Match guards and bindings in Rust

A guard adds a condition to an arm; @ keeps the value you matched on.

fn describe(n: i32) -> String {
    match n {
        0 => "zero".to_string(),
        n @ 1..=9 => format!("single digit {n}"),
        n if n % 2 == 0 => format!("even {n}"),
        10 | 100 | 1000 => "round".to_string(),
        _ => "odd and large".to_string(),
    }
}

fn main() {
    for n in [0, 7, 42, 100, 33] {
        println!("{}", describe(n));
    }
}

How it works

1. if after a pattern is the guard, tested only on a match.
2. n @ 1..=9 binds the value and checks the range.
3. | joins alternative patterns in one arm.

Keywords and builtins used here

• String
• describe
• fn
• for
• i32
• if
• in
• main
• match

The run, in numbers

Lines

15

Characters to type

303

Tokens

117

Three-star pace

90 tpm

At the three-star pace of 90 tokens a minute, this run takes about 78 seconds.

Type this snippet

Step 3 of 4 in Pattern matching, step 21 of 39 in Language basics.

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