draft proposal for Sum Types for D

[Basile.B]

On Tuesday, 29 November 2022 at 20:50:15 UTC, H. S. Teoh wrote:
> On Tue, Nov 29, 2022 at 08:37:57PM +0000, Jacob Carlborg via 
> Digitalmars-d wrote: [...]
>> ### ABI
>> 
>> I think a section on ABI is missing. It should specify the 
>> memory layout of sumtypes, just as for existing language 
>> constructs [1].
>
> Agreed.
>
>
>> What's even more important than the memory layout is the value 
>> of the tag. Something needs to be specified in regards to the 
>> value of the tag. One needs to be able to draw some 
>> conclusions about what happens with the value if members are 
>> added, removed or reordered. Ideally the tag value of existing 
>> members should not change. These are important things when 
>> using sumtypes at ABI boundaries.
>
> You can't dynamically change a sumtype (add/remove/reorder its 
> constituents) after the fact, because that may change their 
> size, which implies an ABI change.

You can add members, Crystal does that all the time.