const debacle

[BCS]

Reply to Walter,

> Steven Schveighoffer wrote:
> 
>> I want to specify that the function does not change the input buffer,
>> and returns the same type as passed in (without duping the input).
>> 
> Look at it another way. You want to declare a filter that does not
> change the input, but can pass it along to another function that will.
> I
> suspect this might be a fundamentally broken concept:
> T[] f(const T[]);
> void g(T[]);
> ...
> 
> T[] t;
> g(f(t));
>
> What's happened here? Although f() promised it wouldn't modify t, it
> got modified anyway, because f() passed t to g() which modified it.
> Far from being a weakness in D's const regime, I think the fact that
> such cannot be implemented without casting away const at some point
> shows that the const system is sound, and trying to implement such a
> filter is unsound.
> 

I think what is being asked for is a function that asserts that it won't 
change what it is given but, you can do anything with the output that you 
could do to the input. Inside it would not be allowed to change it's arg 
but can return it. This should be safe because anything you do the the return 
value could be done to the argument giving the same end effect.