Hi, i wrote a program that doesn't behave as i wanted. So i tried to putStrLn my data structures to see what happened. Since i did this, all my functions changed type, so i had to modify them (main and calcul : use <- instead of let , add returns) and so on for each function called. Debugging this way causes more trouble than it should save. What is the way to trace data thru function recursive calls without changing program structures? Is putStrLn a good idea ? I'm not used to ghci debug and i find it hard to manage (:break, :trace, :cont ...). Is the 'debugged' version as efficient as the first one, due to impure functional code? What if i want to remove 'debug code' ? Should i modify back my functions ? Here are two very simplified versions of my program, the first one without 'trace', the second modified to include putStrLn as wanted. Thanks in advance, Didier First version ------------- main=do let valeurs=[0,1,3,0] let (valeurs_new)=calcul valeurs afficher_resultat valeurs afficher_resultat valeurs_new calcul :: [Int] -> [Int] calcul xv | nblibr == 0 = map (*2) xv | otherwise = map (+1) xv where libres = [ x | x <- [1..length xv] , xv !! (x-1) == 0] nblibr=length libres afficher_resultat xv = do putStrLn (show xv) Second version ---------------- main=do let valeurs=[0,1,3,0] valeurs_new <- calcul valeurs afficher_resultat valeurs afficher_resultat valeurs_new calcul :: [Int] -> IO [Int] calcul xv | nblibr == 0 = do putStrLn "ok" return (map (*2) xv) | otherwise = return (map (+1) xv) where libres = [ x | x <- [1..length xv] , xv !! (x-1) == 0] nblibr=length libres afficher_resultat xv = do putStrLn (show xv)