pbostrom

(def x (ref 0))
(def a (agent nil))

(defn alter-and-send-side-effects
  "Alters refs then sends println actions to agent with new values"
  []
 (dosync
   (let [newx (alter x inc)])
   (send a (fn [_] (println "x is" newx)))))

pbostrom

(defn primes3 [max]
  (let [enqueue (fn [sieve n factor]
                  (let [m (+ n (+ factor factor))]
                    (if (sieve m)
                      (recur sieve m factor)
                      (assoc sieve m factor))))
        next-sieve (fn [sieve candidate]
                     (if-let [factor (sieve candidate)]
                       (-> sieve
                         (dissoc candidate)