draft proposal for Sum Types for D

[Basile.B]

On Wednesday, 30 November 2022 at 14:22:08 UTC, Basile.B wrote:
> On Wednesday, 30 November 2022 at 08:49:00 UTC, Andrey Zherikov 
> wrote:
>> [...]
>
> That would not generally work, or that would not be efficient. 
> In the background "optional access" allocates a default value 
> at the use site, meanings an alloca, and then you select the 
> default or the right one if evaluation has succeed. Optional 
> access is more tied to the language. Optional access does not 
> generate good unoptimized code, let's not making its D version 
> worst that what already exists.
>
> You see a thing like
>
>     a?.b = c
>
> is like
>
>     typeof(a.b) fallback;
>     (a.b ? a.b : fallback) = c;
>
> where the ternary yields a lvalue.
>
> This causes problems with members that will be eventually used 
> in optional accesses. The default value must be related to that 
> particular expression.
>
> Using a sum type for optional access is still a hack.

TLDR; I mean the member of an aggregate is not necessarily a 
sumtype, but you want optional access on it. Because of that I 
think that optional access should be a well defined expression in 
a language. Let's not merge two things because they partially 
intersect.