struct Mul<A> {
    f: Box<dyn Fn(A) -> i32>
}

impl<A> Mul<A> {
    fn call(&self, a: A) {
        println!("Result: {:?}", (self.f)(a));
    }
}

fn create<A>(f: Box<dyn Fn(A) -> i32>) -> Mul<A> {
    Mul { f }
}

fn main() {
    let double_mul = create(Box::new(|x: &i32| *x * 2));
    let arg = 333;
    double_mul.call(&arg);
}