Poll in std::task - Rust
Description: Indicates whether a value is available or if the current task has been scheduled to receive a wakeup instead.

## Poll

# Enum 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.

This is returned by `Future::poll`.

## Variants§

§1.36.0

### Ready(T)

Represents that a value is immediately ready.

§1.36.0

### 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§

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### impl<T> Poll<T>

1.36.0 · Source

#### 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));

1.36.0 (const: 1.49.0) · Source

#### 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);

1.36.0 (const: 1.49.0) · Source

#### 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);

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

1.36.0 · Source

#### 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)));

1.36.0 · Source

#### 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, U>>` 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)));

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

1.51.0 · Source

#### 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))));

1.51.0 · Source

#### 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§

1.36.0 · Source§

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

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

Returns a duplicate of the value. Read more
1.0.0 · Source§

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

Performs copy-assignment from `source`. Read more
1.36.0 · Source§

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

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

Formats the value using the given formatter. Read more
1.36.0 (const: unstable) · Source§

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

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#### 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));

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

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#### 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. Read more
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### impl<T, E, F> FromResidual<Result<Infallible, E>> for Poll<Result<T, F>>
where F: From<E>,

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#### 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. Read more
1.36.0 · Source§

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

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

Feeds this value into the given `Hasher`. Read more
1.3.0 · Source§

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

Feeds a slice of this type into the given `Hasher`. Read more
1.36.0 · Source§

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

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

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · Source§

#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
1.21.0 · Source§

#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
1.50.0 · Source§

#### fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized,

Restrict a value to a certain interval. Read more
1.36.0 · Source§

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

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

Tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · Source§

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

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.36.0 · Source§

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

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

This method returns an ordering between `self` and `other` values if one exists. Read more
1.0.0 · Source§

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

Tests less than (for `self` and `other`) and is used by the `<` operator. Read more
1.0.0 · Source§

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

Tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
1.0.0 · Source§

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

Tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
1.0.0 · Source§

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

Tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
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### impl<T, E> Try for Poll<Option<Result<T, E>>>

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#### 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.
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#### 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. Read more
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#### 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. Read more
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#### 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`). Read more
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### impl<T, E> Try for Poll<Result<T, E>>

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#### 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.
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#### 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. Read more
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#### 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. Read more
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#### 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`). Read more
1.36.0 · Source§

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

1.36.0 · Source§

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

1.36.0 · Source§

### impl<T> StructuralPartialEq for Poll<T>

## Auto Trait Implementations§

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### impl<T> Freeze for Poll<T>
where T: Freeze,

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### impl<T> RefUnwindSafe for Poll<T>
where T: RefUnwindSafe,

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### impl<T> Send for Poll<T>
where T: Send,

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### impl<T> Sync for Poll<T>
where T: Sync,

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### impl<T> Unpin for Poll<T>
where T: Unpin,

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### impl<T> UnwindSafe for Poll<T>
where T: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<!> for T

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#### fn from(t: !) -> T

Converts to this type from the input type.
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

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

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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.