TypeFunction example: ImplictConvTargets

[Paul Backus]

On Tuesday, 6 October 2020 at 18:30:15 UTC, Steven Schveighoffer 
wrote:
> It comes down to one thing -- arrays vs. tuples. In type 
> functions, a tuple is an array, and you can do all the things 
> you can do with a normal array: mutate, sort, shrink, grow, 
> loop, use as a range, etc.
>
> With a Tuple, everything is immutable, and each change needs to 
> go across a new template boundary. Even a loop is not really a 
> loop.

It seems to me like maybe the most obvious way from point A to 
point B is to lift these limitations on tuples (and aliases, and 
manifest constants). Then we could write, for example:

template Filter(alias pred, Args...)
{
     enum pos = 0;
     foreach (Arg; Args) {
         static if (Pred!Arg) {
             Args[pos] := Arg;
             pos := pos + 1;
         }
     }
     alias Filter = Args[0 .. pos];
}

You would still pay for the performance overhead of tuple foreach 
and static if, so maybe it's not that big a win, but it's still 
an improvement over recursive templates and CTFE+mixins.