(defn scheduler []
  "Create an executor for executing fns in an own thread."
  (let [executor (Executors/newSingleThreadScheduledExecutor)]
    (fn
      ([k]
       (cond
         (= k :stop) (.shutdown executor)
         (= k :stop-now) (.shutdownNow executor)))
      ([k f d]
       (cond
         (= k :once) (.schedule executor f d TimeUnit/MILLISECONDS)
         (= k :repeat) (.scheduleAtFixedRate executor f 0 d TimeUnit/MILLISECONDS)))
      ([k f id d]
       (cond
         (= k :repeat) (.scheduleAtFixedRate executor f id d TimeUnit/MILLISECONDS))))))