It would be better to produce a triple (as was suggested) loop = [(x, y, x&&y) | x <- [True, False], y <- [True, False]] The you can map a function that prints over that. Or even better map a function that generates the strings, and the use one big putStr at the end. Always try to separate IO from computation. -- Lennart On Feb 6, 2007, at 10:18 , 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> ...

Thanks.

phiroc

Quoting Ketil Malde :

...

phiroc@free.fr wrote:

...

I would like to create a Haskell function that generates a truth table, for all Boolean values, say, using the following "and" function :

and :: Bool -> Bool -> Bool and a b = a && b

What is the type of the resulting table?

...

I have tried creating a second function called "loop", which repeatedly calls "and", but it did not work, because, for some reason unknown to me, "do" does not like repeated function calls

loop = do and True True and True False

I'm not sure I understand what you expected here. The 'do' syntax is for monadic code.

...

Is there a better way to repeatedly call "and"?

If you want your table in list for, I'd suggest using a list comprehension.

Here's how you'd calculate squares of numbers, for instance:

squares = [ x^2 | x <- [1..5] ]

...

Furthermore, is there a way in Haskell to loop through the Boolean values (True and False)

Since there are only two values, you can just feed a list comprehension with [True,False].

...

Last but not least, in the "loop" function above, assuming that there is a way to repeatedly call the "and" function, how could you intersperse "printStr"s between the "and" calls?

I would't - keep the the calculation and the output separate instead.

-k

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On 2/6/07, phiroc@free.fr wrote:

Hello,

how would you convert Boolean triples to strings, in the IO function?

printStrings :: (Bool,Bool,Bool) -> IO ()

Take a look at 'show': Prelude> show True "True" martin

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

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 wrote:

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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