fn main() {
    let vec = vec![Ok(1), Ok(2), Err("oops"), Ok(4), Err("Oh my buddha")];
    let sum_res = vec.into_iter()
        .try_fold(
            0,
            |sum, item| item.map(|v| sum + v)
        );
    assert_eq!(sum_res, Err("oops"));
}