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Una operación select espera hasta que un conjunto de futuros esté listo y responde al resultado de ese futuro. En JavaScript, esto es similar a Promise.race. En Python, se compara con 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::{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("No se ha podido enviar el gato.");
    });
    tokio::spawn(async move {
        sleep(Duration::from_millis(50)).await;
        dog_sender.send(String::from("Rex")).await.expect("No se ha podido enviar el perro.");
    });

    let winner = first_animal_to_finish_race(cat_receiver, dog_receiver)
        .await
        .expect("No se ha podido recibir el ganador");

    println!("El ganador es {winner:?}");
}