map!.clj

(letfn [(->iter [coll]
          (clojure.lang.RT/iter coll))
        (has-next? [iter]
          (.hasNext ^java.util.Iterator iter))
        (next! [iter]
          (.next ^java.util.Iterator iter))]
  (defn map!
    ([f coll]
     (mapv f coll))
    ([f coll & colls]
     (let [iters (map ->iter (cons coll colls))]
       (loop [acc! (transient [])]
         (if (every? has-next? iters)
           (let [item (apply f (map next! iters))]
             (recur (conj! acc! item)))
           (persistent! acc!)))))))