The Status of Const

[Michel Fortin]

On 2010-08-16 08:08:21 -0400, "Steven Schveighoffer" 
<schveiguy at yahoo.com> said:

> On Fri, 13 Aug 2010 23:20:55 -0400, Andrei Alexandrescu  
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> On 08/13/2010 06:35 PM, Walter Bright wrote:
>>> Steven Schveighoffer wrote:
>>>> I like how it reads naturally. I think it's also syntactically
>>>> unambiguous. Walter, please give this one some attention, I'd love to
>>>> see this fixed.
>>> 
>>> This was endlessly discussed maybe 3 years ago. I probably invested over
>>> a hundred hours in trying to make it work.
>>> 
>>> It doesn't work.
> 
> Perhaps part of this is the reluctance to reexamine something which was 
>  hard to prove correct given the ideas at the time?
> 
> However, the idea is attainable.  For the simple fact that we have  
> tail-const references in other parts of the language.  All that is 
> missing  is syntax.
> 
>>> It wasn't the syntax. There were many syntaxes proposed. The type system
>>> loses its coherency with such a special case in it. Generic code has
>>> weird problems, type deduction gets strange, types lose their
>>> composability, etc.
> 
> The fact that all syntaxes proposed back then didn't cut the mustard 
> does  not mean that no possible syntax exists.  This is not NP != P.  
> Given that  we have syntax that works for pointers, it's logical and 
> reasonable to  assume that some syntax can work for generic 
> tail-constness of  everything.  In fact, we had syntax that *worked* it 
> was just confusing.

Indeed. This reflects my opinion too.

I'm not saying much about this issue right now because I feel it's no 
use, or to be precise, so much time convincing is needed to bring 
something useful I'd rather do something else. I've taken the route 
where I'll wait until others (Andrei?) hit the same problems and put 
enough pressure to force the design to be revisited. Hopefully this 
will happen sooner rather than later.

Same situation for @safe, same for pure, same for shared. Which means I 
won't be adopting D2's most interesting features for some time.


> BTW, I like Tomek's idea with @tail const/immutable/shared.  It's not a 
>  special case, it's a generic case, you can apply it to any type.

The idea's not bad, but I think it needs some improvments. For 
instance, in a struct, I might want some members to be part of the tail 
and some other not. I believe that should be allowed somehow.


-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/