Re: [boost] [outcome] Second high level summary of review feedback accepted so far

Subject: Re: [boost] [outcome] Second high level summary of review feedback accepted so far
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-05-30 21:19:19

Le 30/05/2017 à 19:53, Niall Douglas via Boost a écrit :
> On 30/05/2017 18:43, Peter Dimov via Boost wrote:
>> Niall Douglas wrote:
>>
>>> Do you accept that the static checked and runtime checked varieties
>>> are orthogonal user bases? There is a camp of users who strongly
>>> prefer no runtime overhead and static checking.
>> Why are their needs not served by value_if?
>>
>>     auto r = function();
>>
>>     if( auto* p = r.value_if() )
>>     {
>>         // use *p
>>         // no runtime overhead on using *p
>>         // static checkers know use of *p implies p != nullptr
>>     }
>>
>> Between this pattern and using `assert( r.has_value() )` directly to
>> advise the static checker, are we not covered?
> I'm still pondering your idea on this for the runtime checked editions.
> So far I am liking it, but I need to sleep on it some more.
>
> But there was still a large minority of folk who want all-narrow
> observers. They haven't voiced anything to say they have changed their
> minds.
>
> I currently, roughly speaking, find approx 50% in favour of a runtime
> checked edition, approx 40% in favour of a statically checked edition.
>
> Emil and Vicente make up the 10% of folk who want something completely
> different.
>
I say that we don't have to choose. We can provide as we have already 
done in a lot of libraries provide wide and narrow functions. No need to 
split the world on two parts. No silos.

value_if could be a complementary/helper interface. We don't need to 
provide them on all the types, but as algorithms. The implementation is 
the same for optional, expected, result, ....

The same for
T nv = value_or(pv, v);
E ne = error_or(pe, e);
bool b = has_error(pe, e);

IMHO, most of the wide function constracts can be provided this way. 
There are some exceptions e.g. value that throw a different exception 
depending on the type, but this is because we don't have found yet the 
minimal interface.
E.g. If we had a pe.rethrow() function, we could define value by in 
function of has_value, operator* and rethrow.

Vicente