fn main() {
    println!("{}", (1, (2, (4, Empty()))).get::<Succ<Succ<Zero>>>());
}

struct Empty ();

trait Cons<T> {
    type Tail: Cons<T>;
    
    fn get_head(&self) -> &T;
    fn get_tail(&self) -> &Self::Tail;
    
    fn get<P: Peano>(&self) -> &T where Self: std::marker::Sized {
        P::get(self)
    }
}

impl<T, C:Cons<T>> Cons<T> for (T, C) {
    type Tail = C;
    
    fn get_head(&self) -> &T {
        &self.0
    }
    fn get_tail(&self) -> &Self::Tail {
        &self.1
    }
}

impl<T> Cons<T> for Empty {
    type Tail = Empty;
    
    fn get_head(&self) -> &T {
        panic!()
    }
    fn get_tail(&self) -> &Self::Tail {
        panic!()
    }
}

enum Zero {}

trait Peano {
    fn get<T, C: Cons<T>>(cons: &C) -> &T;
}

impl Peano for Zero {
    
    fn get<T, C: Cons<T>>(cons: &C) -> &T {
        cons.get_head()
    }
}

struct Succ<T>(T);

impl<P: Peano> Peano for Succ<P> {
    fn get<T, C: Cons<T>>(cons: &C) -> &T {
        P::get(cons.get_tail())
    }
}