ControlFlow in std::ops - Rust (original) (raw)

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 falsemean 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.

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

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

Exit the operation without running subsequent phases.

Returns true if this is a Break variant.

#![feature(control_flow_enum)]
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.

#![feature(control_flow_enum)]
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 theControlFlow was Break and None otherwise.

#![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)

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.

#![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.

#![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

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

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

This method tests for !=.

🔬 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)

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

impl Any for T where

T: 'static + ?Sized,

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

impl From for T

impl<T, U> Into for T where

U: From,

The resulting type after obtaining ownership.

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

🔬 This is a nightly-only experimental API. (toowned_clone_into #41263)

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.