Two solutions using immutable and mutable arrays and no unsafe operations: ---- module Main where import Control.Monad.ST import Data.Ix import Data.Array import Data.Array.MArray import Data.Array.ST -- using immutable arrays hist1 :: String -> Array Char Int hist1 str = accumArray (+) 0 ('\0','\255') [(c,1) | c<-str] freq1 :: String -> [(Char,Int)] freq1 = assocs . hist1 -- using mutable ST arrays hist2 :: String -> STArray s Char Int -> ST s () hist2 str arr = sequence_ [do { i<-readArray arr c; writeArray arr c (1+i) } | c<-str] freq2 :: String -> [(Char,Int)] freq2 str = runST (do { arr<-newArray ('\0', '\255') 0 ; hist2 str arr ; getAssocs arr }) -- Cheers, Pedro