"a*, error" has 4 different members, if you consider just "exists" vs "doesn't exist" for each. Of those, "nil, nil" and "&a{}, errors.New("whatever")" are broken. Fully half of the things you can represent are just wrong.
(Yes there are times when those are valid returns, but easily 99% of the time they're not. The type system should be able to represent both ways.)
Language design choices have to make sense. And this doesn't make practical sense to me since I've written code, just in the past 24 hours, that contradicts your assertion. And no, I don't think the code would be better if we robbed Go of expressive power - adding complexity elsewhere is just moving the problem if poor discipline around.
In cases where it's valid, you use a type that makes it valid. Much of the point of having a type system is that the return type tells you exactly what the valid things for the function to return are.
Having an explicit declaration of which functions might return both a result and an error and which functions will definitely return one or the other but not both makes your code easier to understand and work with. As it stands, I would bet that most Go programmers ignore the possibility of returning both a result and an error even in functions that do it deliberately, because the idiomatic thing a function with that signature does is return one or the other but not both.
If anyone has an example of this being a pattern used somewhere that they think is good I'd love for them to share it.
Probably bad example off the top of my head: You're returning say a "User" which has a user_id, name, and a list of recent purchases.
If the function errors out on the recent purchases, the caller may prefer that it still gets back a "User" object with the other information filled in, and then an "error" also set so the caller knows the information isn't complete.
The contract is always right … how well it is expressed through language mechanics is maybe another way to contrast things here. (FWIW I think Go could tighten up a few things - dropping or shadowing errors is a bit worrying - but I’m fairly convinced sum vs product is a little too harsh of a reduction; a useful distinction but not a complete one.)
That's why I wrote: Yes there are times when those are valid returns, but easily 99% of the time they're not. The type system should be able to represent both ways.
The golang type system can _already_ encode the case you're talking about, and that should stay. My complaint is that is the _only_ case it can represent, it cannot represent the more common case.
Go's type system can already do that, the (x, error) idiom is just standard practice in the absence of Generics and Throwables. Adding generics simply flips the default and exception.
Currently, the exception is to define a custom type which captures and constrains your possible return states, and the default is to return a value plus an error. When that flips, and the default is to collapse error states into a single return value, that will cease to give me the same signals (or at least make them less obvious) as to whether or not my abstraction sucks, which I will sorely miss :P
Like, you can create an interface that's read only for the return result like:
interface ReadableResult { HasResult() bool Result() whatever Error() error }
Then to create the result you have a writable interface for setting values, and enforce the 'only one of the two' semantics from the setters:
interface WritableResult { SetResult(whatever) SetError(error) }
Struct MyResult{ err error result whatever }
.... Implementation here.
It's extra code, but it should be a rare necessity, at least in my experience. Res, err := getResult() always worked fine for me.