Enum 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);Move on to the next phase of the operation as normal.
Exit the operation without running subsequent phases.
Source§
1.59.0 (const: unstable) · Source
Returns true if this is a Break variant.
§Examples
use std::ops::ControlFlow;
assert!(ControlFlow::<&str, i32>::Break("Stop right there!").is_break());
assert!(!ControlFlow::<&str, i32>::Continue(3).is_break());1.59.0 (const: unstable) · Source
Returns true if this is a Continue variant.
§Examples
use std::ops::ControlFlow;
assert!(!ControlFlow::<&str, i32>::Break("Stop right there!").is_continue());
assert!(ControlFlow::<&str, i32>::Continue(3).is_continue());1.83.0 (const: unstable) · Source
Converts the ControlFlow into an Option which is Some if the
ControlFlow was Break and None otherwise.
§Examples
use std::ops::ControlFlow;
assert_eq!(ControlFlow::<&str, i32>::Break("Stop right there!").break_value(), Some("Stop right there!"));
assert_eq!(ControlFlow::<&str, i32>::Continue(3).break_value(), None);Source 🔬This is a nightly-only experimental API. (control_flow_ok #140266)
control_flow_ok #140266)Converts the ControlFlow into an Result which is Ok if the
ControlFlow was Break and Err if otherwise.
§Examples
#![feature(control_flow_ok)]
use std::ops::ControlFlow;
struct TreeNode<T> {
value: T,
left: Option<Box<TreeNode<T>>>,
right: Option<Box<TreeNode<T>>>,
}
impl<T> TreeNode<T> {
fn find<'a>(&'a self, mut predicate: impl FnMut(&T) -> bool) -> Result<&'a T, ()> {
let mut f = |t: &'a T| -> ControlFlow<&'a T> {
if predicate(t) {
ControlFlow::Break(t)
} else {
ControlFlow::Continue(())
}
};
self.traverse_inorder(&mut f).break_ok()
}
fn traverse_inorder<'a, B>(
&'a self,
f: &mut impl FnMut(&'a 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 res = node.find(|val: &i32| *val > 3);
assert_eq!(res, Ok(&5));1.83.0 (const: unstable) · Source
Maps ControlFlow<B, C> to ControlFlow<T, C> by applying a function
to the break value in case it exists.
1.83.0 (const: unstable) · Source
Converts the ControlFlow into an Option which is Some if the
ControlFlow was Continue and None otherwise.
§Examples
use std::ops::ControlFlow;
assert_eq!(ControlFlow::<&str, i32>::Break("Stop right there!").continue_value(), None);
assert_eq!(ControlFlow::<&str, i32>::Continue(3).continue_value(), Some(3));Source 🔬This is a nightly-only experimental API. (control_flow_ok #140266)
control_flow_ok #140266)Converts the ControlFlow into an Result which is Ok if the
ControlFlow was Continue and Err if otherwise.
§Examples
#![feature(control_flow_ok)]
use std::ops::ControlFlow;
struct TreeNode<T> {
value: T,
left: Option<Box<TreeNode<T>>>,
right: Option<Box<TreeNode<T>>>,
}
impl<T> TreeNode<T> {
fn validate<B>(&self, f: &mut impl FnMut(&T) -> ControlFlow<B>) -> Result<(), B> {
self.traverse_inorder(f).continue_ok()
}
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 res = node.validate(&mut |val| {
if *val < 0 {
return ControlFlow::Break("negative value detected");
}
if *val > 4 {
return ControlFlow::Break("too big value detected");
}
ControlFlow::Continue(())
});
assert_eq!(res, Err("too big value detected"));Source§
Source 🔬This is a nightly-only experimental API. (control_flow_into_value #137461)
control_flow_into_value #137461)Extracts the value T that is wrapped by ControlFlow<T, T>.
§Examples
#![feature(control_flow_into_value)]
use std::ops::ControlFlow;
assert_eq!(ControlFlow::<i32, i32>::Break(1024).into_value(), 1024);
assert_eq!(ControlFlow::<i32, i32>::Continue(512).into_value(), 512);Source§
Source§
🔬This is a nightly-only experimental API. (try_trait_v2 #84277)
The type of the value produced by ? when not short-circuiting.
Source§
🔬This is a nightly-only experimental API. (try_trait_v2 #84277)
Constructs the type from its Output type. Read more
Source§
🔬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