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Rust Match Syntax

beps/docs/proposals/BEP-002-match/context/rust.md

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Rust Match Syntax

Rust's match is a powerful control flow operator that allows you to compare a value against a series of patterns and then execute code based on which pattern matches. Patterns can be made up of literal values, variable names, wildcards, and many other things.

Basic Syntax

rust
enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter,
}

fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => 1,
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter => 25,
    }
}

Key Features

1. Exhaustiveness

match arms must cover every possibility. If you miss a case, the compiler will error.

rust
// Error: pattern `None` not covered
match Some(5) {
    Some(x) => println!("{}", x),
}

2. Destructuring

You can destructure structs, enums, tuples, and references.

rust
enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

match msg {
    Message::Quit => println!("Quit"),
    Message::Move { x, y } => println!("Move to {}, {}", x, y),
    Message::Write(text) => println!("Text: {}", text),
    Message::ChangeColor(r, g, b) => println!("Color: {}, {}, {}", r, g, b),
}

3. Match Guards

You can add an if condition to a match arm.

rust
match num {
    Some(x) if x < 5 => println!("less than five: {}", x),
    Some(x) => println!("{}", x),
    None => (),
}

4. Binding with @

The @ operator lets you create a variable that holds a value at the same time as you're testing that value for a pattern match.

rust
match msg {
    Message::Hello { id: id_variable @ 3..=7 } => {
        println!("Found an id in range: {}", id_variable)
    },
    Message::Hello { id: 10..=12 } => {
        println!("Found an id in another range")
    },
    Message::Hello { id } => {
        println!("Found some other id: {}", id)
    },
}

5. Expression-based

match is an expression, meaning it returns a value.

rust
let boolean = true;
let binary = match boolean {
    false => 0,
    true => 1,
};