Iteration and Concurrency

Similar to synchronous Iterators, there are many different ways to iterate over and process the values in a Stream. There are combinator-style methods such as map, filter, and fold, and their early-exit-on-error cousins try_map, try_filter, and try_fold.

Unfortunately, for loops are not usable with Streams, but for imperative-style code, while let and the next/try_next functions can be used:


#![allow(unused_variables)]
fn main() {
async fn sum_with_next(mut stream: Pin<&mut dyn Stream<Item = i32>>) -> i32 {
    use futures::stream::StreamExt; // for `next`
    let mut sum = 0;
    while let Some(item) = stream.next().await {
        sum += item;
    }
    sum
}

async fn sum_with_try_next(
    mut stream: Pin<&mut dyn Stream<Item = Result<i32, io::Error>>>,
) -> Result<i32, io::Error> {
    use futures::stream::TryStreamExt; // for `try_next`
    let mut sum = 0;
    while let Some(item) = stream.try_next().await? {
        sum += item;
    }
    Ok(sum)
}
}

However, if we're just processing one element at a time, we're potentially leaving behind opportunity for concurrency, which is, after all, why we're writing async code in the first place. To process multiple items from a stream concurrently, use the for_each_concurrent and try_for_each_concurrent methods:


#![allow(unused_variables)]
fn main() {
async fn jump_around(
    mut stream: Pin<&mut dyn Stream<Item = Result<u8, io::Error>>>,
) -> Result<(), io::Error> {
    use futures::stream::TryStreamExt; // for `try_for_each_concurrent`
    const MAX_CONCURRENT_JUMPERS: usize = 100;

    stream.try_for_each_concurrent(MAX_CONCURRENT_JUMPERS, |num| async move {
        jump_n_times(num).await?;
        report_n_jumps(num).await?;
        Ok(())
    }).await?;

    Ok(())
}
}