How to exhaust a channel's values and then return the result (ClojureScript)?

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Suppose that channel chan has the values "1" and "2" on queue.

Goal: Make a function which takes chan and returns the vector [1 2]. Note that I am totally fine if this function has to block for some time before its value is returned.

Attempt:

(defn chan->vector
  [chan]  
  (let [a (atom true) v []]
    (while (not-nil? @a)
      (go
        (reset! a (<! chan))
        (into v @a)
        (reset! a (<! chan))
      )
    ) v
  )
)

Result: My REPL freezes and eventually spits out a huge error. I have come to realize that this is because the (go ...) block is asynchronous, and so immediately returns. Thus the atom іn my (while ...) loop is never given a chance to be set to nil and the loop can never terminate.

So how do I accomplish the desired result? In case it's relevant, I'm using ClojureScript and targetting nodejs.

you should use alts! from core.async to fulfill this task (https://clojure.github.io/core.async/#clojure.core.async/alts!):

(def x (chan 10))

(go (>! x 1)
    (>! x 2)
    (>! x 3))

(defn read-all [from-chan]
  (<!! (go-loop [res []]
           (let [[v _] (alts! [from-chan] :default :complete)]
             (if (= v :complete)
               res
               (recur (conj res v)))))))

(read-all x) 
;; output: [1 2 3]

(read-all x)
;; output: []

(go (>! x 10)
    (>! x 20)
    (>! x 30)
    (>! x 40))

(read-all x)
;; output: [10 20 30 40]

inside the go-loop this (a/alts! [from-chan] :default :complete) tries to read any value from channel, and in case there are no value available at once it emits the default value, so you will see you should break the loop and return accumulated values.

update: as the blocking read (<!!) is absent in cljs, you can rewrite it the following way:

(defn read-all [from-chan]
  (go-loop [res []]
    (let [[v _] (alts! [from-chan] :default :complete)]
      (if (= v :complete)
        res
        (recur (conj res v)))))))

so it will return the channel, and then just read one value from there:

(go (let [res (<! (read-all x))]
      (println res)
      ;; do something else
      ))

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You can use clojure.core.async/reduce:

;; demo setup
(def ch (async/chan 2))
(async/>!! ch :foo)
(async/>!! ch :bar)

;; background thread to print reduction result
(async/thread
  (prn (async/<!! (async/reduce conj [] ch))))

;; closing the channel…
(async/close! ch)

;; …terminates the reduction and the result gets printed out:
;; [:foo :bar]

clojure.core.async/reduce returns a channel that will produce a value if and when the original channel closes. Internally it uses a go block and will release control in between taking elements from the original channel.

If you want to produce a value after a certain amount of time passes whether or not the original channel closes, you can either wrap the original channel in a pass-through channel that will close itself after a timeout passes or you can use a custom approach to the reduction step (perhaps the approach suggested by @leetwinski).

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Use into

Returns a channel containing the single (collection) result of the items taken from the channel conjoined to the supplied collection. ch must close before into produces a result.

Something like this should work (it should print the events from events-chan given events-chan closes when it is done publishing events):

(go
  (println (<! (into [] events-chan))))

The source channel needs to end (close), otherwise you can't put all events into a collection.

Edit:

Re-read your question, and it is not very clear what you want to accomplish. Whatever you want to do, chan->vector needs to return a channel so that whoever calls it can wait for the result. In fact, chan->vector is exactly into:

; chan->vector ch:Chan<Event> -> Chan<Vector[Event]>
(defn chan->vector [ch]
  (into [] ch))

(go
  (let [events (<! (chan->vector events-chan))]
    (println events) ; Do whatever with the events vector
    ))

As I mentioned above, if the events chan never closes, then you have to do more thinking about how to consume the events. There is no magic solution. Do you want to batch the events by time intervals? By number of events? By a combination of those?

In summary, as mentioned above, chan->vector is into.

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While possible in Clojure and many other languages, what you want to do is not possible in ClojureScript.

You want a function that blocks while listening to a channel. However, ClojureScript's version of core.async doesn't include the blocking operators. Why? Because ClojureScript doesn't block.

I couldn't find a reliable source to back that last sentence. There seems to be a lot of confusion around this topic on the web. However, I'm pretty sure of what I'm saying because ClojureScript ultimately becomes JavaScript, and that's how JavaScript works.

Indeed, JavaScript never blocks, neither on the browser nor in Node.js. Why? As far as I understand, it uses a single thread, so if it were to block, the user would be unable to do anything in the browser.

So it's impossible to do what you want. This is by design, because it could have disastrous UX effects. ClojureScript channels are like JavaScript events; in the same way you don't want an event listener to block the user interface while waiting for an event to happen, you also shouldn't want a channel to block while waiting for new values.

Instead, try using a callback function that gets called whenever a new value is delivered.

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Comments
  • At the first line of read-all you use the <!! with two !'s. In ClojureScript this operation isn't allowed (AFAIK). Does this change anything?
  • @leetwinski although the Clojure version returns an array, the ClojureScript version doesn't. Is there any way to actually return the array instead of printing it?
  • @e18r , since you can't move out of the async context here (which is really close to how the promises and async/await is designed in js), the most logical thing you can do, is to pass this array to some kind of a callback. Well.. that's what the line "do something here" means. Sorry to disappoint you, but that's how async programming paradigm works.