Fogus.scala

import shapeless._

// fogus wanted this:
// [a] -> [b] -> [a b a b ...]

// Follow up question: "Do you know a dependent language with my interleave?"
// Yes. Scala and shapeless even make it a single line!

trait AB[-L <: HList, A, B]

object AB {
  implicit def hnilAB[A, B] = new AB[HNil, A, B] {}
  implicit def hconsAB[A, B, T <: HList](implicit abt: AB[T, B, A]) = new AB[A :: T, A, B] {}
}

object Fogus {
  val a = 1 :: 2 :: 3 :: HNil
  val b = "one" :: "two" :: "three" :: HNil
 
  val interleaved = (a :: b :: HNil).transpose.flatMap(identity)
  // 1 :: one :: 2 :: two :: 3 :: three :: HNil

  // proof that interleaved alternates Int and Strings
  implicitly[AB[interleaved.type, Int, String]]
}