What are AST Macros?

[David Bennett]

On Friday, 13 April 2018 at 11:54:12 UTC, David Bennett wrote:
> Also the other idea I had was to have mixin functions that only 
> take compiletime args (they are UFCS-able though, unlike 
> templates) and mixin themselves when called like:
>
> ---
> mixin add1Xtimes(alias int a, alias int t){
>     uint i=t;
>     do{a++;}while(--i);
>     return a;
> }
> @safe unittest{
>     uint n=0;
>     n.add1Xtimes!(4).add1Xtimes!(6);
>     assert(n==10);
> }
> ---
>
> [snip]
>
> I haven't really given much thought to the implementation of 
> this though.

With more thought thats probably not a good solution as its 
different from the rest of the language and would be harder then 
it needs to be in order to integrate with the front-end.

What could work though is having a new AutoMixinDecloration that 
was inited with an expression. This expression would contain a 
call chain to regular functions, templates, other 
AutoMixinDecloration etc. Then when it's called it dups the 
contents of these functions into the callers scope.

---
auto ref incXTimesAndCount(string cident, alias uint csym, uint 
times)(auto ref uint a=0){
     uint t = times;
     do{
         mixin(cident~"++;");
         csym++;
         a++;
     }while(--t);
     return a;
}

auto mixin incXAndXTimesAndCount(string cident, alias uint csym, 
uint t1, uint t2)
                 = incXTimesAndCount!(cident, csym, 
t1).incXTimesAndCount!(cident, csym, t2);

@safe unittest{
     uint n=10;
     uint c1=0;
     uint c2=0;
     uint r = n.incXAndXTimesAndCount!("c1", c2, 4, 6);
     assert(n==20);
     assert(c1==10);
     assert(c2==10);
     assert(r==20);
}
---

And it lowers to this in the front end:

---
@safe unittest{
     uint n=10;
     uint c1=0;
     uint c2=0;
     uint __m0_t=4;
     do{n++; c1++; c2++;}while(--__m0_t);
     uint __m1_t=6;
     do{n++; c1++; c2++;}while(--__m1_t);
     uint r = n;
     assert(n==20);
     assert(c1==10);
     assert(c2==10);
     assert(r==20);
}
---

Or it might be even possible to make the current 
TemplateMixinDeclaration to act like this if you add the auto 
keyword to the declaration with a symbol with the same name in 
the scope and the above was just sugar to this:

---
auto mixin template incXAndXTimesAndCount(string cident, alias 
uint csym, uint t1, uint t2)
{
     alias incXAndXTimesAndCount
             = incXTimesAndCount!(cident, csym, 
t1).incXTimesAndCount!(cident, csym, t2);
}
---