How to use variadic macros to call nested constructors? - macros

Question

I'm trying to create a macro in Rust that lets me write
make_list!(1, 2, 3)
instead of
Node::new(1, Node::new(2, Node::new(3, None)))
which should work for an arbitrary number of "parameters" including zero. This is what I have so far:
macro_rules! make_list(
() => (
None
);
( $x:expr, $( $more:expr ),* ) => (
Node::new($x, make_list!( $( $more ),* ))
)
);
but I get the following error:
error: unexpected end of macro invocation
--> src/main.rs:19:42
|
19 | Node::new($x, make_list!( $( $more ),* ))
| ^^^^^
I can't make much sense of this. From what I can tell, it should work. What did I do wrong?
The complete code:
type List<T> = Option<Box<Node<T>>>;
struct Node<T> {
value: T,
tail: List<T>,
}
impl<T> Node<T> {
fn new(val: T, tai: List<T>) -> List<T> {
Some(Box::new(Node::<T> {
value: val,
tail: tai,
}))
}
}
macro_rules! make_list(
() => (
None
);
( $x:expr, $( $more:expr ),* ) => (
Node::new($x, make_list!( $( $more ),* ))
)
);
fn main() {
let _list: List<i32> = make_list!(1, 2, 3, 4, 5, 6, 7, 8, 9);
}

Answer 1

Expanding on the error: you get down to the case where there is only one value, and so it writes make_list!(1). However, there is no rule that will match that, for the second rule, after consuming the expression x, wants a comma, which is not provided.
So you need to make it so that it will work for make_list!(1) and not just (in fact, just not) make_list!(1,). To achieve this, get the comma inside the repeating part, like this:
macro_rules! make_list(
() => (
None
);
( $x:expr $( , $more:expr )* ) => (
Node::new($x, make_list!( $( $more ),* ))
)
);
Bonus: you can write make_list![1, 2, 3] instead of make_list!(1, 2, 3) if you want.

Answer 2

As noted by #chris-morgan's answer, expanding the single argument case isn't accounted for.
So you can either include comma in the expansion, or add a single case in the macro:
Example of both, single argument:
macro_rules! make_list {
() => (
None
);
($x:expr) => (
Node::new($x, None)
);
($x:expr, $($more:expr),+) => (
Node::new($x, make_list!($($more),*))
);
}
Including the comma in the expansion:
macro_rules! make_list {
() => (
None
);
($x:expr $(, $more:expr)*) => (
Node::new($x, make_list!($($more),*))
);
}
Here is a fully working example based on the question and updated for Rust 1.14:
type List<T> = Option<Box<Node<T>>>;
#[derive(Debug)]
struct Node<T> {
value: T,
tail: List<T>
}
impl<T> Node<T> {
fn new(val: T, tai: List<T>) -> List<T> {
Some(Box::new(Node::<T> { value: val, tail: tai }))
}
}
macro_rules! make_list {
() => (
None
);
($x:expr $(, $more:expr)*) => (
Node::new($x, make_list!($($more),*))
);
}
fn main() {
let list: List<i64> = make_list!();
println!("{:?}", list);
let list: List<i64> = make_list!(1);
println!("{:?}", list);
let list: List<i64> = make_list!(1,2,3,4,5,6,7,8,9);
println!("{:?}", list);
}