Cancelling on Result Err

[PSeitz]

Hi I have a function, which returns an Result<(), Error>. I want to early return on errors.

I saw in result.rs something, that uses while_some may be the right method here, although I don't know how to use that. I couldn't find something in the documentation.

Also a while_ok method would probably be nice.

[cuviper]

I saw in result.rs something

You're on the right track. FromParallelIterator is used by ParallelIterator::collect(), just like the standard FromIterator and Iterator::collect(). You'll often need to provide a type hint if it's not clear from context. So you could do something like:

let r: Result<Vec<_>, _> = collection.par_iter().map(fn_to_result).collect()

This will collect the Ok items into a Vec, or short-circuit on Err.

This reminds me, I should add FromParallelIterator<()> for () like rust-lang/rust#45379.

Also a while_ok method would probably be nice.

Well, while_some is basically just a pass-through of the unwrapped Some values, and doesn't need to keep the None that ends it. It's not clear what to do with an Err if we had while_ok. If you want to throw that away and just return early, you can use .map(Result::ok).while_some(). If you want to keep the error, then I think the collect model is better.

[PSeitz]

Thanks, I wasn't aware of the type hint, but seems pretty obvious in hindsight.

[WilliamVenner]

while_ok would allow you to have a filter_map that stops depending on a predicate.

while_some does not work with filter_map for obvious reasons 😁

[ghost]

Hello, I'm doing

fn main() -> anyhow::Result<()> {
    0..123123
        .into_par_iter()
        .map(|num| {
            /* something with lots of foo()? */
            println!("Hi from {num}");
            anyhow::Ok(())
        })
        .collect::<Result<Vec<_>, _>>()?
    ;
    Ok(())
}

And then running with cargo run | head -n 10. So when Bash closes the pipe to head after 10 successful printlns, errors start getting throw for every println with the broken pipe error, as expected. However, this doesn't seem to stop Rayon, as it keeps dumping panic errors on stderr non-stop.

What am I doing wrong please?

Rayon 1.6.1.

[cuviper]

@YuriGeinishO the println! macro doesn't return any errors, even though it reports them. Try something like this:

use std::io::{stdout, Write};
// ...
            writeln!(stdout(), "Hi from {num}")?;