I have a function that returns a Result
:
fn find(id: &Id) -> Result<Item, ItemError> {
// ...
}
Then another using it like this:
let parent_items: Vec<Item> = parent_ids.iter()
.map(|id| find(id).unwrap())
.collect();
How do I handle the case of failure inside any of the map
iterations?
I know I could use flat_map
and in this case the error results would be ignored:
let parent_items: Vec<Item> = parent_ids.iter()
.flat_map(|id| find(id).into_iter())
.collect();
Result
's iterator has either 0 or 1 items depending on the success state, and flat_map
will filter it out if it's 0.
However, I don't want to ignore errors, I want to instead make the whole code block just stop and return a new error (based on the error that came up within the map, or just forward the existing error).
How do I best handle this in Rust?
Result
implements FromIterator
, so you can move the Result
outside and iterators will take care of the rest (including stopping iteration if an error is found).
#[derive(Debug)]
struct Item;
type Id = String;
fn find(id: &Id) -> Result<Item, String> {
Err(format!("Not found: {:?}", id))
}
fn main() {
let s = |s: &str| s.to_string();
let ids = vec![s("1"), s("2"), s("3")];
let items: Result<Vec<_>, _> = ids.iter().map(find).collect();
println!("Result: {:?}", items);
}