draft proposal for Sum Types for D

[deadalnix]

On Tuesday, 29 November 2022 at 06:26:20 UTC, Walter Bright wrote:
> Go ahead, Make My Day! Destroy!
>
> https://github.com/WalterBright/DIPs/blob/sumtypes/DIPs/1NNN-(wgb).md

So I spent the last working on something that effectively boils 
down to sum types with a couple of twists. While having this DIP 
on hand would like have made part of the work easier, it doesn't 
address the main challenges I ran into (one of them is solved, 
the other one is still up in the air).

The first problem that needed to be solved is automated 
conversion between sum types members. Think if you are trying to 
run this operation that exist on A but not on B, and the sum 
value currently contains a B, then convert that B to an A and 
then run the operation.

The second one is that it ends up replacing builtin types in the 
codebase by custom types, most notably dynamic arrays (which I'll 
call slices). Doing so de facto breaks most of the codebase in 
ways that cannot be easily fixed, because there is no way to have 
a builtin type that behave like a slice.

That second problem is the most important one. As of now, it has 
occupied roughly half the time I spent on the project as a whole, 
and it is still not even close to be solved. I was able to 
develop a custom sum type, which includes custom hash table and 
clever transform to convert the types into one another, and it's 
bonkers that this isn't dominating the dev time I have to spend 
on this.