draft proposal for Sum Types for D

[Timon Gehr]

On 11/29/22 07:26, Walter Bright wrote:
> Go ahead, Make My Day! Destroy!
> 
> https://github.com/WalterBright/DIPs/blob/sumtypes/DIPs/1NNN-(wgb).md


Nice! I think this general design, where it behaves just like a tagged 
union but @safe, makes sense for D. I _really_ wish bad element access 
resulted in a compile-time error instead of a runtime error though.

Basically, you could treat

if(?s.member){

}

and

assert(?s.member);

specially, and only allow accesses to s.member if they are guarded by 
one of them. By default, accessing members is disallowed. The user can 
then choose between:

if(?s.member){
     writeln(s.member);
}

and

assert(?s.member);
writeln(s.member);

To either check the tag manually or opt into the runtime error very 
explicitly.

I think catching errors early during type checking is one of the most 
compelling things about sum types in other languages, and it would be 
great if D could get that in some way. The analysis does not have to be 
particularly sophisticated. In particular, no control flow analysis is 
required.