The arguments of a macro are prefixed by a dollar sign $ and type annotated
with a designator:
macro_rules! create_function {
// This macro takes an argument of designator `ident` and
// creates a function named `$func_name`.
// The `ident` designator is used for variable/function names.
($func_name:ident) => (
fn $func_name() {
// The `stringify!` macro converts an `ident` into a string.
println!("You called {:?}()",
stringify!($func_name))
}
)
}
// Create functions named `foo` and `bar` with the above macro.
create_function!(foo);
create_function!(bar);
macro_rules! print_result {
// This macro takes an expression of type `expr` and prints
// it as a string along with its result.
// The `expr` designator is used for expressions.
($expression:expr) => (
// `stringify!` will convert the expression *as it is* into a string.
println!("{:?} = {:?}",
stringify!($expression),
$expression)
)
}
fn main() {
foo();
bar();
print_result!(1u32 + 1);
// Recall that blocks are expressions too!
print_result!({
let x = 1u32;
x * x + 2 * x - 1
});
}