Macro select

macro_rules! select {
    ($($tokens:tt)*) => { ... };
}

Polls multiple futures and streams simultaneously, executing the branch for the future that finishes first. If multiple futures are ready, one will be pseudo-randomly selected at runtime. Futures directly passed to select! must be Unpin and implement FusedFuture.

If an expression which yields a Future is passed to select! (e.g. an async fn call) instead of a Future by name the Unpin requirement is relaxed, since the macro will pin the resulting Future on the stack. However the Future returned by the expression must still implement FusedFuture.

Futures and streams which are not already fused can be fused using the .fuse() method. Note, though, that fusing a future or stream directly in the call to select! will not be enough to prevent it from being polled after completion if the select! call is in a loop, so when select!ing in a loop, users should take care to fuse() outside of the loop.

select! can be used as an expression and will return the return value of the selected branch. For this reason the return type of every branch in a select! must be the same.

This macro is only usable inside of async functions, closures, and blocks. It is also gated behind the async-await feature of this library, which is activated by default.

Examples

use futures::future;
use futures::select;
let mut a = future::ready(4);
let mut b = future::pending::<()>();

let res = select! {
    a_res = a => a_res + 1,
    _ = b => 0,
};
assert_eq!(res, 5);

use futures::future;
use futures::stream::{self, StreamExt};
use futures::select;
let mut st = stream::iter(vec![2]).fuse();
let mut fut = future::pending::<()>();

select! {
    x = st.next() => assert_eq!(Some(2), x),
    _ = fut => panic!(),
};

As described earlier, select can directly select on expressions which return Futures - even if those do not implement Unpin:

use futures::future::FutureExt;
use futures::select;

// Calling the following async fn returns a Future which does not
// implement Unpin
async fn async_identity_fn(arg: usize) -> usize {
    arg
}

let res = select! {
    a_res = async_identity_fn(62).fuse() => a_res + 1,
    b_res = async_identity_fn(13).fuse() => b_res,
};
assert!(res == 63 || res == 13);

If a similar async function is called outside of select to produce a Future, the Future must be pinned in order to be able to pass it to select. This can be achieved via Box::pin for pinning a Future on the heap or the pin_mut! macro for pinning a Future on the stack.

use futures::future::FutureExt;
use futures::select;
use futures::pin_mut;

// Calling the following async fn returns a Future which does not
// implement Unpin
async fn async_identity_fn(arg: usize) -> usize {
    arg
}

let fut_1 = async_identity_fn(1).fuse();
let fut_2 = async_identity_fn(2).fuse();
let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
pin_mut!(fut_2); // Pins the Future on the stack

let res = select! {
    a_res = fut_1 => a_res,
    b_res = fut_2 => b_res,
};
assert!(res == 1 || res == 2);

select also accepts a complete branch and a default branch. complete will run if all futures and streams have already been exhausted. default will run if no futures or streams are immediately ready. complete takes priority over default in the case where all futures have completed. A motivating use-case for passing Futures by name as well as for complete blocks is to call select! in a loop, which is demonstrated in the following example:

use futures::future;
use futures::select;
let mut a_fut = future::ready(4);
let mut b_fut = future::ready(6);
let mut total = 0;

loop {
    select! {
        a = a_fut => total += a,
        b = b_fut => total += b,
        complete => break,
        default => panic!(), // never runs (futures run first, then complete)
    };
}
assert_eq!(total, 10);

Note that the futures that have been matched over can still be mutated from inside the select! block’s branches. This can be used to implement more complex behavior such as timer resets or writing into the head of a stream.