为什么在Clojure中从catch部分重现是非法的

Question

至少已经有一个关于异常复发的问题.我的问题是为什么recurClojure编译器不接受这个

(loop []
  (try
    (catch Exception _ex
      (recur))))

错误"只能从尾部位置重复".

根据控制流程,这recur 是在尾部位置,因为代码在recur内部loop形式之后什么都不做.

Answer 1

如果在询问“为什么会这样？”时，您是在询问历史记录，则将此问题发布到 Clojure 邮件列表 (网址为 ) 可能会得到更好的答案clojure@googlegroups.com。我怀疑原因是编译器作者从未考虑过这个用例。

您可以很容易地模拟您想要的行为，如本示例所示。它不像上一个问题的大多数答案那样使用宏：

    (loop [count 5]
      (newline)
      (println "top of loop; count=" count)
      (let [caught-ex (try
                        (println "in try")
                        (/ 1 0)
                        false
                        (catch Exception ex
                          (println "   in catch; count=" count "   cause:  " (.getMessage ex))
                          true))]
           (when (and caught-ex
                   (pos? count))
             (recur (dec count)))))

结果：

top of loop; count= 5
in try
   in catch; count= 5    cause:   Divide by zero

top of loop; count= 4
in try
   in catch; count= 4    cause:   Divide by zero

top of loop; count= 3
in try
   in catch; count= 3    cause:   Divide by zero

top of loop; count= 2
in try
   in catch; count= 2    cause:   Divide by zero

top of loop; count= 1
in try
   in catch; count= 1    cause:   Divide by zero

top of loop; count= 0
in try
   in catch; count= 0    cause:   Divide by zero