Status check: Typed throws

toph42 · September 22, 2023, 9:53pm

(I’m going to reply over here since the discussion from the spun-off topic came back around to typed throws)

Structural Sum Types (used to be Anonymous Union Types)

They're still retaining the exact error that occurred without necessarily having to care about what the error was, but they're at least no longer mindlessly rethrowing an unknown error, because they can't be bothered to actually think about proper error propagation. But even then, I'd prefer this instead:
enum MyError<FirstPartFailure, SecondPartFailure> where
    FirstPartFailure: Error,
    SecondPartFailure: Error
{
    case firstPartFailure(FirstPartFailure)
    case secondPartFailure(SecondPartFailure)
    // ...
}
Because again, they have no business arbitrarily losing type information just because they're too lazy to actually think things through.

I think “too lazy to actually think things through” is an unfair characterization. The way I see Swift exceptions (and I am, again, no expert in CS) is that when you are performing operations that could fail in various ways those operations should throw an error, some of which may be recoverable and others won’t be.

What I, as the programmer, should do is reason about what makes sense for me to handle and look for that, and propagate the rest “up the chain” (if there is an “up”) for anything else that can handle it (or bail out completely, if that’s more appropriate). In that way, only the errors that I can do something about matter. The current state of Swift’s Error works well in this scenario as I can catch whatever I know I can handle and either throw or crash for the rest.

I’ve been participating in discussions here for several years now and the topic of typed throws has cropped up several times, and it genuinely excites a handful of community members each time, but I have never seen anyone actually enumerate the benefits of having a throw with a concrete type before this thread. Even then, the only specific benefit I’ve seen clearly communicated here is that it’s necessary for Embedded Swift.