如何通过枚举变体“切换”？

Question

我想编写一个函数，将提供的值切换/切换到枚举中的下一个值并在末尾环绕：

enum Direction { NORTH, SOUTH, EAST, WEST }

例如，NORTH => SOUTH、SOUTH => EAST、EAST => WEST、WEST => NORTH。

有没有比In Rust, is there a way to iterate through the values of an enum? 中描述的手动创建静态数组更简单的方法

use Direction::*;
static DIRECTIONS: [Direction; 4] = [NORTH, SOUTH, EAST, WEST];

枚举不应该被“枚举”吗？我依稀记得以前在 Rust 中看到过一个例子，但我似乎找不到它。由于 Rust 枚举更像联合/变体，我想这会使事情复杂化。

Answer 1

Vladimir 的回答是正确的，但程序员必须记住在向enum 添加新成员时更改幻数“4”。 turn 的这个定义应该更容易维护：

#[derive(Debug, Copy, Clone, FromPrimitive)]
enum Direction {
    NORTH = 0,
    SOUTH,
    EAST,
    WEST,
}

fn turn(d: Direction) -> Direction {
    match FromPrimitive::from_u8(d as u8 + 1) {
        Some(d2) => d2,
        None => FromPrimitive::from_u8(0).unwrap(),
    }
}

Answer 2

我想这样的事情可以解决问题：

#[macro_use]
extern crate num_derive;
extern crate num_traits;

use num_traits::FromPrimitive;

#[derive(Debug, Copy, Clone, FromPrimitive)]
enum Direction {
    NORTH = 0,
    SOUTH,
    EAST,
    WEST,
}

fn turn(d: Direction) -> Direction {
    FromPrimitive::from_u8((d as u8 + 1) % 4).unwrap()
}

fn main() {
    use Direction::*;
    for &d in [NORTH, SOUTH, EAST, WEST].iter() {
        println!("{:?} -> {:?}", d, turn(d));
    }
}

这不需要unsafe，因为它使用自动派生的FromPrimitive trait。

Answer 3

我更愿意通过match 语句显式编码下一个方向：

#[derive(Debug)]
enum Direction {
    North,
    South,
    East,
    West,
}

impl Direction {
    fn turn(&self) -> Self {
        use Direction::*;
        match *self {
            North => South,
            South => East,
            East => West,
            West => North,
        }
    }
}

fn main() {
    use Direction::*;

    for i in &[North, South, East, West] {
        println!("{:?} -> {:?}", i, i.turn());
    }
}