Repetitions

Learn repetition patterns with $(...),*.

Repetitions

The most powerful part of macros is repetition patterns: $( ... ),* means "zero or more, comma-separated", $( ... ),+ "one or more", $( ... )? "optional". In the body, the same pattern repeats for each match:

macro_rules! topla {
    ($($x:expr),*) => {
        0 $(+ $x)*
    };
}

topla!(1, 2, 3)   // 0 + 1 + 2 + 3
topla!()          // 0

The separator is part of the pattern: $(..),* expects commas, $(..);* semicolons.

Multiple arms

A macro can have several arms; the first match is used. Complex patterns can be built with recursion:

macro_rules! en_buyuk {
    ($x:expr) => { $x };
    ($x:expr, $($kalan:expr),+) => {
        { let a = $x; let b = en_buyuk!($($kalan),+); if a > b { a } else { b } }
    };
}

en_buyuk!(3, 9, 4)   // 9

Exercise Variadic sum

Write the topla! macro above. Print the results of topla!(1, 2, 3) and topla!() separated by a space.

Show example solution

macro_rules! topla {
    ($($x:expr),*) => {
        0 $(+ $x)*
    };
}

fn main() {
    println!("{} {}", topla!(1, 2, 3), topla!());
}

Test yourself

Does <code>topla!(1, 2, 3,)</code> (trailing comma) match this macro?
- It warns
- Panic
- No; $(,)? must be added
- Yes
Show answer

No; $(,)? must be added — To accept a trailing comma, the pattern is written as ($($x:expr),* $(,)?); vec! is defined this way.

Repetition pattern

macro_rules! topla {
    ($($x:expr),____) => {
        0 $(+ $x)*
    };
}

Show answer

macro_rules! topla {
    ($($x:expr),*) => {
        0 $(+ $x)*
    };
}

Pattern and meaning

$(..),*

Zero or more
$(..),+

At least one
$(..)?

Optional
$(,)?

Trailing comma

Next lesson: Types

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