Clojure - generating a random password

gistfile1.clj

(defn random-letter []
  (-> (rand-int 26) (+ 97) char))

(defn random-digit []
  (rand-int 10))                                                                       

; for example, (generate-random-password "a1aa1a11") returns something like "z5qr9s42"
(defn generate-random-password [spec]
  (-> #(if (= % \a) (random-letter) (random-digit)) (map spec) (apply str)))