My proposal would be to add a function like \begin{code} replaceBy :: ([a] -> (b, [a])) -> [a] -> [b] replaceBy splt l = case l of [] -> [] _ -> let (ft, rt) = splt l in ft : replaceBy splt rt \end{code} that takes a function "splt" that splits a non-empty list and returns a shorter list as second component (to ensure termination). I've used this function to implement a replace function (below), but the simplest application is: \begin{code} splitAts :: Int -> [a] -> [[a]] splitAts n | n > 0 = replaceBy (splitAt n) \end{code} or with Control.Applicative: \begin{code} breaks :: (a -> Bool) -> [a] -> [[a]] breaks p = replaceBy $ second (drop 1) . break p \end{code} This "breaks" function ignores a final separator! breaks (',' ==) "123,456,,78" == ["123","456","","78"] breaks (',' ==) "123,456,78," == ["123","456","78"] groupBy could be implemented as follows: \begin{code} groupBy eq = replaceBy $ \ (h : r) -> first (h :) $ span (eq h) r \end{code} And given a function to split of a single "run", "runs" (with the same type as groupBy) is simply: \begin{code} runs :: (a -> a -> Bool) -> [a] -> [[a]] runs eq = replaceBy (run eq) run :: (a -> a -> Bool) -> [a] -> ([a], [a]) run eq l = case l of x : r@(y : s) | eq x y -> first (x :) $ run eq r x : r -> ([x], r) [] -> ([], []) \end{code} My original application for replacing a substring by a special character is defined by: \begin{code} replace :: Eq a => [a] -> a -> [a] -> [a] replace sl@(_ : _) r = replaceBy $ \ l@(hd : tl) -> case stripPrefix sl l of Nothing -> (hd, tl) Just rt -> (r, rt) \end{code} Cheers Christian