No, it is not. Your "if lo" branch implies that the communication happened
- e.g. the sender was already unblocked. A `select` would not unblock the
other side unless that's the actual branch taken.
On Thu, May 6, 2021 at 7:32 PM Robert Engels <reng...@ix.netcom.com> wrote:
> I already showed you - just change it to
>
> Select hi
> Default:
> Select hi,lo
> If lo:
> Select hi
> Default :
> Pass
>
> And enqueue the lo if a hi and lo are read.
>
> That is all that is needed.
>
>
>
> On May 6, 2021, at 10:28 AM, 'Axel Wagner' via golang-nuts <
> golang-nuts@googlegroups.com> wrote:
>
>
>
>
> On Thu, May 6, 2021 at 4:43 PM roger peppe <rogpe...@gmail.com> wrote:
>
>>
>> On Thu, 6 May 2021 at 14:41, 'Axel Wagner' via golang-nuts <
>> golang-nuts@googlegroups.com> wrote:
>>
>>> PS: And I'm not saying there is no argument. Maybe "select is not
>>> atomic" is such an argument. But if there is an argument and/or if this is
>>> that argument, I don't fully understand it myself.
>>>
>>
>> One reason is that the semantics can conflict. Consider this code, for
>> example (assuming a hypothetical "pri select" statement that chooses the
>> first ready arm of the select) - the priorities conflict. I suspect Occam
>> doesn't encounter that issue because it only allows (or at least, it did
>> back when I used Occam) select on input, not output. I believe that
>> restriction was due to the difficulty of implementing bidirectional select
>> between actual distributed hardware processors, but I'm sure Øyvind knows
>> better.
>>
>> func main() {
>> c1, c2, c3 := make(chan int), make(chan int), make(chan int)
>>
>> go func() {
>> pri select {
>> case c1 <- 1:
>> case v := <-c2:
>> c3 <- v
>> }
>> }()
>> go func() {
>> pri select {
>> case c2 <- 2:
>> case v := <-c1:
>> c3 <- v
>> }
>> }()
>> fmt.Println(<-c3)
>> }
>>
>
> Interesting case. I would argue, though, that there is no happens-before
> edge here to order the cases and I was only considering providing a
> guarantee if there is one.
>
>
>> That said, I suspect that the semantics could be ironed out, and the real
>> reason for Go's lack is that it's not actually that useful; that it would
>> be one more feature; and that in practice a random choice makes sense
>> almost all the time.
>>
>
> As I said, this would certainly satisfy me as an answer :)
>
>
>>
>>
>> On Thu, May 6, 2021 at 3:40 PM Axel Wagner <axel.wagner...@googlemail.com>
>>> wrote:
>>>
>>>> FWIW after all this discussion I *am* curious about a more detailed
>>>> argument for why we can't have a priority select that guarantees that
>>>> *if* the high-priority case becomes ready before the low-priority one
>>>> (in the sense of "there exists a happens-before edge according to the
>>>> memory model"), the high-priority will always be chosen.
>>>>
>>>> That is, in the example I posted above
>>>> <https://play.golang.org/p/UUA7nRFdyJE>, we *do* know that `hi`
>>>> becoming readable happens-before `lo` becoming readable, so a true
>>>> prioritized select would always choose `hi` and never return. The construct
>>>> we presented *does* return.
>>>>
>>>> Now, I do 100% agree that it's not possible to have a select that
>>>> guarantees that `hi` will be read if both *become readable
>>>> concurrently*. But I don't see a *fundamental* issue with having a
>>>> select that always chooses `hi` if `*hi` becoming readable
>>>> happens-before `lo` becoming readable*.
>>>>
>>>> And to be clear, I also kinda like that we don't have that - I think
>>>> the value provided by the pseudo-random choice in preventing starvation is
>>>> worth not having an "ideal" priority select construct in the language. But
>>>> I couldn't really make a good case why we *can't* have it.
>>>>
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>>>
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