mutable compile-time data proposal

[Andrei Alexandrescu]

On 12/28/11 12:22 PM, Martin Nowak wrote:
> On Wed, 28 Dec 2011 15:00:10 +0100, Gor Gyolchanyan
> <gor.f.gyolchanyan at gmail.com> wrote:
>
>> This is something I was thinking about for a long time now.
>>
>> There seems to be absolutely no difference between T, const(T) and
>> immutable(T) if T is a compile-time value (a enum or a local in CTFE).
>> The possibility to mutate compile-time values (at compile time, of
>> course) would allow very convenient techniques to be used for a number
>> of purposes.
>> And since in my hypothetical D compile-time data can be mutated, then
>> a compile-time static this is required (especially for the next
>> example).
>> For instance, here's something I'd love to be able to do:
>>
>> class Base
>> {
>> mixin template Register()
>> {
>> ctStaticThis() // a placeholder for the real compile-time static this
>> {
>> Derived = TypeTuple!(Derived, typeof(this));
>> }
>> }
>>
>> enum Derived = TypeTuple!();
>> }
>>
>> class Derived1: Base
>> {
>> mixin Register;
>> }
>>
>> class Derived2: Base
>> {
>> mixin Register;
>> }
>>
>> static assert(is(Base.Derived == TypeTuple!(Base, Derived1, Derived2)));
>>
>> Similar things would allow to quickly build extremely powerful and
>> useful compile-time information, which is currently not possible to
>> build.
>> If mutable compile-time data is implemented, the entire compile-time
>> computation in D would become just like the run-time one.
>>
>
> I'd really like to see such a construct.
>
> enum __iota;
>
> enum Masks
> {
> A = (1 << __iota++),
> B = (1 << __iota++),
> C = (1 << __iota++),
>
> DMask = (0b11 << __iota),
> D0 = (1 << __iota++),
> D1 = (1 << __iota++),
> }
>
> static assert(__iota <= 32);
>
> Now this would depend on the order of semantic so it's not feasible.

enum Masks
{
     __iota = __LINE__, A = (1 << 0),
     B = (1 << __LINE__ - __iota),
     C = (1 << __LINE__ - __iota),
     DMask = (0b11 << __LINE__ - __iota), D0 = (1 << __LINE__ - __iota),
     D1 = (1 << __LINE__ - __iota),
}

Yeah, I'm jesting. Or am I???


Andrei