let 控制流

Rust 有几个与其他语言不同的控制流结构。它们用于模式匹配：

• if let 表达式
• while let expressions
• match 表达式

if let 表达式

if let 表达式 能让你根据某个值是否与模式相匹配来执行不同的代码：

fn sleep_for(secs: f32) {
    let dur = if let Ok(dur) = std::time::Duration::try_from_secs_f32(secs) {
        dur
    } else {
        std::time::Duration::from_millis(500)
    };
    std::thread::sleep(dur);
    println!("slept for {:?}", dur);
}

fn main() {
    sleep_for(-10.0);
    sleep_for(0.8);
}

let else expressions

如需了解匹配模式并从函数返回的常见情况，请使用 let else。“else” 分支必须执行不同的结束方式（例如，return、break 或 panic，但不能直接执行到代码块的末尾）。

fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
    let s = if let Some(s) = maybe_string {
        s
    } else {
        return Err(String::from("got None"));
    };

    let first_byte_char = if let Some(first_byte_char) = s.chars().next() {
        first_byte_char
    } else {
        return Err(String::from("got empty string"));
    };

    if let Some(digit) = first_byte_char.to_digit(16) {
        Ok(digit)
    } else {
        Err(String::from("not a hex digit"))
    }
}

fn main() {
    println!("result: {:?}", hex_or_die_trying(Some(String::from("foo"))));
}

与 if let 一样，while let 变体会针对一个模式重复测试一个值：

fn main() {
    let mut name = String::from("Comprehensive Rust 🦀");
    while let Some(c) = name.pop() {
        println!("character: {c}");
    }
    // (There are more efficient ways to reverse a string!)
}

Here String::pop returns Some(c) until the string is empty, after which it will return None. The while let lets us keep iterating through all items.

#![allow(unused)]
fn main() {
fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
    let Some(s) = maybe_string else {
        return Err(String::from("got None"));
    };

    let Some(first_byte_char) = s.chars().next() else {
        return Err(String::from("got empty string"));
    };

    let Some(digit) = first_byte_char.to_digit(16) else {
        return Err(String::from("not a hex digit"));
    };

    return Ok(digit);
}
}