Subject: Re: [boost] [outcome] On the design and documentation
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-05-26 12:45:09
2017-05-26 9:03 GMT+02:00 Vicente J. Botet Escriba via Boost <
boost_at_[hidden]>:
> Le 26/05/2017 à 08:22, Thomas Heller a écrit :
>
>> On 05/25/2017 07:28 AM, Vicente J. Botet Escriba wrote:
>>
>>> Le 24/05/2017 à 21:44, Thomas Heller via Boost a écrit :
>>>
>>
>>
>>>> First of all, I don't agree with the strong (conceptual) relationship
>>>> between
>>>> optional (be it boost:: or std::experimental) in such a way that
>>>> expected is a
>>>> generalization of it. From what I understand, the purpose of
>>>> expected/outcome/
>>>> result is to be used as a mechanism to return the result of a
>>>> function. As
>>>> such it should also expose the semantics of it. Fortunately, we
>>>> already have a
>>>> (asynchronous) return object standardized (std::future). And this is
>>>> my basic
>>>> disagreement here: Why not model expected in the lines of future? With
>>>> the
>>>> main points being:
>>>> - expected being movable only
>>>> - expected<T,E>::value() should always return by value and
>>>> invalidate this.
>>>> - (I would really prefer the color .get though ;))
>>>> So the question to Vicente/Niall is: what is the motivation to make it
>>>> "optional-ish"? Do we have use cases which make this design undesirable?
>>>>
>>> expected is a generalization of optional as it is synchronous and could
>>> return more information about why the the value is not there.
>>>
>>
>> Right, I can see this argument now, let me try to rephrase it a little
>> bit (correct me if I got something wrong please):
>> We want to be able to have a mechanism to return (or store) a value which
>> might not be there, and we need to know why it is not there. The class
>> that's currently available which almost does what we want is optional, in
>> fact, it is already used in such situations, so what we miss is the
>> possible error. So here we are, and then we naturally end up with something
>> like variant<T, E>. Makes perfect sense.
>>
>> My line of thought was mostly influenced by the property of being solely
>> used as a return object. And well, we already have the asynchronous return
>> objects, so why not go with something a synchronous return object which
>> represents a similar interface/semantics.
>>
>> With that being said, I am still not sure if the result of both ways to
>> look at it should converged into the same result.
>>
>> expected could be seen as the ready storage of a future.
>>> future::get block until the future is ready and then returns by
>>> reference :)
>>>
>>
>> Except not quite ;)
>> excepted indeed sounds like the perfect fit for the value store in the
>> share state. The only problem here, is that it really requires some kind of
>> empty (or uninitialized) since the value is only computed some time in the
>> future (this is motivation #1 for the proposed default constructed
>> semantics), having either a default constructed T or E doesn't make sense
>> in that scenario.
>> So it is more like a variant<monostate, T, E>.
>>
> From a high level I will see it as optional<expected<T,exception_pr>>. We
> optional because we have in addition the not-yet-ready state. Once the
> future becomes ready we have expected<T,exception_ptr>.
> I understand that the implementation optional<expected<T,exception_pr>>
> could be variant<monostate, T, exception_ptr>. The implementation of
> expected<T,exception_pr> could store as well variant<T,exception_pr>, but
> this makes difficult to extract the ready value as variant<T,exception_pr>
> is not convertible from variant<monostate, T,exception_pr>. So there are
> two possibilities for future::then
> * status-quo pass the future as parameter of the continuation
> * make it possible to cast variant<T,exception_pr> from variant<monostate,
> T,exception_pr> and pass expected to the continuation
>
> I proposed other future functions (next and catch_exception) long time ago
> to manage with the success and failure branches, so that we don't add any
> constraint in the way the contents is stored.
>
> Resuming, you are right expected<T,E> will not help as a value type
> representing the ready state without major changes.
>
BTW, is the scope of use cases we want to cover with `expected` outlined
clearly? People arrive at different optimum solutions when they start with
different goals in mind.