Sum types can handle multiple return values seamlessly in a typesafe way as a special case, but are not limited to that because they may have different data shapes other than simple products, and callers can be checked to deal with each possible shape at each call site by the compiler.
On a side note, the same people who claim that sum types are "better" are never able to come up with a constructive proposal how sum types could be integrated into Go in an elegant way.
They are "better" in that they are, in fact, more constrained; only when the error case arises will there be any accessible error value; otherwise, the actual expected value will be found. Since go uses an ad hoc product type, you always get an error value and the return value, even if they are mutually exclusive most of the time.
Also, they are both ways to build larger types from smaller ones, and the way they go about doing it is rather obvious from their names, and thus the contrast.
> On a side note, the same people who claim that sum types are "better" are never able to come up with a constructive proposal how sum types could be integrated into Go in an elegant way.
Forgo the cutesy anonymous members for the massive benefits of sum types? For a team which prides itself for its ability to perform trade-offs, they sure were rigid in this stance.
But multiple return values are there for much more than returned result and error. You conflate that with the specific use of returning result and error, and based on that, you claim that sum types are better. That's a straw man par excellence.
I am not a language designer, but I have become interested in languages in the past couple of years. Sum types require some sort of generics implementation, which Go does not have. I think the design choices the authors made regarding the language have made adding generics that much harder, that now they are struggling to find the "Go way" of fitting them into the language.