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Subject: Re: [boost] Variant and visitation (was: [optional] Safe optional)
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2014-11-18 17:59:50

Cool, thanks for starting this thread.

On Tue, Nov 18, 2014 at 1:40 PM, Nevin Liber <nevin_at_[hidden]> wrote:
> It's the inversion of control that people just don't like.

Perhaps harsh, but this just sounds like a general dislike of high-order
functions, which is somewhat absurd to me. How is it any more problematic
from simply dispatching a function over fully known types at compile time,
or a high-order function that takes a function object and n arguments, then
forwards them along to the function object /without/ visitation? All
variant visitation logically does extra is transform the arguments based on
their discriminator before passing them along.

Perhaps to make things more clear, what if instead of type transformations
of variant to underlying type they were simply value transformations. In
other words, would you have a problem with a high order function that takes
a function object and 10 ints, but it incremented each int by 5 before
passing each one along to the other function object? Would there be a
problem with this? What if instead of incrementing by 5, it cast each
argument to float? If people don't have a problem with any of these
situations I simply do not see why they'd have a problem with something
like apply_visitor.

Further, visitation of a variant gives you /exactly/ the same overload
resolution that you'd get for the underlying type of the variant type(s)
being known at compile time (I.E. ordering, template pattern matching,
etc.). Because of that, it is exactly as capable as compile-time overload
resolution, plain and simple. It has no more or fewer problems in terms of
capability with respect to basic overload resolution because that's exactly
how it is implemented. I wouldn't consider it lacking unless it is also
considered that basic overload resolution is lacking. While /that/ may be
true, it doesn't make sense to expect visitation to change that (nor should
it, IMO). People understand basic overload resolution so they understand
the matching that takes place when doing visitation.

All that said, I do agree with problems of visitation when you do not have
a closed set of types, but that simply is not the case with a variant,
which is why it can be as powerful as it is in practice. Visitation of a
variant really does do exactly what the user wants (and nothing more). It's
also telling that the visitation is implemented in essentially the same way
that someone would manually implement it if the abstraction were not there.

Anyway, I've heard this argument before, and I just don't understand it. As
someone who really does use variants every single day, I have not
understood the criticisms at all. It just sounds as though /valid/
criticisms of visitation in something like a class hierarchy are now being
erroneously applied to visitation in the context of a discriminated union,
which just doesn't really make sense. We're not talking about visitation
into a class hierarchy or visitation into an "any."

The discussion in Urbana on n4218
> <>
> (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 <
> and possibly variant as a language feature. That being said, no one has
> proposed it.

I've read that paper before and will read it again tonight, but briefly
IIRC it was concerned with open sets of types. If you applied the approach
to variants all you'd get is a less efficient solution than visitation and
I simply don't buy the gains.

Again, I'll go back and reread the paper as I don't want to be too critical
without it fresh in my mind, but IMO it just did not apply to discriminated
unions, which is what we have with a variant.

-Matt Calabrese

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