Binary Search in Clojure

core.clj

(defn binary-search [n coll]
  (loop [upper (dec (count coll))
         lower 0]
    (let [mid (int (Math/ceil (/ (+ upper lower) 2)))]
      (if (= upper lower)
        nil
        (cond (= n (nth coll mid)) mid
              (< n (nth coll mid)) (recur (dec mid) lower)
              (>= n (nth coll mid)) (recur upper mid))))))

(binary-search 490 (vec (range 1 50)))
(binary-search 49 (vec (range 1 50)))
(binary-search 2 (vec (range 1 50)))