Select

Selectという操作では、futureの集合のうち、いずれか１つの準備が整うまで待機し、そのfutureが提供する結果に対して応答します。これはJavaScriptにおけるPromise.raceに似ています。また、Pythonにおける asyncio.wait(task_set, return_when=asyncio.FIRST_COMPLETED)と比べることができます。

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::{self, Receiver};
use tokio::time::{sleep, Duration};

#[derive(Debug, PartialEq)]
enum Animal {
    Cat { name: String },
    Dog { name: String },
}

async fn first_animal_to_finish_race(
    mut cat_rcv: Receiver<String>,
    mut dog_rcv: Receiver<String>,
) -> Option<Animal> {
    tokio::select! {
        cat_name = cat_rcv.recv() => Some(Animal::Cat { name: cat_name? }),
        dog_name = dog_rcv.recv() => Some(Animal::Dog { name: dog_name? })
    }
}

#[tokio::main]
async fn main() {
    let (cat_sender, cat_receiver) = mpsc::channel(32);
    let (dog_sender, dog_receiver) = mpsc::channel(32);
    tokio::spawn(async move {
        sleep(Duration::from_millis(500)).await;
        cat_sender.send(String::from("Felix")).await.expect("Failed to send cat.");
    });
    tokio::spawn(async move {
        sleep(Duration::from_millis(50)).await;
        dog_sender.send(String::from("Rex")).await.expect("Failed to send dog.");
    });

    let winner = first_animal_to_finish_race(cat_receiver, dog_receiver)
        .await
        .expect("Failed to receive winner");

    println!("Winner is {winner:?}");
}