Collect errors, too

[blazing ether]

I recently found myself in a situation where I had to either collect all values if none of them had an error, or collect all errors. Since I didn't want to fast-fail through collecting to Result<Vec<T>, E>, I made a function which collects to Result<Vec<T>, Vec<E>>:

fn collect_or_errors<I, T, E>(iter: I) -> Result<Vec<T>, Vec<E>>
where
    I: Iterator<Item = Result<T, E>>,
{
    iter.fold(Ok(Vec::new()), |mut until_now, new| {
        match (&mut until_now, new) {
            (Ok(okays), Ok(new)) => okays.push(new),
            (Err(errors), Err(new)) => errors.push(new),
            (Ok(_), Err(error)) => until_now = Err(vec![error]),
            _ => (),
        };
        until_now
    })
}

fn main() {
    let iter = vec![Err("meow"), Ok(2), Err("doge"), Ok(7), Ok(30)].into_iter();

    let _ = dbg!(collect_or_errors(iter));
}

Two specific things, feel free to note any else, too:

• Can I make this function more generic, like allowing FromIterator instead of the Vecs? I don't think so, since I need some kind of buffer to actuallly push to, but that may just be me.
• The match looks pretty convoluted, can I make it any simpler than it already is?

[drifting ravine]

Itertools has a .partition_result() which looks like it does exactly what you want. You would just see if the error collection has any items in it

[blazing ether]

...can't seem to find it, could you link it? https://docs.rs/itertools/0.10.3/itertools/fn.process_results.html doesn't really seem like what I want

[drifting ravine]

https://docs.rs/itertools/latest/itertools/trait.Itertools.html#method.partition_result

[blazing ether]

Okay, thanks! That may suffice, though I'll ignore one of those values a lot

(also my version allows me to .map_err into an appropiate error and then ? nicely)

[drifting ravine]

That's fine. Don't prematurely optimize it

[drifting ravine]

you can do that before the .partition_result()

[blazing ether]

Not necessarily, if I need all errors collected before throwing them into a compile error, for example

[drifting ravine]

I am confused

fn collect_or_errors<A, B, T, E, I>(iter: I) -> Result<A, B>
where
  I: Iterator<Item = Result<T, E>>,
  A: Default + Extend<T>,
  B: Default + Extend<T>,
 {
  let (ok, err) = iter.partition_result();
  if !err.is_empty() {
    Err(err)
  } else {
    Ok(ok)
  }
}

[blazing ether]

...alright

Thanks!