draft proposal for Sum Types for D

[Andrey Zherikov]

On Tuesday, 29 November 2022 at 06:26:20 UTC, Walter Bright wrote:

This is very nice!

Few comments:

This looks like a typo making recursive grammar:
```d
SumTypeMembers:
     SumTypeMembers     // should it be SumTypeMember?
     SumTypeMember `,`
     SumTypeMember `,` SumTypeMembers
```

> `?x.allow ? x.allow : Xyzzy.busy`

Can this be shortened to `?x.allow : Xyzzy.busy`?

> But since a subtype with only enum members can be implemented 
> as an enum, the compiler should do that rewrite. Similarly, a 
> SumType with only one field declaration should be rewritten as 
> a struct (and the tag can be omitted).

How will `?x.allow` be rewritten in these cases?

Other questions:
How will this work with Enum Type Inference?
Can this be used as an implementation of optional built-in type 
in the future? (IMO this should be another DIP but would like to 
get your thoughts)