Optional arguments in Rust macro definition

Here is a simple example that showing a rust macro that accept optional arguments(arguments number is not fixed/variable arguments)

macro_rules! foo {
    ($var:ident $(, $check:expr )?) => {
        let val = $var;
        let mut ok = true;
        $ ( ok = $check(val); )?
        if ok {
            println!("var is: {}", val);
        }
    };
}

fn main() {
    let x = 10;
    let y = 20;

    foo!(x, |v| v > 10);
    foo!(y, |v| v > 10);
}

First, the ($var:ident $(, $check:expr )?) is pattern matching part of a macro.

? in $(, $check:expr )? means the $check has zero or one occurrences.

The repetition operators are:

• * — indicates any number of repetitions.
• + — indicates any number but at least one.
• ? — indicates an optional fragment with zero or one occurrences.

Next, $ ( ok = $check(val); )? is in format $( xxx )?, means that the statement will execute zero or one time.