Asynchronous Naiveté

This post has been heavily edited. Initially it was describing some behavior I didn't expect using core.async in both ClojureScript and Clojure. It turns out the behavior I observed in ClojureScript was a bug, that has now been fixed. The Clojure and ClojureScript versions can still behave differently than each other, depending on the lifetime of the main thread in Clojure. I have rewritten this to focus on that.

I created a buffered channel in ClojureScript, and tried to put 3 values into it:

(let [c (chan 1)]
  (go
   (.log js/console (<! c)))
  (go
   (>! c 1)
   (.log js/console "put one")
   (>! c 2)
   (.log js/console "put two")
   (>! c 3)
   (.log js/console "Put all my values on the channel")))

The result:

put one
put two
1

Running the equivalent code in a Clojure REPL

  (let [c (chan 1)]
    (go
     (println (<! c)))
    (go
     (>! c 1)
     (println "put one")
     (>! c 2)
     (println "put two")
     (>! c 3)
     (println "put all of my values on the channel")))

Gives the same 3 lines printed out though which line has the number 1 can change from invocation to invocation.

put one
1
put two  

When I put this in a -main method and ran it with lein run, I got no results at all.

Adding Thread/sleep the -main function causes the expected results to return. Though, again which order the 1 gets printed in can vary.

(defn -main [& args]
  (let [c (chan 1)]
    (go
     (println (<! c)))
    (go
     (>! c 1)
     (println "put one")
     (>! c 2)
     (println "put two")
     (>! c 3)
     (println "put all of my values on the channel"))
    (Thread/sleep 1000)))

put one
put two  
1

Putting the main thread to sleep gives the go blocks time to execute. For confirmation of what is happening, we can check what thread each operation is happening on.

(defn -main [& args]
  (let [c (chan 1)]
    (go
     (println (<! c))
     (println  (str "out " (.getId (Thread/currentThread)))))
    (go
     (>! c 1)
     (println "put one")
     (println (str "in1 " (.getId (Thread/currentThread))))
     (>! c 2)
     (println "put two")
     (println (str "in2 " (.getId (Thread/currentThread))))
     (>! c 3)
     (println "put all of my values on the channel"))
    (Thread/sleep 1000)
    (println (str "main  " (.getId (Thread/currentThread))))))

When I ran it I had the first go block on thread 10, the second on 11 and the main thread of execution on thread 1.

put one
in1 11
put two
in2 11
1
out 10
main  1

If we remove the Thread/sleep expression, but leave the calls to println, our output is:

main  1

This is just demo code. I don't know when in production systems your main thread would be likely to exit before asynchronous tasks are complete.

I do think it is interesting that the blocks of code I have run have executed in the same order every time in ClojureScript, but that the order can vary when running on the JVM. I do not know if that would be true of more complicated examples or on different browsers. I have done all of my tests with Google Chrome.