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 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
method can provide a
Waker, which is a handle for waking up the current
When using a future, you generally won't call
poll directly, but instead
.await the value.
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Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.
This function returns:
Poll::Pendingif the future is not ready yet
Poll::Ready(val)with the result
valof this future if it finished successfully.
Once a future has finished, clients should not
poll it again.
When a future is not ready yet,
stores a clone of the
Waker copied from the current
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.
Futures alone are inert; they must be actively
polled to make
progress, meaning that each time the current task is woken up, it should
poll pending futures that it still has an interest in.
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
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
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
Once a future has completed (returned
poll), calling its
poll method again may panic, block forever, or cause other kinds of
Future trait places no requirements on the effects of
such a call. However, as the
poll method is not marked
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.