alternate periodic function

gistfile1.clj

(def ^{:dynamic true} *print-progress* true)

(defmacro defn-do-periodic [name args [var period] body]
  `(let [counter# (atom 0)]
     (defn ~name [~@args]
       (swap! counter# inc)
       (when (and *print-progress*
                  (zero? (mod @counter# ~period)))
         (let [~var @counter#]
           ~body)))))

;; Let's define a periodic logger
(defn-do-periodic log-progress [merchant] [n 100]
  (println (str "we've processed" n "widgets for" merchant)))

;; Usage
(doseq [widget (get-widgets)]
  (process-widget widget)
  (log-progress some-merchant))