Functor trait and example rust

functor.rs

trait Functor <A, B, C, F> where F : Fn(&A) -> B {
    fn fmap(&self, f: F) ->  C;
}

enum List<T> {
    Nil,
    List(T,  Box<List<T>>)
}

impl<A, B, F> Functor<A, B, List<B>, F> for List<A> 
where F : Fn(&A) -> B {
    fn fmap(&self, f: F) -> List<B> {
        match self {
            List::Nil => {return List::Nil},
            List::List(v, next) => {
                List::List(
                    f(v),
                    Box::new(next.fmap(f))
                )
            }
        }
    }
}