use crate::future::{Either, TryFutureExt};
use core::pin::Pin;
use futures_core::future::{Future, TryFuture};
use futures_core::task::{Context, Poll};
/// Future for the [`try_select()`] function.
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[derive(Debug)]
pub struct TrySelect<A, B> {
inner: Option<(A, B)>,
}
impl<A: Unpin, B: Unpin> Unpin for TrySelect<A, B> {}
type EitherOk<A, B> = Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>;
type EitherErr<A, B> = Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>;
/// 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)),
/// }
/// })
/// }
/// ```
pub fn try_select<A, B>(future1: A, future2: B) -> TrySelect<A, B>
where
A: TryFuture + Unpin,
B: TryFuture + Unpin,
{
super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect {
inner: Some((future1, future2)),
})
}
impl<A: Unpin, B: Unpin> Future for TrySelect<A, B>
where
A: TryFuture,
B: TryFuture,
{
type Output = Result<EitherOk<A, B>, EitherErr<A, B>>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let (mut a, mut b) = self.inner.take().expect("cannot poll Select twice");
match a.try_poll_unpin(cx) {
Poll::Ready(Err(x)) => Poll::Ready(Err(Either::Left((x, b)))),
Poll::Ready(Ok(x)) => Poll::Ready(Ok(Either::Left((x, b)))),
Poll::Pending => match b.try_poll_unpin(cx) {
Poll::Ready(Err(x)) => Poll::Ready(Err(Either::Right((x, a)))),
Poll::Ready(Ok(x)) => Poll::Ready(Ok(Either::Right((x, a)))),
Poll::Pending => {
self.inner = Some((a, b));
Poll::Pending
}
},
}
}
}
