dropping-transform

(require '[clojure.java.io :as io])

(defn dropping-transform [fn coll]
  (loop [coll coll]
    (when-let [[first & rest] (seq coll)]
      (let [val (fn first)]
        (if (nil? val)
          (recur rest)
          (cons val rest))))))

(def inputs (atom ["/home/xgp/tmp/1.clj" "/home/xgp/tmp/2.clj"]))

(defn interested-line? [line]
  true)

(defn read-next-line [inputs]
  (dropping-transform (fn [input]
                        (let [reader (if (string? input)
                                       (io/reader input)
                                       (first input))
                              lines (dropping-transform (fn [line]
                                                          (when (interested-line? line)
                                                            line))
                                                        (line-seq reader))]
                          (if (seq lines)
                            [reader lines]
                            (do (.close reader)
                                nil))))
                      inputs))

(loop []
  (when-let [v (seq (swap! inputs read-next-line))]
    (let [[[_ [line]]] v]
      (println line)
      (recur))))