Macro tokio::try_join

macro_rules! try_join {
    (@ {
        // One `_` for each branch in the `try_join!` macro. This is not used once
        // normalization is complete.
        ( $($count:tt)* )

        // The expression `0+1+1+ ... +1` equal to the number of branches.
        ( $($total:tt)* )

        // Normalized try_join! branches
        $( ( $($skip:tt)* ) $e:expr, )*

    }) => { ... };
    (@ { ( $($s:tt)* ) ( $($n:tt)* ) $($t:tt)* } $e:expr, $($r:tt)* ) => { ... };
    ( $($e:expr),* $(,)?) => { ... };
}

Available on crate feature macros only.

Waits on multiple concurrent branches, returning when all branches complete with Ok(_) or on the first Err(_).

The try_join! macro must be used inside of async functions, closures, and blocks.

Similar to join!, the try_join! macro takes a list of async expressions and evaluates them concurrently on the same task. Each async expression evaluates to a future and the futures from each expression are multiplexed on the current task. The try_join! macro returns when all branches return with Ok or when the first branch returns with Err.

Notes

The supplied futures are stored inline and does not require allocating a Vec.

Runtime characteristics

By running all async expressions on the current task, the expressions are able to run concurrently but not in parallel. This means all expressions are run on the same thread and if one branch blocks the thread, all other expressions will be unable to continue. If parallelism is required, spawn each async expression using tokio::spawn and pass the join handle to try_join!.

Examples

Basic try_join with two branches.

async fn do_stuff_async() -> Result<(), &'static str> {
    // async work
}

async fn more_async_work() -> Result<(), &'static str> {
    // more here
}

#[tokio::main]
async fn main() {
    let res = tokio::try_join!(
        do_stuff_async(),
        more_async_work());

    match res {
         Ok((first, second)) => {
             // do something with the values
         }
         Err(err) => {
            println!("processing failed; error = {}", err);
         }
    }
}

Using try_join! with spawned tasks.

use tokio::task::JoinHandle;

async fn do_stuff_async() -> Result<(), &'static str> {
    // async work
}

async fn more_async_work() -> Result<(), &'static str> {
    // more here
}

async fn flatten<T>(handle: JoinHandle<Result<T, &'static str>>) -> Result<T, &'static str> {
    match handle.await {
        Ok(Ok(result)) => Ok(result),
        Ok(Err(err)) => Err(err),
        Err(err) => Err("handling failed"),
    }
}

#[tokio::main]
async fn main() {
    let handle1 = tokio::spawn(do_stuff_async());
    let handle2 = tokio::spawn(more_async_work());
    match tokio::try_join!(flatten(handle1), flatten(handle2)) {
        Ok(val) => {
            // do something with the values
        }
        Err(err) => {
            println!("Failed with {}.", err);
        }
    }
}