merge sort in Clojure

merge.clj

(ns sort.merge)

(defn combine
  "Combines sorted collections c1 and c2"
  [c1 c2]
  (loop [a c1 b c2 sorted []]
    (if (some empty? [a b])
      (concat sorted a b)
      (if (<= (first a) (first b))
        (recur (rest a) b (conj sorted (first a)))
        (recur a (rest b) (conj sorted (first b)))))))

(defn merge-sort
  [x]
  (let [n (count x)
        [left right] (split-at (/ n 2) x)]
    (if (== n 1)
      x
      (combine (merge-sort left) (merge-sort right)))))

(defn -main
  "Reads in file with a number on each line and outputs sorted list."
  [& args]
  (let [unsorted (-> args first clojure.java.io/reader line-seq)]
    (doseq [number (merge-sort (map read-string unsorted))]
      (println number))))