gistfile1.clj

(defn label* [thunk]
  (let [NONE (Object.)
        payload (clojure.lang.Box. NONE)
        the-e (Exception.)]
    (try
     (thunk (fn [ret]
              (if (identical? NONE (.val payload))
                (do (set! (.val payload) ret)
                    (throw the-e))
                (throw (Exception. "Tried to jump outside extent.")))))
     (catch Exception e
       (if (identical? the-e e)
         (.val payload)
         (throw e)))
     (finally (set! (.val payload) nil)))))

(defmacro label [name & body]
  `(label* (fn [~name] ~@body)))