fold_results.rs

use std::error::Error;

pub trait ResultIterator {
    type ValueType;
    type ErrorType;
    fn fold_results<Acc, Fun>(self, init: Acc, fun: Fun) -> Result<Acc, Self::ErrorType>
    where
        Fun: Fn(Acc, &Self::ValueType) -> Acc;
}

impl<It, V, E> ResultIterator for It
where
    It: Iterator<Item = Result<V, E>>,
{
    type ValueType = V;
    type ErrorType = E;

    fn fold_results<Acc, Fun>(self, init: Acc, fun: Fun) -> Result<Acc, Self::ErrorType>
    where
        Fun: Fn(Acc, &Self::ValueType) -> Acc,
    {
        let mut acc = init;
        for item in self {
            match item {
                Ok(x) => acc = fun(acc, &x),
                Err(e) => return Err(e),
            }
        }
        Ok(acc)
    }
}

#[test]
fn test_add() -> Result<(), Box<Error>> {
    assert_eq!(
        vec!["1", "2", "3"]
            .iter()
            .map(|i| i.parse::<f64>())
            .fold_results(0.0, |a, b| a + b)?,
        6.0
    );

    assert!(
        vec!["one", "2", "3"]
            .iter()
            .map(|i| i.parse::<f64>())
            .fold_results(0.0, |a, b| a + b)
            .is_err()
    );

    let data: Vec<Result<f64, Box<Error>>> = vec![Ok(1.0), Ok(2.0), Ok(3.0)];
    let iter = data.iter();
    assert_eq!(iter.fold_results(0.0, |x, y| x + y)?, 6.0);
    Ok(())
}

Errors:

   Compiling fold_results v0.1.0 (/home/florian/dev/rust/fold_results)
error[E0599]: no method named `fold_results` found for type `std::slice::Iter<'_, std::result::Result<f64, std::boxed::Box<dyn std::error::Error>>>` in the current scope
  --> src/main.rs:53:21
   |
53 |     assert_eq!(iter.fold_results(0.0, |x, y| x + y)?, 6.0);
   |                     ^^^^^^^^^^^^
   |
   = note: the method `fold_results` exists but the following trait bounds were not satisfied:
           `std::slice::Iter<'_, std::result::Result<f64, std::boxed::Box<dyn std::error::Error>>> : ResultIterator`
           `&std::slice::Iter<'_, std::result::Result<f64, std::boxed::Box<dyn std::error::Error>>> : ResultIterator`
           `&mut std::slice::Iter<'_, std::result::Result<f64, std::boxed::Box<dyn std::error::Error>>> : ResultIterator`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `fold_results`, perhaps you need to implement it:
           candidate #1: `ResultIterator`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: Could not compile `fold_results`.