <: HList, F[_], L <: HList, T]
  (l: L, z: F[T])
  (implicit folder: RightFolder.Aux[L, F[T], applicativeFolder.type, F[OL]]) = l.foldRight(z)(applicativeFolder)