prepend.rs

use std::util::replace;

enum List<T> {
    Cons(T, ~List<T>),
    Nil
}

impl<T> List<T> {
    fn prepend(&mut self, elem: T) {
        *self = Cons(elem, ~replace(self, Nil));
    }

    fn tail(&self) -> &List<T> {
        match *self {
            Cons(_, ref tail) -> tail,
            Nil -> fail!("empty list")
        }
    }
}