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Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-06-01 08:20:50

2017-05-31 22:17 GMT+02:00 Niall Douglas via Boost <boost_at_[hidden]>:

> On 31/05/2017 14:59, Andrzej Krzemienski via Boost wrote:
> > 2017-05-31 15:45 GMT+02:00 Niall Douglas via Boost <
> boost_at_[hidden]>:
> > Errors -- usual failure, Exceptions -- exceptional failure. Ok. But when
> I
> > call `error()` it is like saying "tell me if a *usual* failure ocurred",
> I
> > get no exception, fine. When I call `exception()` it is like saying "tell
> > me if an *exceptional* failure ocurred", and you are giving me a
> > non-exceptional one.
> >
> > This does not (yet) sound consistent to me.
> It makes sense when one assumes that outcome<T> accumulates result<T>,
> which it does. Outcome's design assumes that all outcome types
> eventually end up in an outcome<T>.
> The programmer, when working with outcome<T>, knows that calling
> .exception() will return the exact same exception as would be thrown
> when calling .value(), thus saving wrapping .value() in a try...catch
> just to capture the exception_ptr emitted.

Ok, this is important. And not obvious from the docs. But I do not think it
is true in the face of the potential empty state.

> The actually odd one out here is .error(), as Emil correctly spotted. It
> is there basically for the programmer to more efficiently intercept when
> an error code to exception throw conversion would occur, which is
> expensive.
> So, for outcome<T>, start with assuming that .value() and .exception()
> fully represent the twin sides of behaviour, and .error() is an early
> out mechanism.
> For result<T>, it's basically a castrated outcome<T> used only for
> performance sensitive code, or rather, to say "this function takes
> performance very seriously".

Is it also true that when I call `exception()` it will give me the
error_code_extended wrapped into an exception_ptr, if there is one?

> Does this make more sense?

This is my current understanding: When inspecting `outcome`, I should use
`exception()` to get information about any "failure" (either reported by
exception_ptr or by error_code_extended). Function `error()` is either for
performance or to discriminate between exception_ptr or by

Did I get it right?

(I hope so, and note that in order to describe that one does not have to
say that "some failures are expected, and others are not expected")


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