core.async: terminate go loop after timeout

Question

I have a function which processes a stream of values from a shared channel and looks for a value which satisfies a specific predicate:

(defn find-my-object [my-channel]
  (go-loop []
    (when-some [value (<! my-channel)]
      (if (some-predicate? value)
        value
        (recur)

Now I want this function to return some 'failure' value after waiting for a timeout, which I have implemented like this:

(alts! [(find-my-object my-channel) (timeout 1000)])

The problem with this is that the go-loop above continues to execute after the timeout. I want find-my-object to drain values from my-channel while the search is ongoing, but I don't want to close my-channel on timeout, since it's used in some other places. What is the idiomatic way to implement this functionality?

amalloy · Accepted Answer · 2017-04-27 22:23:55Z

2

Read from the timeout channel inside of find-my-object, not outside of it. You can hardcode that if you like, or make the function take a second channel argument, which is the "please stop reading things" channel. Then you can pass it a timeout channel to get the behavior you want now, or pass it some other kind of channel to control it in a different way.

answered Apr 27, 2017 at 22:23

amalloy

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2 Comments

Derek Thurn Over a year ago

Is there a way to tell that the timeout channel is closed and that the loop should be terminated? There isn't a "closed?" function or anything.

amalloy Over a year ago

alts! returns to you which channel it read from. If that was the timeout channel, then the timeout has expired.

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