Iterating over Results
An Iter::map operation might fail, for example:
fn main() { let strings = vec!["tofu", "93", "18"]; let possible_numbers: Vec<_> = strings .into_iter() .map(|s| s.parse::<i32>()) .collect(); println!("Results: {:?}", possible_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() .map(|s| s.parse::<i32>()) .filter_map(Result::ok) .collect(); println!("Results: {:?}", numbers); }
Fail the entire operation with collect()
Result implements FromIter 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); }