Allow MultiUnzip on Result<tuple, E> with behaviour like collect()?

[shahn]

I haven't found a way to make a pattern like this work:

let inputs = vec![(Ok(1), 2, 3), (Ok(4), 5, 6), (Ok(7), 8, 9)];
    
    let (a, b, c): (Vec<_>, Vec<_>, Vec<_>) = inputs.
        into_iter()
        .map(|(x, y, z)| {
            x.map(|x| (x, y, z))
        })
        .multiunzip()?;

It would be great if this could work like collect() to allow short circuiting on an error

[phimuemue]

Hi there, it seems you'd like to collect into three vectors, throwing away all of them if you encounter an Err in any tuple component.

If so, you probably want try_collect, and your example boiled down to 2-tuples could look as follows:

    let inputs = vec![(Ok::<usize, ()>(1), 2), (Ok(4), 5), (Ok(7), 8)];
    let _: Result<(Vec<_>, Vec<_>), ()> = dbg!(
        inputs.into_iter()
            .map(|(x, y)| x.map(|x| (x, y)))
            .try_collect()
    );

The remaining problem then lies in the fact that FromIterator is only implemented for 2-tuples, but you want 3-tuples. Packing the 3-tuples into nested 2-tuples works around the problem:

    let inputs = vec![(Ok::<usize, ()>(1), 2, 3), (Ok(4), 5, 6), (Ok(7), 8, 9)];
    let (a, (b, c)): (Vec<_>, (Vec<_>, Vec<_>)) = dbg!(
        inputs.into_iter()
            .map(|(x, y, z)| x.map(|x| (x, (y, z))))
            .try_collect()
    ).unwrap();

But I think the adequate way to solve this would be to implement FromIterator (resp. Extend) for 3-tuples. (I'm kinda surprised to see it's only implemented for 2-tuples, because std usually goes up to 12-tuples. Maybe @jswrenn or @scottmcm know the reasons for this limitation.)

[phimuemue]

All this raises the question why multiunzip exists in the first place. I guess it's mainly to mirror multizip similar to how unzip mirrors zip.

[phimuemue]

Closing this as per https://github.com/rust-lang/rust/pull/85835#issuecomment-2434600625 and https://github.com/rust-lang/rust/pull/85835#issuecomment-2434603162.