diff --git a/src/Control/Abstract/Evaluator.hs b/src/Control/Abstract/Evaluator.hs
index 80b0617a6..62528a324 100644
--- a/src/Control/Abstract/Evaluator.hs
+++ b/src/Control/Abstract/Evaluator.hs
@@ -39,6 +39,7 @@ deriving instance Member NonDet effects => Alternative (Evaluator term address v
 deriving instance Member (Lift IO) effects => MonadIO (Evaluator term address value effects)
 
 
+-- | An open-recursive function.
 type Open a = a -> a