Propagating Errors with ?

The try-operator ? is used to return errors to the caller. It lets you turn the common

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

into the much simpler

some_expression?

We can use this to simplify our error handing code:

use std::fs;
use std::io::{self, Read};
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(e) => return Err(e),
    };
    let mut username = String::new();
    match username_file.read_to_string(&mut username) {
        Ok(_) => Ok(username),
        Err(e) => Err(e),
    }
}
fn main() {
    //fs::write("config.dat", "alice").unwrap();
    let username = read_username("config.dat");
    println!("username or error: {username:?}");
}

Key points:

• The username variable can be either Ok(string) or Err(error).
• Use the fs::write call to test out the different scenarios: no file, empty file, file with username.