[TWiD] static foreach loop variable

[Adam D. Ruppe]

On Tuesday, 28 May 2019 at 13:43:45 UTC, Nick Treleaven wrote:
> Hi,
> Last week's TWiD had a tip that didn't make sense:
> http://dpldocs.info/this-week-in-d/Blog.Posted_2019_05_20.html#tip-of-the-week
>
> template Locals(int i) {
>     alias Whatever = int;
> }
>
> static foreach(i; [1, 2, 3]) {
>    Locals!i.Whatever;
> }

> The body is just `int;`. Not sure how to reach Adam. What was 
> intended?

Yes, intended, but also incomplete. The idea there is to use the 
template as a namespace for whatever local stuff you had. (I 
adapted this from a reflection thing where Whatever would have 
been an alias to a member type, and then in simplifying to get to 
just the new idea - and in a rush to type it up that day - I 
removed like all the context).

So it might actually be more like (still an incomplete example 
but more complete):

void test(T)() {
   template Locals(int i) {
      static if(is(typeof(__traits(getMember, T, 
__traits(allMembers, T)[i])) Ret == return))
        alias ReturnValue = Ret;
   }

   static foreach(idx, t; __traits(allMembers, T)) {
      mixin("auto " ~ t ~ "() { return 
Locals!idx.ReturnValue.init; }");
   }
}


So now the Locals template wraps a bunch of aliases so we can 
refer to them more succinctly later.

A regular alias defined inside that static foreach would end up 
being duplicated in future iterations. Putting the extra {} 
around it would mean the mixed in function would not be visible 
outside.

so the Locals template is just a collection of convenience 
aliases in a namespace.