On 11/29/10 12:56 PM, Michel Fortin wrote:
On 2010-11-29 13:08:54 -0500, Andrei Alexandrescu
<[email protected]> said:
On 11/29/10 10:54 AM, Michel Fortin wrote:
The biggest problem I see is with generic programming. Say I pass
Rebindable!(const C) as a template argument, then the template wants to
make it const: should it become const(Rebindable!(const C)), or will
this be resolved to const(C)?
const should follow normal logic, and const(Rebindable!(const C))
should behave as expected.
I think we agree on that.
The problem is that you now have two distinct types to mean a const
reference to a const object. You have "const(Rebindable!(const C))" and
const "const(C)", whichever you get depends on where the type comes
from. Type equality won't work between the two, and you won't be able to
concatenate two arrays one containing rebindables and the other (for
instance). It's like a special case in the type system where constness
is implemented using a template while the rest is a language feature, it
it thus behave differently and code that might work for one
implementation of constness might break with the other, because the
don't play with each other very well. Add immutable to the mix and it
becomes even stranger.
For instance, here's a tricky question: should Unqual!(const C) give you
a const(C), a Rebindable!(const C), or simply C?
C.
Now, what should
const(Unqual!(immutable C)) give you?
const(C).
I know most of the problems are going to be edge cases most people don't
care about. But on the whole, creating a fragile and complex solution to
work around the lack of an inspiring syntax seems rather silly. I would
immensely prefer having an ugly syntax that works as it should. If it's
too ugly, we can still hide that syntax behind Rebindable by defining it
as a simple type alias.
Rebindable, if properly executed, is not fragile and complex. Instead it
is a simple solution for a specific problem: I want a class reference
that I need to bind to different const class objects. If you apply const
to Rebindable, normal rules enter in action. I agree that a built-in
component can be made nicer by adding extra short-circuit rules.
Andrei
