Permutations of a list...

P26.scala

object P26 extends App {
  def combinations[T](n: Int, l: List[T]): List[List[T]] = {
    if(l.size < n) {
      Nil
    } else {
	  n match {
	    case 0 => Nil
	    case 1 => l.map{List(_)}
	    case _ => combinations(n-1,l.tail).map{ l.head :: _} ::: combinations(n, l.tail)
	  }
    }
  }
  println(combinations(3, List('a, 'b, 'c, 'd, 'e, 'f)))
}

//Output:
// List(List('a, 'b, 'c), List('a, 'b, 'd), List('a, 'b, 'e), List('a, 'b, 'f), List('a, 'c, 'd), List('a, 'c, 'e), List('a, 'c, 'f), List('a, 'd, 'e), List('a, 'd, 'f), List('a, 'e, 'f), List('b, 'c, 'd), List('b, 'c, 'e), List('b, 'c, 'f), List('b, 'd, 'e), List('b, 'd, 'f), List('b, 'e, 'f), List('c, 'd, 'e), List('c, 'd, 'f), List('c, 'e, 'f), List('d, 'e, 'f))