Propagating errors with ? in Rust - Mini Project: Build & Apply

Propagating errors with ? in Rust

📖 Scenario: You are writing a small Rust program that reads a username from a file and prints it. Sometimes the file might not exist or be unreadable, so you need to handle errors properly.

🎯 Goal: Build a Rust program that reads the username from a file called username.txt and prints it. Use the ? operator to propagate errors instead of handling them manually.

📋 What You'll Learn

Create a function called read_username that returns a Result

Use std::fs::read_to_string inside read_username to read the file

Use the ? operator to propagate errors in read_username

Call read_username from main and handle the result

Print the username if reading is successful, or print an error message if it fails

💡 Why This Matters

🌍 Real World

Reading files and handling errors is common in programs that work with user data or configuration files.

💼 Career

Understanding error propagation with ? is essential for writing robust Rust applications in software development jobs.

1

Create the read_username function

Write a function called read_username that returns Result. Inside it, use std::fs::read_to_string to read the file "username.txt" and return the result directly.

Rust

# Your code here

Hint

Use fs::read_to_string("username.txt") and return it directly from the function.

2

Use the ? operator to propagate errors

Modify the read_username function to use the ? operator with fs::read_to_string to propagate errors instead of returning the result directly.

Rust

use std::fs;
use std::io;

fn read_username() -> Result {
    let username = fs::read_to_string("username.txt")?;
    // Your code here
}

Hint

Use let username = fs::read_to_string("username.txt")?; and then return Ok(username).

3

Call read_username from main and handle errors

Write a main function that calls read_username. Use a match statement on the result to print the username if successful, or print Error reading username if there is an error.

Rust

use std::fs;
use std::io;

fn read_username() -> Result {
    let username = fs::read_to_string("username.txt")?;
    Ok(username)
}

fn main() {
    // Your code here
}

Hint

Use match read_username() and print the username or an error message accordingly.

4

Print the username or error message

Run the program. It should print the username from username.txt if the file exists, or print Error reading username if it does not.

Rust

use std::fs;
use std::io;

fn read_username() -> Result {
    let username = fs::read_to_string("username.txt")?;
    Ok(username)
}

fn main() {
    match read_username() {
        Ok(name) => println!("Username: {}", name),
        Err(_) => println!("Error reading username"),
    }
}

// Run the program to see the output

Hint

Make sure the file username.txt exists with content like Alice to see the username printed.