A fast, non-lazy 'repeatedly' implementation

repeatedly.clj

(require '[criterium.core :as c])

(set! *warn-on-reflection* true)
(set! *unchecked-math* :warn-on-boxed)

(defn repeatedly!
  ([^long n f]
   (loop [i 0
          v (transient [])]
     (if (== i n)
       (persistent! v)
       (recur (inc i) (conj! v (f))))))
  ([^long n kf vf]
   (loop [i 0
          m (transient {})]
     (if (== i n)
       (persistent! m)
       (recur (inc i) (assoc! m (kf) (vf)))))))

;; 1.13 us
(c/with-progress-reporting
  (c/quick-bench
    (repeatedly! 80 #(+ 1 1))))

;; 8.17 us
(c/with-progress-reporting
  (c/quick-bench
    (doall (repeatedly 80 #(+ 1 1)))))

;; 650 ns
(c/with-progress-reporting
  (c/quick-bench
    (repeatedly! 80 #(+ 1 1) #(+ 1 1))))

;; 13.68 us
(c/with-progress-reporting
  (c/quick-bench
    (->> (repeatedly 80 (fn [] [2 2]))
         (into {}))))