How to use try_fold in place of fold_while?

[NoraCodes]

fold_while has been deprecated in favor of try_fold, and will eventually be removed ( #223 ). However, there is little documentation on how to actually do this in an idiomatic way.

For example, this playground comparison is the best I could do and it looks awful. The iterator finishing early isn't an error - it's a totally acceptable condition.

I'm sure there's a better way to do this, but it would be nice to document that in fold_while or, perhaps, in try_fold (I'll be happy to open a MR for that if someone can give me a good example.)

[scottmcm]

It's true that a different impl Try makes this much nicer -- I added one to libcore that's used internally to make things like this more readable:

https://github.com/rust-lang/rust/blob/515c9fa505e18a65d7f61bc3e9eb833b79a68618/library/core/src/iter/mod.rs#L373-L378

[NoraCodes]

Oh yeah, that's much nicer! Would we be amenable to including such a construct in itertools since it's such a common case?

[NoraCodes]

Actually, never mind, that's silly.

[jswrenn]

@NoraCodes I'll hold off on removing fold_while for the foreseeable future, at least until a more ergonomic alternative is available.

[NoraCodes]

Thank you 🙏

[jswrenn]

This feels like an abuse of import renaming, but...

fn main() {
    use Result::{
        self as LoopState,
        Ok as Continue,
        Err as Break,
    };

    println!("{}", (1..10).try_fold(0, |mut acc, v| {
        acc += v;
        if (acc >= 6) {
            Break(acc)
        } else {
            Continue(acc)
        }
    }).unwrap_or_else(|v| v));
}

I think this makes for a good demonstration of how to translate from fold_while to try_fold.

[NoraCodes]

I actually just committed almost that exact change to our codebase. It works great, except that .unwrap_or_else(|x| x) is a little janky. But it's totally workable until something else comes along.

[jswrenn]

except that .unwrap_or_else(|x| x) is a little janky.

Agreed. There's this old PR introducing Result::into_inner for situations like this, but it was unfortunately rejected: rust-lang/rust#54219

[jswrenn]

fold_while will be un-deprecated (and slightly optimized!) our next release (0.10.0) with #476.

[NoraCodes]

Awesome! The stdlib feature has been merged, too, so you can use that at least on nightly.

[scottmcm]

...and I now have rust-lang/rfcs#3058 open to start moving ControlFlow towards stabilization.

[scottmcm]

Stabilization is now approved, but it'll take until September 9th to reach stable, so there's still a few months before any more work can happen here.