Clojure Sieve of Eratosthenes - Christophe Grand

sieve.clj

(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)
                         (enqueue candidate factor))
                       (enqueue sieve candidate candidate)))]
    (cons 2 (vals (reduce next-sieve {} (range 3 max 2))))))

See http://clj-me.cgrand.net/index.php?s=Everybody%20loves%20the%20Sieve%20of%20Eratosthenes