Function eventuals::map_with_retry

pub fn map_with_retry<Ok, Err, F, Fut, E, FutE, R>(
    source: R, 
    f: F, 
    on_err: E
) -> Eventual<Ok> where
    R: IntoReader,
    F: 'static + Send + FnMut(R::Output) -> Fut,
    E: 'static + Send + Sync + FnMut(Err) -> FutE,
    Ok: Value,
    Err: Value,
    Fut: Send + Future<Output = Result<Ok, Err>>,
    FutE: Send + Future<Output = ()>, 

Ensure that a fallible map operation will succeed eventually. For example given map_with_retry([“url_1”, “url_2”], fallibly_get_data, sleep) may produce [“data_1”, “data_2”] or just [“data_2”]. The difference between map_with_retry and something like map(source, retry(fallibly_get_data, on_err)) is that the former supports ‘moving on’ to “url_2” even if “url_1” is in a retry state, whereas the latter would have to complete one item fully before progressing. It is because of this distinction that map_with_retry is allowed to retry forever instead of giving up after a set number of attempts.