控制流程

Rust 的某些控制流程結構與其他程式語言不同。這些結構會用於模式配對：

• if let 運算式
• while let 運算式
• match 運算式

if let 運算式

if let 運算式可讓您根據值是否符合模式，執行不同的程式碼：

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 運算式

如果是要配對模式並從函式傳回的常見情況，請使用 let else。如果是「其他」情況，則必須發散 (return、break 或恐慌，也就是落在區塊結尾之外的任何情況)。

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"))));
}

和 if let 的情況一樣，有一個 while let 變數可針對模式重複測試值：

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);
}
}