loop-recur 怎么可能抛出 StackOverflowError？

Question

我一直在尝试实现 chainl1 的尾递归版本，但即使使用循环递归，它也会抛出 StackOverflowError。这怎么可能？我能做些什么来改变它？

(defn atest [state]
  (when-not (and (= "" state) (not (= (first state) \a))) 
      (list (first state) (. state (substring 1)))))
(defn op [state]
  (when-not (and (= "" state) (not (= (first state) \a)))
    (list #(list :| %1 %2) (. state (substring 1)))))
(defn chainl1-helper [x p op]
  (fn [state]
    (loop [x x
           state state]
      (if-let [xs (op state)]
        (when-let [xs2 (p (second xs))]
          (recur ((first xs) x (first xs2)) (second xs2)))
        (list x state)))))

(defn chainl1 [p op]
  (fn [state]
    (when-let [[v s] (p state)]
      ((chainl1-helper v p op) s))))
(def test-parse (chainl1 atest op))
(defn stress-test [n] (test-parse (apply str (take n (interleave (repeat "a") (repeat "+"))))))
(stress-test 99999)

Answer 1

它打印出堆栈的最终结果 所以这是 REPL 而不是你的代码。

将最后一行替换为

(count (stress-test 99999))

然后就结束了

堆栈跟踪的这种模式重复了很多次：

 13:    core_print.clj:58 clojure.core/print-sequential
 14:    core_print.clj:140 clojure.core/fn
 15:      MultiFn.java:167 clojure.lang.MultiFn.invoke

编辑：LDomagala 指出 print-level 是针对此类崩溃的安全措施

user>  (set! *print-level* 20)
20
user> (stress-test 9999)
((:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f (:f # \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) \a) "")
user>