src/future/try_select.rs

use core::pin::Pin;
use futures_core::future::{Future, TryFuture};
use futures_core::task::{Context, Poll};
use crate::future::{Either, TryFutureExt};

/// 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> {}

/// 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
{
    TrySelect { inner: Some((future1, future2)) }
}

impl<A: Unpin, B: Unpin> Future for TrySelect<A, B>
    where A: TryFuture, B: TryFuture
{
    #[allow(clippy::type_complexity)]
    type Output = Result<
        Either<(A::Ok, B), (B::Ok, A)>,
        Either<(A::Error, B), (B::Error, A)>,
    >;

    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
                }
            }
        }
    }
}