First cut implementation of `with-progress` and `report!`

progress.clj

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

(defmacro with-progress
  "Bind a `reportfn` function, and evaluate `body` wherein
  calling (report!) will invoke the report function with the current
  state of the iteration."
  [reportfn & body]
  `(let [iter# (atom 0)
         reporter# ~reportfn]
     (letfn [(~'report! []
               (when *print-progress*
                 (swap! iter# inc)
                 (reporter# (deref iter#))))]
       ~@body)))

(defn fprint [& more]
  "Same as print but explicitly flushes *out*."
  (apply print more)
  (flush))

(defn fprintln
  "Same as println but explicitly flushes *out*."
  [& more]
  (apply println more)
  (flush))

(defn make-standard-reporter
  "Generate a standard reporter function which can be passed to
  `with-progress`."
  [n & {:keys [ncols line-handler]}]
  (fn [i]
    (cond
      (zero? (mod i n))
      (fprintln (format "%,8d rows%s"
                        i
                        (if line-handler (str " " (line-handler i)) "")))

      (zero? (mod i (int (/ n (or ncols 70))))) (fprint "."))))