Midway through chapter 4

NOTES.md

 Sometimes you just have to get your hands dirty and do some lifetimes, just a fact of life, get used to it son.
 Revise some of these impl types, to understand better how they function with std library collections.
 
-# Lessons from Persistent Stack
+# Lessons from Stack 3
 
 and_then(<closure>) allows you to map over a value returning an option.
-Useful for when the type mapping is an option.
+Useful for when the type mapping is an option.
+
+# Lessons from Stack 4
+
+It's important to conceptualise the invariants before you write code to validate assumptions and edge cases.
+borrow() and borrow_mut() are important methods when using RefCell.
+Drop can be implemented with `popping` until no elements are left.

lists/src/fourth.rs

+use std::rc::Rc;
+use std::cell::RefCell;
+
+pub struct List<T> {
+    head: Link<T>,
+    tail: Link<T>,
+}
+
+type Link<T> = Option<Rc<RefCell<Node<T>>>>;
+
+struct Node<T> {
+    elem: T,
+    next: Link<T>,
+    prev: Link<T>,
+}
+
+impl<T> Node<T> {
+    fn new(elem: T) -> Rc<RefCell<Self>> {
+        Rc::new(RefCell::new(Node {
+            elem,
+            prev: None,
+            next: None,
+        }))
+    }
+}
+
+impl<T> List<T> {
+
+    pub fn new() -> Self {
+        List { head: None, tail: None }
+    }
+
+    pub fn push_front(&mut self, elem: T) {
+        let new_head = Node::new(elem);
+        match self.head.take() {
+            Some(old_head) => {
+                old_head.borrow_mut().prev = Some(new_head.clone());
+                new_head.borrow_mut().next = Some(old_head);
+                self.head = Some(new_head);
+            }
+            None => {
+                self.tail = Some(new_head.clone());
+                self.head = Some(new_head);
+            }
+        }
+    }
+
+    pub fn pop_front(&mut self) -> Option<T> {
+        self.head.take().map(|old_head| {
+            match old_head.borrow_mut().next.take() {
+                Some(new_head) => {
+                    new_head.borrow_mut().prev.take();
+                    self.head = Some(new_head);
+                }
+                None => {
+                    self.tail.take();
+                }
+            }
+            Rc::try_unwrap(old_head).ok().unwrap().into_inner().elem
+        })
+    }
+}
+
+impl<T> Drop for List<T> {
+    fn drop(&mut self) {
+        while self.pop_front().is_some() {}
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::List;
+
+    #[test]
+    fn basics() {
+        let mut list = List::new();
+
+        assert_eq!(list.pop_front(), None);
+
+        list.push_front(1);
+        list.push_front(2);
+        list.push_front(3);
+
+        assert_eq!(list.pop_front(), Some(3));
+        assert_eq!(list.pop_front(), Some(2));
+
+        list.push_front(4);
+        list.push_front(5);
+
+        assert_eq!(list.pop_front(), Some(5));
+        assert_eq!(list.pop_front(), Some(4));
+
+        assert_eq!(list.pop_front(), Some(1));
+        assert_eq!(list.pop_front(), None);
+    }
+}
+
+

lists/src/lib.rs

 pub mod first;
 pub mod second;
-pub mod third;
+pub mod third;
+pub mod fourth;