try-continue

try-continue provides one method, try_continue, which allows you to work with iterators of type Result<T, _>, as if they were simply iterators of type T. this is is implemented for all iterators providing a Result. This is particularly useful if you need to map to a fallible function, and would like to continue using the iterator API to process the elements, but still know if the mapped function fails.

For instance, consider a simple parser where you are provided a list of integers as strings, and you would like to count all the strings that hold even numbers. If you wanted to work with the iterator API exclusively, it may get a bit cumbersome to pass along the Result if an element failed to parse. Worse, doing so may preclude you from using methods such as Iterator::count, as this would actually attempt to count the Results, forcing you to re-implement the counting with Iterator::fold. Using the try_continue method will allow you to work with an iterator of the parsed numbers directly.

use std::str::FromStr;
use try_continue::TryContinue;

fn count_even_number_strings(elements: &[&str]) -> Result<usize, <u8 as FromStr>::Err> {
    elements
       .iter()
       .map(|&s| s.parse::<u8>())
       .try_continue(|iter| iter.filter(|n| n % 2 == 0).count())
}

let num_evens_result = count_even_number_strings(&vec!["1", "2", "3", "24", "28"]);
assert_eq!(3, num_evens_result.unwrap());

let num_evens_bad_result = count_even_number_strings(&vec!["1", "2", "three", "-4", "28"]);
assert!(num_evens_bad_result.is_err());