Enums and Pattern Matching in Rust

Alex Garella

9th November 2023

Rust stands out for its approach to safety and expressiveness, with enums and pattern matching being prime examples of these features. Enums allow you to define a type by enumerating its possible variants, and pattern matching is a way to execute code based on which variant a value is. Let's take a closer look at these powerful constructs.

Understanding Enums

In Rust, an enum (short for enumeration) is a type that can be any one of several variants. This is different from other languages where enums are essentially a set of named constants. Rust’s enums are more akin to algebraic data types in functional languages.

Here's a simple example:

enum Direction { Up, Down, Left, Right, }

Each variant of the Direction enum is a different possible value for a Direction type variable.

Leveraging Pattern Matching

Pattern matching in Rust is done primarily through the match expression, which is similar to a switch statement in other languages but far more powerful.

Here's how you might use a match expression with the Direction enum:

fn match_direction(dir: Direction) { match dir { Direction::Up => println!("We're heading up!"), Direction::Down => println!("We're going down!"), Direction::Left => println!("We're going left!"), Direction::Right => println!("We're going right!"), } }

When the match_direction function is called with a Direction, the match expression determines which variant it is and executes the corresponding code.

Enums with Data

Rust enums can also hold data. For instance:

enum WebEvent { PageLoad, PageUnload, KeyPress(char), Paste(String), Click { x: i64, y: i64 }, }

This WebEvent enum has variants without data, like PageLoad, and variants with data, like KeyPress(char).

Matching with Enums and Data

When you use match with such enums, you can destructure the variants to access their data:

fn match_web_event(event: WebEvent) { match event { WebEvent::PageLoad => println!("page loaded"), WebEvent::PageUnload => println!("page unloaded"), WebEvent::KeyPress(c) => println!("pressed '{}'.", c), WebEvent::Paste(s) => println!("pasted \"{}\".", s), WebEvent::Click { x, y } => println!("clicked at x={}, y={}.", x, y), } } fn main() { match_web_event(WebEvent::PageLoad) // Output: page loaded }

Here, the match expression checks the variant of WebEvent and, for KeyPress, Paste, and Click, destructures them to work with the data they contain.

if let Expressions in Pattern Matching

While match expressions are a cornerstone of pattern matching in Rust, there's another tool in the Rustacean's toolbox: the if let expression. This feature shines when you're interested in only one pattern and wish to ignore the rest. It offers a more concise syntax, eliminating the need for a full match when it's not necessary.

Suppose you have a WebEvent enum and want to execute code only for the KeyPress variant:

fn main() { let event = WebEvent::KeyPress('x'); if let WebEvent::KeyPress(c) = event { println!("Key press event with char: {}", c); } else { println!("Some other event."); } // Output: Key press event with char: x }

This if let expression checks for the KeyPress variant of WebEvent. If it matches, it executes the block of code, binding the character to c. If it doesn't match, the code in the else block is executed. The if let thus provides a streamlined alternative to match, reducing boilerplate while maintaining clarity when handling single cases. It's a subtle, yet powerful, syntax that exemplifies Rust's commitment to concise, expressive code.

Advanced Pattern Matching

Rust pattern matching can also be used with guards, bindings, and more, providing a way to handle complex logic in a clear and concise manner.

For example, you can match several patterns at once:

fn main() { let some_value = 2; match some_value { 1 | 2 => println!("one or two"), other => println!("The number is {}", other), } // Output: one or two }

Or you can use guards to add conditions to patterns:

fn main() { let some_tuple = (1, 1); match some_tuple { (x, y) if x == y => println!("These are equal"), (x, y) if x > y => println!("Greater"), _ => println!("No match"), } // Output: These are equal }


Enums and pattern matching in Rust offer a robust way to handle different types and the logic associated with them. Enums provide the structure to define complex data types with variants, and pattern matching offers the tools to handle these variants elegantly. Together, they give Rust developers a high level of control over the flow of their programs, ensuring that all cases are handled and that the code remains clear and concise.

By understanding and utilizing these features, you can write Rust code that is both expressive and safe, harnessing the full power of the language to write better, more maintainable applications.

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