Re: [boost] [outcome] How to drop the formal empty state

Subject: Re: [boost] [outcome] How to drop the formal empty state
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-05-25 05:54:21

Le 24/05/2017 à 21:43, Niall Douglas via Boost a écrit :
> There has been a fair bit of review feedback that people don't like the
> formal empty state in outcome<T> and result<T>. I still think this
> opinion daft, just because it's there doesn't mean you have to use it,
> but here are some options:
>
> 1. Like Expected, we could require the E types to provide a nothrow move
> constructor. This would allow us to guarantee no empty state, not ever.
> Default constructing an outcome<T> or result<T> would set a default
> constructed T state, just like Expected.
We could as well choose to don't initialize at all. I want to open this 
debate in Toronto.
>
> The only remaining difference now between result<T> and expected<T>
> would be the wide vs narrow observer functions. outcome<T> still can
> also carry a std::exception_ptr.
>
> 2. We could do the above, but default constructor constructs to some
> constexpr undefined state that is not state T nor E.
Agreed, we could as well choose to don't initialize it at all. I want to 
open this debate in Toronto.
I don't want this to be observable.
>
> 3. We could only provide a guarantee of no empty state if the E types
> provide nothrow move construction, but if they do not we fall back to an
> empty state arising if throws occur during assignment.
This is a *valueless-by-exception* as for variant. When do you want to 
report an error that has a move constructor that can throw?
What would you report at the end. Sorry I can not report to you the 
reason for failure as an exception was throws while constructing the 
report :(
If we want to KISS Error types must be no-throw movable.
>
> 4. The option for an empty state becomes a template parameter defaulted
> to false i.e. disabled. This can be combined with any of the above.
No thanks.

I don't see yet the need for this empty state.

Vicente

P.S. I expect to have in C++20 a std::variant that don't have this 
*valueless-by-exception* state when the conditions are satisfied.