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Subject: Re: [std-proposals] Meta types
From: Jake Arkinstall (jake.arkinstall_at_[hidden])
Date: 2020-02-05 17:05:45


This proposal in isolation is purely about a function being able to return
multiple times and with multiple different types, and the call site
offering behaviours for each. In precisely the same way that a function can
be generated by a call to a template function, the return path could be
generated by a yield from a function such as I propose. The idea is simply
to provide what callback structs provide with a more intuitive interface.

The sender/receiver would simply be an application of this. A sender has a
list of defined types to transmit, and a receiver has a list of defined
types to receive (this can be figured out by the compiler). The current
library implementations of this concept are inelegant compared with a
language approach, and come with more bootstrap than you can shake a stick
at. They also tend to focus on the async side of things, which isn't my
intention.

On Wed, 5 Feb 2020, 13:19 Gašper Ažman via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Jake,
>
> Seems to me like doing what you describe well is exactly what the
> executors' sender/receiver model is built for. Look at libunifex for an
> example.
>
> On Wed, Feb 5, 2020 at 12:27 AM Garrett May via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> On Tue, 4 Feb 2020, 11:27 am Jake Arkinstall via Std-Proposals, <
>> std-proposals_at_[hidden]> wrote:
>>
>> This proposal will likely also include a mechanism for switching on
>> types. I'm also thinking of using template/concept syntax in place of var.
>>
>>
>> I don't believe that template/concept syntax would be appropriate here
>> unless changes were made to what templates/concepts can do. That's because
>> what we have here is return types that are not dependent on templates, nor
>> on concepts, but instead on *branches*.
>>
>> Because this would be designed to be "switching on types", perhaps the
>> following would be appropriate syntax:
>>
>> switch(auto) halve(unsigned int i){
>> if(i % 2 == 0){
>> // branch for unsigned int
>> return i / 2;
>> } else{
>> // branch for double
>> return i * 0.5;
>> }
>> }
>>
>> And probably for consistency, a switch(decltype(auto)) would be available.
>>
>> As for the yielding concept, this seems like a second proposal, because
>> its basis is mainly on the ability to yield (a lazy idea) rather than
>> "switching on types".
>>
>> This is just my two pence. It seems along the same lines as more recent
>> proposals, such as expansion statements (template for).
>>
>>
>> On Tue, 4 Feb 2020, 4:18 pm Jake Arkinstall via Std-Proposals, <
>> std-proposals_at_[hidden]> wrote:
>>
>>> The '14 version you have provided is exactly what my proposal generates,
>>> but without the indirection. Much of my code involves exactly this pattern,
>>> stitched together with a pipeline builder struct, cringy use of the >>
>>> operator and helper macros for yield, etc. It suffices, but it's crying out
>>> for either a new language, a preprocessor, or a new language feature.
>>>
>>> You'll also note that debugging through the callback approach is nothing
>>> less than horrific, especially if any pipeline building code is involved
>>> (just like with variants). I can't imagine this changing while the code
>>> being written is in the form of callbacks.
>>>
>>> The reason is that given N components of the pipeline (each yielding a
>>> new value to be passed to the next), an unhandled type from component N-1
>>> to component N bubbles an error generating from the first call to component
>>> 0. Deciphering the message that comes out is fun, to say the least, and
>>> while static_assert is unable to add typenames etc there's no real way
>>> around it.
>>>
>>> As a language feature, it's easy. The compiler is aware of what you're
>>> trying to do, and can provide useful information when you get it wrong.
>>>
>>> On Tue, 4 Feb 2020, 15:16 Arthur O'Dwyer via Std-Proposals, <
>>> std-proposals_at_[hidden]> wrote:
>>>
>>>> On Tue, Feb 4, 2020 at 8:16 AM Михаил Найденов via Std-Proposals <
>>>> std-proposals_at_[hidden]> wrote:
>>>>
>>>>> This definitely steps into both "language variant" and Pattern
>>>>> Matching territory.
>>>>> I am not sure what optimization we can expect from "language variant",
>>>>> but PM should give us close to your ideal code
>>>>>
>>>>> void main(int argc, char const* const* argv){
>>>>> inspect(halve(args))
>>>>> Numeric x => std::cout << "You passed 2*(" << x << ")
>>>>> arguments\n";
>>>>> return 0;
>>>>> }
>>>>>
>>>>
>>>> The current Pattern Matching proposal doesn't give the programmer any
>>>> way to write `halve` so that it can actually return either of two
>>>> alternative types (`unsigned` or `double`) depending on a *runtime*
>>>> condition (the evenness of the runtime argument).
>>>>
>>>> However, I don't see anything wrong with the existing C++14 language
>>>> solution to OP's problem. It's not worth pursuing any crazy core-language
>>>> gymnastics unless you can provide a use-case that isn't already solvable
>>>> idiomatically in C++14 (17, 20). Here's the '14 solution:
>>>>
>>>> template<class F>
>>>> void halve(unsigned i, const F& f) {
>>>> if (i % 2 == 0) {
>>>> f(i / 2);
>>>> } else {
>>>> f(i * 0.5);
>>>> }
>>>> }
>>>>
>>>> int main(int argc, char**) {
>>>> halve(argc, [](auto x) {
>>>> std::cout << "You passed 2*(" << x << ") arguments\n";
>>>> });
>>>> return 0;
>>>> }
>>>>
>>>> Short, readable, relatively easy to understand. Certainly easi*er* to
>>>> understand than anything proposed in the OP.
>>>>
>>>> –Arthur
>>>> --
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>>>>
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