Enum std::task::Poll

1.36.0 · source · []
pub enum Poll<T> {
    Ready(T),
    Pending,
}
Expand description

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

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));
Run

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);
Run

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);
Run
🔬 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(())
}
Run

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)));
Run

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)));
Run

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))));
Run

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))));
Run

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

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

Example
assert_eq!(Poll::from(true), Poll::Ready(true));
Run
🔬 This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from a compatible Residual type. Read more

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

Constructs the type from a compatible Residual type. Read more

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

Constructs the type from a compatible Residual type. Read more

Feeds this value into the given Hasher. Read more

Feeds a slice of this type into the given Hasher. Read more

This method returns an Ordering between self and other. Read more

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

Restrict a value to a certain interval. Read more

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

This method tests for !=.

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

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

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

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

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

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

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

🔬 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. Read more

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

Constructs the type from its Output type. Read more

🔬 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). Read more

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

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

🔬 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. Read more

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

Constructs the type from its Output type. Read more

🔬 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). Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Converts to this type from the input type.

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.