Re: [boost] Outcome v2

Subject: Re: [boost] Outcome v2
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-07-12 17:23:37

2017-07-12 19:11 GMT+02:00 Emil Dotchevski via Boost <boost_at_[hidden]>
:

> On Tue, Jul 11, 2017 at 3:16 PM, Emil Dotchevski <emildotchevski_at_[hidden]
> >
> wrote:
>
> > On Tue, Jul 11, 2017 at 2:11 PM, Niall Douglas via Boost <
> > boost_at_[hidden]> wrote:
> >
> >> > Outcome does not solve the most important problem that needs solving
> by
> >> an
> >> > error handling library.
> >>
> >> Does Expected?
> >>
> >
> > Nope, but the original outcome<T> was closer. I thought, perhaps it is
> > possible to make it able to transport arbitrary error types, and not by
> > exception_ptr. My attempt at solving that problem lead me to TLS but
> maybe
> > there are other ways.
> >
>
> Allow me to clarify. Suppose I have a function foo which returns FILE * on
> success, some EC1 on failure, and another function bar(), which calls foo
> and returns an int on success, some EC2 on failure. I believe in terms of
> Outcome this would be:
>
> outcome::result<FILE *,EC1> foo() noexcept;
>
> outcome::result<int,EC2> bar() noexcept {
>   if( auto r=foo() ) {
>     //no error, use r.value(), produce the int result or return EC2(x).
>   } else {
>     return ______;
>   }
> }
>
> What do you recommend in place of _____?
>
> Here is the same code in terms of Noexcept:
>
> FILE * foo() noexcept;
>
> int bar() noexcept {
>   if( FILE * r=foo() ) {
>     //no error, use r, produce the int result or return throw_(EC2(x)).
>   } else {
>     return throw_();
>   }
> }
>
> That is, with Noexcept, bar() would not care what error types propagate out
> of foo because it can't handle any errors anyway. Whatever the error is, it
> is simply passed to the caller.
>

The default  EC type in outcome::result<> is std::error_code, and it is
possible to use only this type throughout entire program (similarly to
std::unique_ptr<T>: you can use it all around the program and not even be
aware that it had a second template parameter).

With std::error_code you do acheive this guarantee that the original error
inofrmation is always preserved as the control goes throughout the layers
of the program.

Of course, you cannot carry arbitrary payload now, but given that these
errors are to be handled immediatly in the next layer, it should not be
that much of the problem.

Regards,
&rzej;