疊代器

連線遭拒或找不到檔案等執行階段錯誤，都是透過 Result 型別來處理，但每次呼叫時都比對此類型可能相當麻煩。try 運算子 ? 的用途是將錯誤傳回呼叫端，可讓您將下列常見的程式碼

match some_expression {
    Ok(value) => value,
    Err(err) => return Err(err),
}

轉換成以下較簡潔的程式碼：

some_expression?

We can use this to simplify our error handling code:

use std::io::Read;
use std::{fs, io};

fn read_username(path: &str) -> Result<String, io::Error> {
    let username_file_result = fs::File::open(path);
    let mut username_file = match username_file_result {
        Ok(file) => file,
        Err(err) => return Err(err),
    };

    let mut username = String::new();
    match username_file.read_to_string(&mut username) {
        Ok(_) => Ok(username),
        Err(err) => Err(err),
    }
}

fn main() {
    //fs::write("config.dat", "alice").unwrap();
    let username = read_username("config.dat");
    println!("username or error: {username:?}");
}