[boost] Variant and visitation (was: [optional] Safe optional)

Subject: [boost] Variant and visitation (was: [optional] Safe optional)
From: Nevin Liber (nevin_at_[hidden])
Date: 2014-11-18 16:40:37

On 18 November 2014 15:02, Matt Calabrese <rivorus_at_[hidden]> wrote:

>
> > > For a while I assumed exactly that, but he actually specifically voiced
> > > that he does not like visitation on variant and thinks of it as a hack.
> > > It's sad :/
> > >
> >
> > FWIW:  I think he is (a) correct, but (b) we have nothing better to
> replace
> > it, so we still need it.
> >
>
> We should really start a thread about this. I'm very curious to see an
> actual objective rationale both for why visitation over a closed set of
> types known at compile time is in any way a "hack,"

It's the inversion of control that people just don't like.

> along with a realistic
> alternative.

As I already said, I don't know of one, and having it is better than not
having it.

The discussion in Urbana on n4218
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4218.pdf>
(variant), which didn't propose visitation, basically amounted to we don't
like visitation but we need it unless something better comes along.

> It really is a fundamental operation of a discriminated union.
> As someone who uses variants pretty much as the "default" for run-time
> polymorphism in day-to-day coding, I have never seen an actual explanation
> for what someone might consider bad about variant visitation, neither from
> Bjarne nor anyone else.
>

While I don't want to speak for Bjarne, I believe he would rather have some
form of Pattern Matching <http://www.stroustrup.com/OpenPatternMatching.pdf>
and possibly variant as a language feature.  That being said, no one has
proposed it.
-- 
 Nevin ":-)" Liber  <mailto:nevin_at_[hidden]>  [phone]