Working with core.async: Exceptions in go blocks

Dealing with exceptions in go blocks/threads is different from normal Clojure core. This gotcha is very common when moving your code into core.async go blocks – all your exceptions are gone! Since the body of a go block is run on a thread pool, there’s not much we can do with an exception, thus core.async will just eat them and close the channel. That’s what happened in the second snippet in this post. The nil result occurs because the channel we read from is closed.

I find myself wanting to know the cause of problem at the consumer side of a channel. That means the go block needs to catch the exception, put it (the exception) on the channel before it dies. David Nolen has written about this pattern, and I’ve been using the proposed <? quite happily.

(defn throw-err [e]
  (when (instance? Throwable e) (throw e))
  e)

(defmacro <? [ch]
  `(throw-err (async/<! ~ch)))

(defn get-data [s]
  (async/go (try
              (client/get (str "http://some.query?q=s") {:throw-exceptions true})
              ;; catch and put exception on the channel
              (catch Exception e
                e))))

(async/go (try
            (let [data      (<? (get-data "clojure"))
                  more-data (<? (get-data "core.async")]
              ;; process all data
              )
            ;; Handle exceptions for all '<? calls'
            (catch Exception e
              (log/error "error getting data"))))

If you’re interested in how some Go examples convert to core.async, check out this repo.