You were very close! Try
mapM putStrLn pImpliesQAndQImpliesRLoopShow >> return ()
Note the M on in mapM. That should do it for you.
On 2/6/07, Philippe de Rochambeau
Hello,
the following code
------------------------------------------------------------------------ ------- import Stdm -- From "Discrete Mathematics Using a Computer", by Cordelia Hall and John O'Donnell.
pImpliesQAndR :: [Bool] -> Bool pImpliesQAndR (p:q:r:[]) = p ==> ( q /\ r )
pImpliesQAndQImpliesR :: [Bool] -> Bool pImpliesQAndQImpliesR (p:q:r:[]) = ( p ==> q ) /\ ( q ==> r )
pImpliesQAndRLoop = [ ( (p, q, r), pImpliesQAndR [p, q, r] ) | p <- [True, False], q <- [True, False], r <- [True,False] ] pImpliesQAndQImpliesRLoop = [ ( (p, q, r), pImpliesQAndQImpliesR [p, q, r] ) | p <- [True, False], q <- [True, False], r <- [True,False] ]
pImpliesQAndRLoopShow = [ "pImpliesQAndR" ++ show x ++ " = " ++ show xs | (x,xs) <- pImpliesQAndRLoop ] pImpliesQAndQImpliesRLoopShow = [ "pImpliesQAndQImpliesR" ++ show x + + " = " ++ show xs | (x,xs) <- pImpliesQAndQImpliesRLoop ] ------------------------------------------------------------------------ -------
returns
------------------------------------------------------------------------ ------- ["pImpliesQAndR(True,True,True) = True","pImpliesQAndR (True,True,False) = False","pImpliesQAndR(True,False,True) = False","pImpliesQAndR(True,False,False) = False","pImpliesQAndR (False,True,True) = True","pImpliesQAndR(False,True,False) = True","pImpliesQAndR(False,False,True) = True","pImpliesQAndR (False,False,False) = True"] ------------------------------------------------------------------------ -------
when I type
------------------------------------------------------------------------ ------- pImpliesQAndRLoopShow ------------------------------------------------------------------------ -------
I still can't figure out how to write a function that loops through the string items in the list returned by pImpliesQAndRLoopShow and pImpliesQAndQImpliesRLoopShow, and print them separated by carriage returns, thus
pImpliesQAndR(True,True,True) = True pImpliesQAndR(True,True,False) = False ....
I have tried
------------------------------------------------------------------------ ------- map (putStrLn) pImpliesQAndRLoopShow ------------------------------------------------------------------------ -------
but that results in the following error message:
------------------------------------------------------------------------ ------- <interactive>:1:0: No instance for (Show (IO ())) arising from use of `print' at <interactive>:1:0-33 Possible fix: add an instance declaration for (Show (IO ())) In the expression: print it In a 'do' expression: print it *Main> ------------------------------------------------------------------------ -------
Any help would be appreciated.
Cheers,
phiroc
On 6 févr. 07, at 10:32, Ketil Malde wrote:
phiroc@free.fr wrote:
Ketil,
thanks for you help.
Here's the code:
and2 :: Bool -> Bool -> Bool and2 a b = a && b
loop = [ and2 x y | x <- [True,False], y <- [True,False] ]
Now, how do I have Haskell print
printStrLn("True and True = ") + <result of calling and2 True True> printStrLn("True and False = ") + <result of calling and2 True False>
Well, if you keep x and y in the table as well, either as a triple (x, y, and2 x y) or a nested tuple ((x,y), and2 x y) (which would let you use the lookup function), you can define a function that converts such a tuple to the approprate string. Keep in mind that Bool has a Show instance, so that e.g. show True => "True", and use ++ to join two strings.
Then, you can 'map' this function on the table to get a list of strings corresponding to table rows, and use 'concat' or 'unlines'. When you have a result you're happy with, you feed it to putStrLn.
-k
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