Head Const

[Jonathan M Davis via Digitalmars-d]

On Wednesday, 17 February 2016 at 22:44:27 UTC, Walter Bright 
wrote:

> It would seem that implementing headconst as a type constructor 
> would let people who wanted mutable members have their way, 
> without introducing backdoors in const.

I really don't see how that's the same thing at all. The main 
problems with const pop up when what folks need is something like 
C++'s mutable, because they need to be able to do something like 
a reference count or a mutex which is not really part of the 
logical state of the object but is still part of the object. 
Having some form of non-transitive const allows for things like a 
const pointer to non-const data, but it doesn't help at all when 
what you need to do is treat most of the object as const while 
treating a small portion of it as mutable. Sure, that's not 
completely transitive, because parts of the object are mutable, 
but it's specific parts of the object, not which part of a 
pointer declaration is const and which isn't. Maybe I'm missing 
something, but I don't see how adding non-transitive const to D 
in order to solve the C++ declaration problem would help at all 
with the complaints that folks have with D's const. The 
complaints with D's const are almost entirely about the lack of 
backdoors (be they well-defined like a mutable attribute or a 
free-for-all like casting away const and mutating).

- Jonathan M Davis