Proposal: First class types (at compiletime)

[Quirin Schroll]

On Friday, 21 July 2023 at 08:52:47 UTC, Quirin Schroll wrote:
> On Thursday, 20 July 2023 at 13:44:15 UTC, Commander Zot wrote:
>> wouldn't it be possible to have something like first class 
>> types (at compile time) to replace tamplates for type logic 
>> with regular functions executed at CTFE?
>
> Of course, that’s possible. You’d have a type called `Type` 
> that represents types. Of course, `Type` does not actually 
> exist, a function that takes `Type` parameters or returns 
> `Type` cannot end up in the object file. But for CTFE, the 
> compiler can pretend that `Type` is just a type like `Object`.
>
> […]

Note that, in my understanding of “first-class,” this isn’t 
actually first-class types. You still have transformations. 
`Type` is a first-class type, and `Type` objects are first-class 
objects – at least in CTFE –, but types (as per D grammar) are 
not first-class things.

If you had first-class types, you could do something like:
```d
Type[] sort(Type[] types) { … }
auto Integers = sort(int, long, short);
Integers[0] little;
Integers[1] middle;
Integers[2] large;
```
I guess parsing any expression as a type is out of reach and thus 
first-class types are as well.