validate.clj

  ;; Any type of error class, as CLJS uses js/Error
  (defn e! [fmt & args]
    (throw (AssertionError. (apply format fmt args))))
  …
  ;; Exception message takes precedence over :validation-msg
  ["-i" "--input-directory DIR"
   :validate-fn #(cond (not (ok-path? %))
                       (e! "Input directory path %s is malformed" (pr-str %))

                       (let [f (File. %)]
                         (and (.exists f) (not (.isDirectory f))))
                       (e! "Input directory %s is not a directory" (pr-str %))

                       :else true)]