Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. I came up with the following tail-recursive generalization of `foldl` that allows exiting the computation prematurely: foldlE :: (a -> c) -> (a -> b -> Either c a) -> Either c a -> [b] -> c foldlE f g = fld where fld (Left c) _ = c fld (Right a) [] = f a fld (Right a) (x:xs) = fld (g a x) xs `foldl` can be defined from it as foldl'' :: (a -> b -> a) -> a -> [b] -> a foldl'' f z = foldlE id ((Right .) . f) (Right z) and `!!` as: -- Checks for a negative index omitted for brevity. index :: Int -> [a] -> a index i = foldlE (error $ "No such index") f (Right i) where f 0 x = Left x f n _ = Right (n - 1) Is something like that already available somewhere? Best regards, Petr Pudlak