Clojure, console: println output not always visible

Question

Subj. There's a working program, which basically copies filesystem trees recursively. Somehow println from inside the recursive function won't show any output.

build-album calls traverse-dir; I can see the "10" in the console, but never any "11"s -- should be a lot of them. (println "11") can't possibly miss the path of execution, since files get really copied (the line above). This is not quite nice, since the project is meant as a console application, reporting to the user each copied file, lest he should suspect freezing. This is no joke, because the app is intended to upload albums to mobile phones.

(defn traverse-dir
  "Traverses the (source) directory, preorder"
  [src-dir dst-step]
  (let [{:keys [options arguments]} *parsed-args*
        dst-root (arguments 1)
        [dirs files] (list-dir-groomed (fs/list-dir src-dir))

        dir-handler  (fn [dir-obj]
                       "Processes the current directory, source side;
                       creates properly named directory destination side, if necessary"
                       (let [dir (.getPath dir-obj)
                             step (str dst-step *nix-sep* (fs/base-name dir-obj))]
                         (fs/mkdir (str dst-root step))
                         (traverse-dir dir step)))

        file-handler (fn [file-obj]
                       "Copies the current file, properly named and tagged"
                       (let [dst-path (str dst-root dst-step *nix-sep* (.getName file-obj))]
                         (fs/copy file-obj (fs/file dst-path))
                         (println "11")
                         dst-path))]

    (concat (map dir-handler dirs) (map file-handler files))))

(defn build-album
  "Copy source files to destination according
  to command line options"
  []
  (let [{:keys [options arguments]} *parsed-args*
        output (traverse-dir (arguments 0) "")]
    (println "10")
    output))

Answer 1

Might be the problem with lazy sequences: you build a lazy seq which is never realized and thus the code never executes. Try calling doall on the result of traverse-dir:

(doall (concat (map dir-handler dirs) (map file-handler files))))