Enumerated Unions (sum types)

[Dukc]

Paul Backus kirjoitti 12.9.2024 klo 5.42:
> https://gist.github.com/pbackus/28e7f5668219ce83467c83c347ec7202
> 
> This DIP proposes a conservative design for sum types that aims to be 
> consistent with existing D syntax and semantics. It does not discuss 
> pattern matching.

You're leaving the implementation of tag field undefined. I think it's 
better to define it. That way sum types can be used in foreign language 
APIs, and `sizeof(ASumType)` stays predictable.

Also I propose that the tag type is enumerated type that is implicitly 
convertible to `int`. The base type is an unsigned integer type big 
enough to hold all the field types. Default value and the enumerated 
values (other than having one for each type) can remain unspecified though.

The user should not have to write that `get` function of the example 
themselves. Otherwise we can't use the sum type from fully `@safe` code 
if any of it's fields can have unsafe values. Likewise, we should have a 
generic setter function for all the sum types that type safely checks 
the tag, and runs the destructor of the old object if the assignment 
changes the type.

I propose the DIP explicitly allows `void` fields, that work the same 
way as `Void` fields of TaggedAlgebraic. The void field could be 
initialised with either `void.init` or `void`. The return reference from 
`get`ting a void field would point to null. Setting a sumtype to void 
would simply wouldn't take a value argument.

Overall I like this approach though. Way better than the sum type 
proposals of Walter and Rikki.