label.clj

(defn label* [body]
  (let [payload (atom nil)]
    (try
     (let [result (body (fn [& [x]]
                          (if (compare-and-set! payload nil [x])
                            (throw (Exception.)) ;; It's a pain in the ass to gen-class custom Exception subclass.
                            (throw (Exception. "Tried to jump outside extent.")))))]
       (swap! payload (fn [_] []))
       result)
     (catch Exception e
       (if (seq @payload)
         (first @payload)
         (throw e))))))

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

;; Examples

(label break
  (println "before")
  (break 42)
  (println "after"))

(label break
  (doseq [x (range 10)]
    (label continue
      (doseq [y (range 10)]
        (if (>= y 5)
          (if (>= x 5)
            (break 42)
            (continue))
          (println "x:" x "y:" y))))))

((label break break) 42) ;; "Tried to jump outside extent."