Clojure: How to to recur upon exception? - clojure

Question

I am trying to execute a func several times before giving up upon exceptions.
But it is not valid in Clojure to recur from catch block.
How can this be achieved ?
(loop [tries 10]
(try
(might-throw-exception)
(catch Exception e
(when (pos? tries) (recur (dec tries))))))
java.lang.UnsupportedOperationException: Cannot recur from catch/finally
The best I could find is the following clumsy solution (wrapping in func and calling it)
(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

Macros are calling...
How about this:
(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)))

Answer 2

kotarak's idea is the way to go, but this question tickled my fancy so I'd like to provide a riff on the same theme that I prefer because it doesn't use loop/recur:
(defn try-times* [thunk times]
(let [res (first (drop-while #{::fail}
(repeatedly times
#(try (thunk)
(catch Throwable _ ::fail)))))]
(when-not (= ::fail res)
res)))
And leave the try-times macro as it is.
If you want to allow the thunk to return nil, you can drop the let/when pair, and let ::fail represent "the function failed n times", while nil means "the function returned nil". This behavior would be more flexible but less convenient (the caller has to check for ::fail to see if it worked rather than just nil), so perhaps it would be best implemented as an optional second parameter:
(defn try-times* [thunk n & fail-value]
(first (drop-while #{fail-value} ...)))

Answer 3

A try-times macro is elegant, but for a one-off, just pull your when out of the try block:
(loop [tries 10]
(when (try
(might-throw-exception)
false ; so 'when' is false, whatever 'might-throw-exception' returned
(catch Exception e
(pos? tries)))
(recur (dec tries))))

Answer 4

My proposal:
(defmacro try-times
"Retries expr for times times,
then throws exception or returns evaluated value of expr"
[times & expr]
`(loop [err# (dec ~times)]
(let [[result# no-retry#] (try [(do ~#expr) true]
(catch Exception e#
(when (zero? err#)
(throw e#))
[nil false]))]
(if no-retry#
result#
(recur (dec err#))))))
Will print "no errors here" once:
(try-times 3 (println "no errors here") 42)
Will print "trying" 3 times, then throw Divide by zero:
(try-times 3 (println "trying") (/ 1 0))

Answer 5

One more solution, without macro
(defn retry [& {:keys [fun waits ex-handler]
:or {ex-handler #(log/error (.getMessage %))}}]
(fn [ctx]
(loop [[time & rem] waits]
(let [{:keys [res ex]} (try
{:res (fun ctx)}
(catch Exception e
(when ex-handler
(ex-handler e))
{:ex e}))]
(if-not ex
res
(do
(Thread/sleep time)
(if (seq rem)
(recur rem)
(throw ex))))))))

Answer 6

This allows catching multiple more then one exception and provides some feedback about the causes for the retries.
(defmacro try-n-times
"Try running the body `n` times, catching listed exceptions."
{:style/indent [2 :form :form [1]]}
[n exceptions & body]
`(loop [n# ~n
causes# []]
(if (> n# 0)
(let [result#
(try
~#body
~#(map (partial apply list 'catch) exceptions (repeat `(e# e#))))]
(if (some #(instance? % result#) ~exceptions)
(recur (dec n#) (conj causes# result#))
result#))
(throw (ex-info "Maximum retries exceeded!"
{:retries ~n
:causes causes#})))))

Answer 7

If you add a result arg to your loop, you can nest the (try) block inside of the (recur). I solved it like this:
(loop [result nil tries 10]
(cond (some? result) result
(neg? tries) nil
:else (recur (try (might-throw-exception)
(catch Exception e nil))
(dec tries))))

Answer 8

Here's yet another approach:
(loop [tries 10]
(let [res (try
(might-throw-exception)
(catch Exception e
(if (pos? tries)
::retry
(throw e))))]
(if (#{::retry} res)
(recur (dec tries))
res)))
But may I also recommend a cool little trick, instead of having a number of retries, provide a seq of times to sleep for:
(loop [tries [10 10 100 1000]]
(let [res (try
(might-throw-exception)
(catch Exception e
(if tries
::retry
(throw e))))]
(if (#{::retry} res)
(do
(Thread/sleep (first tries))
(recur (next tries)))
res)))
And finally put it all into a macro if you want it to be less verbose:
(defmacro with-retries
[retries & body]
`(loop [retries# ~retries]
(let [res# (try ~#body
(catch Exception e#
(if retries#
'retry#
(throw e#))))]
(if (= 'retry# res#)
(do (Thread/sleep (first retries#))
(recur (next retries#)))
res#))))
(with-retries [10 10 100 1000]
(might-throw-exception))