watch.clj

(defn watch
  "Returns (running) thread which observes the value of f every delay
  milliseconds, and calls the callback fn if the value changes. callback
  will be called with the old and new values from f"
  ([f callback delay]
     (let [proc (fn []
		  (loop [x1 ::watch-init]
		    (let [x2 (f)]
		      (when (not= x1 x2)
			(callback x1 x2))
		      (Thread/sleep delay)
		      (recur x2))))
	   thread (Thread. proc)]
       (.start thread)
       thread))
  ([f callback]
     (watch f callback 5000)))