3 /// Used to tell an operation whether it should exit early or go on as usual.
5 /// This is used when exposing things (like graph traversals or visitors) where
6 /// you want the user to be able to choose whether to exit early.
7 /// Having the enum makes it clearer -- no more wondering "wait, what did `false`
8 /// mean again?" -- and allows including a value.
12 /// Early-exiting from [`Iterator::try_for_each`]:
14 /// #![feature(control_flow_enum)]
15 /// use std::ops::ControlFlow;
17 /// let r = (2..100).try_for_each(|x| {
19 /// return ControlFlow::Break(x)
22 /// ControlFlow::Continue(())
24 /// assert_eq!(r, ControlFlow::Break(13));
27 /// A basic tree traversal:
29 /// #![feature(control_flow_enum)]
30 /// use std::ops::ControlFlow;
32 /// pub struct TreeNode<T> {
34 /// left: Option<Box<TreeNode<T>>>,
35 /// right: Option<Box<TreeNode<T>>>,
38 /// impl<T> TreeNode<T> {
39 /// pub fn traverse_inorder<B>(&self, mut f: impl FnMut(&T) -> ControlFlow<B>) -> ControlFlow<B> {
40 /// if let Some(left) = &self.left {
41 /// left.traverse_inorder(&mut f)?;
44 /// if let Some(right) = &self.right {
45 /// right.traverse_inorder(&mut f)?;
47 /// ControlFlow::Continue(())
51 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
52 #[derive(Debug, Clone, Copy, PartialEq)]
53 pub enum ControlFlow<B, C = ()> {
54 /// Move on to the next phase of the operation as normal.
56 /// Exit the operation without running subsequent phases.
58 // Yes, the order of the variants doesn't match the type parameters.
59 // They're in this order so that `ControlFlow<A, B>` <-> `Result<B, A>`
60 // is a no-op conversion in the `Try` implementation.
63 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
64 impl<B, C> Try for ControlFlow<B, C> {
68 fn into_result(self) -> Result<Self::Ok, Self::Error> {
70 ControlFlow::Continue(y) => Ok(y),
71 ControlFlow::Break(x) => Err(x),
75 fn from_error(v: Self::Error) -> Self {
79 fn from_ok(v: Self::Ok) -> Self {
80 ControlFlow::Continue(v)
84 impl<B, C> ControlFlow<B, C> {
85 /// Returns `true` if this is a `Break` variant.
90 /// #![feature(control_flow_enum)]
91 /// use std::ops::ControlFlow;
93 /// assert!(ControlFlow::<i32, String>::Break(3).is_break());
94 /// assert!(!ControlFlow::<String, i32>::Continue(3).is_break());
97 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
98 pub fn is_break(&self) -> bool {
99 matches!(*self, ControlFlow::Break(_))
102 /// Returns `true` if this is a `Continue` variant.
107 /// #![feature(control_flow_enum)]
108 /// use std::ops::ControlFlow;
110 /// assert!(!ControlFlow::<i32, String>::Break(3).is_continue());
111 /// assert!(ControlFlow::<String, i32>::Continue(3).is_continue());
114 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
115 pub fn is_continue(&self) -> bool {
116 matches!(*self, ControlFlow::Continue(_))
119 /// Converts the `ControlFlow` into an `Option` which is `Some` if the
120 /// `ControlFlow` was `Break` and `None` otherwise.
125 /// #![feature(control_flow_enum)]
126 /// use std::ops::ControlFlow;
128 /// assert_eq!(ControlFlow::<i32, String>::Break(3).break_value(), Some(3));
129 /// assert_eq!(ControlFlow::<String, i32>::Continue(3).break_value(), None);
132 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
133 pub fn break_value(self) -> Option<B> {
135 ControlFlow::Continue(..) => None,
136 ControlFlow::Break(x) => Some(x),
140 /// Maps `ControlFlow<B, C>` to `ControlFlow<T, C>` by applying a function
141 /// to the break value in case it exists.
143 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
144 pub fn map_break<T, F>(self, f: F) -> ControlFlow<T, C>
149 ControlFlow::Continue(x) => ControlFlow::Continue(x),
150 ControlFlow::Break(x) => ControlFlow::Break(f(x)),
155 impl<R: Try> ControlFlow<R, R::Ok> {
156 /// Create a `ControlFlow` from any type implementing `Try`.
157 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
159 pub fn from_try(r: R) -> Self {
160 match Try::into_result(r) {
161 Ok(v) => ControlFlow::Continue(v),
162 Err(v) => ControlFlow::Break(Try::from_error(v)),
166 /// Convert a `ControlFlow` into any type implementing `Try`;
167 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
169 pub fn into_try(self) -> R {
171 ControlFlow::Continue(v) => Try::from_ok(v),
172 ControlFlow::Break(v) => v,
177 impl<B> ControlFlow<B, ()> {
178 /// It's frequently the case that there's no value needed with `Continue`,
179 /// so this provides a way to avoid typing `(())`, if you prefer it.
184 /// #![feature(control_flow_enum)]
185 /// use std::ops::ControlFlow;
187 /// let mut partial_sum = 0;
188 /// let last_used = (1..10).chain(20..25).try_for_each(|x| {
189 /// partial_sum += x;
190 /// if partial_sum > 100 { ControlFlow::Break(x) }
191 /// else { ControlFlow::CONTINUE }
193 /// assert_eq!(last_used.break_value(), Some(22));
195 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
196 pub const CONTINUE: Self = ControlFlow::Continue(());
199 impl<C> ControlFlow<(), C> {
200 /// APIs like `try_for_each` don't need values with `Break`,
201 /// so this provides a way to avoid typing `(())`, if you prefer it.
206 /// #![feature(control_flow_enum)]
207 /// use std::ops::ControlFlow;
209 /// let mut partial_sum = 0;
210 /// (1..10).chain(20..25).try_for_each(|x| {
211 /// if partial_sum > 100 { ControlFlow::BREAK }
212 /// else { partial_sum += x; ControlFlow::CONTINUE }
214 /// assert_eq!(partial_sum, 108);
216 #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
217 pub const BREAK: Self = ControlFlow::Break(());