Am Samstag 30 Januar 2010 23:01:57 schrieb Michael Snoyman:
> 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.
Yes. Definitely.
As a further tip:
import Debug.Trace
infixl 0 `debug`
debug :: a -> String -> a
debug = flip trace
(of course you can call it something else)
With 'debug', it's more convenient to add and comment out tracing output:
fonction x = trace ("fonction " ++ show x) (travail x)
~> fonction x = {- trace ("fonction " ++ show x) -} (travail x)
vs.
fonction x = travail x `debug` "fonction " ++ show x
~> fonction x = travail x -- `debug` "fonction " ++ show x
>
> Let me know if you have any questions,
> Michael
>
> On Sun, Jan 31, 2010 at 12:00 AM, legajid <legajid at 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)
> >
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