Subject: Re: [boost] Noexcept
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-06-13 05:54:13
Le 13/06/2017 Ã 01:01, Niall Douglas via Boost a Ã©crit :
>> Still, I fail to see how Noexcept differs from Outcome in this aspect.
>> Semantically the only difference is that Noexcept doesn't force users to
>> use a special template in return types, but that's a good thing. If it's
>> preferable, they can still use a special template, and if they do, it's
>> trivial to design because it doesn't have to transport errors -- Noexcept
>> takes care of that for you.
> You *want* APIs to clearly indicate their failure contract.
> Relying on TLS trickery hides control flow paths. And if people fail to
> write the check, errors get lost or pop out in the wrong locations.
> Forcing a wrapper type to be used also allows [[nodiscard]] to be
> leveraged, and in the future static analysis to be applied. Neither
> works with your scheme, which is why I rejected it very early on.
> Finally, Rust and Swift have adopted a Result<T, E> model. It is
> generally viewed as a good design choice for its problem domain. Varying
> significantly from what the other system languages are doing needs to
> have very strong rationale.
AFAIK , the proposed library and Swift error handling mechanism are
very close. Swift has alternatively also used Result<T,E> as we could
The main difference I see is that one is library based and the other
In Swift you signal that a function can throw adding throws() to the
signature. Swift has builtin optionals and adding throw is almost like
declaring it to return T? (optional<T>.
You cannot call this function without using try, try! or try?
IIUC, with Noexcept, you cannot require this as it is a library. However
when the user uses try_ it is able to control whether the call succeeds
In order to force it, the closer is to use a return type that tell you
that there could be errors, as return_<T>.
I will say that if Noexcept required this return_<T> type, it will be
like outcome<T>, except that the error is transported using TLS instead
of using the stack (please let me know if I'm wrong)
However if Noexcept doesn't require a return_<T> then it is much
difficult to force the use of the try functions. But it works yet.
I see advantages in this approach and I don't know which one is more
efficient in the success and failure cases. Some measures will be more
I'll rename Noexcept to ErrorTLS
do we want an error handling mechanism in C++ based on Swift error
handling ;-) ? Do we want a library that emulates it as Boost.Noexcept
do we want a monadic error handling in C++ as Result<T,E> in Boost?
do we want both in Boost?
I believe both merit to be tried.
P.S. I've not read the full documentation yet, but this seams promising.
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