How to create a core.async channel that will drop messages if no consumer is currently reading from the channel?

[Yehonathan Sharvit]

Hello,

I would like to create a channel that will drop messages if no consumer is currently reading from the channel.

Something like a channel with a dropping-buffer of size 0.

The problem is that the API doesn't allow to create a dropping-buffer of size 0.

With the following code snippet, I would like the reader to read nothing. 

(def c (create-unbuffered-chan)) 

(put! c :ok)

(go (println (<! c))); should not print anything

Your help is appreciated.

Thanks,

Yehonathan.

[Laurent PETIT]

Hello, 

I'm still a newbie with core.async myself, but I'll try to answer anyway, learning on the way ;-)

My idea is that you could leverage alts! which can return a default value immediately if none of the operations is "ready". I think "ready" means "would not wait/park" (there's either a value to be read in an input channel, or room in a channel buffer of an output channel).

So it's not really a channel that will have the feature, but rather a process. I guess it is easy to encapsulate the desired behaviour by hiding the process behind a function taking the desired output channel, and returning a channel exhibiting the encaspulated behavior:

(defn put-or-drop-channel [target-channel]

  (let [input-channel (chan)]

    (go-loop

      (when-let [v (<! input-channel)]

        (alt! [target-channel v] nil :default nil))))

?

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Laurent Petit

[Laurent PETIT]

Hmmm, I'm really not convinced by the answer I gave you ...

I hope someone else with better xp than me with core.async (not hard !) will answer to you and correct me. 

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Laurent Petit

[Jozef Wagner]

Implement a custom buffer type, or use mult. With mult, use tap/untap to control reading. Items sent to mult are dropped if there are no taps.

Jozef

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[Yehonathan Sharvit]

What is the reason for the dropping buffer not supporting size 0?

Why the most trivial option of "no buffering" is not supported out of the box by core.async?

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[Jozef Wagner]

From my understanding of how core.async is implemented, if channel has a buffer, every message goes through it, regardless of whether there are takes waiting or not. This is due to transducers that are applied only on buffer items. None messages would thus pass through with a 0 size dropping buffer.

Jozef

[Yehonathan Sharvit]

Anyway, I was able to implement only one oof your suggested solutions. The tap/untsp one.

I was unable to implement a buffer of size 0.

I have tried, this:

(deftype NoBuffer [state]

  impl/UnblockingBuffer

  impl/Buffer

  (full? [this] false)

  (remove! [this])

  (add!* [this itm])

  clojure.lang.Counted

  (count [this] 0))

But it didn’t work at all!!

Please help.

Thanks.

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