pub trait State {}
pub struct Fun;
impl State for Fun {}
pub struct Sad;
impl State for Sad {}

pub struct J<T: State> {
    s: T
}

impl J<Sad> {
    fn new() -> J<Sad> {
        J{s: Sad}
    }
}

impl <T: State>  J<T> {
    fn change<TT: State>(self, t: TT) -> J<TT> {
        J { s: t}
    }
}

pub trait Funny {
    fn fun(&mut self);
}

pub struct JuJu<T: State> { /// don't want to have type parameter here
    j: J<T>
}

impl <T: State> Funny for JuJu<T> {
    fn fun(&mut self) {
        self.j = self.j.change(Fun)
    }
}

fn main() {
    println!("Hello, world!");
}