Add future::try_select

Author: taiki-e

futures-util/src/try_future/mod.rs

@@ -21,9 +21,8 @@ mod try_join_all;
 #[cfg(feature = "alloc")]
 pub use self::try_join_all::{try_join_all, TryJoinAll};
 
-// TODO
-// mod try_select;
-// pub use self::try_select::{try_select, TrySelect};
+mod try_select;
+pub use self::try_select::{try_select, TrySelect};
 
 #[cfg(feature = "alloc")]
 mod select_ok;
@@ -319,50 +318,6 @@ pub trait TryFutureExt: TryFuture {
         OrElse::new(self, f)
     }
 
-    /* TODO
-    /// Waits for either one of two differently-typed futures to complete.
-    ///
-    /// This function will return a new future which awaits for either this or
-    /// the `other` future 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::split` method to
-    /// conveniently extract out the value at the end.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use futures::future::{self, Either};
-    ///
-    /// // A poor-man's join implemented on top of select
-    ///
-    /// fn join<A, B, E>(a: A, b: B) -> Box<Future<Item=(A::Item, B::Item), Error=E>>
-    ///     where A: Future<Error = E> + 'static,
-    ///           B: Future<Error = E> + 'static,
-    ///           E: 'static,
-    /// {
-    ///     Box::new(a.select(b).then(|res| -> Box<Future<Item=_, Error=_>> {
-    ///         match res {
-    ///             Ok(Either::Left((x, b))) => Box::new(b.map(move |y| (x, y))),
-    ///             Ok(Either::Right((y, a))) => Box::new(a.map(move |x| (x, y))),
-    ///             Err(Either::Left((e, _))) => Box::new(future::err(e)),
-    ///             Err(Either::Right((e, _))) => Box::new(future::err(e)),
-    ///         }
-    ///     }))
-    /// }}
-    /// ```
-    fn select<B>(self, other: B) -> Select<Self, B::Future>
-        where B: IntoFuture, Self: Sized
-    {
-        select::new(self, other.into_future())
-    }
-*/
-
     /// Unwraps this future's ouput, producing a future with this future's
     /// [`Ok`](TryFuture::Ok) type as its
     /// [`Output`](std::future::Future::Output) type.

futures-util/src/try_future/try_select.rs

@@ -0,0 +1,80 @@
+use core::pin::Pin;
+use futures_core::future::{Future, TryFuture};
+use futures_core::task::{Context, Poll};
+use crate::future::Either;
+
+/// Future for the [`try_select()`] function.
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+#[derive(Debug)]
+pub struct TrySelect<A: Unpin, B: Unpin> {
+    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 Pin::new(&mut a).try_poll(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 Pin::new(&mut b).try_poll(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
+                }
+            }
+        }
+    }
+}

futures/src/lib.rs

@@ -247,6 +247,7 @@ pub mod future {
     pub use futures_util::try_future::{
         try_join, try_join3, try_join4, try_join5,
         TryJoin, TryJoin3, TryJoin4, TryJoin5,
+        try_select, TrySelect,
 
         TryFutureExt,
         AndThen, ErrInto, FlattenSink, IntoFuture, MapErr, MapOk, OrElse,