It seems like the fastest way to build a list of frequency from a string is this : import Data.Array (assocs, accumArray, Array) frequency :: String -> [(Char,Int)] frequency = filter (\f -> snd f>0) . assocs . accumArray (+) 0 ('\0', '\255') . map (\x->(x,1)) -- (~1.3 sec on a string of 700ko) But if we want to make this polymorphic we have to do this : import Data.Ix (Ix) frequency :: (Ix a, Bounded a) => [a] -> [(a,Int)] frequency = filter (\f -> snd f>0) . assocs . accumArray (+) 0 (minBound, maxBound) . map (\x->(x,1)) -- (>5 sec on a string of 700ko) but it's really much slower, it seems like min/maxBound are check each time or I dont know, and it's restrictive on the type of the elements of the list. The best polymorphic version seems to be : import Data.Map as Map (lookup, assocs, empty, insertWith) frequency :: (Ord a) => [a] -> [(a,Int)] frequency xs = Map.assocs $ foldl f Map.empty xs where f m x = let m' = Map.insertWith (+) x 1 m Just v = Map.lookup x m' in seq v m' -- (~2.1 sec on a string of 700ko) Which is not as fast as the specialized version on string, but really faster and less restrictive than the second. Thanks everyone for all your answers ! Charles