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Mastering Rust Error Handling - A Practical Guide with Real-World Examples
Anubhav Gain
2025-01-10
Programming
rust
/
programming
/
error-handling
/
best-practices

Mastering Rust Error Handling: A Practical Guide with Real-World Examples#

Error handling is one of Rust’s most powerful features, setting it apart from many other programming languages. Unlike languages that rely on exceptions, Rust forces you to handle errors explicitly at compile time, making your programs more robust and predictable. In this comprehensive guide, we’ll explore Rust’s error handling mechanisms with practical, real-world examples.

Table of Contents#

  1. Understanding Result and Option Types
  2. Pattern Matching for Error Handling
  3. The Question Mark Operator
  4. Creating Custom Error Types
  5. Combinator Methods
  6. Real-World Examples
  7. Best Practices

Understanding Result and Option Types#

Rust provides two fundamental types for handling scenarios where operations might fail or values might be absent.

The Option Type#

Option<T> represents a value that might or might not exist:

enum Option<T> {
    Some(T),  // Contains a value
    None,     // No value present
}

Practical Example: Safe Division

fn safe_divide(numerator: f64, denominator: f64) -> Option<f64> {
    if denominator == 0.0 {
        None
    } else {
        Some(numerator / denominator)
    }
}


fn main() {
    match safe_divide(10.0, 2.0) {
        Some(result) => println!("Result: {}", result),
        None => println!("Cannot divide by zero!"),
    }


    // Using if let for simpler cases
    if let Some(result) = safe_divide(20.0, 4.0) {
        println!("20 / 4 = {}", result);
    }
}

The Result Type#

Result<T, E> is used for operations that can succeed with a value of type T or fail with an error of type E:

enum Result<T, E> {
    Ok(T),   // Success with value
    Err(E),  // Error occurred
}

Practical Example: File Reading

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


fn read_username_from_file(path: &str) -> Result<String, io::Error> {
    let mut file = File::open(path)?;
    let mut username = String::new();
    file.read_to_string(&mut username)?;
    Ok(username)
}


fn main() {
    match read_username_from_file("user.txt") {
        Ok(username) => println!("Username: {}", username),
        Err(error) => eprintln!("Error reading file: {}", error),
    }
}

Pattern Matching for Error Handling#

Pattern matching is Rust’s Swiss Army knife for handling different cases elegantly.

Basic Pattern Matching#

use std::num::ParseIntError;


fn parse_and_double(input: &str) -> Result<i32, ParseIntError> {
    match input.parse::<i32>() {
        Ok(num) => Ok(num * 2),
        Err(e) => Err(e),
    }
}


fn main() {
    let inputs = vec!["42", "abc", "100", "12.5"];


    for input in inputs {
        match parse_and_double(input) {
            Ok(result) => println!("{} doubled is {}", input, result),
            Err(_) => println!("{} is not a valid integer", input),
        }
    }
}

Nested Pattern Matching#

#[derive(Debug)]
enum DatabaseError {
    ConnectionFailed,
    QueryFailed(String),
    NotFound,
}


fn get_user_age(user_id: u32) -> Result<Option<u8>, DatabaseError> {
    // Simulating database operations
    if user_id == 0 {
        Err(DatabaseError::ConnectionFailed)
    } else if user_id == 999 {
        Err(DatabaseError::QueryFailed("Invalid query".to_string()))
    } else if user_id == 100 {
        Ok(None) // User exists but age not set
    } else {
        Ok(Some(25)) // User exists with age
    }
}


fn main() {
    let user_ids = vec![1, 100, 999, 0];


    for id in user_ids {
        match get_user_age(id) {
            Ok(Some(age)) => println!("User {} is {} years old", id, age),
            Ok(None) => println!("User {} exists but age is not set", id),
            Err(DatabaseError::NotFound) => println!("User {} not found", id),
            Err(DatabaseError::ConnectionFailed) => {
                println!("Failed to connect to database for user {}", id)
            }
            Err(DatabaseError::QueryFailed(msg)) => {
                println!("Query failed for user {}: {}", id, msg)
            }
        }
    }
}

The Question Mark Operator#

The ? operator is syntactic sugar that makes error propagation cleaner and more readable.

Before and After Using ?#

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


// Without ? operator - verbose
fn read_file_verbose(path: &str) -> Result<String, io::Error> {
    let file_result = File::open(path);
    let mut file = match file_result {
        Ok(file) => file,
        Err(e) => return Err(e),
    };


    let mut contents = String::new();
    match file.read_to_string(&mut contents) {
        Ok(_) => Ok(contents),
        Err(e) => Err(e),
    }
}


// With ? operator - concise
fn read_file_concise(path: &str) -> Result<String, io::Error> {
    let mut file = File::open(path)?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    Ok(contents)
}


// Even more concise with chaining
fn read_file_chain(path: &str) -> Result<String, io::Error> {
    std::fs::read_to_string(path)
}

Chaining Operations with ?#

use std::error::Error;
use std::fs::File;
use std::io::{BufRead, BufReader};


fn count_lines_with_word(path: &str, word: &str) -> Result<usize, Box<dyn Error>> {
    let file = File::open(path)?;
    let reader = BufReader::new(file);


    let count = reader
        .lines()
        .filter_map(Result::ok)
        .filter(|line| line.contains(word))
        .count();


    Ok(count)
}


fn main() -> Result<(), Box<dyn Error>> {
    let count = count_lines_with_word("example.txt", "rust")?;
    println!("Found {} lines containing 'rust'", count);
    Ok(())
}

Creating Custom Error Types#

For complex applications, custom error types provide better error handling and more meaningful error messages.

Simple Custom Error#

use std::fmt;


#[derive(Debug)]
enum ValidationError {
    TooShort,
    TooLong,
    InvalidCharacters,
}


impl fmt::Display for ValidationError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            ValidationError::TooShort => write!(f, "Input is too short"),
            ValidationError::TooLong => write!(f, "Input is too long"),
            ValidationError::InvalidCharacters => write!(f, "Input contains invalid characters"),
        }
    }
}


impl std::error::Error for ValidationError {}


fn validate_username(username: &str) -> Result<(), ValidationError> {
    if username.len() < 3 {
        return Err(ValidationError::TooShort);
    }
    if username.len() > 20 {
        return Err(ValidationError::TooLong);
    }
    if !username.chars().all(|c| c.is_alphanumeric() || c == '_') {
        return Err(ValidationError::InvalidCharacters);
    }
    Ok(())
}


fn main() {
    let usernames = vec!["ab", "valid_user", "very_long_username_that_exceeds_limit", "invalid-chars!"];


    for username in usernames {
        match validate_username(username) {
            Ok(()) => println!("'{}' is valid", username),
            Err(e) => println!("'{}' is invalid: {}", username, e),
        }
    }
}

Using thiserror for Convenient Error Types#

// Add to Cargo.toml: thiserror = "1.0"
use thiserror::Error;


#[derive(Error, Debug)]
enum AppError {
    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),


    #[error("Parse error: {0}")]
    Parse(#[from] std::num::ParseIntError),


    #[error("Custom error: {message}")]
    Custom { message: String },


    #[error("Not found: {0}")]
    NotFound(String),
}


fn process_config_file(path: &str) -> Result<i32, AppError> {
    let contents = std::fs::read_to_string(path)?;
    let value: i32 = contents.trim().parse()?;


    if value < 0 {
        return Err(AppError::Custom {
            message: "Value must be positive".to_string(),
        });
    }


    Ok(value)
}

Combinator Methods#

Result and Option types come with powerful methods that allow for functional-style error handling.

Working with Option#

fn main() {
    let numbers = vec!["42", "93", "invalid", "18"];


    // Using map and filter
    let valid_numbers: Vec<i32> = numbers
        .iter()
        .filter_map(|s| s.parse::<i32>().ok())
        .map(|n| n * 2)
        .collect();


    println!("Doubled valid numbers: {:?}", valid_numbers);


    // Using unwrap_or and unwrap_or_else
    let config_value = std::env::var("MY_CONFIG")
        .ok()
        .and_then(|s| s.parse::<i32>().ok())
        .unwrap_or(42);


    println!("Config value: {}", config_value);
}

Working with Result#

use std::fs;
use std::path::Path;


fn process_files(paths: &[&str]) -> Vec<String> {
    paths
        .iter()
        .filter_map(|path| {
            fs::read_to_string(path)
                .map_err(|e| eprintln!("Failed to read {}: {}", path, e))
                .ok()
        })
        .map(|content| content.to_uppercase())
        .collect()
}


fn main() {
    let files = vec!["file1.txt", "file2.txt", "nonexistent.txt"];
    let processed = process_files(&files);


    for content in processed {
        println!("Processed content: {}", content);
    }
}

Real-World Examples#

Example 1: Configuration File Parser#

use std::collections::HashMap;
use std::fs;
use std::io;
use thiserror::Error;


#[derive(Error, Debug)]
enum ConfigError {
    #[error("IO error: {0}")]
    Io(#[from] io::Error),


    #[error("Parse error on line {line}: {message}")]
    Parse { line: usize, message: String },


    #[error("Missing required field: {0}")]
    MissingField(String),
}


struct Config {
    database_url: String,
    port: u16,
    debug: bool,
}


fn parse_config(path: &str) -> Result<Config, ConfigError> {
    let contents = fs::read_to_string(path)?;
    let mut settings = HashMap::new();


    for (line_num, line) in contents.lines().enumerate() {
        if line.is_empty() || line.starts_with('#') {
            continue;
        }


        let parts: Vec<&str> = line.splitn(2, '=').collect();
        if parts.len() != 2 {
            return Err(ConfigError::Parse {
                line: line_num + 1,
                message: "Invalid format, expected KEY=VALUE".to_string(),
            });
        }


        settings.insert(parts[0].trim(), parts[1].trim());
    }


    let database_url = settings
        .get("database_url")
        .ok_or(ConfigError::MissingField("database_url".to_string()))?
        .to_string();


    let port = settings
        .get("port")
        .ok_or(ConfigError::MissingField("port".to_string()))?
        .parse::<u16>()
        .map_err(|_| ConfigError::Parse {
            line: 0,
            message: "Invalid port number".to_string(),
        })?;


    let debug = settings
        .get("debug")
        .map(|s| s == "true")
        .unwrap_or(false);


    Ok(Config {
        database_url,
        port,
        debug,
    })
}

Example 2: HTTP Client with Retry Logic#

use std::time::Duration;
use std::thread;


#[derive(Debug)]
enum HttpError {
    Timeout,
    ConnectionFailed,
    ServerError(u16),
}


fn make_request(url: &str) -> Result<String, HttpError> {
    // Simulating HTTP request
    use rand::Rng;
    let mut rng = rand::thread_rng();
    let random = rng.gen_range(0..10);


    if random < 3 {
        Err(HttpError::Timeout)
    } else if random < 5 {
        Err(HttpError::ConnectionFailed)
    } else if random < 7 {
        Err(HttpError::ServerError(500))
    } else {
        Ok("Response data".to_string())
    }
}


fn request_with_retry(url: &str, max_retries: u32) -> Result<String, HttpError> {
    let mut attempts = 0;


    loop {
        match make_request(url) {
            Ok(response) => return Ok(response),
            Err(e) if attempts >= max_retries => return Err(e),
            Err(HttpError::ServerError(code)) if code >= 500 => {
                attempts += 1;
                println!("Server error ({}), retrying... ({}/{})", code, attempts, max_retries);
                thread::sleep(Duration::from_secs(1));
            }
            Err(HttpError::Timeout) | Err(HttpError::ConnectionFailed) => {
                attempts += 1;
                println!("Connection issue, retrying... ({}/{})", attempts, max_retries);
                thread::sleep(Duration::from_secs(1));
            }
            Err(e) => return Err(e),
        }
    }
}

Best Practices#

1. Use Descriptive Error Messages#

// Bad
Err("error")


// Good
Err(format!("Failed to parse config file '{}': invalid port number on line {}", path, line_num))

2. Document Error Conditions#

/// Parses a configuration file and returns a Config struct.
///
/// # Errors
///
/// This function will return an error if:
/// * The file cannot be read
/// * The file format is invalid
/// * Required fields are missing
pub fn parse_config(path: &str) -> Result<Config, ConfigError> {
    // Implementation
}

3. Avoid unwrap() in Production Code#

// Bad - will panic on error
let file = File::open("config.txt").unwrap();


// Good - handle the error
let file = File::open("config.txt")
    .expect("Failed to open config.txt - make sure it exists");


// Better - propagate the error
let file = File::open("config.txt")?;

4. Use Type Aliases for Complex Results#

type DbResult<T> = Result<T, DatabaseError>;


fn get_user(id: u32) -> DbResult<User> {
    // Implementation
}


fn update_user(user: &User) -> DbResult<()> {
    // Implementation
}

5. Consider anyhow for Applications#

// Add to Cargo.toml: anyhow = "1.0"
use anyhow::{Context, Result};


fn process_data(path: &str) -> Result<()> {
    let data = std::fs::read_to_string(path)
        .context("Failed to read input file")?;


    let parsed: Value = serde_json::from_str(&data)
        .context("Failed to parse JSON")?;


    // Process data...
    Ok(())
}

Conclusion#

Rust’s error handling system, while initially seeming verbose, provides unparalleled safety and expressiveness. By leveraging Result, Option, pattern matching, and the ? operator, you can write robust code that handles errors gracefully and predictably.

Key takeaways:

  • Always handle errors explicitly - the compiler won’t let you forget
  • Use ? for cleaner error propagation
  • Create custom error types for domain-specific errors
  • Leverage combinator methods for functional-style error handling
  • Document error conditions in your public APIs
  • Choose the right error handling crate (thiserror for libraries, anyhow for applications)

With these patterns and practices, you’ll write Rust code that’s not just safe, but also maintainable and easy to reason about.

Next Steps#

Ready to dive deeper? Check out these resources:

Happy error handling!

Mastering Rust Error Handling - A Practical Guide with Real-World Examples
https://mranv.pages.dev/posts/rust-error-handling-practical-guide/
Author
Anubhav Gain
Published at
2025-01-10
License
CC BY-NC-SA 4.0
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