build.boot

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

(deftask demo []
  (let [tmp     (tmp-dir!)                                            ; Anonymous, boot-managed temp directory.
        prev-fs (atom nil)]                                           ; Atom where the previous fileset is stored.
    (with-pre-wrap [fs]
      (let [diff   (fileset-diff @prev-fs (reset! prev-fs fs))        ; Fileset containing only changed files.
            inputs (->> (input-files diff)                            ; Sequence of [String, java.io.File]
                        (by-ext [".c"])                               ; pairs, the classpath path and File
                        (map (juxt tmp-path tmp-file)))]              ; for files with given extensions.
        (doseq [[inpath infile] inputs]
          (let [outpath (.replaceAll inpath "\\.c$" ".o")             ; The output file classpath path.
                outfile (doto (io/file tmp outpath) io/make-parents)] ; The output File (in the temp dir).
            (cc infile outfile)))                                     ; The cc fn is assumed to be defined elsewhere.
        (-> fs (add-resource tmp) (commit!))))))                      ; Add temp dir to fileset, and commit changes.