Interleaving HLists in Scala and shapeless.

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!

object Fogus {
  val a = 1 :: true :: "three" :: HNil
  val b = "four" :: 5 :: false :: HNil

  (a :: b :: HNil).transpose.flatMap(identity)
  // 1 :: four :: true :: 5 :: three :: false :: HNil
}