std.sumtype?

[Paul Backus]

On Tuesday, 30 March 2021 at 17:01:29 UTC, Steven Schveighoffer 
wrote:
> What might be nice is to have some simplifiers for match to 
> prevent having to jump through the hoops.
>
> For example, a match flavor that throws by default, or one that 
> ignores unhandled types. This way, you don't have to write 
> stuff like `(_) {}` at the end of your handlers.
>
> FWIW, taggedalgebraic has `visit` to enforce all items are 
> handled, and `tryVisit` that throws if at runtime it determines 
> no visitors match the current type.

sumtype has had tryMatch [1] since version 0.4.0 (released May 
2018).

For ignoring types, I have found the following utility function 
handy:

     void ignore(T)(auto ref T) {}

You can instantiate it explicitly to ignore a specific type:

     obj.match!(
         ignore!A,
         (B b) { doSomething1; },
         (C c) { doSomething2; }
     );

Or you can use it as a catch-all handler:

     obj.match!(
         (A a) { doSomethingWith(a); },
         ignore
     );

[1] https://pbackus.github.io/sumtype/sumtype.tryMatch.html