std.concurrency & immutable classes...

[Steven Schveighoffer]

On Thu, 10 Feb 2011 14:45:14 -0500, Tomek Sowiński <just at ask.me> wrote:

> Michel Fortin napisał:
>
>> > Thanks for doing this. Is it approved by Walter?
>>
>> Depends on what you mean by "approved".
>>
>> He commented once on the newsgroup after I posted an earlier version of
>> the patch, saying I should add tests for type deduction and some other
>> stuff. This change his something he attempted to do in the past and
>> failed, I expect him to be skeptical.
>
> It would be much easier if he provided the specific case(s) which broke  
> his teeth. Then we'll all know where's the problem. If it's soluble,  
> it'll open the door to tail type modifiers in general, not just in  
> classes. It's a burning issue e.g. with ranges (mostly struct).
>
> http://d.puremagic.com/issues/show_bug.cgi?id=5377
>
> Look at the attachment to get a feel of what hoops we'll have to jump  
> through to side-step lack of tail X.

I've worked through this very same problem (a few months back), thinking  
that we need a general solution to tail-const.  The large issue with  
tail-const for structs in the general case is that you cannot control the  
type of 'this'.  It's always ref.  This might seem like a very  
inconsequential detail, but I realized that a ref to X does not implicitly  
convert to a ref to a tail-const X.  This violates a rule of two  
indirections, in which case you are not able to implicitly convert the  
indirect type, even if the indirect type would implicitly convert outside  
the reference.

A simple example, you cannot convert an int** to a const(int)**.  Reason  
being, then you could change the indirect pointer to point to something  
that's immutable, and the original int ** now points to immutable data.

The same is for tail-const structs, because you go through one ref via  
'this' and the other ref via the referring member.

What does this all mean?  It basically means that you have to define  
*separate* functions for tail-const and const, and separate functions for  
tail-immutable and immutable.  This is untenable.

You might ask "why doesn't this problem occur with tail-const arrays?",  
well because you *don't pass them by ref*.  With structs we have no choice.

I think what we need is a way to define two different structs as being the  
tail-const version of the other, with some compiler help, and then we do  
not need to define a new flavor of const functions.  We still need to  
define these "tail-const" functions, but it comes in a more understandable  
form.  But importantly, the implicit cast makes a *temporary* copy of the  
struct, allowing the cast to work.

-Steve