Apart from token tree types, we also need a way to repeatedly generate certain parts of our code. One of the practical examples from the standard library is the vec![] macro, which relies on repetition to give an illusion of variadic arguments, and allows you to create Vecs in any of the following manners:
vec![1, 2, 3]; vec![9, 8, 7, 6, 5, 4];
Let's see how vec! does this. Here's vec's macro_rules! definition from the standard library:
macro_rules! vec {
($elem:expr; $n:expr) => (
$crate::vec::from_elem($elem, $n)
);
($($x:expr),*) => (
<[_]>::into_vec(box [$($x),*])
);
($($x:expr,)*) => (vec![$($x),*])
}By ignoring the details to the right of => and focusing on the last two matching rules on the left-hand side, we can see something new in these rules:
($($x:expr),*) ($($x:expr,)*)
These are repeating rules. The repeating pattern rule follows:
- pattern:
$($var:type)*. Notice the$()*. For the sake of referring to them...