How to use try_fold in place of fold_while?

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.)

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

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

Actually, never mind, that's silly.

@NoraCodes

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

Thank you 🙏

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.

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.

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

undeprecate and optimize `fold_while`

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

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

Merge #475 #476 #479

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

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.

Rollup merge of rust-lang#85608 - scottmcm:stabilize-control-flow-enu…

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How to use `try_fold` in place of `fold_while`? #469

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How to use try_fold in place of fold_while? #469

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scottmcm commented on Aug 18, 2020

NoraCodes commented on Aug 20, 2020

NoraCodes commented on Aug 20, 2020

jswrenn commented on Aug 20, 2020

NoraCodes commented on Aug 20, 2020

jswrenn commented on Aug 21, 2020

NoraCodes commented on Aug 21, 2020

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NoraCodes commented on Sep 5, 2020

scottmcm commented on Feb 7, 2021

scottmcm commented on Jun 14, 2021

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How to use `try_fold` in place of `fold_while`? #469