切片模式

Minimum Rust version: 1.26

你有没有试过,用模式匹配去匹配切片的内容和结构? Rust 2018 将让你做到这一点。

例如,我们想要接受一个名单列表并回复问候语。使用切片模式,我们可以用以下方式轻松完成:

fn main() {
    greet(&[]);
    // output: Bummer, there's no one here :(
    greet(&["Alan"]);
    // output: Hey, there Alan! You seem to be alone.
    greet(&["Joan", "Hugh"]);
    // output: Hello, Joan and Hugh. Nice to see you are at least 2!
    greet(&["John", "Peter", "Stewart"]);
    // output: Hey everyone, we seem to be 3 here today.
}

fn greet(people: &[&str]) {
    match people {
        [] => println!("Bummer, there's no one here :("),
        [only_one] => println!("Hey, there {}! You seem to be alone.", only_one),
        [first, second] => println!(
            "Hello, {} and {}. Nice to see you are at least 2!",
            first, second
        ),
        _ => println!("Hey everyone, we seem to be {} here today.", people.len()),
    }
}

现在,你不必检查长度了。

你也可以匹配 array 如下:


#![allow(unused_variables)]
fn main() {
let arr = [1, 2, 3];

assert_eq!("ends with 3", match arr {
    [_, _, 3] => "ends with 3",
    [a, b, c] => "ends with something else",
});
}

更多的细节

穷举模式

在第一个例子中,注意匹配的 _ => ...。 如果开始匹配,那么将会匹配一切情况,所以有“穷尽所有模式”的处理方式。 如果我们忘记使用 _ => ... 或者 identifier => ... 模式,我们会得到如下的错误提醒:

error[E0004]: non-exhaustive patterns: `&[_, _, _]` not covered

如果我们再增加一项,我们将得到如下:

error[E0004]: non-exhaustive patterns: `&[_, _, _, _]` not covered

如此。

数组和精确的长度

在第二个例子中,数组是有固定长度的,我们需要匹配所有长度项,如果只匹配2,4项的话,会报错:

error[E0527]: pattern requires 2 elements but array has 3

error[E0527]: pattern requires 4 elements but array has 3

管道中

在切片模式方面,计划采用更先进的形式,但尚未稳定。要了解更多信息,请跟踪 the tracking issue