combinations.scala

val combinationsLetters = (for {
    (i, idi) <- List("A", "B", "C", "D", "E").zipWithIndex
    (j, idj) <- List("A", "B", "C", "D", "E").zipWithIndex.take(idi)
    (k, _) <- List("A", "B", "C", "D", "E").zipWithIndex.take(idj)
  } yield (i, j, k))

  val combinationsNumbers = (for {
    (i, idi) <- List("1", "2", "3").zipWithIndex
    (j, _) <- List("1", "2", "3").zipWithIndex.take(idi)
  } yield (i, j))

  val combinationsAll = (for {
    i <- combinationsLetters
    j <- combinationsNumbers
  } yield (i, j))