March 2005 - Haskell-Cafe

Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

Curiouser and curiouser... expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t + e) +++ return t solves the undefined variable problem but introduces a new 'Last operator in do {...} must be an expression' error, which then disappears if I explicitly return e expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return e return (t + e) +++ return t to give me the original undefined variable t error at the line expr = do t <- term . It looks in scope to me... :( Thanks, Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: [Haskell-cafe] Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

perhaps like this: > expr = do t <- term > (do symbol "+" > e <- expr > return (t+e) > ) > +++ > (return t) > > >although I think you may also want a 'try' before the first alternative. No, that still gives the same undefined variable error. :( Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: [Haskell-cafe] wxHaskell htmlWindow usage
by Bayley, Alistair 15 Mar '05

15 Mar '05

> From: David Owen [mailto:d_w_o@hotmail.com] > > htmlWindowSetPage might be the wrong function to use for this > or I might > just be using it incorrectly. This may well be more > complicated than I'm > hoping it is too! I-Am-Not-A-WxHaskell-Expert, but... a quick google finds this page: http://www.wxwidgets.org/manuals/2.4.2/wx499.htm#wxhtml ... where is is explained that wxHtmlWindow.LoadPage is used to load a page from a file, while wxHtmlWindow.SetPage is used to display a literal HTML string in the widget. And in the wxHaskell docs there are two matching functions: - htmlWindowSetPage - htmlWindowLoadPage which probably correspond. So, I guess you want to use htmlWindowLoadPage. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Parity of the number of inversions of a permutation
by Henning Thielemann 15 Mar '05

15 Mar '05

I think it is a quite popular problem. I have a permutation and I want to count how often a big number is left from a smaller one. More precisely I'm interested in whether this number is even or odd. That's for instance the criterion to decide whether Lloyd's shuffle puzzle is solvable or not. Example: 1 4 3 2 I can choose six pairs (respecting the order) of numbers out of it, namely (1,4) (1,3) (1,2) (4,3) (4,2) (3,2), where in the last three pairs the first member is greater than the second one. Thus I have 3 inversions and an odd parity. I' searching for a function which sorts the numbers and determines the parity of the number of inversions. I assume that there are elegant and fast algorithms for this problem (n * log n time steps), e.g. a merge sort algorithm. A brute force solution with quadratic time consumption is countInversions :: (Ord a) => [a] -> Int countInversions = sum . map (\(x:xs) -> length (filter (x>) xs)) . init . tails

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Parser question
by Nicola Whitehead 15 Mar '05

15 Mar '05

Hi folks, I have a parser problem. I have a basic calculator program (Graham Hutton's from Nottingham) which contains the following code: -- Define a parser to handle the input expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t + e) +++ return t term :: Parser Int term = do f <- factor do symbol "*" e <- expr return (f * t) +++ return f factor :: Parser Int factor = do symbol "(" e <- expr symbol ")" return e +++ natural symbol and natural are defined elsewhere and work fine, but when I compile it I get the error ERROR "C:/HUGS/Calculator.hs":66 - Undefined variable "t" I suspect I'm missing something obvious, but for the life of me I can't see it. Any suggestions? Thanks, Nik (Trying to keep a couple of weeks ahead of her students) Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: Newbern's Example 7
by Alson Kemp 13 Mar '05

13 Mar '05

Jeff, Perfect explanation. I got gummed up in the syntactic sugar and thought that >>= was sugar for the "do" notation, not vice versa. Thank you for the reminder and clarification. - Alson

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Solution to Thompson's Exercise 4.4
by Kaoru Hosokawa 13 Mar '05

13 Mar '05

I have been working through the exercises in Thompson's The Craft of Functional Programming 2nd Ed book. I am looking for a solution web site for Thompson's book. Or maybe the people here can help. In exercise 4.4, I am asked to define a function howManyOfFourEqual :: Int -> Int -> Int -> Int -> Int which returns the number of integers that are equal to each other. For example, howManyOfFourEqual 1 1 1 1 = 4 howManyOfFourEqual 1 2 3 1 = 2 howManyOfFourEqual 1 2 3 4 = 0 This is my solution. I give it here, since it's not an elegant solution. howManyOfFourEqual :: Int -> Int -> Int -> Int -> Int howManyOfFourEqual a b c d | a == b && howManyEqual b c d /= 0 = howManyEqual b c d + 1 | a == c && howManyEqual b c d /= 0 = howManyEqual b c d + 1 | a == d && howManyEqual b c d /= 0 = howManyEqual b c d + 1 | a == b && howManyEqual b c d == 0 = 2 | a == c && howManyEqual b c d == 0 = 2 | a == d && howManyEqual b c d == 0 = 2 | otherwise = howManyEqual b c d howManyEqual is a function from a previous exercise. howManyEqual :: Int -> Int -> Int -> Int howManyEqual a b c | a == b && b == c = 3 | a /= b && b /= c && a /= c = 0 | otherwise = 2 I hope to find a better solution. I googled but couldn't find the answer. Kaoru Hosokawa khosokawa(a)gmail.com

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RE: Newbern's Example 7
by Alson Kemp 13 Mar '05

13 Mar '05

Jeff, Thank you for your reply. >> After fn is lifted, where is "bind" used in allCombinations? >The bind happens inside the liftM2 function, which can be defined as >(taken from http://www.haskell.org/onlinereport/monad.html) >liftM2 f = \a b -> do { a' <- a; b' <- b; return (f a' b') } Using the above, I think that allCombinations can be rewritten: allCombinations fn (l:ls) = foldl (\a b -> do { a' <- a; b' <- b; return (fn a' b') }) l ls I'm not sure how "allCombinations (+) [[1,2][2,3]]" gets to the final result, since the above would seem to yield: allCombinations (+) [1,2]:[[2,3]] = foldl (\a b -> do { a' <- a; b' <- b; return (fn a' b') }) [1,2] [[2,3]] allCombinations (+) [1,2]:[[2,3]] = return ((+) [1,2] [2,3]) You mentioned on your site that the operation of the List monad is key to comprehending this function, but I still wasn't able to grok it. Any pointers would be appreciated. - Alson

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hFileSize vs length
by S. Alexander Jacobson 12 Mar '05

12 Mar '05

I am using GHC 6.2 on windows and am finding that when I open a file and use hFileSize I get a different number than I get from reading in the file and calculating the length. I assume this is not a bug, but I don't know why its happening. Also, why isn't there getFileSize function in System.Directory? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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fgl and Windows
by Tom Spencer 11 Mar '05

11 Mar '05

Does anyone have a build.bat that they have used to install fgl on windows that they could donate to me? Thanks Tom Spencer

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