Process a zipper structure

process_zipper.clj

(defn render-element [open-or-close node]
  (let [slash (if (= open-or-close :open)
                ""
                "/")]
    (if (map? node)
      (str "<" slash (name (:tag node)) ">")
      node)))

(defn traverse [root arrive-fn leave-fn]
  (loop [loc root
         nodes []
         direction :arrive]
    (let [node-fn (if (= direction :arrive) arrive-fn leave-fn)
          new-nodes (conj nodes (node-fn (zip/node loc)))]
      (if (and (= loc root) (= direction :leave))
        new-nodes
        (let [child-loc (zip/down loc)
              right-loc (zip/right loc)]
          (if (or (nil? child-loc) (= direction :leave))
            (if (nil? right-loc)
              (recur (zip/up loc) new-nodes :leave)
              (recur right-loc new-nodes :arrive))
            (recur child-loc new-nodes :arrive)))))))