Iterating over Results

An Iter::map operation might fail, for example:

fn main() {
    let strings = vec!["tofu", "93", "18"];
    let numbers: Vec<_> = strings
        .into_iter()
        .map(|s| s.parse::<i32>())
        .collect();
    println!("Results: {:?}", numbers);
}

Let's step through strategies for handling this.

Ignore the failed items with filter_map()

filter_map calls a function and filters out the results that are None.

fn main() {
    let strings = vec!["tofu", "93", "18"];
    let numbers: Vec<_> = strings
        .into_iter()
        .filter_map(|s| s.parse::<i32>().ok())
        .collect();
    println!("Results: {:?}", numbers);
}

Collect the failed items with map_err() and filter_map()

map_err calls a function with the error, so by adding that to the previous filter_map solution we can save them off to the side while iterating.

fn main() {
    let strings = vec!["42", "tofu", "93", "999", "18"];
    let mut errors = vec![];
    let numbers: Vec<_> = strings
        .into_iter()
        .map(|s| s.parse::<u8>())
        .filter_map(|r| r.map_err(|e| errors.push(e)).ok())
        .collect();
    println!("Numbers: {:?}", numbers);
    println!("Errors: {:?}", errors);
}

Fail the entire operation with collect()

Result implements FromIterator so that a vector of results (Vec<Result<T, E>>) can be turned into a result with a vector (Result<Vec<T>, E>). Once an Result::Err is found, the iteration will terminate.

fn main() {
    let strings = vec!["tofu", "93", "18"];
    let numbers: Result<Vec<_>, _> = strings
        .into_iter()
        .map(|s| s.parse::<i32>())
        .collect();
    println!("Results: {:?}", numbers);
}

This same technique can be used with Option.

Collect all valid values and failures with partition()

fn main() {
    let strings = vec!["tofu", "93", "18"];
    let (numbers, errors): (Vec<_>, Vec<_>) = strings
        .into_iter()
        .map(|s| s.parse::<i32>())
        .partition(Result::is_ok);
    println!("Numbers: {:?}", numbers);
    println!("Errors: {:?}", errors);
}

When you look at the results, you'll note that everything is still wrapped in Result. A little more boilerplate is needed for this.

fn main() {
    let strings = vec!["tofu", "93", "18"];
    let (numbers, errors): (Vec<_>, Vec<_>) = strings
        .into_iter()
        .map(|s| s.parse::<i32>())
        .partition(Result::is_ok);
    let numbers: Vec<_> = numbers.into_iter().map(Result::unwrap).collect();
    let errors: Vec<_> = errors.into_iter().map(Result::unwrap_err).collect();
    println!("Numbers: {:?}", numbers);
    println!("Errors: {:?}", errors);
}