added stacks example

Author: BartMassey

README.md

@@ -26,6 +26,7 @@ in a controlled repository now.
 * `addy.rs`: trait def/impl example
 * `bno-salad.rs`: trait salad example from Blandy and Orendorff
 * `matmul.rs`: example of overloading multiplication for matrices
+* `stacks.rs`: example defining a stack trait
 * `phantom.rs`: phantom type example
 * `deref.rs`: deref trait example
 * `from.rs`: from / try_from trait example

stacks.rs

@@ -0,0 +1,53 @@
+use std::collections::LinkedList;
+
+pub trait Stack<T> {
+    fn spush(&mut self, v: T);
+    fn spop(&mut self) -> Option<T>;
+}
+
+impl<'a, T: ?Sized> Stack<&'a T> for Vec<&'a T> {
+    fn spush(&mut self, v: &'a T) {
+        self.push(v);
+    }
+
+    fn spop(&mut self) -> Option<&'a T> {
+        self.pop()
+    }
+}
+
+impl<'a, T: ?Sized> Stack<&'a T> for LinkedList<&'a T> {
+    fn spush(&mut self, v: &'a T) {
+        self.push_front(v);
+    }
+
+    fn spop(&mut self) -> Option<&'a T> {
+        self.pop_front()
+    }
+}
+
+pub fn dup<'a, 'b: 'a, S, T>(stack: &mut S)
+    where S: Stack<&'b T> + 'a, T: 'b + ?Sized
+{
+    let v = stack.spop().unwrap();
+    stack.spush(v);
+    stack.spush(v);
+}
+
+#[cfg(test)]
+mod test {
+    use crate::*;
+
+    fn test<S>(mut s: S) where S: Stack<&'static str> {
+        s.spush("hello");
+        dup(&mut s);
+        assert_eq!(s.spop().unwrap(), "hello");
+        assert_eq!(s.spop().unwrap(), "hello");
+        assert_eq!(s.spop(), None);
+    }
+
+    #[test]
+    fn test_impls() {
+        test(Vec::new());
+        test(LinkedList::new());
+    }
+}