An automatic cached protocol impl I wrote but didn't end up using.

cached_protocol.clj

;;  (:import
;;   (java.nio.file Path FileSystems StandardWatchEventKinds))

(def ^:private all-events
  (object-array
   [StandardWatchEventKinds/ENTRY_CREATE
    StandardWatchEventKinds/ENTRY_DELETE
    StandardWatchEventKinds/ENTRY_MODIFY]))

(defn ^:private watch!
  [^Path path callback]
  (let [svc (-> (FileSystems/getDefault) .newWatchService)]
    (.register path svc all-events)
    (future
      (loop []
        (let [key (.take svc)]
          (callback)
          (.reset key))))))

(defmacro ^:private cached-protocol-impl
  [proto backing-store cache]
  `(reify ~proto
     ~@(for [{:keys [name arglists]} (-> proto resolve deref :sigs vals)
             arglist arglists
             :let [cache-key (into [(keyword name)] (rest arglist))]]
         `(~name [~@arglist]
           (or (-> ~cache deref (get-in ~cache-key))
               (let [actual# (~name ~backing-store ~@(rest arglist))]
                 (swap! ~cache assoc-in ~cache-key actual#)
                 actual#))))))