D const design rationale

[Bruno Medeiros]

Sean Kelly wrote:
> 
>>> I can appreciate that 'invariant' may be of tremendous use to the 
>>> compiler, but I balk at the notion of adding language features that 
>>> seem largely intended as compiler "hints."
>>
>> It's a lot more than that. First, there's the self-documenting aspect 
>> of it. Second, it opens the way for functional programming, which can 
>> be of huge importance.
> 
> Could you explain?  I've been trying to determine how I would write a 
> library using these new features, and so far, 'invariant' simply seems 
> more an obstacle to maintainability (because of the necessary code 
> duplication), than it is an aid to programming.
> 

In which situation does having 'invariant' lead to code duplication? I 
can see the common situation where you have class getter methods, or 
other similiar methods, but even without 'invariant' and only with const 
and non-const you have that problem.

> 
>>> The compiler can inspect the code however, and a global const is as 
>>> good as an invariant for optimization (as far as I know).  As for the 
>>> rest, I think the majority of remaining cases aren't ones where 
>>> 'invariant' would apply anyway: dynamic buffers whose contents are 
>>> guaranteed not to change either in word or by design, etc.
>>
>> But they do apply - that's the whole array optimization thing. You're 
>> not just going to have global arrays.
> 
> Perhaps I misunderstood the "must be known at compile-time" clause.  Can 
> 'invariant' apply to dynamic arrays that will remain unchanged once 
> initialized?  In my work I use almost no static data--it's all generated 
> on the fly or loaded from some data source.  Will 'invariant' help to 
> make my code more optimal?
> 
> 
> Sean

Of course, that's the whole point of invariant. The 'invariant' keyword 
is being used for two things, compile-time values, and invariant data. 
You obviously can't do what you mention with compile-time values, but 
you can with invariant data (which is created at runtime). (or have I 
misunderstood you, and you already knew that?)

-- 
Bruno Medeiros - MSc in CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D