1use crate::future::{Either, TryFutureExt};
2use core::pin::Pin;
3use futures_core::future::{Future, TryFuture};
4use futures_core::task::{Context, Poll};
5
6/// Future for the [`try_select()`] function.
7#[must_use = "futures do nothing unless you `.await` or poll them"]
8#[derive(Debug)]
9pub struct TrySelect<A, B> {
10 inner: Option<(A, B)>,
11}
12
13impl<A: Unpin, B: Unpin> Unpin for TrySelect<A, B> {}
14
15type EitherOk<A, B> = Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>;
16type EitherErr<A, B> = Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>;
17
18/// Waits for either one of two differently-typed futures to complete.
19///
20/// This function will return a new future which awaits for either one of both
21/// futures to complete. The returned future will finish with both the value
22/// resolved and a future representing the completion of the other work.
23///
24/// Note that this function consumes the receiving futures and returns a
25/// wrapped version of them.
26///
27/// Also note that if both this and the second future have the same
28/// success/error type you can use the `Either::factor_first` method to
29/// conveniently extract out the value at the end.
30///
31/// # Examples
32///
33/// ```
34/// use futures::future::{self, Either, Future, FutureExt, TryFuture, TryFutureExt};
35///
36/// // A poor-man's try_join implemented on top of select
37///
38/// fn try_join<A, B, E>(a: A, b: B) -> impl TryFuture<Ok=(A::Ok, B::Ok), Error=E>
39/// where A: TryFuture<Error = E> + Unpin + 'static,
40/// B: TryFuture<Error = E> + Unpin + 'static,
41/// E: 'static,
42/// {
43/// future::try_select(a, b).then(|res| -> Box<dyn Future<Output = Result<_, _>> + Unpin> {
44/// match res {
45/// Ok(Either::Left((x, b))) => Box::new(b.map_ok(move |y| (x, y))),
46/// Ok(Either::Right((y, a))) => Box::new(a.map_ok(move |x| (x, y))),
47/// Err(Either::Left((e, _))) => Box::new(future::err(e)),
48/// Err(Either::Right((e, _))) => Box::new(future::err(e)),
49/// }
50/// })
51/// }
52/// ```
53pub fn try_select<A, B>(future1: A, future2: B) -> TrySelect<A, B>
54where
55 A: TryFuture + Unpin,
56 B: TryFuture + Unpin,
57{
58 super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect {
59 inner: Some((future1, future2)),
60 })
61}
62
63impl<A: Unpin, B: Unpin> Future for TrySelect<A, B>
64where
65 A: TryFuture,
66 B: TryFuture,
67{
68 type Output = Result<EitherOk<A, B>, EitherErr<A, B>>;
69
70 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
71 let (mut a, mut b) = self.inner.take().expect("cannot poll Select twice");
72 match a.try_poll_unpin(cx) {
73 Poll::Ready(Err(x)) => Poll::Ready(Err(Either::Left((x, b)))),
74 Poll::Ready(Ok(x)) => Poll::Ready(Ok(Either::Left((x, b)))),
75 Poll::Pending => match b.try_poll_unpin(cx) {
76 Poll::Ready(Err(x)) => Poll::Ready(Err(Either::Right((x, a)))),
77 Poll::Ready(Ok(x)) => Poll::Ready(Ok(Either::Right((x, a)))),
78 Poll::Pending => {
79 self.inner = Some((a, b));
80 Poll::Pending
81 }
82 },
83 }
84 }
85}
86