Rust 如何通过枚举变体 toggle

发布于09月16日

我想写一个函数，将提供的值切换到枚举中的下一个值，并在末尾换行:

enum Direction { NORTH, SOUTH, EAST, WEST }

For example, NORTH => SOUTH, SOUTH => EAST, EAST => WEST, WEST => NORTH.

有没有比手动创建静态数组更简单的方法

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

枚举不是应该被"枚举"吗？我模糊地记得以前在《铁 rust 》中看到过一个例子，但我似乎找不到它.由于 rust 迹枚举更像联合/变体，我想这会使事情复杂化.

推荐答案

我想像这样的事情会奏效:

#[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特征.