[types] Test for displaying empty Pair

types/src/object.rs

@@ -39,6 +39,7 @@ impl Obj {
     }
 
     pub fn take(&mut self) -> Obj {
+        // Stole Option's implementation of this. Handy. :D
         mem::replace(self, Obj::Null)
     }
 

types/src/pair.rs

@@ -49,3 +49,14 @@ impl fmt::Display for Pair {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::Pair;
+
+    #[test]
+    fn display_empty_pair() {
+        let empty = Pair::empty();
+        let disp = format!("{}", empty);
+        assert_eq!(disp, "(())");
+    }
+}