Re: [boost] [review] Review of Outcome (starts Fri-19-May)

Subject: Re: [boost] [review] Review of Outcome (starts Fri-19-May)
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-05-16 07:21:07

2017-05-16 1:47 GMT+02:00 Peter Dimov via Boost <boost_at_[hidden]>:

> Gavin Lambert wrote:
>
> Maybe I'm missing some context, but unless T is void, making a void get()
>> method seems very peculiar to me.  Logically a get method should actually
>> get something.
>>
>
> get() is not a very good name, it's more like ensure(), and the intended
> use is in the transition from an outcome<>-using layer to a throwing layer:
>
> T f() // throws
> {
>    outcome<X> o1 = f1_impl(); // doesn't throw
>    o1.ensure(); // ensure we have a value
>
>    outcoume<Y> o2 = f2_impl();
>    o2.ensure();
>
>    return { *o1, *o2 };
> }
>
> The reference access paradigm implies that we want to keep the values in
> the outcomes, so there's no need to actually extract them, we can use them
> in-place.
>
> In contrast, the value return paradigm implies that in this scenario we
> want to extract the values from the outcomes and throw the outcomes away:
>
> T f()
> {
>    X x = f1_impl().value();
>    Y y = f2_impl().value();
>
>    return { x, y };
> }
>
> No need for a void get() here. Or, if we want to be verbose:
>
> T f()
> {
>    outcome<X> o1 = f1_impl();
>    X x = std::move(o1).value();
>
>    outcome<Y> o2 = f2_impl();
>    Y y = std::move(o2).value();
>
>    return { x, y };
> }
>
> although in this case o1 and o2 stay in scope and we prefer them not to
> because we don't want to access them after the values have been extracted.
> It's more explicit though so to each his own.

It's interesting. Only after Peter's explanation I am able to see the
usefulness of outcome<>, at least this part of the interface. It looks like
you - Peter - have the understanding of Niall's intentions. And I am
clearly lacking them. I would not be able to figure out this use case from
the tutorials of the reference.

This gives me the impression that the library has a big potential, but it
may not succeed because of the failure to explain its power to potential
users.

It would really be helpful to see a section in the docs with a number of
examples simple as the ones above, explaining different use cases. I am
still missing how the tribool logic can be used: in if-statements?

```
if (o1 || o2) ... // is that it?
```

Regards,
&rzej;