Answer by noisesmith for clojure - refactor from atom into immutable

;; for starters, we can pass the counter through each reduce(defn build-tree [node xs]  (let [[counter tree] (reduce (fn [[counter tree] x]                                 (reduce (fn [[counter' t] l]                                           (if-not (= (:val l) -1)                                             (let [next-branch (nth xs counter)                                                   ... ...                                                   t' ...]                                               [(inc counter') t'])                                             [counter' t])                                           [counter t]                                           (map-indexed                                            (fn [idx itm]                                              (let [side (if (= 0 idx) :left :right)]                                                {:side side :index idx :val itm}))                                            x))))                               [0 node]                               xs)]));; the code becomes much clearer with apropriate let bindings(defn build-tree  [node xs]  ;; the optional name arg to an anonymous function makes stack traces much easier to read  (let [process-branch (fn process-branch [[counter t] l]                         (if-not (= (:val l) -1)                           (let [next-branch (nth xs counter)                                 ... ...                                 t' ...]                             [(inc counter) t'])                           [counter t]))        mark-branch (fn mark-branch [x]                      (map-indexed                       (fn [idx itm]                         (let [side (if (= 0 idx) :left :right)]                           {:side side :index idx :val itm}))                       x))        [counter tree] (reduce (fn [[counter tree] x]                                 (reduce process-branch                                         [counter t]                                         (mark-branch x)))                               [0 node]                               xs)]    tree))