Function select

pub fn select<A, B>(future1: A, future2: B) -> Select<A, B> 
where
    A: Future + Unpin,
    B: Future + Unpin,

Waits for either one of two differently-typed futures to complete.

This function will return a new future which awaits for either one of both futures to complete. The returned future will finish with both the value resolved and a future representing the completion of the other work.

Note that this function consumes the receiving futures and returns a wrapped version of them.

Also note that if both this and the second future have the same output type you can use the Either::factor_first method to conveniently extract out the value at the end.

Examples

A simple example

use futures::{
    pin_mut,
    future::Either,
    future::self,
};

// These two futures have different types even though their outputs have the same type.
let future1 = async {
    future::pending::<()>().await; // will never finish
    1
};
let future2 = async {
    future::ready(2).await
};

// 'select' requires Future + Unpin bounds
pin_mut!(future1);
pin_mut!(future2);

let value = match future::select(future1, future2).await {
    Either::Left((value1, _)) => value1,  // `value1` is resolved from `future1`
                                          // `_` represents `future2`
    Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
                                          // `_` represents `future1`
};

assert!(value == 2);

A more complex example

use futures::future::{self, Either, Future, FutureExt};

// A poor-man's join implemented on top of select

fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
    where A: Future + Unpin,
          B: Future + Unpin,
{
    future::select(a, b).then(|either| {
        match either {
            Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
            Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
        }
    })
}