# Enum std::ops::ControlFlow

1.55.0 · source ·

    pub enum ControlFlow<B, C = ()> {
        Continue(C),
        Break(B),
    }

Expand description

Used to tell an operation whether it should exit early or go on as usual.

This is used when exposing things (like graph traversals or visitors) where you want the user to be able to choose whether to exit early. Having the enum makes it clearer – no more wondering “wait, what did `false` mean again?” – and allows including a value.

Similar to `Option` and `Result`, this enum can be used with the `?` operator to return immediately if the `Break` variant is present or otherwise continue normally with the value inside the `Continue` variant.

## Examples

Early-exiting from `Iterator::try_for_each`:

    use std::ops::ControlFlow;

    let r = (2..100).try_for_each(|x| {
        if 403 % x == 0 {
            return ControlFlow::Break(x)
        }

        ControlFlow::Continue(())
    });
    assert_eq!(r, ControlFlow::Break(13));

Run

A basic tree traversal:

    use std::ops::ControlFlow;

    pub struct TreeNode<T> {
        value: T,
        left: Option<Box<TreeNode<T>>>,
        right: Option<Box<TreeNode<T>>>,
    }

    impl<T> TreeNode<T> {
        pub fn traverse_inorder<B>(&self, f: &mut impl FnMut(&T) -> ControlFlow<B>) -> ControlFlow<B> {
            if let Some(left) = &self.left {
                left.traverse_inorder(f)?;
            }
            f(&self.value)?;
            if let Some(right) = &self.right {
                right.traverse_inorder(f)?;
            }
            ControlFlow::Continue(())
        }
        fn leaf(value: T) -> Option<Box<TreeNode<T>>> {
            Some(Box::new(Self { value, left: None, right: None }))
        }
    }

    let node = TreeNode {
        value: 0,
        left: TreeNode::leaf(1),
        right: Some(Box::new(TreeNode {
            value: -1,
            left: TreeNode::leaf(5),
            right: TreeNode::leaf(2),
        }))
    };
    let mut sum = 0;

    let res = node.traverse_inorder(&mut |val| {
        if *val < 0 {
            ControlFlow::Break(*val)
        } else {
            sum += *val;
            ControlFlow::Continue(())
        }
    });
    assert_eq!(res, ControlFlow::Break(-1));
    assert_eq!(sum, 6);

Run

## Variants§

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### Continue(C)

Move on to the next phase of the operation as normal.

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### Break(B)

Exit the operation without running subsequent phases.

## Implementations§

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### impl<B, C> ControlFlow<B, C>

1.59.0 · source

#### pub fn is_break(&self) -> bool

Returns `true` if this is a `Break` variant.

##### Examples

    use std::ops::ControlFlow;

    assert!(ControlFlow::<i32, String>::Break(3).is_break());
    assert!(!ControlFlow::<String, i32>::Continue(3).is_break());

Run
1.59.0 · source

#### pub fn is_continue(&self) -> bool

Returns `true` if this is a `Continue` variant.

##### Examples

    use std::ops::ControlFlow;

    assert!(!ControlFlow::<i32, String>::Break(3).is_continue());
    assert!(ControlFlow::<String, i32>::Continue(3).is_continue());

Run
source

#### pub fn break_value(self) -> Option<B>

🔬This is a nightly-only experimental API. (`control_flow_enum` #75744)

Converts the `ControlFlow` into an `Option` which is `Some` if the `ControlFlow` was `Break` and `None` otherwise.

##### Examples

    #![feature(control_flow_enum)]
    use std::ops::ControlFlow;

    assert_eq!(ControlFlow::<i32, String>::Break(3).break_value(), Some(3));
    assert_eq!(ControlFlow::<String, i32>::Continue(3).break_value(), None);

Run
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#### pub fn map_break<T, F>(self, f: F) -> ControlFlow<T, C>where F: FnOnce(B) -> T,

🔬This is a nightly-only experimental API. (`control_flow_enum` #75744)

Maps `ControlFlow<B, C>` to `ControlFlow<T, C>` by applying a function to the break value in case it exists.

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#### pub fn continue_value(self) -> Option<C>

🔬This is a nightly-only experimental API. (`control_flow_enum` #75744)

Converts the `ControlFlow` into an `Option` which is `Some` if the `ControlFlow` was `Continue` and `None` otherwise.

##### Examples

    #![feature(control_flow_enum)]
    use std::ops::ControlFlow;

    assert_eq!(ControlFlow::<i32, String>::Break(3).continue_value(), None);
    assert_eq!(ControlFlow::<String, i32>::Continue(3).continue_value(), Some(3));

Run
source

#### pub fn map_continue<T, F>(self, f: F) -> ControlFlow<B, T>where F: FnOnce(C) -> T,

🔬This is a nightly-only experimental API. (`control_flow_enum` #75744)

Maps `ControlFlow<B, C>` to `ControlFlow<B, T>` by applying a function to the continue value in case it exists.

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### impl<R> ControlFlow<R, <R as Try>::Output>where R: Try,

This impl block contains no items.

These are used only as part of implementing the iterator adapters. They have mediocre names and non-obvious semantics, so aren’t currently on a path to potential stabilization.

## Trait Implementations§

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### impl<B, C> Clone for ControlFlow<B, C>where B: Clone, C: Clone,

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#### fn clone(&self) -> ControlFlow<B, C>

Returns a copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from `source`. Read more
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### impl<B, C> Debug for ControlFlow<B, C>where B: Debug, C: Debug,

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

Formats the value using the given formatter. Read more
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### impl<B, C> FromResidual for ControlFlow<B, C>

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#### fn from_residual(residual: ControlFlow<B, Infallible>) -> ControlFlow<B, C>

🔬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<B, C> Hash for ControlFlow<B, C>where B: Hash, C: 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
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### impl<B, C> PartialEq for ControlFlow<B, C>where B: PartialEq, C: PartialEq,

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#### fn eq(&self, other: &ControlFlow<B, C>) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · source§

#### 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.
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### impl<B, C> Residual<C> for ControlFlow<B, Infallible>

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#### type TryType = ControlFlow<B, C>

🔬This is a nightly-only experimental API. (`try_trait_v2_residual` #91285)
The “return” type of this meta-function.
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### impl<B, C> Try for ControlFlow<B, C>

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#### type Output = C

🔬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 = ControlFlow<B, Infallible>

🔬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(output: <ControlFlow<B, C> as Try>::Output) -> ControlFlow<B, C>

🔬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<<ControlFlow<B, C> as Try>::Residual, <ControlFlow<B, C> 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<B, C> Copy for ControlFlow<B, C>where B: Copy, C: Copy,

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### impl<B, C> Eq for ControlFlow<B, C>where B: Eq, C: Eq,

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### impl<B, C> StructuralEq for ControlFlow<B, C>

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### impl<B, C> StructuralPartialEq for ControlFlow<B, C>

## Auto Trait Implementations§

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### impl<B, C> RefUnwindSafe for ControlFlow<B, C>where B: RefUnwindSafe, C: RefUnwindSafe,

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### impl<B, C> Send for ControlFlow<B, C>where B: Send, C: Send,

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### impl<B, C> Sync for ControlFlow<B, C>where B: Sync, C: Sync,

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### impl<B, C> Unpin for ControlFlow<B, C>where B: Unpin, C: Unpin,

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### impl<B, C> UnwindSafe for ControlFlow<B, C>where B: UnwindSafe, C: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for Twhere 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 Twhere 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 Twhere 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> 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 Twhere 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 Twhere 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 Twhere 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 Twhere 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.