Fixing const arrays

[Jonathan M Davis]

On Sunday, December 11, 2011 10:34:40 Andrei Alexandrescu wrote:
> On 12/11/11 9:46 AM, dsimcha wrote:
> > On 12/10/2011 4:47 PM, Andrei Alexandrescu wrote:
> >> We decided to fix this issue by automatically shedding the top-level
> >> const when passing an array or a pointer by value into a function.
> > 
> > Really silly question: Why not do the same for primitives (int, float,
> > char, etc.) or even structs without indirection? I've seen plenty of
> > code that blows up when passed an immutable double because it tries to
> > mutate its arguments. About 1.5 years ago I fixed a bug like this in
> > std.math.pow().
> 
> Yes, that would be good to do as well.

Actually, that could be a problem for some stuff. It might be an acceptable 
problem, but it creates a problem nonetheless. What about containers? You can 
have arrays with immutable elements, but if you made it so that immutable int 
and int were the same as far as templates were concerned, then it would be 
impossible to have a container which held immutable elements. How big of a 
problem that is, I don't know, but I'd be concerned about some of the side 
effects.

Another concern would be what would happen with primitives when they're inside 
of arrays. e.g.

void func(T)(T[] arr)

Having T lose its constness would be a big problem here.

Now, if we're talking only IFTI - such func(cast(immutable int)2) instantiates 
with int and func!(immutable int)(2) still instantiates with immutable int - 
and it's only for primitives by themselves (e.g. func above is unaffected), 
then it likely isn't a problem. But we need to really look at the side effects 
of trying to treat primitives as mutable in templates.

And if you're going to do that with primitives, it also opens up the question 
as to what to do with structs which are value types. Should they be treated 
the same?

In theory, I think that stripping const and immutable from primitives at the 
top level could be a nice thing to have, but I'd be very concerned about how 
that would be implemented and what it would affect. We risk losing valuable 
type information and breaking existing code if it's not done right.

- Jonathan M Davis