Quicksort with clojure

sort.clj

(ns sort)

(defn qsort [l]
  (if (first l)
    (concat
     (qsort (filter #(< (first l) %) (rest l)))
     [(first l)]
     (qsort (filter #(>= (first l) %) (rest l))))
    ()))

(defn qsort2 [[x & xs]]
  (if x
    (concat
     (qsort2 (filter #(< x %) xs))
     [x]
     (qsort2 (filter #(>= x %) xs)))
    ()))

(defn qsort3 [[x & xs]]
  (if x
    (letfn [(greater [y] (< x y))]
      (concat
       (qsort3 (filter greater xs))
       [x]
       (qsort3 (filter (complement greater) xs))))
    ()))

(defn qsort4 [[x & xs]]
  (if x
    (letfn [(greater [y] (< x y))]
      (lazy-cat
       (qsort4 (filter greater xs))
       [x]
       (qsort4 (filter (complement greater) xs))))
    ()))

(defn random [n]
  (take n (repeatedly #(rand-int 1000))))

(defn -main []
  (let [n 100000
        [r1 r2] [(random n) (random n)]]
    (time (take 50 (qsort r1))) ; 4623 msec
    (time (take 50 (qsort4 r2)))) ; 0.47 msec
  )