1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
use crate::{convert, ops};

/// 用于告诉操作是应该提前退出还是像往常一样继续操作。
///
/// 在将您希望用户能够选择是否提前退出的事物 (例如图形遍历或访问者) 公开时使用。
/// 有了枚举可以使它更清晰 - 不必再奇怪 "wait, what did `false` mean again?" 了 - 并允许包含一个值。
///
/// 与 [`Option`] 和 [`Result`] 类似,此枚举可与 `?` 运算符一起使用,以便在 [`Break`] 变体存在时立即返回,或者以其他方式正常继续使用 [`Continue`] 变体中的值。
///
///
/// # Examples
///
/// 从 [`Iterator::try_for_each`] 提前退出:
///
/// ```
/// 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));
/// ```
///
/// 一个基本的树遍历:
///
/// ```
/// 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);
/// ```
///
/// [`Break`]: ControlFlow::Break
/// [`Continue`]: ControlFlow::Continue
///
#[stable(feature = "control_flow_enum_type", since = "1.55.0")]
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ControlFlow<B, C = ()> {
    /// 照常进行下一阶段的操作。
    #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
    #[lang = "Continue"]
    Continue(C),
    /// 退出操作而不运行后续阶段。
    #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
    #[lang = "Break"]
    Break(B),
    // 是的,变体的顺序与类型参数不匹配。
    // 它们是按此顺序排列的,因此 `ControlFlow<A, B>` <-> `Result<B, A>` 是 `Try` 实现中的无操作转换。
    //
}

#[unstable(feature = "try_trait_v2", issue = "84277")]
impl<B, C> ops::Try for ControlFlow<B, C> {
    type Output = C;
    type Residual = ControlFlow<B, convert::Infallible>;

    #[inline]
    fn from_output(output: Self::Output) -> Self {
        ControlFlow::Continue(output)
    }

    #[inline]
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
        match self {
            ControlFlow::Continue(c) => ControlFlow::Continue(c),
            ControlFlow::Break(b) => ControlFlow::Break(ControlFlow::Break(b)),
        }
    }
}

#[unstable(feature = "try_trait_v2", issue = "84277")]
impl<B, C> ops::FromResidual for ControlFlow<B, C> {
    #[inline]
    fn from_residual(residual: ControlFlow<B, convert::Infallible>) -> Self {
        match residual {
            ControlFlow::Break(b) => ControlFlow::Break(b),
        }
    }
}

impl<B, C> ControlFlow<B, C> {
    /// 如果这是 `Break` 变体,则返回 `true`。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// assert!(ControlFlow::<i32, String>::Break(3).is_break());
    /// assert!(!ControlFlow::<String, i32>::Continue(3).is_break());
    /// ```
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn is_break(&self) -> bool {
        matches!(*self, ControlFlow::Break(_))
    }

    /// 如果这是 `Continue` 变体,则返回 `true`。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// assert!(!ControlFlow::<i32, String>::Break(3).is_continue());
    /// assert!(ControlFlow::<String, i32>::Continue(3).is_continue());
    /// ```
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn is_continue(&self) -> bool {
        matches!(*self, ControlFlow::Continue(_))
    }

    /// 如果 `ControlFlow` 为 `Break`,则将 `ControlFlow` 转换为 `Some`,否则为 `None`。
    ///
    ///
    /// # 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);
    /// ```
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn break_value(self) -> Option<B> {
        match self {
            ControlFlow::Continue(..) => None,
            ControlFlow::Break(x) => Some(x),
        }
    }

    /// Maps `ControlFlow<B, C>` 到 `ControlFlow<T, C>` 通过在中断值 (如果存在) 上应用函数来实现。
    ///
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn map_break<T, F>(self, f: F) -> ControlFlow<T, C>
    where
        F: FnOnce(B) -> T,
    {
        match self {
            ControlFlow::Continue(x) => ControlFlow::Continue(x),
            ControlFlow::Break(x) => ControlFlow::Break(f(x)),
        }
    }
}

/// 这些仅用作实现迭代器适配器的一部分。
/// 它们具有普通的名称和不明显的语义,因此目前还没有走上潜在的稳定之路。
///
impl<R: ops::Try> ControlFlow<R, R::Output> {
    /// 从实现 `Try` 的任何类型创建 `ControlFlow`。
    #[inline]
    pub(crate) fn from_try(r: R) -> Self {
        match R::branch(r) {
            ControlFlow::Continue(v) => ControlFlow::Continue(v),
            ControlFlow::Break(v) => ControlFlow::Break(R::from_residual(v)),
        }
    }

    /// 将 `ControlFlow` 转换为实现 `Try` 的任何类型;
    #[inline]
    pub(crate) fn into_try(self) -> R {
        match self {
            ControlFlow::Continue(v) => R::from_output(v),
            ControlFlow::Break(v) => v,
        }
    }
}

impl<B> ControlFlow<B, ()> {
    /// 通常,`Continue` 不需要任何值,因此,如果愿意,这提供了一种避免输入 `(())` 的方法。
    ///
    ///
    /// # 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));
    /// ```
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub const CONTINUE: Self = ControlFlow::Continue(());
}

impl<C> ControlFlow<(), C> {
    /// 像 `try_for_each` 这样的 API 不需要 `Break` 的值,因此,如果您愿意的话,这提供了一种避免输入 `(())` 的方法。
    ///
    ///
    /// # 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);
    /// ```
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub const BREAK: Self = ControlFlow::Break(());
}