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)
}
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.
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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