Error handling best practices in Rust - Mini Project: Build & Apply

Error handling best practices

📖 Scenario: You are building a simple Rust program that reads a username from a file and prints a greeting. Handling errors properly is important to make your program reliable and user-friendly.

🎯 Goal: Learn how to handle errors in Rust using Result, match, and the ? operator to write clean and safe code.

📋 What You'll Learn

Create a function to read a username from a file

Use a configuration variable for the filename

Handle errors using match and Result

Use the ? operator for error propagation

Print the greeting or an error message

💡 Why This Matters

🌍 Real World

Reading configuration or user data from files is common in many programs. Handling errors properly prevents crashes and helps users understand what went wrong.

💼 Career

Rust developers must write safe and reliable code. Understanding error handling with Result, match, and the ? operator is essential for building robust applications.

1

DATA SETUP: Create a function to read username from a file

Write a function called read_username that returns a Result. Inside, use std::fs::read_to_string to read from a file named "user.txt" and return the content.

Rust

fn read_username() -> Result {
    // Your code here
}

Hint

Use std::fs::read_to_string("user.txt") to read the file content and return it directly.

2

CONFIGURATION: Add a variable for the filename

Create a variable called filename and set it to "user.txt". Update the read_username function to use this filename variable instead of the hardcoded string.

Rust

const filename: &str = "user.txt";

fn read_username() -> Result {
    // Your code here
}

Hint

Define filename outside the function and use it inside read_username.

3

CORE LOGIC: Use match to handle errors when calling read_username

Call read_username() and use a match statement with variables username and error to handle the Ok and Err cases. In Ok, print "Hello, {username}!". In Err, print "Failed to read username: {error}".

Rust

fn main() {
    let result = read_username();

    // Your code here
}

Hint

Use match result with Ok(username) and Err(error) arms to print messages.

4

OUTPUT: Use the ? operator in read_username and print the greeting in main

Rewrite read_username to use the ? operator for error propagation. Then, in main, call read_username() and use match with variables username and error to print "Hello, {username}!" or "Failed to read username: {error}".

Rust

fn read_username() -> Result {
    // Your code here
}

fn main() {
    // Your code here
}

Hint

Use ? operator inside read_username to return errors quickly. Then handle the result in main with match.

Make sure the file user.txt contains Alice for testing.

Practice

1. What is the primary purpose of using Result in Rust error handling?

easy

A. To convert a string into a number

B. To represent either success or failure of an operation

C. To define a constant value in the program

D. To create a new thread for parallel execution

Solution

1. Step 1: Understand the role of Result

   Result is an enum used to represent either success (Ok) or failure (Err) in Rust operations.

2. Step 2: Identify the purpose in error handling

   It helps safely handle errors by forcing the programmer to consider both success and failure cases explicitly.

3. Final Answer:

   To represent either success or failure of an operation -> Option B

4. Quick Check:

   Result = success or failure [OK]

Hint: Result means success or error outcome [OK]

Common Mistakes:

• Confusing Result with threading or constants
• Thinking Result automatically fixes errors
• Using Result only for numbers or strings

2. Which of the following is the correct syntax to use the ? operator in a function that returns Result<T, E>?

easy

A. let value = some_function()?;

B. let value = some_function!();

C. let value = some_function();?

D. let value = ?some_function();

Solution

1. Step 1: Recall the ? operator usage

   The ? operator is placed immediately after a function call that returns a Result to propagate errors easily.

2. Step 2: Check syntax correctness

   let value = some_function()?; correctly places ? after the function call: some_function()?. Other options misuse the operator placement.

3. Final Answer:

   let value = some_function()?; -> Option A

4. Quick Check:

   ? goes right after function call [OK]

Hint: Put ? right after the function call [OK]

Common Mistakes:

• Placing ? before the function call
• Using ! instead of ?
• Adding ? after semicolon

3. What will be the output of this Rust code snippet?

fn divide(a: i32, b: i32) -> Result {
    if b == 0 {
        Err(String::from("division by zero"))
    } else {
        Ok(a / b)
    }
}

fn main() {
    match divide(10, 0) {
        Ok(result) => println!("Result: {}", result),
        Err(e) => println!("Error: {}", e),
    }
}

medium

A. Result: 0

B. Panic at runtime

C. Error: division by zero

D. No output

Solution

1. Step 1: Analyze the divide function call

   The function is called with b = 0, triggering the error branch returning Err("division by zero").

2. Step 2: Check the match handling in main

   The Err case prints "Error: division by zero" to the console.

3. Final Answer:

   Error: division by zero -> Option C

4. Quick Check:

   Error message prints on zero divisor [OK]

Hint: Check if divisor is zero to get error output [OK]

Common Mistakes:

• Assuming division returns 0 instead of error
• Expecting panic instead of handled error
• Ignoring match arms

4. Identify the error in this Rust code snippet related to error handling:

fn read_file() -> Result {
    let content = std::fs::read_to_string("file.txt");
    Ok(content)
}

medium

A. File path should be a variable, not a string literal

B. Function should return String not Result

C. Using Ok instead of Err

D. Missing ? operator to propagate error

Solution

1. Step 1: Understand the return type and function call

   The function returns Result<String, std::io::Error>, but read_to_string returns a Result itself.

2. Step 2: Identify missing error propagation

   The code assigns content as a Result but wraps it again in Ok without handling errors. The ? operator is needed to propagate errors properly.

3. Final Answer:

   Missing ? operator to propagate error -> Option D

4. Quick Check:

   Use ? to handle Result errors [OK]

Hint: Use ? to unwrap or propagate Result errors [OK]

Common Mistakes:

• Wrapping Result inside Ok again
• Ignoring error propagation
• Changing return type incorrectly

5. You want to write a function that reads two files and returns the combined content as a String. Which is the best way to handle errors using Result and the ? operator? Choose the correct implementation:

fn read_two_files() -> Result {
    // Option A
    let content1 = std::fs::read_to_string("file1.txt")?;
    let content2 = std::fs::read_to_string("file2.txt")?;
    Ok(content1 + &content2)

    // Option B
    let content1 = std::fs::read_to_string("file1.txt").unwrap();
    let content2 = std::fs::read_to_string("file2.txt").unwrap();
    Ok(content1 + &content2)

    // Option C
    let content1 = std::fs::read_to_string("file1.txt").expect("file1 missing");
    let content2 = std::fs::read_to_string("file2.txt").expect("file2 missing");
    Ok(content1 + &content2)

    // Option D
    let content1 = std::fs::read_to_string("file1.txt");
    let content2 = std::fs::read_to_string("file2.txt");
    Ok(content1.unwrap() + &content2.unwrap())
}

hard

A. Use ? operator to propagate errors cleanly

B. Use unwrap() which panics on error

C. Use expect() which panics with message

D. Call unwrap() on Result without error handling

Solution

1. Step 1: Understand error propagation best practice

   Using the ? operator propagates errors to the caller without panicking, making the function safe and clear.

2. Step 2: Compare with other options

   The other options use unwrap() or expect(), which cause the program to panic on error, not recommended for robust error handling.

3. Final Answer:

   Use ? operator to propagate errors cleanly -> Option A

4. Quick Check:

   ? propagates errors safely [OK]

Hint: Use ? to avoid panics and propagate errors [OK]

Common Mistakes:

• Using unwrap or expect causing panics
• Not propagating errors properly
• Ignoring error handling for multiple calls