Hello viktor import Data.List data T = T String Int Int deriving Show l :: [T] l = [ T "1" 0 30 , T "2" 0 40 ] tweight :: T -> Int tweight (T n f t) = t - f chunks :: Int -> (a -> Int) -> [a] -> [[a]] chunks target weight xs = go 0 xs where go _ [] = [] go n (h:ts) | null ts = [h] : [] | let w = n + weight h, w < target = cons1 h $ go w ts | otherwise = [h] : go 0 ts cons1 h ~(a:b) = (h : a) : b main :: IO () main = do let cs = chunks 10 tweight l print cs seems to work, but this is not what I want :)) picca@cush:~$ runhaskell test.hs [[T "1" 0 30],[T "2" 0 40]] I want at the end to split each of my T into chunk of length target. like this [[T "1" 0 10], [T "1" 10 20], [T "1" 20 30], [T "2" 0 10], ...] So a split function like this should be used split :: Int -> a -> (a, a) split s (T n f t) = (T n f s, T n s t) Cheers Frederic