Trait async_std::future::Future

#[must_use = "futures do nothing unless you `.await` or poll them"]
#[lang = "future_trait"]pub trait Future {
    type Output;
#[lang = "poll"]    pub fn poll(
        self: Pin<&mut Self>, 
        cx: &mut Context<'_>
    ) -> Poll<Self::Output>;
}

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 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.

Associated Types

type Output

The type of value produced on completion.

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Required methods

#[lang = "poll"]pub 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:

• Poll::Pending if the future is not ready yet
• Poll::Ready(val) with the result val of this future if it finished successfully.

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.

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Implementations on Foreign Types

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

type Output = <F as Future>::Output

pub fn poll(
    self: Pin<&mut AssertUnwindSafe<F>>,
    cx: &mut Context<'_>
) -> Poll<<AssertUnwindSafe<F> as Future>::Output>

impl<T> Future for Ready<T>

type Output = T

pub fn poll(self: Pin<&mut Ready<T>>, _cx: &mut Context<'_>) -> Poll<T>

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

type Output = T

pub fn poll(self: Pin<&mut PollFn<F>>, cx: &mut Context<'_>) -> Poll<T>

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

type Output = <F as Future>::Output

pub fn poll(
    self: Pin<&mut &'_ mut F>,
    cx: &mut Context<'_>
) -> Poll<<&'_ mut F as Future>::Output>

impl<T> Future for Pending<T>

type Output = T

pub fn poll(self: Pin<&mut Pending<T>>, &mut Context<'_>) -> Poll<T>

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

type Output = <F as Future>::Output

pub fn poll(
    self: Pin<&mut Box<F, A>>,
    cx: &mut Context<'_>
) -> Poll<<Box<F, A> as Future>::Output>

impl Future for EventListener

type Output = ()

pub fn poll(
    self: Pin<&mut EventListener>,
    cx: &mut Context<'_>
) -> Poll<<EventListener as Future>::Output>

impl Future for Timer

type Output = Instant

pub fn poll(
    self: Pin<&mut Timer>,
    cx: &mut Context<'_>
) -> Poll<<Timer as Future>::Output>

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

type Output = bool

pub fn poll(
    self: Pin<&mut AllFuture<'_, S, P>>,
    cx: &mut Context<'_>
) -> Poll<<AllFuture<'_, S, P> as Future>::Output>

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

type Output = Option<usize>

pub fn poll(
    self: Pin<&mut PositionFuture<'a, S, P>>,
    cx: &mut Context<'_>
) -> Poll<<PositionFuture<'a, S, P> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut FindFuture<'a, S, P>>,
    cx: &mut Context<'_>
) -> Poll<<FindFuture<'a, S, P> as Future>::Output>

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

type Output = T

pub fn poll(self: Pin<&mut PollFn<F>>, cx: &mut Context<'_>) -> Poll<T>

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

type Output = ()

pub fn poll(
    self: Pin<&mut ForEachFuture<S, F>>,
    cx: &mut Context<'_>
) -> Poll<<ForEachFuture<S, F> as Future>::Output>

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

type Output = Result<(), Error>

pub fn poll(
    self: Pin<&mut CloseFuture<'_, W>>,
    cx: &mut Context<'_>
) -> Poll<<CloseFuture<'_, W> as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut ReadToStringFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadToStringFuture<'_, R> as Future>::Output>

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

type Output = T

pub fn poll(
    self: Pin<&mut Race<F1, F2>>,
    cx: &mut Context<'_>
) -> Poll<<Race<F1, F2> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut Zip<F1, F2>>,
    cx: &mut Context<'_>
) -> Poll<<Zip<F1, F2> as Future>::Output>

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

type Output = usize

pub fn poll(
    self: Pin<&mut CountFuture<S>>,
    cx: &mut Context<'_>
) -> Poll<<CountFuture<S> as Future>::Output>

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

type Output = Result<C, E>

pub fn poll(
    self: Pin<&mut TryCollectFuture<S, C>>,
    cx: &mut Context<'_>
) -> Poll<<TryCollectFuture<S, C> as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut ReadLineFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadLineFuture<'_, R> as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut ReadUntilFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadUntilFuture<'_, R> as Future>::Output>

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

type Output = Option<B>

pub fn poll(
    self: Pin<&mut FindMapFuture<'a, S, F>>,
    cx: &mut Context<'_>
) -> Poll<<FindMapFuture<'a, S, F> as Future>::Output>

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

type Output = T

pub fn poll(
    self: Pin<&mut FoldFuture<S, F, T>>,
    cx: &mut Context<'_>
) -> Poll<<FoldFuture<S, F, T> as Future>::Output>

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

type Output = Option<T>

pub fn poll(
    self: Pin<&mut PollOnce<F>>,
    cx: &mut Context<'_>
) -> Poll<<PollOnce<F> as Future>::Output>

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

type Output = (B, B)

pub fn poll(
    self: Pin<&mut PartitionFuture<S, P, B>>,
    cx: &mut Context<'_>
) -> Poll<<PartitionFuture<S, P, B> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut NextFuture<'_, S>>,
    cx: &mut Context<'_>
) -> Poll<<NextFuture<'_, S> as Future>::Output>

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

type Output = Result<(), Error>

pub fn poll(
    self: Pin<&mut FlushFuture<'_, W>>,
    cx: &mut Context<'_>
) -> Poll<<FlushFuture<'_, W> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut NthFuture<'a, S>>,
    cx: &mut Context<'_>
) -> Poll<<NthFuture<'a, S> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut FillBuf<'a, R>>,
    cx: &mut Context<'_>
) -> Poll<<FillBuf<'a, R> as Future>::Output>

impl Future for YieldNow

type Output = ()

pub fn poll(
    self: Pin<&mut YieldNow>,
    cx: &mut Context<'_>
) -> Poll<<YieldNow as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut ReadToEndFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadToEndFuture<'_, R> as Future>::Output>

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

type Output = Result<(), Error>

pub fn poll(
    self: Pin<&mut WriteAllFuture<'_, W>>,
    cx: &mut Context<'_>
) -> Poll<<WriteAllFuture<'_, W> as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut ReadFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadFuture<'_, R> as Future>::Output>

impl<T> Future for Pending<T>

type Output = T

pub fn poll(self: Pin<&mut Pending<T>>, &mut Context<'_>) -> Poll<T>

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

type Output = Result<B, E>

pub fn poll(
    self: Pin<&mut TryFoldFuture<'a, S, F, B>>,
    cx: &mut Context<'_>
) -> Poll<<TryFoldFuture<'a, S, F, B> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut TryNextFuture<'_, S>>,
    cx: &mut Context<'_>
) -> Poll<<TryNextFuture<'_, S> as Future>::Output>

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

type Output = C

pub fn poll(
    self: Pin<&mut CollectFuture<S, C>>,
    cx: &mut Context<'_>
) -> Poll<C>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut ReadVectoredFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadVectoredFuture<'_, R> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut CatchUnwind<F>>,
    cx: &mut Context<'_>
) -> Poll<<CatchUnwind<F> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut LastFuture<S>>,
    cx: &mut Context<'_>
) -> Poll<<LastFuture<S> as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut WriteFuture<'_, W>>,
    cx: &mut Context<'_>
) -> Poll<<WriteFuture<'_, W> as Future>::Output>

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

type Output = T

pub fn poll(
    self: Pin<&mut Or<F1, F2>>,
    cx: &mut Context<'_>
) -> Poll<<Or<F1, F2> as Future>::Output>

impl<T> Future for Ready<T>

type Output = T

pub fn poll(self: Pin<&mut Ready<T>>, _cx: &mut Context<'_>) -> Poll<T>

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

type Output = Result<(), E>

pub fn poll(
    self: Pin<&mut TryForEachFuture<'a, S, F>>,
    cx: &mut Context<'_>
) -> Poll<<TryForEachFuture<'a, S, F> as Future>::Output>

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

type Output = Result<(T1, T2), E>

pub fn poll(
    self: Pin<&mut TryZip<F1, F2>>,
    cx: &mut Context<'_>
) -> Poll<<TryZip<F1, F2> as Future>::Output>

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

type Output = Result<(), Error>

pub fn poll(
    self: Pin<&mut ReadExactFuture<'_, R>>,
    cx: &mut Context<'_>
) -> Poll<<ReadExactFuture<'_, R> as Future>::Output>

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

type Output = Result<usize, Error>

pub fn poll(
    self: Pin<&mut WriteVectoredFuture<'_, W>>,
    cx: &mut Context<'_>
) -> Poll<<WriteVectoredFuture<'_, W> as Future>::Output>

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

type Output = (FromA, FromB)

pub fn poll(
    self: Pin<&mut UnzipFuture<S, FromA, FromB>>,
    cx: &mut Context<'_>
) -> Poll<<UnzipFuture<S, FromA, FromB> as Future>::Output>

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

type Output = Result<u64, Error>

pub fn poll(
    self: Pin<&mut SeekFuture<'_, S>>,
    cx: &mut Context<'_>
) -> Poll<<SeekFuture<'_, S> as Future>::Output>

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

type Output = bool

pub fn poll(
    self: Pin<&mut AnyFuture<'_, S, P>>,
    cx: &mut Context<'_>
) -> Poll<<AnyFuture<'_, S, P> as Future>::Output>

impl<T> Future for Task<T>

type Output = T

pub fn poll(
    self: Pin<&mut Task<T>>,
    cx: &mut Context<'_>
) -> Poll<<Task<T> as Future>::Output>

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Implementors

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

type Output = Result<T, RecvError>

pub fn poll(
    self: Pin<&mut Recv<'a, T>>,
    cx: &mut Context<'_>
) -> Poll<<Recv<'a, T> as Future>::Output>

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

type Output = Result<(), SendError<T>>

pub fn poll(
    self: Pin<&mut Send<'a, T>>,
    cx: &mut Context<'_>
) -> Poll<<Send<'a, T> as Future>::Output>

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

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

pub fn poll(
    self: Pin<&mut Pin<P>>,
    cx: &mut Context<'_>
) -> Poll<<Pin<P> as Future>::Output>

impl<T> Future for JoinHandle<T>

type Output = T

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

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