Hello, I'd like to add the mapAccumL function to the vector package. Specifically the Data.Vector.Storable module, but it would also be useful other vector modules. This is my attempt at an implementation: {-# LANGUAGE ScopedTypeVariables #-} mapAccumL :: forall a b c. (Storable b, Storable c) => (a -> b -> (a, c)) -> a -> DVS.Vector b -> (a, DVS.Vector c) mapAccumL f a vb = DVS.createT $ do vc <- DVSM.unsafeNew (DVS.length vb) a' <- go 0 a vc return (a', vc) where go :: Int -> a -> DVS.MVector s c -> ST s a go i a0 vc = if i < DVS.length vb then do let (a1, c1) = f a0 (DVS.unsafeIndex vb i) DVSM.unsafeWrite vc i c1 go (i + 1) a1 vc else return a0 {-# INLINE mapAccumL #-} The implementation should obey the following law: import qualified Data.List as L import qualified Data.Vector.Storable as DVS (DVS.toList <$> DVS.mapAccumL f a (DVS.fromList bs)) === L.mapAccumL f a bs Cheers, -John