Hi, all When I fold a list to update Data.Array, memory usage is very high. Consider following source, which counts occurence of each element in a list (1): import Data.Array import Data.List import Control.DeepSeq go :: [Int] -> [Int] go = elems . foldl' update (array (0,99) [(i,0) | i <- [0..99]]) where update acc x = acc // [(x, acc ! x + 1)] main = putStrLn . unwords . map show . go . concat . replicate 5000 $ [0..99] Above program uses about 350MB at heap. Memory usage is same if I try to force strictness in array update with `seq` (2) : where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `seq` a' However, when I use `deepseq`, memory usage is gone (around 35Kbyte) (3): where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `deepseq` a' What's the missing part in (2)? At (2), evaluation of updated array a' is forced and the value of array cell is also evaluated forcefully with `seq`. Any help would be appreciated deeply. -- Regards, Chul-Woong Yang