Enum core::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));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);Variants
Continue(C)
Move on to the next phase of the operation as normal.
Break(B)
Exit the operation without running subsequent phases.
Implementations
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);Maps ControlFlow<B, C> to ControlFlow<T, C> by applying a function
to the break value in case it exists.
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));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);Trait Implementations
This method tests for self and other values to be equal, and is used
by ==. Read more
This method tests for !=.
type Residual = ControlFlow<B, Infallible>
type Residual = ControlFlow<B, Infallible>
The type of the value passed to FromResidual::from_residual
as part of ? when short-circuiting. Read more
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