Transpose a List of HLists.

Transpose.scala

//see original SO answer: https://stackoverflow.com/a/49184270/1927425

import shapeless.ops.hlist._
import shapeless.{::, _}

trait EmptyOf[T <: HList, AsList <: HList] {
  def empty(): AsList
}

object EmptyOf {

  implicit val emptyOfHNil: EmptyOf[HNil, HNil] = () => HNil

  implicit def emptyOfList[H, T <: HList, TEmpty <: HList](
    implicit ev: EmptyOf[T, TEmpty]): EmptyOf[H :: T, List[H] :: TEmpty] =
    () => List.empty[H] :: ev.empty()
}

object Sample {

  object concat extends Poly2 {
    implicit def prepend[S] = at[List[S], S]((list, s) => s :: list)
  }

  def transpose[T <: HList, U <: HList](lists: List[T])(implicit
                                                                      emptyOf: EmptyOf[T, U],
                                                                      zip: ZipWith.Aux[U, T, concat.type, U]): U =
    lists.foldLeft(emptyOf.empty())(_.zipWith(_)(concat))

  def main(args: Array[String]): Unit = {
    val myList: List[Int :: String :: Boolean :: HNil] = List(
      1 :: "sample" :: true :: HNil,
      2 :: "sample2" :: false :: HNil,
      3 :: "sample3" :: true :: HNil
    )

    val actual: List[Int] :: List[String] :: List[Boolean] :: HNil = transpose(myList)

    val expected: List[Int] :: List[String] :: List[Boolean] :: HNil =
      List(3, 2, 1) ::
        List("sample3", "sample2", "sample") ::
        List(true, false, true) ::
        HNil

    println(actual == expected)
  }
}