“If lo” means that if the lo channel was read.
This code will prevent a lo from being processed if a hi is available at the
happens before moment of a value being ready.
That moment is somewhat arbitrary so it is only mildly different than
Select hi:
Default:
Select hi
Select lo
Btw using indents rather than brackets in the above - maybe that is causing the
confusion.
> On May 6, 2021, at 12:37 PM, 'Axel Wagner' via golang-nuts
> <golang-nuts@googlegroups.com> wrote:
>
>
> 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, 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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