// Enum that works like a constructed function
enum Func {
    Add(i32, Box<Func>),
    Sub(i32, Box<Func>),
    Nil
}

impl Func {
    fn call(&self, value:i32) -> i32 {
        match self {
            Func::Nil => value,
            Func::Add(num, next) => next.call(value + num),
            Func::Sub(num, next) => next.call(value - num)
        }
    }
}

fn main() {
   let foo = Func::Add(40, Box::new(Func::Sub(3, Box::new(Func::Nil))));
   println!("{}", foo.call(5));
}