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

fn main() {
    // --- Basic Result Handling ---
    let f = File::open("hello.txt");
    
    let f = match f {
        Ok(file) => file,
        Err(error) => {
            println!("Problem opening the file: {:?}", error);
            return; // Exit main
        },
    };
    
    println!("File opened successfully: {:?}", f);
    
    // --- Propagating Errors ---
    match read_username_from_file() {
        Ok(username) => println!("Username: {}", username),
        Err(e) => println!("Error reading username: {}", e),
    }
}

// Function that returns a Result
fn read_username_from_file() -> Result<String, io::Error> {
    // The '?' operator is a shortcut for propagating errors
    // If File::open fails, '?' returns the Err immediately.
    // If it succeeds, it unwraps the Ok value into 'f'.
    let mut f = File::open("username.txt")?;
    
    let mut s = String::new();
    
    // Same here: if read_to_string fails, return Err.
    f.read_to_string(&mut s)?;
    
    // If we got here, everything is fine. Return Ok.
    Ok(s)
    
    // The above can be shortened to:
    // File::open("username.txt")?.read_to_string(&mut s)?;
    // Ok(s)
}