Hi, it works fine, except (xv1,ok1) = calcul'' pxv c vallib `debug` (" " ++ show xv1) `debug` (" Calcul' ok1=" ++ show ok1 ++ " c:cs= " ++ show (c:cs)) that causes an infinite loop, cause xv1 and ok1 are just being calculated. Thank you for your clear explanation of do statements. Didier. Daniel Fischer a écrit :
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 " "