clojure - 从原子重构为不可变

Question

我有一个嵌套的 reduce 函数，我用一个名为 counter 的原子来在每次满足条件时增加一个计数

(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)))

我不喜欢在函数中使用可变 ref。有没有一种方法可以在不使用 ref 的情况下实现相同的行为？

Answer 1

;; 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))