How to write a nested loop over several arguments in a macro? - rust

Question

The following code
macro_rules! test {
( $( $x1:expr ),*; blub $( $x2:expr ),* ) => {
$(
println!("{} * {} = {}", $x1, $x2, $x1 * $x2);
)*
}
}
fn main() {
test!{1, 2, 3; blub 4, 5, 6};
}
prints:
1 * 4 = 4
2 * 5 = 10
3 * 6 = 18
However I want to loop over both lists seperately like a nested loop. It should print:
1 * 4 = 4
1 * 5 = 5
1 * 6 = 6
2 * 4 = 8
2 * 5 = 10
2 * 6 = 12
3 * 4 = 12
3 * 5 = 15
3 * 6 = 18
How do I do that?

Answer 1

The only way I found was to cheat a little by using a token tree as one of the arguments like this:
macro_rules! test {
// secondary invocation with a `[]` delimited list of parameters
// as the first arguments and a single second argument.
( [ $( $x1:expr),* ] ; $x2:expr ) => {
$(
println!("{:?} * {:?} = {:?}", $x1, $x2, $x1 * $x2);
)*
};
// the main invocation of the macro, takes a token tree `x1`
// and a `[]` delimited `,` separated list of arguments for
// each of which it calls itself again with `x1` as first
// parameter and the element of the list as the second
( $x1:tt [ $( $x2:expr ),* ] ) => {
$(
test!($x1; $x2);
)*
};
}
fn main() {
test!{
[1, 2, 3]
[4, 5, 6]
};
}

Answer 2

You'll need to use a nested loop in some form or another because you need to iterate over x2 a variable number of times. You can expand each of your repeated tokens separately into an array, and loop over it the same way you would any iterable collection:
macro_rules! test {
( $($x1:expr ),*; blub $($x2:expr ),* ) => {
for x1 in [$($x1),*] {
for x2 in [$($x2),*] {
println!("{} * {} = {}", x1, x2, x1 * x2);
}
}
}
}
fn main() {
test!{1, 2, 3; blub 4, 5, 6};
}