a persistent immutable linked list made with Arcs (can be replaced with Rcs) (chapter 3)

src/lib.rs

@@ -1,2 +1,3 @@
 pub mod bad;
 pub mod ok;
+pub mod persistent;

src/persistent.rs

@@ -0,0 +1,105 @@
+use std::sync::Arc;
+
+pub struct List <T> {
+    head: Link<T>,
+}
+
+type Link <T> = Option<Arc<Node<T>>>;
+
+struct Node <T> {
+    value: T,
+    next: Link<T>,
+}
+
+impl <T> List <T> {
+    pub fn new () -> Self {
+        List { head: None }
+    }
+
+    pub fn prepend (&self, value: T) -> List<T> {
+        List { head: Some(Arc::new(Node {
+            value, 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.value)
+    }
+
+    pub fn iter (&self) -> Iter<T> {
+        Iter { next: self.head.as_deref()}
+    }
+}
+
+pub struct Iter <'a, T> {
+    next: Option<&'a Node<T>>,
+}
+
+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_deref();
+            &node.value
+        })
+    }
+}
+
+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) = Arc::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.prepend(1).prepend(2).prepend(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().prepend(1).prepend(2).prepend(3);
+
+        let mut iter = list.iter();
+        assert_eq!(iter.next(), Some(&3));
+        assert_eq!(iter.next(), Some(&2));
+        assert_eq!(iter.next(), Some(&1));
+    }
+
+
+}