Re: [boost] [outcome v2] Please comment on new result<T, EC> reference API docs

Subject: Re: [boost] [outcome v2] Please comment on new result<T, EC> reference API docs
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-06-22 02:23:56

On 22/06/2017 02:08, Peter Dimov via Boost wrote:
> Niall Douglas wrote:
>> The idea, since we no longer have variant storage, is to see if we'd
>> like to implement
>>
>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0262r0.html
>> (except with the parameter order reversed).
> 
> I'm not particularly enthusiastic about status_value (and the argument
> given in the paper is unconvincing.) Neither am I thrilled by the idea
> of result<R, void> (an optional) or result<void, void> (something like a
> glorified pair<bool, bool>.)

I've fixed the problems you raised at
https://dedi4.nedprod.com/static/result-v2/doc_result.html and moved the
implementation stuff into a impl namespace so standardese will still
show the docs for the composited types, but not be usable by end users.

> result<T, EC> in its original form represented a very clear concept: the
> type of a function that must either return a value, or a reason for the
> lack of value. All of the above do not fit this concept and to me ring
> hollow.

Andrzej feels similarly to you.

But I don't think things are as bad as both of you feel. In v2 result<T,
EC> you now can have:

1. Successful: T

2. Failure: EC

3. Successful + Status: T + EC

The reason I don't think this is a problem is because you can't
accidentally return a wrong result<T, EC> in practice. Either you're
using result to return Success + Status, or you're using result to
return Success | Failure. It is very hard to accidentally write code
which does the wrong thing here, almost certainly the type system would
refuse to compile incorrect code.

Niall

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