I am one of the 90% of programmers who cannot write binary search: http://www.cn-cuckoo.com/2010/04/20/are-you-one-of-the-10-percent-of-programmers-who-can-write-a-binary-search-1530.html

bsearch.clj

(defn binary-search [items value]
  (letfn
    [(do-search
      [items offset]
      (let [/         (comp long /)
            mid       (/ (dec (count items)) 2)
            mid-value (items mid)]
        (cond
         ;; found
         (= value mid-value) (+ offset mid)
         ;; not found
         (= 1 (count items)) -1
         ;; less than
         (< value mid-value)
         (let [items (subvec items 0 mid)]
           (recur items offset))
         ;; greater than
         (> value mid-value)
         (let [items  (subvec items (inc mid))
               offset (+ offset (inc mid))]
           (recur items offset)))))]
    (do-search items 0)))

;;; a little test
(let
  [xs (->> (range 1000) (filter even?) vec)]
  (every? #(= % (* 2 (binary-search xs %)))
          xs))