struct Optimisation {
    objective : Box< dyn Fn(f64) -> f64 + 'static >,
    diff_objective : Option<Box<dyn Fn(f64) -> f64 + 'static >>
}

fn main() {

    let opt = Optimisation {
        objective: Box::new(|x: f64| -x*x),
        diff_objective: None
    };

    let opt_closed_form = Optimisation {
        objective: Box::new(|x: f64| -x*x),
        diff_objective: Some(Box::new(|x| -2.0*x))
    };
}