Range Redesign: Empty Ranges

[Paul Backus]

On Wednesday, 6 March 2024 at 17:32:17 UTC, H. S. Teoh wrote:
> Every time this topic comes up, class-based ranges become the 
> whipping boy of range design woes.  Actually, they serve an 
> extremely important role: type erasure, which is critical when 
> you have code like this:
>
> 	auto myRangeFunc(R,S)(R range1, S range2) {
> 		if (runtimeDecision()) {
> 			return range1;
> 		} else {
> 			return range2;
> 		}
> 	}
>
> [...]
>
> Note that you can't use a struct wrapper here, because R and S 
> have different ABIs; the only way to correctly forward range 
> methods to R or S is to use overridden base class methods. IOW, 
> the type erasure of R and S is unavoidable for this code to 
> work.

This is what std.range.choose [1] is for. Internally, it's 
implemented as a tagged union of R and S.

[1] https://dlang.org/library/std/range/choose.html