How to find the index of an element in a Vec of Results, stopping if an Err is found?

Question

Suppose I have a collection of Result items, like:

let items: &[Result<&str, u32>] = &[Ok("foo"), Err(444), Ok("bar")];

I need to find the index of the first element with the value "bar". My first attempt looks like:

let bar_idx = items.iter()
    .position(|item| item? == "bar")?;

This won't work because of the ? operator inside the closure.

I'm aware of the try_for_each method, which fails the entire collection if an Err is returned, but there is no such equivalent for position, which is what I'm trying to do.

How can I implement this search?

EDIT:

I opened an issue at Rust repo, because I believe try_position should exist.

Answer 1

You can build one on top of try_for_each():

use std::ops::ControlFlow;
let item: ControlFlow<Result<usize, u32>> =
    items
        .iter()
        .enumerate()
        .try_for_each(|(idx, item)| match item {
            Ok(v) if *v == "bar" => ControlFlow::Break(Ok(idx)),
            Ok(_) => ControlFlow::Continue(()),
            Err(err) => ControlFlow::Break(Err(*err)),
        });
// Can replace by `ControlFlow::break_value()` once stabilized.
let item = match item {
    ControlFlow::Break(v) => Some(v),
    ControlFlow::Continue(()) => None,
};

But frankly, it's likely better to use a simple for loop:

let mut result = None;
for (idx, item) in items.iter().enumerate() {
    match item {
        Ok(v) if *v == "bar" => {
            result = Some(Ok(idx));
            break;
        }
        Ok(_) => {}
        Err(err) => {
            result = Some(Err(*err));
            break;
        }
    }
}