clojure - 为什么从 Clojure 中的 catch 部分重复是非法的

Question

至少已经有 one answered question关于异常复发。我的问题是为什么这个 recur Clojure 编译器不接受

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

错误“只能从尾部位置重复”。

根据控制流程这个 recur处于尾部位置，因为代码在 recur 之后什么也不做内 loop形式。

Answer 1

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

如本示例所示，您可以很容易地模拟您想要的行为。它不像之前链接的问题中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