Rust Generic Enums
Introduction
Generic enums are a powerful feature in Rust that allow you to create enumeration types that can work with different data types. Just like generic functions and structs, generic enums help you write flexible, reusable code without sacrificing Rust's strong type safety. In this tutorial, we'll explore how to define and use generic enums, understand their benefits, and see practical examples of how they solve real programming problems.
What are Generic Enums?
An enum (short for enumeration) in Rust is a type that can be one of several variants. When we make an enum generic, we're essentially saying: "this enum can work with different types, which we'll specify when we use it."
Let's start with a basic example to understand the syntax:
enum Result<T, E> {
Ok(T),
Err(E),
}
In this example, Result is a generic enum with two type parameters: T and E. The Ok variant holds a value of type T, and the Err variant holds a value of type E. This means we can use Result with any combination of types!
Basic Generic Enum Examples
Option Enum
One of the most commonly used generic enums in Rust is Option<T>:
enum Option<T> {
Some(T),
None,
}
Here's how you might use it:
fn main() {
// Option with an integer
let some_number: Option<i32> = Some(5);
// Option with a string
let some_text: Option<String> = Some(String::from("Hello"));
// Option with no value
let absent_number: Option<i32> = None;
// Print the values
println!("some_number: {:?}", some_number); // Output: some_number: Some(5)
println!("some_text: {:?}", some_text); // Output: some_text: Some("Hello")
println!("absent_number: {:?}", absent_number); // Output: absent_number: None
}
The Option<T> enum is so fundamental to Rust that it's included in the prelude, meaning you don't need to explicitly import it.
Custom Generic Enum
Let's create our own generic enum to represent different types of messages:
enum Message<T> {
Text(String),
Data(T),
Quit,
}
fn main() {
// Message with integer data
let msg1 = Message::Text(String::from("Hello, Rust!"));
let msg2 = Message::Data(42);
let msg3 = Message::Quit;
process_message(msg1);
process_message(msg2);
process_message(msg3);
}
fn process_message<T: std::fmt::Debug>(msg: Message<T>) {
match msg {
Message::Text(text) => println!("Text message: {}", text),
Message::Data(data) => println!("Data received: {:?}", data),
Message::Quit => println!("Quit command received"),
}
}
Output:
Text message: Hello, Rust!
Data received: 42
Quit command received
Multiple Type Parameters
Just like with generic functions and structs, enums can have multiple type parameters:
enum Either<L, R> {
Left(L),
Right(R),
}
fn main() {
let left = Either::Left::<i32, String>(42);
let right = Either::Right::<i32, String>(String::from("Hello"));
match left {
Either::Left(value) => println!("Left value: {}", value),
Either::Right(value) => println!("Right value: {}", value),
}
match right {
Either::Left(value) => println!("Left value: {}", value),
Either::Right(value) => println!("Right value: {}", value),
}
}
Output:
Left value: 42
Right value: Hello
Generic Enums with Trait Bounds
You can also add trait bounds to your generic enums, ensuring that the type parameters implement specific traits:
enum Status<T: std::fmt::Display> {
Success(T),
Failure(String),
}
fn main() {
// This works because i32 implements Display
let status1 = Status::Success(200);
// This works because String implements Display
let status2 = Status::Success(String::from("OK"));
// This would NOT compile:
// let status3 = Status::Success(vec![1, 2, 3]);
// Because Vec<i32> doesn't implement Display
print_status(status1);
print_status(status2);
}
fn print_status<T: std::fmt::Display>(status: Status<T>) {
match status {
Status::Success(value) => println!("Success: {}", value),
Status::Failure(err) => println!("Failure: {}", err),
}
}
Output:
Success: 200
Success: OK
Where to Use Generic Enums
Generic enums are particularly useful in the following scenarios:
- Error handling: The
Result<T, E>enum is perfect for functions that might fail. - Optional values:
Option<T>elegantly handles the case where a value might be absent. - State machines: Different states can hold different types of data.
- Message passing: When different message types need to be handled by the same system.
Practical Example: Building a Simple Cache
Let's build a simple cache that can store different types of values:
enum CacheEntry<T> {
Empty,
Filled(T, u64), // Value and timestamp
}
struct Cache<T> {
entries: Vec<CacheEntry<T>>,
}
impl<T: Clone> Cache<T> {
fn new(size: usize) -> Self {
let mut entries = Vec::with_capacity(size);
for _ in 0..size {
entries.push(CacheEntry::Empty);
}
Cache { entries }
}
fn store(&mut self, index: usize, value: T) {
if index < self.entries.len() {
// Store with current timestamp (simplified)
let timestamp = std::time::SystemTime::now()
.duration_since(std::time::UNIX_EPOCH)
.unwrap()
.as_secs();
self.entries[index] = CacheEntry::Filled(value, timestamp);
}
}
fn retrieve(&self, index: usize) -> Option<T> {
if index >= self.entries.len() {
return None;
}
match &self.entries[index] {
CacheEntry::Filled(value, _) => Some(value.clone()),
CacheEntry::Empty => None,
}
}
}
fn main() {
// Cache for integers
let mut int_cache = Cache::new(5);
int_cache.store(0, 42);
int_cache.store(2, 123);
println!("Value at index 0: {:?}", int_cache.retrieve(0)); // Some(42)
println!("Value at index 1: {:?}", int_cache.retrieve(1)); // None
println!("Value at index 2: {:?}", int_cache.retrieve(2)); // Some(123)
// Cache for strings
let mut string_cache = Cache::new(3);
string_cache.store(0, String::from("Hello"));
string_cache.store(1, String::from("Rust"));
println!("String at index 0: {:?}", string_cache.retrieve(0)); // Some("Hello")
println!("String at index 1: {:?}", string_cache.retrieve(1)); // Some("Rust")
println!("String at index 2: {:?}", string_cache.retrieve(2)); // None
}
This cache example demonstrates how generic enums can be used to represent different states of entries (empty or filled) while also being flexible about what type of data is stored.
Understanding the Flow Control with Generic Enums
One of the most powerful aspects of Rust's enums is pattern matching. Let's visualize how flow control works with a generic enum:
This pattern of explicitly handling all possible variants makes your code more robust by forcing you to consider all cases.
Common Patterns with Generic Enums
The Map Pattern
One common pattern is to transform the value inside an enum without unwrapping it:
fn main() {
let number = Some(5);
// Transform the value inside Some, if it exists
let doubled = number.map(|n| n * 2);
println!("Original: {:?}", number); // Original: Some(5)
println!("Doubled: {:?}", doubled); // Doubled: Some(10)
// Works with None too
let nothing: Option<i32> = None;
let still_nothing = nothing.map(|n| n * 2);
println!("Nothing: {:?}", still_nothing); // Nothing: None
}
The Unwrap or Default Pattern
Another common pattern is to provide a default value when unwrapping:
fn main() {
let number = Some(5);
let no_number: Option<i32> = None;
// Unwrap with a default
let unwrapped1 = number.unwrap_or(0);
let unwrapped2 = no_number.unwrap_or(0);
println!("Unwrapped Some: {}", unwrapped1); // Unwrapped Some: 5
println!("Unwrapped None: {}", unwrapped2); // Unwrapped None: 0
}
Summary
Generic enums in Rust provide a powerful way to create flexible, type-safe code:
- They allow you to define variants that can work with different types
- They combine the expressiveness of enums with the flexibility of generics
- They're used extensively in Rust's standard library with
Option<T>andResult<T, E> - They can be constrained using trait bounds
- They work well with Rust's pattern matching to ensure all cases are handled
By mastering generic enums, you gain a fundamental tool for writing robust, reusable Rust code that clearly expresses your intent while maintaining Rust's safety guarantees.
Further Exercises
- Create a generic
Tree<T>enum that can represent a binary tree structure. - Implement a
findmethod for your tree that returns anOption<&T>. - Create a generic
LinkedList<T>enum withConsandNilvariants. - Write a function that converts between two different generic enum types.
- Implement the
Either<L, R>enum with methods likemap_leftandmap_right.
Additional Resources
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