Select

A select operation waits until any of a set of futures is ready, and responds to that future's result. In JavaScript, this is similar to Promise.race. In Python, it compares to asyncio.wait(task_set, return_when=asyncio.FIRST_COMPLETED).

Similar to a match statement, the body of select! has a number of arms, each of the form pattern = future => statement. When a future is ready, its return value is destructured by the pattern. The statement is then run with the resulting variables. The statement result becomes the result of the select! macro.

use tokio::sync::mpsc;
use tokio::time::{sleep, Duration};

#[tokio::main]
async fn main() {
    let (tx, mut rx) = mpsc::channel(32);
    let listener = tokio::spawn(async move {
        tokio::select! {
            Some(msg) = rx.recv() => println!("got: {msg}"),
            _ = sleep(Duration::from_millis(50)) => println!("timeout"),
        };
    });
    sleep(Duration::from_millis(10)).await;
    tx.send(String::from("Hello!")).await.expect("Failed to send greeting");

    listener.await.expect("Listener failed");
}