Picture the following: producer ---> go-loop ---> external service
1- The producer puts a value to a unbuffered (chan) by doing (>! c v) 2- The go-loop consumes the value with a take operation, **unblocking** the producer 3- The go-loop contacts the external-service but the external service answers it can't process the value yet 4- The go-loop waits some timeout to retry the request to the external service After step 2 the producer continues to compute (suppose an expensive computing) a new value but the previous one wasn't effectively consumed by the external service. I don't want that, I want to enforce an end-to-end flow-control setup where the producer blocks on (>! c v) (the step 1) until the value is consumed by all parties, Sure, this flow control can be solved adding an ack channel and sending an ack from the go-loop to the producer when the external service effectively consumes the value, previously blocking the producer after step 1 waiting that ack. But I think a peek operation in step 2 will be more elegant. Also, I was curious if the implementation of core.async channels limits in some way adding a peek operation. A take-if with a pure predicate can't solve this, because you need to contact the external service to decide to consume the value or not. Saludos, Nahuel Greco. On Sun, Oct 5, 2014 at 9:52 AM, Fluid Dynamics <a2093...@trbvm.com> wrote: > On Sunday, October 5, 2014 12:51:04 AM UTC-4, Nahuel Greco wrote: >> >> I was thinking in a single-consumer scenario with a buffered chan, in >> which you want to check if you can consume the value before effectively >> consuming it. As you said, a peek operation has no sense if the channel has >> multiple consumers. >> > > And if you can't consume the value, then what? Nothing ever does, and that > channel becomes useless? > > Actually the only "peek" operation that to me makes much sense would be a > (take-if pred chan) or something similar, which atomically tests the next > value with pred and consumes it or not, so, it can't be consumed elsewhere > between the pred test and optional consumption here. And if not consumed, > two behaviors both occur to me as possible -- return nil or some other > sentinel value for "do not want" or block until the unwanted object is > consumed by someone else and then test the next item, etc.; at which point > you've got four versions of take-if you'd want, the inside-go and > outside-go versions cross product with the two when-not-wanted behaviors. > > At that point, you'd probably be better off just writing a consumer that > splits off the pred-matching items into one out channel and feeds > everything else into a second channel, with your original consumer taking > from the first of these and the others taking from the second. That gets > you the block until version of the behavior. The other version can be had > by making the pred-using consumer the sole consumer of the in channel, > which takes a value, applies pred, and branches, on the "want" branch doing > whatever and on the "do not want" branch putting the value onto an out > channel that feeds the other consumers before taking its own "do not want" > actions. > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
