Exercise 7: More Repetition

This exercise is going to also cover writing repetitions, but now involving more than one metavariable. Don't worry: the syntax is the exact same as what you've seen before.

Before you start, let's just quickly cover the different ways you can use a metavariable within a repetition.

Multiple Metavariables in One Repetition

You can indicate that two metavariables should be used in a single repetition.

For example, ( $($i:ident is $e:expr),+ ) would match my_macro!(pi is 3.14, tau is 6.28). You would end up with $i having matched pi and tau; and $e having matched 3.14 and 6.28.

Any repetition in the transcriber can use $i, or $e, or both within the same repetition. So a transcriber could be $(let $i = $e;)+; or let product = $($e)*+

One Metavariable Each, For Two Repetitions

Alternatively, you could specify two different repetitions, each containing their own metavariable. For example, this program will construct two vecs.

macro_rules! two_vecs {
    ($($vec1:expr),+; $($vec2:expr),+) => {
            let mut vec1 = Vec::new();
            let mut vec2 = Vec::new();

            (vec1, vec2)

fn main() {
    let vecs = two_vecs!(1, 2, 3; 'a', 'b');

Importantly, with the above example, you have to be careful about using $vec1 and $vec2 in the same repetition within the transcriber. It is a compiler error to use two metavariables captured a different number of times in the same repetition.

To quote the reference:

Each repetition in the transcriber must contain at least one metavariable to decide how many times to expand it. If multiple metavariables appear in the same repetition, they must be bound to the same number of fragments. For instance, ( $( $i:ident ),* ; $( $j:ident ),* ) => (( $( ($i,$j) ),* )) must bind the same number of $i fragments as $j fragments. This means that invoking the macro with (a, b, c; d, e, f) is legal and expands to ((a,d), (b,e), (c,f)), but (a, b, c; d, e) is illegal because it does not have the same number.

Exercise 7: More Repetition

In this task, you will be creating a hashmap macro. It should consist of comma-separated pairs, of the form literal => expr, This should construct an empty HashMap and insert the relevant key-value pairs.

You may not edit the main function, but it should eventually look like the following:

fn main() {
    let value = "my_string";
    let my_hashmap = {
        let mut hm = HashMap::new();
        hm.insert("hash", "map");
        hm.insert("Key", value);