futures_util/future/
select.rs

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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 futures::{
36///     pin_mut,
37///     future::Either,
38///     future::self,
39/// };
40///
41/// // These two futures have different types even though their outputs have the same type.
42/// let future1 = async {
43///     future::pending::<()>().await; // will never finish
44///     1
45/// };
46/// let future2 = async {
47///     future::ready(2).await
48/// };
49///
50/// // 'select' requires Future + Unpin bounds
51/// pin_mut!(future1);
52/// pin_mut!(future2);
53///
54/// let value = match future::select(future1, future2).await {
55///     Either::Left((value1, _)) => value1,  // `value1` is resolved from `future1`
56///                                           // `_` represents `future2`
57///     Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
58///                                           // `_` represents `future1`
59/// };
60///
61/// assert!(value == 2);
62/// # });
63/// ```
64///
65/// A more complex example
66///
67/// ```
68/// use futures::future::{self, Either, Future, FutureExt};
69///
70/// // A poor-man's join implemented on top of select
71///
72/// fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
73///     where A: Future + Unpin,
74///           B: Future + Unpin,
75/// {
76///     future::select(a, b).then(|either| {
77///         match either {
78///             Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
79///             Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
80///         }
81///     })
82/// }
83/// ```
84pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
85where
86    A: Future + Unpin,
87    B: Future + Unpin,
88{
89    assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
90        inner: Some((future1, future2)),
91    })
92}
93
94impl<A, B> Future for Select<A, B>
95where
96    A: Future + Unpin,
97    B: Future + Unpin,
98{
99    type Output = Either<(A::Output, B), (B::Output, A)>;
100
101    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
102        /// When compiled with `-C opt-level=z`, this function will help the compiler eliminate the `None` branch, where
103        /// `Option::unwrap` does not.
104        #[inline(always)]
105        fn unwrap_option<T>(value: Option<T>) -> T {
106            match value {
107                None => unreachable!(),
108                Some(value) => value,
109            }
110        }
111
112        let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");
113
114        if let Poll::Ready(val) = a.poll_unpin(cx) {
115            return Poll::Ready(Either::Left((val, unwrap_option(self.inner.take()).1)));
116        }
117
118        if let Poll::Ready(val) = b.poll_unpin(cx) {
119            return Poll::Ready(Either::Right((val, unwrap_option(self.inner.take()).0)));
120        }
121
122        Poll::Pending
123    }
124}
125
126impl<A, B> FusedFuture for Select<A, B>
127where
128    A: Future + Unpin,
129    B: Future + Unpin,
130{
131    fn is_terminated(&self) -> bool {
132        self.inner.is_none()
133    }
134}
futures_util/future/select.rs

futures_util/future/
select.rs
