Logical const

[Andrei Alexandrescu]

On 11/29/10 8:56 AM, Steven Schveighoffer wrote:
> On Sat, 20 Nov 2010 09:21:04 -0500, Peter Alexander
> <peter.alexander.au at gmail.com> wrote:
>
>> D does not support logical const due to the weak guarantees that it
>> provides.
>>
>> So, without logical const, how are D users supposed to provide lazy
>> evaluation and memoization in their interfaces, given that the
>> interface should *seem* const, e.g.
>>
>> class Matrix
>> {
>> double getDeterminant() const { /* expensive calculation */ }
>> }
>>
>> If it turns out that getDeterminant is called often with the raw
>> matrix data remaining unchanged, how can we add caching to this class
>> without rewriting the const-ness of all code that touches it?
>
> This has been discussed at length on this newsgroup, and I argued for it
> for a long time. You will not get any traction with Walter, because I've
> already proven that logical const == const, and it still doesn't change
> his mind.
>
> The thing is we *already* have a hidden field that is logically const --
> an object's monitor. Regardless of an object's constancy, you can always
> mutate the monitor. The compiler does it by logically inserting a cast
> away from const, so that's what I'd suggest.
>
> In reality, once you get into the realm of logical const, the compiler
> no longer helps you. Any guarantees are now provided by you, not the
> compiler.

In fact it's possible to provide logical const with guarantees. You need 
a construct that keeps together a bool, a pure function/delegate/method, 
and a value, and works like this:

class X
{
     int x;
     lconst int y = { return x + 42; };
     void m1() { x = 2; invalidate(y); }
     void m2() const { writeln(y); }
}

You can use the intrinsic invalidate() against a non-const lconst field, 
but not against const lconst. Reading that field checks whether the 
field has been invalidated. If so, it writes the field with the result 
of the function/delegate/method and turns the validation flag on.

With this system in place, I think the lconst value is guaranteed to be 
as strong as const.

I recall I managed to convince Walter that the system works, but we 
agreed it would cost too much for what it does. I think it can be 
implemented as a library artifact under certain assumptions.


Andrei