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))

permutation.clj

(defn permute [n lst]
  (let [[x & l] lst]
    (cond
      (or (< (count lst) n) (= n 0)) ()
      (= 1 n) (map #(list %) lst)
      :else (concat (map #(conj % x) (permute (dec n) l)) (permute n l)))))

(permute 3 [:a, :b, :c, :d, :e, :f])          

;OUTPUT
;((:a :b :c) (:a :b :d) (:a :b :e) (:a :b :f) (:a :c :d) (:a :c :e) (:a :c :f) (:a :d :e) (:a :d :f) (:a :e :f) (:b :c :d) (:b ;:c :e) (:b :c :f) (:b :d :e) (:b :d :f) (:b :e :f) (:c :d :e) (:c :d :f) (:c :e :f) (:d :e :f))

permutation2.clj

(defn permute [n l]
  (if (< (count l) n) ()
    (case n
      0 ()
      1 (map #(list %) l)
      (concat (map #(conj % (first l)) (permute (dec n) (rest l))) (permute n (rest l))))))

(permute 3 [:a, :b, :c, :d, :e, :f]) 

;OUTPUT
;((:a :b :c) (:a :b :d) (:a :b :e) (:a :b :f) (:a :c :d) (:a :c :e) (:a :c :f) (:a :d :e) (:a :d :f) (:a :e :f) (:b :c :d) (:b ;:c :e) (:b :c :f) (:b :d :e) (:b :d :f) (:b :e :f) (:c :d :e) (:c :d :f) (:c :e :f) (:d :e :f))