dosync in clojure

(defn traverse [f dat]
  (vec (map #(f %) dat)))

(def data (traverse ref (range 10)))

(-> (fn []
      (dosync (loop [i 0
                     data data]
                (when (seq data)
                  (Thread/sleep 1000)
                  (println (str ">>> " i ": " @(first data)))
                  (recur (+ i 1) (next data))))))
    Thread.
    .start)

(Thread/sleep 1500)
(dosync
 (traverse (fn [ref]
             (alter ref inc))
           data))

(Thread/sleep 1000)
;; This one extra sleep forces the "main" thread sleeping for one
;; second.  Due to some reason I don't know for now, when `dosync'
;; retries, the first sleep will be interrupted.
(Thread/sleep 1000)
(dosync
 (traverse (fn [ref]
             (alter ref inc))
           data))

(traverse #(println (str "--- " @%)) data)