retrying something 3 times before throwing an exception - in clojure

Question

I don\'t know how to implement this piece of Python code in Clojure

for i in range(3):
    try:
        ......
    except e:
        if i == 2:
            raise         


        

Answer 1

Here's one approach:

(defmacro retry
  "Evaluates expr up to cnt + 1 times, retrying if an exception
  is thrown. If an exception is thrown on the final attempt, it
  is allowed to bubble up."
  [cnt expr]
  (letfn [(go [cnt]
            (if (zero? cnt)
              expr
              `(try ~expr
                    (catch Exception e#
                      (retry ~(dec cnt) ~expr)))))]
    (go cnt)))

Example from the REPL:

user> (retry 2 (do (println :foo) (throw (RuntimeException. "foo"))))
:foo
:foo
:foo
; Evaluation aborted.

(Passing 2 to retry asks expr to be retried twice it if fails the first time round, for a total of three attempts. Three :foos are printed, because the println occurs before the throw in the do form passed to retry. The final ; Evaluation aborted. means an exception was thrown.)

Also, about the for loop from your snippet:

If you try looping over a longer range (replace (range 3) with (range 10), say), the output will end after i reaches 3. Also, if you put in a println before the form which throws the exception, it will of course print out whatever you pass to it; if the println occurs after the exception-throwing form, there will be no printout. In any case, at most three calls to println will be executed (assuming an exception is thrown on every iteration).