Is it possible to write a macro that expands an expression into multiple indexed arguments, which can be passed to a function or another macro?
See this simple self contained example.
The aim is to have unpack3 expand v into v[0], v[1], v[2].
macro_rules! elem {
($val:expr, $($var:expr), *) => {
$($val == $var) || *
}
}
// attempt to expand an array.
macro_rules! unpack3 {
($v:expr) => {
$v[0], $v[1], $v[2]
}
}
fn main() {
let a = 2;
let vars = [0, 1, 3];
// works!
if elem!(a, vars[0], vars[1], vars[2]) {
println!("Found!");
}
// fails!
if elem!(a, unpack3!(vars)) {
println!("Found!");
}
}
The second example fails, is it possible to make this work?
Possible solutions could include:
- Changing use of macro grammar.
- Using tuples, then expanding into arguments after.
- Re-arranging the expressions to workaround macro constraints.
Note, this may be related to Escaping commas in macro output but don't think its a duplicate.