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!
>>
>>
>>
>>
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