Do GADT instead

src/Data/Query.hs

@@ -28,10 +28,10 @@ data Field a where
   Url :: Field String
   Title :: Field String
 
-data QueryF f
-  = forall a. StartsWith (Field a) a
-  | forall a. EndsWith (Field a) a
-  | And f f
+data QueryF f where
+  StartsWith :: Field a -> a -> QueryF f
+  EndsWith :: Field a -> a -> QueryF f
+  And :: f -> f -> QueryF f
 
 deriving instance Functor QueryF