futures_util/future/
select.rs

1use super::assert_future;
2use crate::future::{Either, FutureExt};
3use core::pin::Pin;
4use futures_core::future::{FusedFuture, Future};
5use futures_core::task::{Context, Poll};
6
7/// Future for the [`select()`] function.
8#[must_use = "futures do nothing unless you `.await` or poll them"]
9#[derive(Debug)]
10pub struct Select<A, B> {
11    inner: Option<(A, B)>,
12}
13
14impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
15
16/// Waits for either one of two differently-typed futures to complete.
17///
18/// This function will return a new future which awaits for either one of both
19/// futures to complete. The returned future will finish with both the value
20/// resolved and a future representing the completion of the other work.
21///
22/// Note that this function consumes the receiving futures and returns a
23/// wrapped version of them.
24///
25/// Also note that if both this and the second future have the same
26/// output type you can use the `Either::factor_first` method to
27/// conveniently extract out the value at the end.
28///
29/// # Examples
30///
31/// A simple example
32///
33/// ```
34/// # futures::executor::block_on(async {
35/// use core::pin::pin;
36///
37/// use futures::future;
38/// use futures::future::Either;
39///
40/// // These two futures have different types even though their outputs have the same type.
41/// let future1 = async {
42///     future::pending::<()>().await; // will never finish
43///     1
44/// };
45/// let future2 = async {
46///     future::ready(2).await
47/// };
48///
49/// // 'select' requires Future + Unpin bounds
50/// let future1 = pin!(future1);
51/// let future2 = pin!(future2);
52///
53/// let value = match future::select(future1, future2).await {
54///     Either::Left((value1, _)) => value1,  // `value1` is resolved from `future1`
55///                                           // `_` represents `future2`
56///     Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
57///                                           // `_` represents `future1`
58/// };
59///
60/// assert!(value == 2);
61/// # });
62/// ```
63///
64/// A more complex example
65///
66/// ```
67/// use futures::future::{self, Either, Future, FutureExt};
68///
69/// // A poor-man's join implemented on top of select
70///
71/// fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
72///     where A: Future + Unpin,
73///           B: Future + Unpin,
74/// {
75///     future::select(a, b).then(|either| {
76///         match either {
77///             Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
78///             Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
79///         }
80///     })
81/// }
82/// ```
83pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
84where
85    A: Future + Unpin,
86    B: Future + Unpin,
87{
88    assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
89        inner: Some((future1, future2)),
90    })
91}
92
93impl<A, B> Future for Select<A, B>
94where
95    A: Future + Unpin,
96    B: Future + Unpin,
97{
98    type Output = Either<(A::Output, B), (B::Output, A)>;
99
100    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
101        /// When compiled with `-C opt-level=z`, this function will help the compiler eliminate the `None` branch, where
102        /// `Option::unwrap` does not.
103        #[inline(always)]
104        fn unwrap_option<T>(value: Option<T>) -> T {
105            match value {
106                None => unreachable!(),
107                Some(value) => value,
108            }
109        }
110
111        let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");
112
113        if let Poll::Ready(val) = a.poll_unpin(cx) {
114            return Poll::Ready(Either::Left((val, unwrap_option(self.inner.take()).1)));
115        }
116
117        if let Poll::Ready(val) = b.poll_unpin(cx) {
118            return Poll::Ready(Either::Right((val, unwrap_option(self.inner.take()).0)));
119        }
120
121        Poll::Pending
122    }
123}
124
125impl<A, B> FusedFuture for Select<A, B>
126where
127    A: Future + Unpin,
128    B: Future + Unpin,
129{
130    fn is_terminated(&self) -> bool {
131        self.inner.is_none()
132    }
133}
futures_util/future/select.rs

futures_util/future/
select.rs
