Third chapter done

NOTES.md

 take() is handy, read more about this.
 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
+
+and_then(<closure>) allows you to map over a value returning an option.
+Useful for when the type mapping is an option.

lists/src/lib.rs

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

lists/src/third.rs

+use std::rc::Rc;
+
+pub struct List<T> {
+    head: Link<T>,
+}
+
+type Link<T> = Option<Rc<Node<T>>>;
+
+struct Node<T> {
+    elem: T,
+    next: Link<T>,
+}
+
+impl<T> List<T> {
+    pub fn new() -> Self {
+        List { head: None }
+    }
+    pub fn append(&self, elem: T) -> List<T> {
+        List { head: Some(Rc::new(Node {
+            elem,
+            next: self.head.clone(),
+        }))}
+    }
+    pub fn tail(&self) -> List<T> {
+        List { head: self.head.as_ref().and_then(|node| node.next.clone()) }
+    }
+    pub fn head(&self) -> Option<&T> {
+        self.head.as_ref().map(|node| &node.elem )
+    }
+}
+
+pub struct Iter<'a, T> {
+    next: Option<&'a Node<T>>,
+}
+
+impl<T> List<T> {
+    pub fn iter(&self) -> Iter<'_, T> {
+        Iter { next: self.head.as_ref().map(|node| &**node) }
+    }
+}
+
+impl<'a, T> Iterator for Iter<'a, T> {
+    type Item = &'a T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.next.map(|node| {
+            self.next = node.next.as_ref().map(|node| &**node);
+            &node.elem
+        })
+    }
+}
+
+impl<T> Drop for List<T> {
+    fn drop(&mut self) {
+        let mut head = self.head.take();
+        while let Some(node) = head {
+            if let Ok(mut node) = Rc::try_unwrap(node) {
+                head = node.next.take();
+            } else {
+                break;
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::List;
+
+    #[test]
+    fn basics() {
+        let list = List::new();
+        assert_eq!(list.head(), None);
+
+        let list = list.append(1).append(2).append(3);
+        assert_eq!(list.head(), Some(&3));
+
+        let list = list.tail();
+        assert_eq!(list.head(), Some(&2));
+
+        let list = list.tail();
+        assert_eq!(list.head(), Some(&1));
+
+        let list = list.tail();
+        assert_eq!(list.head(), None);
+
+        // Make sure empty tail works
+        let list = list.tail();
+        assert_eq!(list.head(), None);
+
+    }
+
+    #[test]
+    fn iter() {
+        let list = List::new().append(1).append(2).append(3);
+        let mut iter = list.iter();
+        assert_eq!(iter.next(), Some(&3));
+        assert_eq!(iter.next(), Some(&2));
+        assert_eq!(iter.next(), Some(&1));
+    }
+}