Function isSafe for NQueens problem

NQueensProblem.scala

object NQueensProblem {
   def queens(n: Int): List[List[Int]] = {
      def placeQueens(k: Int): List[List[Int]] =
         if (k == 0) List(List())
         else for { queens <- placeQueens(k - 1)
                     column <- List.range(1, n + 1)
                     if isSafe(column, queens, 1)} yield column :: queens
      placeQueens(n)
   }
   def isSafe(col: Int, queens: List[Int], delta: Int): Boolean = queens match {
      case Nil => true
      case x :: xs => col != x && col != x - delta && col != x + delta &&
               isSafe(col, xs, delta + 1)
   }
   
   def main(args: Array[String]) {
      println ("Solving nqueens: ")
      val x = queens(8)
      println("Number of solutions: " + x.length)
      println(x)
   }
}