let else Statements

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

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

    Ok(digit)
}

fn main() {
    println!("result: {:?}", hex_or_die_trying(Some(String::from("foo"))));
}
The rewritten version is: 
#![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"));
    };

    Ok(digit)
}
}

探索更多

  • This early return-based control flow is common in Rust error handling code, where you try to get a value out of a Result, returning an error if the Result was Err.
  • If students ask, you can also demonstrate how real error handling code would be written with ?.