Implementing Binary Search with Clojure

I was trying to implement a simple binary search using a purely functional approach, and after much hacking, googling and wikibooking, came up with this in Clojure.

(defn binarysearch
( [lst n]
(binarysearch lst 0 (dec (count lst)) n))
( [lst lb ub n]
(if (> lb ub) -1 ; this is the case where no element is found
(let [mid (quot (+ lb ub) 2)
mth (nth lst mid)]
; mid > n, so search lower
(> mth n) (recur lst lb (dec mid) n)
; mid < n, search upper (< mth n) (recur lst (inc mid) ub n) ; else, found, return index (= mth n) mid)))))