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§

§

Continue(C)

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

§

Break(B)

Exit the operation without running subsequent phases.

Implementations§

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

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
🔬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
🔬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.

🔬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
🔬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.

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.

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

It’s frequently the case that there’s no value needed with Continue, so this provides a way to avoid typing (()), if you prefer it.

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

let mut partial_sum = 0;
let last_used = (1..10).chain(20..25).try_for_each(|x| {
    partial_sum += x;
    if partial_sum > 100 { ControlFlow::Break(x) }
    else { ControlFlow::CONTINUE }
});
assert_eq!(last_used.break_value(), Some(22));
Run
🔬This is a nightly-only experimental API. (control_flow_enum #75744)

APIs like try_for_each don’t need values with Break, so this provides a way to avoid typing (()), if you prefer it.

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

let mut partial_sum = 0;
(1..10).chain(20..25).try_for_each(|x| {
    if partial_sum > 100 { ControlFlow::BREAK }
    else { partial_sum += x; ControlFlow::CONTINUE }
});
assert_eq!(partial_sum, 108);
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
🔬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 tests for self and other values to be equal, and is used by ==.
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
🔬This is a nightly-only experimental API. (try_trait_v2_residual #91285)
The “return” type of this meta-function.
🔬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

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.