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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.


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