1 | //! Definition of the `Option` (optional step) combinator |
2 | |
3 | use core::pin::Pin; |
4 | use futures_core::future::{FusedFuture, Future}; |
5 | use futures_core::task::{Context, Poll}; |
6 | use pin_project_lite::pin_project; |
7 | |
8 | pin_project! { |
9 | /// A future representing a value which may or may not be present. |
10 | /// |
11 | /// Created by the [`From`] implementation for [`Option`](std::option::Option). |
12 | /// |
13 | /// # Examples |
14 | /// |
15 | /// ``` |
16 | /// # futures::executor::block_on(async { |
17 | /// use futures::future::OptionFuture; |
18 | /// |
19 | /// let mut a: OptionFuture<_> = Some(async { 123 }).into(); |
20 | /// assert_eq!(a.await, Some(123)); |
21 | /// |
22 | /// a = None.into(); |
23 | /// assert_eq!(a.await, None); |
24 | /// # }); |
25 | /// ``` |
26 | #[derive(Debug, Clone)] |
27 | #[must_use = "futures do nothing unless you `.await` or poll them" ] |
28 | pub struct OptionFuture<F> { |
29 | #[pin] |
30 | inner: Option<F>, |
31 | } |
32 | } |
33 | |
34 | impl<F> Default for OptionFuture<F> { |
35 | fn default() -> Self { |
36 | Self { inner: None } |
37 | } |
38 | } |
39 | |
40 | impl<F: Future> Future for OptionFuture<F> { |
41 | type Output = Option<F::Output>; |
42 | |
43 | fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { |
44 | match self.project().inner.as_pin_mut() { |
45 | Some(x: Pin<&mut F>) => x.poll(cx).map(Some), |
46 | None => Poll::Ready(None), |
47 | } |
48 | } |
49 | } |
50 | |
51 | impl<F: FusedFuture> FusedFuture for OptionFuture<F> { |
52 | fn is_terminated(&self) -> bool { |
53 | match &self.inner { |
54 | Some(x: &F) => x.is_terminated(), |
55 | None => true, |
56 | } |
57 | } |
58 | } |
59 | |
60 | impl<T> From<Option<T>> for OptionFuture<T> { |
61 | fn from(option: Option<T>) -> Self { |
62 | Self { inner: option } |
63 | } |
64 | } |
65 | |