[−][src]Trait futures_core::future::Future

#[must_use = "futures do nothing unless polled"]

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output>;
}
🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

[−]

A future represents an asynchronous computation.

A future is a value that may not have finished computing yet. This kind of "asynchronous value" makes it possible for a thread to continue doing useful work while it waits for the value to become available.

The poll method

The core method of future, poll, attempts to resolve the future into a final value. This method does not block if the value is not ready. Instead, the current task is scheduled to be woken up when it's possible to make further progress by polling again. The wake up is performed using the waker argument of the poll() method, which is a handle for waking up the current task.

When using a future, you generally won't call poll directly, but instead await! the value.

Associated Types

type Output[−]

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

The type of value produced on completion.

Required methods

fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output>[−]

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

Return value

This function returns:

Once a future has finished, clients should not poll it again.

When a future is not ready yet, poll returns Poll::Pending and stores a clone of the Waker to be woken once the future can make progress. For example, a future waiting for a socket to become readable would call .clone() on the Waker and store it. When a signal arrives elsewhere indicating that the socket is readable, [Waker::wake] is called and the socket future's task is awoken. Once a task has been woken up, it should attempt to poll the future again, which may or may not produce a final value.

Note that on multiple calls to poll, only the most recent Waker passed to poll should be scheduled to receive a wakeup.

Runtime characteristics

Futures alone are inert; they must be actively polled to make progress, meaning that each time the current task is woken up, it should actively re-poll pending futures that it still has an interest in.

The poll function is not called repeatedly in a tight loop -- instead, it should only be called when the future indicates that it is ready to make progress (by calling wake()). If you're familiar with the poll(2) or select(2) syscalls on Unix it's worth noting that futures typically do not suffer the same problems of "all wakeups must poll all events"; they are more like epoll(4).

An implementation of poll should strive to return quickly, and should not block. Returning quickly prevents unnecessarily clogging up threads or event loops. If it is known ahead of time that a call to poll may end up taking awhile, the work should be offloaded to a thread pool (or something similar) to ensure that poll can return quickly.

An implementation of poll may also never cause memory unsafety.

Panics

Once a future has completed (returned Ready from poll), then any future calls to poll may panic, block forever, or otherwise cause any kind of bad behavior except causing memory unsafety. The Future trait itself provides no guarantees about the behavior of poll after a future has completed.

Implementations on Foreign Types

impl<F> Future for AssertUnwindSafe<F> where
    F: Future, [src][−]

type Output = <F as Future>::Output

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized, [src][−]

type Output = <F as Future>::Output

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

impl<P> Future for Pin<P> where
    P: Unpin + DerefMut,
    <P as Deref>::Target: Future, [src][−]

type Output = <<P as Deref>::Target as Future>::Output

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, [src][−]

type Output = <F as Future>::Output

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

Implementors

impl<'a, T> Future for FutureObj<'a, T>[src][−]

type Output = T

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

impl<'a, T> Future for LocalFutureObj<'a, T>[src][−]

type Output = T

🔬 This is a nightly-only experimental API. (futures_api)

futures in libcore are unstable

impl<T> Future for Receiver<T>

impl<Fut: Future> Future for AssertUnmoved<Fut>

impl<Fut: Future> Future for PendingOnce<Fut>

impl<T> Future for Empty<T>

impl<R, F> Future for Lazy<F> where
    F: FnOnce(&Waker) -> R, 

impl<Fut: Future> Future for MaybeDone<Fut>

impl<F: Future> Future for OptionFuture<F>

impl<T, F> Future for PollFn<F> where
    F: FnMut(&Waker) -> Poll<T>, 

impl<T> Future for Ready<T>

impl<Fut> Future for Flatten<Fut> where
    Fut: Future,
    Fut::Output: Future, 

impl<Fut: Future> Future for Fuse<Fut>

impl<Fut1: Future, Fut2: Future> Future for Join<Fut1, Fut2>

impl<Fut1: Future, Fut2: Future, Fut3: Future> Future for Join3<Fut1, Fut2, Fut3>

impl<Fut1: Future, Fut2: Future, Fut3: Future, Fut4: Future> Future for Join4<Fut1, Fut2, Fut3, Fut4>

impl<Fut1: Future, Fut2: Future, Fut3: Future, Fut4: Future, Fut5: Future> Future for Join5<Fut1, Fut2, Fut3, Fut4, Fut5>

impl<Fut, F, T> Future for Map<Fut, F> where
    Fut: Future,
    F: FnOnce(Fut::Output) -> T, 

impl<Fut1, Fut2, F> Future for Then<Fut1, Fut2, F> where
    Fut1: Future,
    Fut2: Future,
    F: FnOnce(Fut1::Output) -> Fut2, 

impl<Fut, F> Future for Inspect<Fut, F> where
    Fut: Future,
    F: FnOnce(&Fut::Output), 

impl<Fut, T> Future for UnitError<Fut> where
    Fut: Future<Output = T>, 

impl<Fut> Future for Abortable<Fut> where
    Fut: Future, 

impl<Fut> Future for CatchUnwind<Fut> where
    Fut: Future + UnwindSafe, 

impl<T: Send + 'static> Future for RemoteHandle<T>

impl<Fut: Future> Future for Remote<Fut>

impl<F> Future for JoinAll<F> where
    F: Future, 

impl<Fut: Future> Future for Shared<Fut> where
    Fut::Output: Clone, 

impl<Fut1, Fut2, F> Future for AndThen<Fut1, Fut2, F> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>,
    F: FnOnce(Fut1::Ok) -> Fut2, 

impl<Fut, E> Future for ErrInto<Fut, E> where
    Fut: TryFuture,
    Fut::Error: Into<E>, 

impl<Fut1, Fut2> Future for TryJoin<Fut1, Fut2> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>, 

impl<Fut1, Fut2, Fut3> Future for TryJoin3<Fut1, Fut2, Fut3> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>,
    Fut3: TryFuture<Error = Fut1::Error>, 

impl<Fut1, Fut2, Fut3, Fut4> Future for TryJoin4<Fut1, Fut2, Fut3, Fut4> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>,
    Fut3: TryFuture<Error = Fut1::Error>,
    Fut4: TryFuture<Error = Fut1::Error>, 

impl<Fut1, Fut2, Fut3, Fut4, Fut5> Future for TryJoin5<Fut1, Fut2, Fut3, Fut4, Fut5> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>,
    Fut3: TryFuture<Error = Fut1::Error>,
    Fut4: TryFuture<Error = Fut1::Error>,
    Fut5: TryFuture<Error = Fut1::Error>, 

impl<Fut: TryFuture> Future for IntoFuture<Fut>

impl<Fut, F, E> Future for MapErr<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(Fut::Error) -> E, 

impl<Fut, F, T> Future for MapOk<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(Fut::Ok) -> T, 

impl<Fut1, Fut2, F> Future for OrElse<Fut1, Fut2, F> where
    Fut1: TryFuture,
    Fut2: TryFuture<Ok = Fut1::Ok>,
    F: FnOnce(Fut1::Error) -> Fut2, 

impl<Fut, F> Future for UnwrapOrElse<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(Fut::Error) -> Fut::Ok, 

impl<F> Future for TryJoinAll<F> where
    F: TryFuture, 

impl<St, C> Future for Collect<St, C> where
    St: Stream,
    C: Default + Extend<St::Item>, 

impl<St> Future for Concat<St> where
    St: Stream,
    St::Item: Extend<<St::Item as IntoIterator>::Item> + IntoIterator + Default, 

impl<St, Fut, T, F> Future for Fold<St, Fut, T, F> where
    St: Stream,
    F: FnMut(T, St::Item) -> Fut,
    Fut: Future<Output = T>, 

impl<St, Si> Future for Forward<St, Si> where
    Si: Sink + Unpin,
    St: Stream<Item = Result<Si::SinkItem, Si::SinkError>>, 

impl<St, Fut, F> Future for ForEach<St, Fut, F> where
    St: Stream,
    F: FnMut(St::Item) -> Fut,
    Fut: Future<Output = ()>, 

impl<St: Stream + Unpin> Future for StreamFuture<St>

impl<St: Stream + Unpin, '_> Future for Next<'_, St>

impl<'a, St: Stream + FusedStream + Unpin> Future for SelectNextSome<'a, St>

impl<St, Fut, F> Future for ForEachConcurrent<St, Fut, F> where
    St: Stream,
    F: FnMut(St::Item) -> Fut,
    Fut: Future<Output = ()>, 

impl<St: TryStream + Unpin, '_> Future for TryNext<'_, St>

impl<St, Fut, F> Future for TryForEach<St, Fut, F> where
    St: TryStream,
    F: FnMut(St::Ok) -> Fut,
    Fut: TryFuture<Ok = (), Error = St::Error>, 

impl<St> Future for TryConcat<St> where
    St: TryStream,
    St::Ok: Extend<<St::Ok as IntoIterator>::Item> + IntoIterator + Default, 

impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F> where
    St: TryStream,
    F: FnMut(T, St::Ok) -> Fut,
    Fut: TryFuture<Ok = T, Error = St::Error>, 

impl<St, C> Future for TryCollect<St, C> where
    St: TryStream,
    C: Default + Extend<St::Ok>, 

impl<St, Fut, F> Future for TryForEachConcurrent<St, Fut, F> where
    St: TryStream,
    F: FnMut(St::Ok) -> Fut,
    Fut: Future<Output = Result<(), St::Error>>, 

impl<Si: Sink + Unpin + ?Sized, '_> Future for Close<'_, Si>

impl<Si: Sink + Unpin + ?Sized, '_> Future for Flush<'_, Si>

impl<Si: Sink + Unpin + ?Sized, '_> Future for Send<'_, Si>

impl<Si: ?Sized, St: ?Sized, '_> Future for SendAll<'_, Si, St> where
    Si: Sink + Unpin,
    St: Stream<Item = Si::SinkItem> + Unpin, 

impl<Fut: Future01> Future for Compat01As03<Fut>

impl<R: ?Sized, W: ?Sized, '_> Future for CopyInto<'_, R, W> where
    R: AsyncRead,
    W: AsyncWrite, 

impl<W: ?Sized, '_> Future for Flush<'_, W> where
    W: AsyncWrite, 

impl<R: AsyncRead + ?Sized, '_> Future for Read<'_, R>

impl<R: AsyncRead + ?Sized, '_> Future for ReadExact<'_, R>

impl<A: ?Sized, '_> Future for ReadToEnd<'_, A> where
    A: AsyncRead, 

impl<W: AsyncWrite + ?Sized, '_> Future for Close<'_, W>

impl<W: AsyncWrite + ?Sized, '_> Future for WriteAll<'_, W>

impl<'a, T> Future for MutexLockFuture<'a, T>

impl<'a, T> Future for BiLockAcquire<'a, T>