Function futures::future::try_select

Important traits for TrySelect<A, B>

impl<A, B> Future for TrySelect<A, B> where
    A: Unpin + TryFuture,
    B: Unpin + TryFuture,  type Output = Result<Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>, Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>>;

pub fn try_select<A, B>(future1: A, future2: B) -> TrySelect<A, B> where
    A: TryFuture + Unpin,
    B: TryFuture + 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 success/error type you can use the Either::factor_first method to conveniently extract out the value at the end.

Examples

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

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

fn try_join<A, B, E>(a: A, b: B) -> impl TryFuture<Ok=(A::Ok, B::Ok), Error=E>
     where A: TryFuture<Error = E> + Unpin + 'static,
           B: TryFuture<Error = E> + Unpin + 'static,
           E: 'static,
{
    future::try_select(a, b).then(|res| -> Box<dyn Future<Output = Result<_, _>> + Unpin> {
        match res {
            Ok(Either::Left((x, b))) => Box::new(b.map_ok(move |y| (x, y))),
            Ok(Either::Right((y, a))) => Box::new(a.map_ok(move |x| (x, y))),
            Err(Either::Left((e, _))) => Box::new(future::err(e)),
            Err(Either::Right((e, _))) => Box::new(future::err(e)),
        }
    })
}