Re: [Haskell-beginners] Tracing a program with putStrLn

30 Jan 2010


      OK, that's a great help.


Michael Snoyman a écrit :
...
I think you want the Debug.Trace module. The documentation is 
available at 
http://haskell.org/ghc/docs/latest/html/libraries/base/Debug-Trace.html.
Let me know if you have any questions,
Michael
On Sun, Jan 31, 2010 at 12:00 AM, legajid mailto:legajid@free.fr> wrote:
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)
_______________________________________________
    Beginners mailing list
    Beginners@haskell.org mailto:Beginners@haskell.org
    http://www.haskell.org/mailman/listinfo/beginners