Attribute Macro evaluate 

#[evaluate]

Evaluates a zero-argument function and optionally reports when the value changes.

The #[evaluate(print_fn)] attribute macro transforms a function into a reactive evaluator that:

1. Computes the function result on each call.
2. Compares it with the previously computed value.
3. If the value is unchanged, calls the specified print function with a message.

Requirements

• The function must have no parameters.
• The function must return a value (-> T), which must implement Eq + Clone.
• The print function (e.g., print) must be a callable accepting a String.

Examples

use reactive_macros::evaluate;

fn print(msg: String) {
    println!("{}", msg);
}

#[evaluate(print)]
pub fn get_number() -> i32 {
    42
}

fn main() {
    // First call computes the value
    assert_eq!(get_number(), 42);
    // Second call compares with previous; prints message since value didn't change
    assert_eq!(get_number(), 42);
}

SAFETY

This macro uses a static mut internally to store the previous value, so it is not thread-safe. It should only be used in single-threaded contexts.