C++ Logo

STD-PROPOSALS

Advanced search

Subject: Re: [std-proposals] Alternative return-paths
From: Toby Brull (toby.brull_at_[hidden])
Date: 2020-06-08 06:50:12


Yes, sorry, that should've had curly braces around it. Obviously, none of
the above compiles, so I could not verify it through a compiler.

On the other hand, the same way alternative return-paths map to
std::variant, I briefly thought about a similar mechanism that would map to
std::tuple. This would basically mean that any function (and any
return-path) could return multiple values (akin to a return value of
type std::variant<std::tuple,
std::tuple, ...>). So, you could rewrite the above function as

int, int parse_int_square(std::string_view const str)
  ->[parse_error] std::string
{
  auto const i = parse_int(str);
  return i, i * i;
}

int main() {
  // ...
  auto i, i_sq = parse_int_squared(str) ->[parse_error] throw;
  // ...
}

But I'm not proposing this here.

On Mon, 8 Jun 2020 at 12:05, Nikolay Mihaylov <nmmm_at_[hidden]> wrote:

> Since when this compiles?
>
> Isn't it needs {} ?
>
> auto parse_int_square(std::string_view const str)
> -> std::tuple<int, int>
> {
> auto const i = parse_int(str);
> return i, i * i; // <-------
> }
>
>
> On Mon, Jun 8, 2020 at 1:53 PM Toby Brull via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> Hi,
>>
>> I was thinking about error-handling recently (see references [1] and [2]
>> below). Of course, there are several proposals aimed at improving the
>> error-handling mechanisms in C++ (p0709, p1028, p1029, p1144; p0323, p0798).
>>
>> I was particularly interested in p0709 (title: "Zero-overhead
>> deterministic exceptions: Throwing values"). I would summarise this
>> proposal here as making error-handling better by making exceptions better.
>> My thought was that maybe one could tackle the problem of error-handling at
>> a lower level first, and only bring exceptions into the picture later,
>> especially given that some people don’t want to use exceptions.
>>
>> So, my idea was to allow functions to have multiple, alternative
>> return-paths at the language level. Basically, a function declaration would
>> be allowed to declare any number of alternative return-paths (each with an
>> associated alternative return-type). The function implementation could then
>> return along any of those return-paths at any point in its execution. The
>> caller of such a function would always be required to handle all
>> alternative return-paths in some way: either (a) explicitly (e.g. by
>> throwing an exception or handling the error otherwise) or (b) by declaring
>> the same alternative return-path (in which case the mechanism would work
>> similar to stack unwinding). Failure to do either (a) or (b) results in a
>> compile-time error.
>>
>> Here is a short snippet of what it might look like (inspired by [2]):
>>
>> int parse_int(std::string_view const str)
>> ->[parse_error] std::string
>> {
>> std::stringstream ss; ss << str;
>> int i; ss >> i;
>> if (ss.fail()) {
>> return[parse_error] "parse_positive_int: not an int";
>> }
>> else {
>> return i;
>> }
>> }
>>
>> auto parse_int_square(std::string_view const str)
>> -> std::tuple<int, int>
>> ->[parse_error] std::string
>> {
>> auto const i = parse_int(str);
>> return i, i * i;
>> }
>>
>> int main() {
>> std::string input;
>> std::cin >> input;
>>
>> auto const [i, i_sq] = parse_int_square(input)
>> ->[parse_error] auto const& msg {
>> std::cerr << “Error: “ << msg;
>> return 1;
>> };
>>
>> assert(i * i == i_sq);
>> std::cout << "i_sq = " << i_sq << '\n';
>> return 0;
>> }
>>
>> More extensive brain storming here:
>> https://github.com/TobyBrull/scratchpad/blob/master/alternative_return_paths.cpp
>>
>> I imagine all this would still be compatible with p1028/std::error. It
>> might also have other applications besides error-handling; essentially,
>> every function that returns a std::variant could use this mechanism.
>>
>> I don't have any experience working on compilers, but I imagine this
>> should be possible to implement efficiently, although it would require an
>> ABI extension. I imagine the alternative returns-paths to be part of the
>> function type and thus affect name mangling.
>>
>> What do you think?
>>
>> Cheers,
>> Toby
>>
>> References:
>> [1] http://lucteo.ro/2018/04/21/exception-exploration-2/
>> [2] Phil Nash's CppCon 2019 talk,
>> "The Dawn of a New Error" ( https://www.youtube.com/watch?v=ZUH8p1EQswA )
>> --
>> Std-Proposals mailing list
>> Std-Proposals_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
>



STD-PROPOSALS list run by std-proposals-owner@lists.isocpp.org

Standard Proposals Archives on Google Groups