attach debugging info to a var, detach cleanly later

user.clj

(defn set-debug
  [target-var]
  (let [metadata (meta target-var)
        target (::original metadata @target-var)
        debug-data (:debug-trace metadata (atom []))]
    ;; only one layer of debugging, if we were already debugging,
    ;; just replace the previous layer
    (when-not (and (:debug-trace metadata)
                   (::original metadata))
      (alter-meta! target-var assoc
                   ::original target
                   :debug-trace debug-data))
    (alter-var-root
     target-var
     (constantly
      (fn replaced-debugging-function
        [& args]
        (let [result (try (apply target args)
                          (catch Exception e
                            {::error e}))]
          (swap! debug-data conj {:at (java.util.Date.)
                                  :args args
                                  :result result})
          (if-let [error (::error result)]
            (throw error)
            result)))))))

(defn unset-debug
  [target-var]
  (let [metadata (meta target-var)
        debug-data (:debug-trace metadata)]
    (when-let [original (::original metadata)]
      (alter-meta! target-var dissoc ::original :debug-trace)
      (alter-var-root target-var (constantly original)))
    @debug-data))