clojure - 从原子到不可变的重构

Question

我有一个嵌套的reduce函数，我有一个名为counter的原子在每次满足条件时递增一个计数

(defrecord TreeNode [val left right])

(defn build-tree [node xs]
  (let [counter (atom 1)]
   (reduce (fn [t x]
             (reduce (fn [t l]
                       (if-not (= (:val l) -1)
                         (let [next-branch (nth xs @counter)]
                           (swap! counter inc)
                           ;; do stuff
                           ) t)
                       )t (map-indexed
                           (fn [idx itm]
                             (let [side (if (= 0 idx) :left :right)]
                               {:side side :index idx :val itm})) x))) node xs)))

我对在函数中使用可变参考感到不满意。有没有办法在不使用ref的情况下实现相同的行为？