How do I perform iterator computations over iterators of Results without collecting to a temporary vector? - iterator

Question

I'm looking for a way to eliminate the temporary vector allocation in this example:
fn doit<T: Iterator<Item = Result<i32, &'static str>>>(name: &str, iter: T) {
println!(
"{}: {:?}",
name,
iter.collect::<Result<Vec<_>, _>>()
.map(|v| v.into_iter().min())
);
}
fn main() {
let without_errors = vec![Ok(1), Ok(2), Ok(3)];
let with_errors = vec![Ok(1), Err("error"), Ok(2)];
doit("without errors", without_errors.into_iter());
doit("with errors", with_errors.into_iter());
}
This is a variation of the iterator with error handling theme, except that I don't want to create a collection (so collect() doesn't quite do the job), but I want to perform further operations on the elements being iterated over.
Note that this gives the wrong result because Ok is less than Err:
fn doit<T: Iterator<Item = Result<i32, &'static str>>>(name: &str, iter: T) {
println!("{}: {:?}", name, iter.min());
}
It would give the right result for max() by accident, but it would not stop iterating on the first error.

Answer 1

Iterator::try_fold provides the framework for what you need, and it's available since Rust 1.27 (Playground):
fn fold_ok<I, T, E, F>(mut iter: I, f: F) -> Result<Option<T>, E>
where
I: Iterator<Item = Result<T, E>>,
T: Ord,
F: Fn(T, T) -> T,
{
iter.try_fold(None, |r, i| {
let i = i?;
Ok(Some(if let Some(r) = r { f(r, i) } else { i }))
})
}
fn main() {
let without_errors = vec![Ok(1), Ok(2), Ok(3)];
let with_errors = vec![Ok(1), Err("error"), Ok(2)];
fn doit<'r, T>(name: &str, iter: T)
where
T: Iterator<Item = &'r Result<i32, &'static str>> + Clone,
{
println!("{}: {:?}", name, fold_ok(iter.cloned(), ::std::cmp::min));
}
doit("without errors", without_errors.iter());
doit("with errors", with_errors.iter());
}
Before that, I think your only option is manually iterating (Playground)
fn fold_ok<I, T, E, F>(mut iter: I, f: F) -> Result<Option<T>, E>
where
I: Iterator<Item = Result<T, E>>,
T: Ord,
F: Fn(T, T) -> T,
{
let mut result = match iter.next() {
None => return Ok(None),
Some(r) => r?,
};
for item in iter {
result = f(result, item?);
}
Ok(Some(result))
}
fn main() {
let without_errors = vec![Ok(1), Ok(2), Ok(3)];
let with_errors = vec![Ok(1), Err("error"), Ok(2)];
fn doit<'r, T>(name: &str, iter: T)
where
T: Iterator<Item = &'r Result<i32, &'static str>> + Clone,
{
println!(
"{}: {:?}",
name,
fold_ok(iter.clone().cloned(), ::std::cmp::min)
);
}
doit("without errors", without_errors.iter());
doit("with errors", with_errors.iter());
}

Answer 2

"Lifting" a function to handle an iterator of results is a fairly common pattern and, as usual, itertools has a solution — process_results:
use itertools; // 0.8.0
fn doit(name: &str, iter: impl Iterator<Item = Result<i32, &'static str>>) {
let min = itertools::process_results(iter, |i| i.min());
println!("{}: {:?}", name, min);
}
This code began life as ResultShunt in the standard library before being extracted to itertools. It's what underlies the implementation of sum and product for iterators of Result.

Answer 3

It's possible to abuse collect() for this:
pub struct Min<T> {
value: Option<T>,
}
impl<T> Min<T> {
pub fn value(self) -> Option<T> {
self.value
}
}
impl<T> std::iter::FromIterator<T> for Min<T>
where
T: Ord,
{
fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
let mut iter = iter.into_iter();
match iter.next() {
None => Min { value: None },
Some(mut value) => {
for i in iter {
value = std::cmp::min(value, i);
}
Min { value: Some(value) }
}
}
}
}
This can be used via iter.collect::<Min<_>>().value(). This is a lot of machinery, and I don't see a way to abstract over it (so that you only need to supply std::cmp::min or some other semigroup operation).
I didn't look in the direction of Iterator::try_fold, which provides most of the machinery.