Sieve of Eratosthenes

sieve.clj

(defn gen-primes []
  (letfn [(reinsert [table x prime]
            (update-in table [(+ prime x)] conj prime))
          (step [table d]
            (if-let [factors (table d)]
              (recur (reduce #(reinsert %1 d %2) (dissoc table d) factors)
                     (inc d))
              (lazy-seq (cons d
                              (step (assoc table (* d d) (list d)) (inc d))))))]
    (step {} 2)))