[Haskell-beginners] Using Debug.Trace

[Daniel Fischer]

Am Sonntag 31 Januar 2010 10:25:58 schrieb legajid:
> Hi,
>
> Starting with trace, i have trouble  with my calcul'' function while
> calcul' is ok.
> When afftrace in calcul'' is commented, the program compiles.
> When uncommented (            afftrace ("    calcul'' vide") ), i get
> the following messages :
>
> ------------------------------------------------------------------------
>------------------- *Main> :r
> [1 of 1] Compiling Main             ( sud3c.hs, interpreted )
>
> sud3c.hs:62:3:
>     Couldn't match expected type `[Char]'
>            against inferred type `(Plateau, Char)'
>     In a stmt of a 'do' expression: afftrace ("    calcul'' vide")
>     In the expression:
>         do afftrace ("    calcul'' vide")
>            (pxv, False)
>     In the definition of `calcul''':
>         calcul'' pxv _ []
>                    = do afftrace ("    calcul'' vide")
>                         (pxv, False)
> Failed, modules loaded: none.
> ------------------------------------------------------------------------
>-------------------
>
> I don't understand why, in calcul', it's ok and why, in calcul'', it's
> problematic. Because return value of calcul'' is a tuple ?

You defined

> afftrace x= if modetrace then trace x " "
>              else " "

If you ask ghci the type of that, you'll get

ghci> :t afftrace
afftrace :: String -> [Char]

(or afftrace :: [Char] -> [Char], or String -> String)

since

ghci> :t trace
trace :: String -> a -> a

Now you use afftrace in a do-block, which means "afftrace x" must have type

(Monad m) => m a

for some m and a. 
Well, afftrace x has type [Char], so m is [] and a is Char, fine.

That means you can use afftrace in any calculation returning a list of some 
kind (outside of do-blocks, also in other calculations).

But calcul'' doesn't return a list, it returns a pair. So

calcul'' pxv _ [] = afftrace ("  calcul'' vide") >> (pxv,False)

, which is what the first equation of calcul'' is desugared to, isn't well 
typed.

(>>) :: Monad m => m a -> m b -> m b

afftrace "  calcul'' vide" :: [] Char   -- m === [], a === Char

(pxv,False) :: (,) Plateau Bool   -- m === ((,) Plateau), b === Bool


(actually, ((,) Plateau) is indeed a monad, but it's a different one from 
[], so the expression is not well typed).

You could
- modify calcul'' to return [(Plateau,Bool)]
- not use do-blocks just for the sake of tracing and restructure your code
(I recommend the second)

infixl 0 `debug`

debug = flip trace

calcul' pxv [] = pxv `debug` "  Calcul' vide"
calcul' pxv (c:cs)
    | ok1       = calcul' xv1 cs `debug` "  Calcul' suite"
    | otherwise = pxv `debug` "  Calcul' pas de valeur"
      where
        vallib = [1 .. length pxv] ++ [5 .. 7]
        nbvlib = length vallib
        (xv1,ok1) = calcul'' pxv c vallib `debug` ("  " ++ show xv1) 
                             `debug` ("  Calcul' ok1=" ++ show ok1 ++ " 
c:cs= " ++ show (c:cs))

calcul'' pxv _ [] = (pxv,False) `debug` "  calcul'' vide"
calcul'' pxv c (li:lis)
    | False `debug` "  " ++ show pvx ++ "..." = undefined
    | li == 4 || li `elem` pxv  = calcul'' pxv c lis `debug` "quoi?"
    | otherwise     = (calcul'' pxv c li,True) `debug` "   calcul'''"

Now the code reads more natural (except for the "False `debug` ... " to 
produce general debugging output), and removing the debugging output isn't 
any harder.

>
> When my program is ok, should i remove all trace instructions (and
> associated do commands too) or just set  my modetrace value to False ?
>

Remove, resp. comment out.

>
> Thanks for helping,
> Didier
>
> Below my code :
>
>
> calcul' :: Plateau -> [Cellule] -> Plateau
> calcul' pxv  [] = do
>             afftrace ("  Calcul' vide")
>             pxv
> calcul' pxv  (c:cs)=    do
>             afftrace ("  Calcul' ok1="++show ok1++" c:cs= "++show
> (c:cs)) afftrace ("  "++show xv1)
>
>             if ok1  then do
>                     afftrace ("  Calcul' suite")
>                     calcul' xv1 cs
>                 else do
>                     afftrace ("  Calcul' pas de valeur")
>                     pxv
>     where
>         vallib=[1..length pxv]++[5..7]
>         nbvlib=length vallib
>         (xv1,ok1)=calcul'' pxv c vallib
>
>
>
> calcul'' :: Plateau -> Cellule -> [Valeur] -> (Plateau, Bool)
> calcul'' pxv  _ [] = do
>             --afftrace ("    calcul'' vide")
>             (pxv, False)
>
> calcul'' pxv  c (li : lis) = do
>                 --afftrace ("    "++show pxv)
>                 --afftrace ("    "++show c ++ "   "++show(li:lis))
>                 v2
>                 where
>                   v2= if (elem li pxv || li==4)
>                     then
>                         calcul'' pxv  c lis
>                     else do
>                         --afftrace ("      calcul'''")
>                         (calcul''' pxv c li, True)
>
> calcul''' :: Plateau -> Cellule -> Valeur -> Plateau
> calcul''' pxv c li =
>     take (c-1) pxv ++ [li] ++ drop c pxv
>
> afftrace x= if modetrace then trace x " "
>              else " "