Example of potential performance benefit of early exit in reduce

ReturningException.java

/* don't use this, it's just for a timing demonstration */

public class ReturningException extends RuntimeException {
    transient public final Object data;
    transient public final Object context;
    public ReturningException(Object o){
        this(null, o);
    }
    public ReturningException(Object c, Object o){
        super("Not really an exception");
        data = o;
        context = c;
    }
}

shortcut.clj

;this is not a recommended implementation, just a possible way to measure what early-exit can do
;this example lets you time internal-reduce with early exit without changing core code

(defmacro shortcut [sentinel & forms]
  (let [context (gensym)]
    `(try
       ~@(postwalk
          (fn[term]
            (if (and (list? term)
                     (= sentinel (first term)))
              `(throw (ReturningException.
                       '~context ~@(rest term)))
              term))
          forms)
       (catch ReturningException e#
         (if
             (= '~context (.context e#))
           (.data e#)
           (throw e#))))))

(shortcut :return (reduce (fn[a b] (if (> a 50000000) (:return a) (+ a b))) 0 (range)))

(last (take-while #(not= :done %) (reductions (fn[a b] (if (> a 50000000) :done (+ a b))) 0 (range))))