diff --git a/src/func.rs b/src/func.rs
index 9fd2fbb..468989e 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -300,3 +300,9 @@ pub trait SharedFunctor: WeakFunctor {
     where
         Self: 'a;
 }
+
+pub trait CovariantFunctor: WeakFunctor {
+    fn variate<'a: 'b, 'b, A: 'a>(fa: Self::F<'a, A>) -> Self::F<'b, A>
+    where
+        Self: 'a;
+}