For-comprehension

for_comprehension.scala

def f(xs: List[Int]): List[List[(Int, Int)]] = {
  xs match {
    case Nil => List(List()) 
    case x :: xs => for {
      ns <- f(xs)
      i <- 0 to x
      } yield (x, i) :: ns
  }
}

f(List(1, 2, 3))

// f(List(1, 2))
// res0: List[List[(Int, Int)]] = List(List((1,0), (2,0)), List((1,1), (2,0)), List((1,0), (2,1)), List((1,1), (2,1)), List((1,0), (2,2)), List((1,1), (2,2)))

List(('a', 1), ('d', 1), ('l', 1), ('r', 1)).toMap



def substract(xs: List[(Char, Int)], ys: List[(Char, Int)]): List[(Char, Int)] = {
  def substractAux(xs: List[(Char, Int)], ys: List[(Char, Int)]): List[(Char, Int)] = {
    for {
      (char, occ) <- xs
      } yield ys.toMap.get(char) match {
        case Some(o) => (char, occ - o)
        case _ => (char, occ)
        }
  }
  substractAux(xs, ys).filter(_._2 > 0)
}

substract(List(('a', 1), ('d', 1), ('l', 1), ('r', 1)), List(('r', 1)))