You are not wrong. I think it was obvious for the author to consider that >!! will not block if there are waiting offers.
You can see it in the code, if no buffer is set the write method will iterate over takers ready to consume the value. https://github.com/clojure/core.async/blob/91e6971a05fa49ca639fc1b7793141dd5f3d32ce/src/main/clojure/clojure/core/async/impl/channels.clj#L116 Le lundi 20 mai 2019 15:18:17 UTC+2, Brian Beckman a écrit : > > Thanks, Thomas. I shouldn't have included the quoted code about (<!! c) in > my question because it distracts from what I really want to know, and what > I want to know is all about how (go (<! c)) "makes buffer space available" > so that (>!! c 42) doesn't block. > > The following is an attempt to clarify my question. I first (go (<! c)) > and give the name d to the result, which is a channel that I am going > read-from later. Channel d is "connected to" or "relayed from" c. I then > (>!! c 42), and then (<!! d), both on the blocking "UI" thread. Everything > works. I wasted your attention by writing some code that would block (<!! > c) if it weren't quoted. > > Here is the part I don't understand: the documentation says that (>!! c > 42) will block "if there is no buffer space available," and the > documentation does not specify any other conditions. Well, I created c with > no buffer space, so, (>!! c 42) must block unless something else "makes > buffer space available," assuming the documentation is correct. The only > other interaction with c that could possibly be alive at the time when I do > (>!! c 42), is (go (<! c)), so (go (<! c)) must "make buffer space > available," assuming the documentation is correct. My understanding of (<! > c) (and I am suspicious of my understanding), is that (<! c) makes a > rendezvous available, not a buffer. If that understanding is correct, then > (>!! c 42) should block because there is no buffer available. > > On Sunday, May 19, 2019 at 1:48:16 PM UTC-7, Thomas Heller wrote: >> >> (<!! c) will hang because the value you put into c has already been taken >> by the first go (running in a different thread). So it is blocking until >> something puts another value into c. Since nothing ever does your program >> hangs. >> >> If it helps you can read "go" as "please run this somewhere else, >> possibly at a different time" and let the current thread continue after the >> go. >> >> I can't explain this very well but the documentation aspect is accurate. >> >> On Sunday, May 19, 2019 at 7:33:07 PM UTC+2, Brian Beckman wrote: >>> >>> The documentation for >!! reads: >>> >>> ------------------------- >>> clojure.core.async/>!! >>> ([port val]) >>> puts a val into port. nil values are not allowed. Will block if no >>> buffer space is available. Returns true unless port is already closed. >>> >>> >>> I have a case where I believe that the channel has no buffer, I park a >>> "pseudothread" in a go block reading off that channel via <!, and then >>> (lexically, not temporally), put to the unbuffered channel via >!!: >>> >>> (let [c (chan) ;; NO BUFFER! >>> d (go (<! c)) ;; park a pseudothread to read c >>> e (>!! c 42)] ;; blocking write to c, will unpark c's pseudothread >>> (println {:c-coughs-up '(this will hang (<!! c)), >>> :d-coughs-up (<!! d), >>> :what's-e e}) >>> (close! c) (close! d)) >>> >>> {:c-coughs-up (this will hang (<!! c)), :d-coughs-up 42, :what's-e true} >>> >>> >>> This case leads me to wonder whether the documentation might read >>> >>> >!! will block if there is no buffer space available *and* if there is >>> no *rendezvous *available, that is, no pseudothread parked waiting for >>> <!. >>> >>> but it's more likely that I completely misunderstand core.async because >>> I just made up the notion of a pseudothread in my struggle to understand! >>> >>> >>> >>> -- 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. To view this discussion on the web visit https://groups.google.com/d/msgid/clojure/8fbd0b5f-3cf8-4f4c-839b-8fce739dafb7%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
