foldMapWithIndex :: Monoid m => (Int -> a -> m) -> Seq a -> m This would match both Data.Map.foldMapWithKey and Control.Lens.Indexed.ifoldMap, filling out the indexed fold menagerie. Annoyingly, the straightforward implementation I've come up with is rather slow when used with Endo to implement either foldlWithIndex or foldrWithIndex. I suspect the polymorphic recursion inherent in these sequences is blinding GHC's optimizer. Oh well.