sumtypes for D

[Paul Backus]

On Tuesday, 29 November 2022 at 03:03:37 UTC, Steven 
Schveighoffer wrote:
>
> The problem is that there's no syntax to "tail-modify" a type 
> other than a pointer or an array. This is the biggest problem 
> with properly using const on ranges (it doesn't prevent using 
> const containers, or prevent ranges to const data, it just 
> prevents taking a range to a *mutable container* and iterating 
> it via a const parameter).
>
> I think I've been bringing this problem up for 10 years. It 
> would be a huge win to get it solved.

The thing is, we pretty much know what the solution is. In the 
general case, solving this requires user-defined implicit 
conversions, because in the general case only the user has the 
knowledge necessary to establish the correspondence between a 
templated type's *structure* (e.g., "the head-mutable version of 
`const(Foo!T)`") and its *name* (e.g., `Foo!(const(T))`).

The problem is, that solution has been declared categorically off 
the table, so we are stuck going in circles debating the merits 
and shortcoming of various half-measures.