Why MapResult.empty() is not const in Phobos?

[Jonathan M Davis]

On Saturday, July 6, 2019 10:18:49 AM MDT Exil via Digitalmars-d wrote:
> On Saturday, 6 July 2019 at 06:56:22 UTC, Jonathan M Davis wrote:
> > On Friday, July 5, 2019 11:37:10 PM MDT valmat via
> >
> > Digitalmars-d wrote:
> >> Minimal examples when it broke code:
> >> https://run.dlang.io/is/HbEfkJ https://run.dlang.io/is/4h1B6Z
> >
> > In general, it can't be empty, because that would require that
> > the range it's wrapping have a const empty, and most don't. The
> > only way that map (or any wrapper range) can have a const empty
> > would be if it goes to the effort of duplicating the function
> > and uses static ifs to determine whether the version that's
> > const will work or whether the version that's not const needs
> > to be compiled in. And since const ranges are basically
> > unusable, because they can't be iterated, going to that level
> > of effort and making the code messy in that manner is arguably
> > a waste of time.
> >
> > - Jonathan M Davis
>
> So phobos doesn't properly use const either, officially. Just one
> more reason not to use const on the pile.

There really isn't a way to use const "properly" with ranges. The most that
you could possibly do with a const range is query whether it's empty or get
at a const version of front. And that's assuming that the range is
implemented in a way that either of those would work and that any ranges
that it wraps also have const on empty and/or front.

Maybe if we had a generic way to get a tail-const slice of a range, then
const ranges would be more viable, but we don't, and with how restrictive
D's const is, it wouldn't surprise me if plenty of ranges (especially those
over containers) couldn't have tail-const slices anyway due to how they
iterate internally. Either way, we don't have a way to get tail-const ranges
other than with dynamic arrays, and the range API fundamentally requires
that a range be mutable in order to be iterated. So, trying to use const
with ranges quickly becomes a waste of time. Even if we regularly tried to
go as far as we can go with const and the range API, it wouldn't get very
far. D's range API is incompatible with const.

- Jonathan M Davis