Re: [boost] [outcome] Ternary logic -- need an example

Subject: Re: [boost] [outcome] Ternary logic -- need an example
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-05-19 20:53:40

Le 19/05/2017 à 09:01, Andrzej Krzemienski via Boost a écrit :
> 2017-05-19 0:49 GMT+02:00 Niall Douglas via Boost <boost_at_[hidden]>:
>
> No undefined behaviour needed, and you can write your code using
>> Outcome with the hard assumption that o.error() will always return an
>> accurate view of the current state. No need to check .has_error(), or
>> anything like it.
>>
> Modulo this situation with `http_error::ok == 200`. But with this you are
> also saying, the library provides two ways for checking if you have an
> error:
>
> o.has_error();  // option 1
> o.error() == std::error_code{}; // option 2
In the current standard expected proposal, we have an open point about a 
has_error(exp, err) that return true if exp has no value and the errror 
is the provided as parameter.
This has the advantage to not throwing.
> And by describing clear semantics for option 2, you are saying, it is
> equally fine to use option 2 for checking if we have an error. This
> encourages the usage of option 2, but I would not want my colleague
> programmers to start using this syntax, because I then cannot tell proper
> usages from inadvertent omissions. And reading the value of `error()`
> without having confirmed that some error occurred is almost surely a bug,
> even if you can assign a well defined semantics in `boost::outcome` for it.
>
>
>
>> Remember, expected<std::error_code, std::error_code> is legal, though
>> Outcome's Expected doesn't allow it.
Why?

Best,
Vicente