Is Result not just a specialised Either? In what way is Either inferior?

> Is Result not just a specialised Either? In what way is Either inferior?

It's better named when it comes to being a value/error union: Result/Ok/Err is somewhat more obvious than Either/Right/Left, especially for people who don't make the connection between "right" and "correct".

Aside from that, Result was annotated with #[must_use], so the compiler will warn if you ignore a Result return value:

    fn main() {
        f1();
    }

    fn f1() -> Result<(), ()> {
        Ok(())
    }

=>

    > rustc test.rs
    test.rs:2:5: 2:10 warning: unused result which must be used, #[warn(unused_must_use)] on by default
    test.rs:2     f1();
                  ^~~~~

doing that with Either would be weird.

Those are not huge, but they're small tweaks specially making Result a better fit for an exception replacement.

I agree 100% that it is a specialized either.

I would argue that either is inferior because programmers empirically won't be bothered remembering that Left = Err and Right = Ok. Also fixing your Left to conform to a common error type which supports chaining is directly is also pretty important. This can be accomplished in a few different ways, but it's going to be done so often that Either ends up never being used for anything other than Results base class / Result is implemented as a type-class on either, with crappy names for error and ok.

Overally though I dislike Either, because to me, it is a symptom of a language lacking a way to declare anonymous type disjunctions inline[1], just like things like std::pair indicates a language lacks tuples.

1: Something like

def parseToken : ParseError | Int | String

looking at that maybe with inline type disjunctions the need for Result itself melts away.