Enum std::task::Poll

pub enum Poll<T> {
    Ready(T),
    Pending,
}

Indicates whether a value is available or if the current task has been scheduled to receive a wakeup instead.

Variants

Ready(T)

Represents that a value is immediately ready.

Pending

Represents that a value is not ready yet.

When a function returns Pending, the function must also ensure that the current task is scheduled to be awoken when progress can be made.

Implementations

impl<T> Poll<T>

pub fn map<U, F>(self, f: F) -> Poll<U> where
    F: FnOnce(T) -> U, 

Maps a Poll<T> to Poll<U> by applying a function to a contained value.

Examples

Converts a Poll<String> into a Poll<usize>, consuming the original:

let poll_some_string = Poll::Ready(String::from("Hello, World!"));
// `Poll::map` takes self *by value*, consuming `poll_some_string`
let poll_some_len = poll_some_string.map(|s| s.len());

assert_eq!(poll_some_len, Poll::Ready(13));

pub const fn is_ready(&self) -> bool

Returns true if the poll is a Poll::Ready value.

Examples

let x: Poll<u32> = Poll::Ready(2);
assert_eq!(x.is_ready(), true);

let x: Poll<u32> = Poll::Pending;
assert_eq!(x.is_ready(), false);

pub const fn is_pending(&self) -> bool

Returns true if the poll is a Pending value.

Examples

let x: Poll<u32> = Poll::Ready(2);
assert_eq!(x.is_pending(), false);

let x: Poll<u32> = Poll::Pending;
assert_eq!(x.is_pending(), true);

pub fn ready(self) -> Ready<T>

🔬This is a nightly-only experimental API. (poll_ready #89780)

Extracts the successful type of a Poll<T>.

When combined with the ? operator, this function will propagate any Poll::Pending values to the caller, and extract the T from Poll::Ready.

Examples

#![feature(poll_ready)]

use std::task::{Context, Poll};
use std::future::{self, Future};
use std::pin::Pin;

pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
    let mut fut = future::ready(42);
    let fut = Pin::new(&mut fut);

    let num = fut.poll(cx).ready()?;
    // ... use num

    Poll::Ready(())
}

impl<T, E> Poll<Result<T, E>>

pub fn map_ok<U, F>(self, f: F) -> Poll<Result<U, E>> where
    F: FnOnce(T) -> U, 

Maps a Poll<Result<T, E>> to Poll<Result<U, E>> by applying a function to a contained Poll::Ready(Ok) value, leaving all other variants untouched.

This function can be used to compose the results of two functions.

Examples

let res: Poll<Result<u8, _>> = Poll::Ready("12".parse());
let squared = res.map_ok(|n| n * n);
assert_eq!(squared, Poll::Ready(Ok(144)));

pub fn map_err<U, F>(self, f: F) -> Poll<Result<T, U>> where
    F: FnOnce(E) -> U, 

Maps a Poll::Ready<Result<T, E>> to Poll::Ready<Result<T, F>> by applying a function to a contained Poll::Ready(Err) value, leaving all other variants untouched.

This function can be used to pass through a successful result while handling an error.

Examples

let res: Poll<Result<u8, _>> = Poll::Ready("oops".parse());
let res = res.map_err(|_| 0_u8);
assert_eq!(res, Poll::Ready(Err(0)));

impl<T, E> Poll<Option<Result<T, E>>>

pub fn map_ok<U, F>(self, f: F) -> Poll<Option<Result<U, E>>> where
    F: FnOnce(T) -> U, 

Maps a Poll<Option<Result<T, E>>> to Poll<Option<Result<U, E>>> by applying a function to a contained Poll::Ready(Some(Ok)) value, leaving all other variants untouched.

This function can be used to compose the results of two functions.

Examples

let res: Poll<Option<Result<u8, _>>> = Poll::Ready(Some("12".parse()));
let squared = res.map_ok(|n| n * n);
assert_eq!(squared, Poll::Ready(Some(Ok(144))));

pub fn map_err<U, F>(self, f: F) -> Poll<Option<Result<T, U>>> where
    F: FnOnce(E) -> U, 

Maps a Poll::Ready<Option<Result<T, E>>> to Poll::Ready<Option<Result<T, F>>> by applying a function to a contained Poll::Ready(Some(Err)) value, leaving all other variants untouched.

This function can be used to pass through a successful result while handling an error.

Examples

let res: Poll<Option<Result<u8, _>>> = Poll::Ready(Some("oops".parse()));
let res = res.map_err(|_| 0_u8);
assert_eq!(res, Poll::Ready(Some(Err(0))));

Trait Implementations

impl<T> Clone for Poll<T> where
    T: Clone, 

fn clone(&self) -> Poll<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Debug for Poll<T> where
    T: Debug, 

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> From<T> for Poll<T>

fn from(t: T) -> Poll<T>

Moves the value into a Poll::Ready to make a Poll<T>.

Example

assert_eq!(Poll::from(true), Poll::Ready(true));

impl<T> FromResidual<Ready<Infallible>> for Poll<T>

fn from_residual(residual: Ready<Infallible>) -> Poll<T>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from a compatible Residual type.

impl<T, E, F> FromResidual<Result<Infallible, E>> for Poll<Option<Result<T, F>>> where
    F: From<E>, 

fn from_residual(x: Result<Infallible, E>) -> Poll<Option<Result<T, F>>>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from a compatible Residual type.

impl<T, E, F> FromResidual<Result<Infallible, E>> for Poll<Result<T, F>> where
    F: From<E>, 

fn from_residual(x: Result<Infallible, E>) -> Poll<Result<T, F>>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from a compatible Residual type.

impl<T> Hash for Poll<T> where
    T: Hash, 

fn hash<__H>(&self, state: &mut __H) where
    __H: Hasher, 

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl<T> Ord for Poll<T> where
    T: Ord, 

fn cmp(&self, other: &Poll<T>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self where
    Self: PartialOrd<Self>, 

Restrict a value to a certain interval.

impl<T> PartialEq<Poll<T>> for Poll<T> where
    T: PartialEq<T>, 

fn eq(&self, other: &Poll<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T> PartialOrd<Poll<T>> for Poll<T> where
    T: PartialOrd<T>, 

fn partial_cmp(&self, other: &Poll<T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T, E> Try for Poll<Option<Result<T, E>>>

type Output = Poll<Option<T>>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

The type of the value produced by ? when not short-circuiting.

type Residual = Result<Infallible, E>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

The type of the value passed to FromResidual::from_residual as part of ? when short-circuiting.

fn from_output(
    c: <Poll<Option<Result<T, E>>> as Try>::Output
) -> Poll<Option<Result<T, E>>>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from its Output type.

fn branch(
    self
) -> ControlFlow<<Poll<Option<Result<T, E>>> as Try>::Residual, <Poll<Option<Result<T, E>>> as Try>::Output>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Used in ? to decide whether the operator should produce a value (because this returned ControlFlow::Continue) or propagate a value back to the caller (because this returned ControlFlow::Break).

impl<T, E> Try for Poll<Result<T, E>>

type Output = Poll<T>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

The type of the value produced by ? when not short-circuiting.

type Residual = Result<Infallible, E>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

The type of the value passed to FromResidual::from_residual as part of ? when short-circuiting.

fn from_output(c: <Poll<Result<T, E>> as Try>::Output) -> Poll<Result<T, E>>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from its Output type.

fn branch(
    self
) -> ControlFlow<<Poll<Result<T, E>> as Try>::Residual, <Poll<Result<T, E>> as Try>::Output>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Used in ? to decide whether the operator should produce a value (because this returned ControlFlow::Continue) or propagate a value back to the caller (because this returned ControlFlow::Break).

impl<T> Copy for Poll<T> where
    T: Copy, 

impl<T> Eq for Poll<T> where
    T: Eq, 

impl<T> StructuralEq for Poll<T>

impl<T> StructuralPartialEq for Poll<T>