uniqueness

[Fawzi Mohamed]

It is nice that Michel Fortin made the effort to propose a patch  
trying to address the ability to rebind const objects.

Looking at the "uglification" of my code to support const, I saw that  
many cases I actually had a unique type, or partially unique type.

There are several examples of similar attempts, like linear types, or  
various uniqueness types systems (more or less related to the Clean  
example).
It is known that some uniqueness settings are equivalent to shared, so  
maybe using uniqueness with const might be meaningful.

This is a difficult topic, as pushing those concepts into the type  
system is always tricky, and the consequences of various choices are  
often non obvious, anyway here is what I thought.

If one could declare return or out types as unique (note that unique  
is *not* part of the type, it is like the storage attributes), these  
methods could be implicitly castable to const or immutable, allowing  
nicer code.

Constructors *might* return unique objects (an object is unique only  
if all its references are to unique or immutable objects).

In several cases uniqueness could be checked by the compiler. I think  
that such a change would improve part of my code, removing the need  
for several spurious casts, while at the same time making the code  
safer.

I did also think about having a front_unique attribute that can be  
applied to any local variable or argument that would make it tail  
const in the sense discussed previously, and still implicitly castable  
to full const.
In that case the situation is more complex (one should ensure that  
local references cannot spill out, otherwise a full const is needed,  
and for immutable, making it immutable is "irreversible".
The front_unique property can almost always be checked by the  
compiler, but activating it implicitly would have effects that would  
probably deemed surprising by the programmer (front_unique immutable  
objects would be rebindable).
Thus I am not sold on front_unique, but I still find it interesting,  
due to its relationship with tail const.

Fawzi