Clojure:如何在异常时重现？

Question

在因异常而放弃之前，我尝试多次执行函数。但在 Clojure 中从 catch block 中重复是无效的。如何才能实现这一目标？

(loop [tries 10]
  (try
    (might-throw-exception)
    (catch Exception e
      (when (pos? tries) (recur (dec tries))))))

java.lang.UnsupportedOperationException: Cannot recur from catch/finally 

我能找到的最好的是以下笨拙的解决方案(包装在 func 中并调用它)

(defn do-it []
  (try
    (might-throw-exception)
    (catch Exception e nil)))

(loop [times 10]
  (when (and (nil? (do-it)) (pos? times))
    (recur (dec times))))

Answer 1

宏正在调用...

这个怎么样:

(defn try-times*
  "Executes thunk. If an exception is thrown, will retry. At most n retries
  are done. If still some exception is thrown it is bubbled upwards in
  the call chain."
  [n thunk]
  (loop [n n]
    (if-let [result (try
                      [(thunk)]
                      (catch Exception e
                        (when (zero? n)
                          (throw e))))]
      (result 0)
      (recur (dec n)))))

(defmacro try-times
  "Executes body. If an exception is thrown, will retry. At most n retries
  are done. If still some exception is thrown it is bubbled upwards in
  the call chain."
  [n & body]
  `(try-times* ~n (fn [] ~@body)))