Debounce in Clojure on the JVM

debounce.clj

(import '[java.util Timer TimerTask])

(defn debounce
  ([f] (debounce f 1000))
  ([f timeout]
    (let [timer (Timer.)
          task (atom nil)]
      (with-meta
        (fn [& args]
          (when-let [t ^TimerTask @task]
            (.cancel t))
          (let [new-task (proxy [TimerTask] []
                           (run []
                             (apply f args)
                             (reset! task nil)
                             (.purge timer)))]
            (reset! task new-task)
            (.schedule timer new-task timeout)))
        {:task-atom task}))))

;; example usage
(def say-hello (debounce #(println "Hello" %1)))
(say-hello "is it me you're looking for?")
(say-hello "Lionel")
;; one second later...
;; Hello Lionel

Works great!

this throws already cancelled for me once in a while:

java.lang.IllegalStateException: Timer already cancelled.
 at java.util.Timer.sched (Timer.java:408)
    java.util.Timer.schedule (Timer.java:204)
    flower.watch$debounce$fn__12336.doInvoke (watch.clj:41)

this fixed it for me:

   (let [timer (Timer.)
         task (atom nil)]
     (fn [& args]
       (let [new-task (proxy [TimerTask] []
                        (run []
                          (apply f args)
                          (reset! task nil)
                          (.purge timer)))
             old ^TimerTask @task]
         ; NOTE: we never retry this;
         ; an outdated value means we already scheduled a rerun,
         ; and we don't promise that all events get through.
         (when (compare-and-set! task old new-task)
           (when old (.cancel old))
           (.schedule timer new-task ms))))))