On Wednesday, 17 February 2016 at 23:47:54 UTC, Walter Bright
wrote:
On 2/17/2016 3:12 PM, Jonathan M Davis wrote:
On Wednesday, 17 February 2016 at 22:44:27 UTC, Walter Bright
wrote:
It would seem that implementing headconst as a type
constructor would let
people who wanted mutable members have their way, without
introducing
backdoors in const.
I really don't see how that's the same thing at all. The main
problems with
const pop up when what folks need is something like C++'s
mutable, because they
need to be able to do something like a reference count or a
mutex which is not
really part of the logical state of the object but is still
part of the object.
Having some form of non-transitive const allows for things
like a const pointer
to non-const data, but it doesn't help at all when what you
need to do is treat
most of the object as const while treating a small portion of
it as mutable.
Sure, that's not completely transitive, because parts of the
object are mutable,
but it's specific parts of the object, not which part of a
pointer declaration
is const and which isn't. Maybe I'm missing something, but I
don't see how
adding non-transitive const to D in order to solve the C++
declaration problem
would help at all with the complaints that folks have with D's
const. The
complaints with D's const are almost entirely about the lack
of backdoors (be
they well-defined like a mutable attribute or a free-for-all
like casting away
const and mutating).
If the headconst was one level up, the struct can have mutating
members.
Yes, but that really isn't going to help much code. It would be
useless for ref-counting const objects, it wouldn't allow you to
put a mutex in a const object, and you couldn't do anything with
caching calculated properties in a const object. All it means is
that you can do something like have a const pointer to a mutable
object, which in my experience is usually useless. There have
been plenty of complaints about having to use Rebindable instead
of having tail-const for classes, but I've rarely seen anyone
complain about the lack of head-const. And what's usually the
case is that folks want logical const, and I don't see how that's
possible without either having something similar to C++'s mutable
or make it well-defined to cast away const and mutate.
Having something similar to C++'s mutable would at least solve
most of the problem while making it explicit and safe, whereas
allowing casting away const and mutating would just throw all of
the guarantees of const out the window and risk serious problems
with immutable, making it so that all const really did was
prevent accidental mutation and serve as documentation of intent.
Something like @mutable would at least retain the guarantees that
we currently have when @mutable isn't used and restrict the
effects of removing const to very well defined areas such that it
wouldn't run afoul of immutable.
So, if we want to actually solve the problems that folks
typically complain about with const, I think that something like
@mutable is the way to go. Now, we can certainly decide that
that's not worth it and that those idioms simply won't work in D
as long as const is used, but I really don't think that
head-const is going to solve the same problem at all or really
make much of anyone much happier aside from how it helps with C++
interoperability. Adding tail-const for classes to the language
would make a whale of a lot more people happy than head-const
would even though Rebindable ostensibly solves that problem
already.
- Jonathan M Davis
