Trait async_std::future::Future

1.36.0 · source ·
pub trait Future {
    type Output;

    // Required method
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}
Expand description

A future represents an asynchronous computation obtained by use of async.

A future is a value that might 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 context passed to the poll method can provide a Waker, 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.

Required Associated Types§

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type Output

The type of value produced on completion.

Required Methods§

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fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>

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 copied from the current Context. This Waker is then 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 Waker from the Context passed to the most recent call 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.

Panics

Once a future has completed (returned Ready from poll), calling its poll method again may panic, block forever, or cause other kinds of problems; the Future trait places no requirements on the effects of such a call. However, as the poll method is not marked unsafe, Rust’s usual rules apply: calls must never cause undefined behavior (memory corruption, incorrect use of unsafe functions, or the like), regardless of the future’s state.

Implementors§

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impl Future for Timer

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impl Future for EventListener

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type Output = ()

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impl Future for YieldNow

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type Output = ()

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impl<'a, R> Future for FillBuf<'a, R>
where R: AsyncBufRead + Unpin + ?Sized,

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type Output = Result<&'a [u8], Error>

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impl<'a, S> Future for NthFuture<'a, S>
where S: Stream + Unpin + ?Sized,

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type Output = Option<<S as Stream>::Item>

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impl<'a, S, B, F> Future for FindMapFuture<'a, S, F>
where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Option<B>,

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type Output = Option<B>

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impl<'a, S, F, E> Future for TryForEachFuture<'a, S, F>
where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Result<(), E>,

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type Output = Result<(), E>

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impl<'a, S, P> Future for FindFuture<'a, S, P>
where S: Stream + Unpin + ?Sized, P: FnMut(&<S as Stream>::Item) -> bool,

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type Output = Option<<S as Stream>::Item>

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impl<'a, S, P> Future for PositionFuture<'a, S, P>
where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

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

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

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impl<'a, T, E, S, F, B> Future for TryFoldFuture<'a, S, F, B>
where S: Stream<Item = Result<T, E>> + Unpin, F: FnMut(B, T) -> Result<B, E>,

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type Output = Result<B, E>

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impl<F1, F2> Future for Zip<F1, F2>
where F1: Future, F2: Future,

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type Output = (<F1 as Future>::Output, <F2 as Future>::Output)

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impl<F> Future for &mut F
where F: Future + Unpin + ?Sized,

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type Output = <F as Future>::Output

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

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type Output = Result<<F as Future>::Output, Box<dyn Any + Send>>

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

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type Output = <F as Future>::Output

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impl<F, A> Future for Box<F, A>
where F: Future + Unpin + ?Sized, A: Allocator + 'static,

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type Output = <F as Future>::Output

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impl<P> Future for Pin<P>
where P: DerefMut, <P as Deref>::Target: Future,

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

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impl<R> Future for ReadExactFuture<'_, R>
where R: AsyncRead + Unpin + ?Sized,

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impl<R> Future for ReadFuture<'_, R>
where R: AsyncRead + Unpin + ?Sized,

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impl<R> Future for ReadLineFuture<'_, R>
where R: AsyncBufRead + Unpin + ?Sized,

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impl<R> Future for ReadToEndFuture<'_, R>
where R: AsyncRead + Unpin + ?Sized,

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impl<R> Future for ReadToStringFuture<'_, R>
where R: AsyncRead + Unpin + ?Sized,

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impl<R> Future for ReadUntilFuture<'_, R>
where R: AsyncBufRead + Unpin + ?Sized,

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impl<R> Future for ReadVectoredFuture<'_, R>
where R: AsyncRead + Unpin + ?Sized,

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impl<S> Future for SeekFuture<'_, S>
where S: AsyncSeek + Unpin + ?Sized,

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impl<S> Future for CountFuture<S>
where S: Stream + ?Sized,

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impl<S> Future for LastFuture<S>
where S: Stream,

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type Output = Option<<S as Stream>::Item>

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impl<S> Future for NextFuture<'_, S>
where S: Stream + Unpin + ?Sized,

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type Output = Option<<S as Stream>::Item>

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impl<S, A, B, FromA, FromB> Future for UnzipFuture<S, FromA, FromB>
where S: Stream<Item = (A, B)>, FromA: Default + Extend<A>, FromB: Default + Extend<B>,

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impl<S, C> Future for CollectFuture<S, C>
where S: Stream, C: Default + Extend<<S as Stream>::Item>,

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type Output = C

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impl<S, F> Future for ForEachFuture<S, F>
where S: Stream, F: FnMut(<S as Stream>::Item),

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type Output = ()

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impl<S, F, T> Future for FoldFuture<S, F, T>
where S: Stream, F: FnMut(T, <S as Stream>::Item) -> T,

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type Output = T

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impl<S, P> Future for AllFuture<'_, S, P>
where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

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type Output = bool

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impl<S, P> Future for AnyFuture<'_, S, P>
where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

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type Output = bool

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impl<S, P, B> Future for PartitionFuture<S, P, B>
where S: Stream, P: FnMut(&<S as Stream>::Item) -> bool, B: Default + Extend<<S as Stream>::Item>,

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impl<T1, T2, E, F1, F2> Future for TryZip<F1, F2>
where F1: Future<Output = Result<T1, E>>, F2: Future<Output = Result<T2, E>>,

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impl<T> Future for Readable<'_, T>

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impl<T> Future for ReadableOwned<T>

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impl<T> Future for Writable<'_, T>

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impl<T> Future for WritableOwned<T>

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impl<T> Future for FallibleTask<T>

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type Output = Option<T>

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impl<T> Future for Task<T>

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type Output = T

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impl<T> Future for futures_lite::future::Pending<T>

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type Output = T

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impl<T> Future for futures_lite::future::Ready<T>

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type Output = T

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impl<T> Future for JoinHandle<T>

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type Output = T

1.48.0 · source§

impl<T> Future for core::future::pending::Pending<T>

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type Output = T

1.48.0 · source§

impl<T> Future for core::future::ready::Ready<T>

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type Output = T

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impl<T> Future for Exclusive<T>
where T: Future + ?Sized,

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type Output = <T as Future>::Output

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impl<T, E, S> Future for TryNextFuture<'_, S>
where S: Stream<Item = Result<T, E>> + Unpin + ?Sized,

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type Output = Result<Option<T>, E>

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impl<T, E, S, C> Future for TryCollectFuture<S, C>
where S: Stream<Item = Result<T, E>>, C: Default + Extend<T>,

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type Output = Result<C, E>

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impl<T, F1, F2> Future for Or<F1, F2>
where F1: Future<Output = T>, F2: Future<Output = T>,

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type Output = T

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impl<T, F1, F2> Future for Race<F1, F2>
where F1: Future<Output = T>, F2: Future<Output = T>,

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type Output = T

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impl<T, F> Future for futures_lite::future::PollFn<F>
where F: FnMut(&mut Context<'_>) -> Poll<T>,

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type Output = T

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impl<T, F> Future for PollOnce<F>
where F: Future<Output = T>,

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type Output = Option<T>

1.64.0 · source§

impl<T, F> Future for core::future::poll_fn::PollFn<F>
where F: FnMut(&mut Context<'_>) -> Poll<T>,

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type Output = T

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impl<W> Future for CloseFuture<'_, W>
where W: AsyncWrite + Unpin + ?Sized,

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impl<W> Future for FlushFuture<'_, W>
where W: AsyncWrite + Unpin + ?Sized,

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impl<W> Future for WriteAllFuture<'_, W>
where W: AsyncWrite + Unpin + ?Sized,

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impl<W> Future for WriteFuture<'_, W>
where W: AsyncWrite + Unpin + ?Sized,

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impl<W> Future for WriteVectoredFuture<'_, W>
where W: AsyncWrite + Unpin + ?Sized,