Macro tokio::join

macro_rules! join {
    (@ {
        // One `_` for each branch in the `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 join! branches
        $( ( $($skip:tt)* ) $e:expr, )*

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

Waits on multiple concurrent branches, returning when all branches complete.

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

The 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.

When working with async expressions returning Result, join! will wait for all branches complete regardless if any complete with Err. Use try_join! to return early when Err is encountered.

Notes

The supplied futures are stored inline and do 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 join!.

Examples

Basic join with two branches

async fn do_stuff_async() {
    // async work
}

async fn more_async_work() {
    // more here
}

#[tokio::main]
async fn main() {
    let (first, second) = tokio::join!(
        do_stuff_async(),
        more_async_work());

    // do something with the values
}