- Beginners - Haskell.org

Mathematical Blundering
by Jeffrey Drake 18 Oct '08

18 Oct '08

I have defined myself a set of functions to test: fact 1 = 1 fact n = n * (fact $ n - 1) sine x = x - (x^3/(fact 3)) + (x^5/(fact 5)) - (x^7/(fact 7)) Where my code is 'sine' and the prelude's is sin: *Main> sine 1 0.841468253968254 *Main> sin 1 0.8414709848078965 *Main> sine 2 0.9079365079365079 *Main> sin 2 0.9092974268256817 *Main> sine 3 9.107142857142847e-2 *Main> sin 3 0.1411200080598672 *Main> sine 4 -1.3841269841269837 *Main> sin 4 -0.7568024953079282 After 2 they seem to diverge rather rapidly, and I am not sure why. Any ideas? I would have thought that 4 terms would have been enough. - Jeff.

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IO Problem
by Jamie McCloskey 18 Oct '08

18 Oct '08

Hi everyone, I have just started learning Haskell, it is a very interesting language. As a first project, I am building a brainfuck interpreter, but have hit a stumbling block: IO. My program source is located at http://hpaste.org/11219 My program handles the state of a brainfuck program by passing State(lists of ints) objects around. When I added my getIn fuction, however, it needed to return an IO State object. This broke the run function, as it returned a State. No problem, I thought, and changed run to return IO State. This broke my exec function, which calls run on each element in a list. As exec passes on the state returned by the run function to itself, it needs to take an IO State. This causes problems in the loop function, which calls exec on a subprogram. Because exec now takes IO State, loop also needs to. However, loop is called by run, so now run needs to take IO State. Now, all functions called by run need to take IO State. All this creates a whole load of functions with IO, even though it should not be necessary. What I would like to know, is how I can avoid this, possibly by modifying exec to just take a State. Whew! What a long-winded explanation! Any ideas? --Jamie P.S. Please ignore my long-winded list iteration functions -- I'm working on a cleaner version.

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HaXml.SAX successfully parses a malformed XML document
by David Frey 17 Oct '08

17 Oct '08

It seems that saxParse from Text.XML.HaXml.SAX will successfully parse a malformed XML document. Notice that the SubElem opening and closing tag are not matched in the XML document below. --- input.xml --- <RootNode> <ElemOne> <SubElem attr="foo">element data</SubElemBroken> <NoDataElem/> <ElemOne> </RootNode> --- end input.xml --- The Haskell code below runs without error. It prints out the type of elements found during the parse. --- Haskell Code --- module Main where import qualified Text.XML.HaXml.SAX as SAX import Data.Maybe main = let inputFilename = "input.xml" in do content <- readFile inputFilename let (elements, error) = SAX.saxParse inputFilename content if isNothing error then mapM_ putStrLn (summarizeElements elements) else putStrLn $ "ERROR:" ++ (fromJust error) summarizeElements :: [SAX.SaxElement] -> [String] summarizeElements elements = map summarizeElement elements summarizeElement :: SAX.SaxElement -> String summarizeElement element = case element of (SAX.SaxDocTypeDecl d) -> "DocType" (SAX.SaxProcessingInstruction p) -> "Processing Instruction" (SAX.SaxComment s) -> "Comment" (SAX.SaxElementOpen name attrs) -> "Element Open" (SAX.SaxElementClose name) -> "Element Close" (SAX.SaxElementTag name attrs) -> "No Content Element" (SAX.SaxCharData charData) -> "Character Data" (SAX.SaxReference reference) -> "Reference" --- End Haskell Code --- The Python code below throws an exception when parsing the same input document. --- Python Code --- from xml.sax import make_parser from xml.sax.handler import ContentHandler def main(): c = ContentHandler() p = make_parser() p.setContentHandler(c) p.parse(open('input.xml')) if __name__ == '__main__': main() -- End Python Code --- Is this a bug in the HaXml SAX parser? Thanks, David

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better way to create Array defined on all indices
by Larry Evans 16 Oct '08

16 Oct '08

I'd like to have an Array indices values (see http://haskell.org/onlinereport/array.html) defined on [minBound::indices ... maxBound::indices]. The attached does this; however, it also allows duplicate definitions (e.g. in the attached, the value for Term index is defined twice). Is there a better way to do this? Maybe a way that doesn't use all the contexts? Also, is there a way to do it so that the value associated with an index is not redefined? In effect, I'm making a map, but using an Array as the implementation because, I guess, it would be a bit faster. TIA.

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fix
by Matthew J. Williams 16 Oct '08

16 Oct '08

hello listers, a few days ago A fellow lister sent me the following link: http://en.wikibooks.org/wiki/Haskell/Fix_and_recursion The 'fix' function is interesting to say the least. There is one example that I've had difficulty expanding: fix (\rec n -> if n == 0 then 1 else n * rec (n-1)) 5 120 My interpretation: fix (\rec n -> if n == 0 then 1 else n * rec (n-1)) 5 ((\rec n -> if n == 0 then 1 else n * rec (n-1)) (fix (\rec n -> if n == 0 then 1 else n * rec (n-1)) )) 5 . . . Yet, it does not quite explain how 'fix' does not result in infinite recursion. Sincerely Matthew J. Williams

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Problems with ghc
by Paul Johnston 15 Oct '08

15 Oct '08

Apologies if this is not exactly what this list is for but is anyone running ghc on Solaris, specifically Open Solaris 5.11? It seemed to work paulj@hb-0021209.humanities.manchester:~/haskell/aht/3$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.3 paulj@hb-0021209.humanities.manchester:~/haskell/aht/3$ ghci GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> And ghci works as expected but !!! Trying to compile I get: paulj@hb-0021209.humanities.manchester:~/haskell/aht/3$ ghc -o Haq Haq.hs compilation IS NOT required Undefined first referenced symbol in file __gmpz_tdiv_q /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_tdiv_r /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpn_cmp /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_add /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_and /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_gcd /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_ior /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_com /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_mul /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_xor /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_sub /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpn_gcd_1 /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_init /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_fdiv_qr /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmp_set_memory_functions /usr/local/lib/ghc-6.8.3/libHSrts.a(Storage.o) __gmpz_divexact /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) __gmpz_tdiv_qr /usr/local/lib/ghc-6.8.3/libHSrts.a(PrimOps.o) ld: fatal: Symbol referencing errors. No output written to Haq collect2: ld returned 1 exit status I get this trying to build hmake with the new configure file supplied by Malcolm Wallace. Once again sorry if its off topic but wondered if it's only me :-) Cheers Paul

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variables in haskell
by Matthew J. Williams 15 Oct '08

15 Oct '08

Hello listers, would one be correct in thinking that 'bound variables' such as those used in haskell were in fact constants? Sincerely Matthew Williams

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Can we specify the module of the function being referred
by Amit Kumar Dhar 14 Oct '08

14 Oct '08

Dear listers, I was writing a program which defines a function named "show". But at each of the recursive calls to itself "ghci" is showing the following message: ' Ambiguous occurrence `show' It could refer to either `show', defined at temp.hs:5:0 or `show', imported from Prelude ' I don't want to import the 'show' function from prelude. Is there any way out or I have to declare instance of the 'Show'? Can we specify the module of a function from which it will be referred? -- Amit kumar Dhar

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How to explain this behaviour of ghc ?
by sylvain 11 Oct '08

11 Oct '08

Hi, when loaded in ghci (version 6.8.2) the following code snippet data Test = A Int instance Show Test main = print (A 1) leads to: *Main> main *** Exception: stack overflow How to explain that ? Thanks, Sylvain

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Call for Contributions - Haskell Communities and Activities Report, November 2008 edition
by Janis Voigtlaender 10 Oct '08

10 Oct '08

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, I would very much like to collect contributions for the 15th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 31 October 2008 (please send your contributions to hcar(a)haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the May 2008 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The best format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2008/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2008/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should have received your old entry as a template already (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Do not worry about writing correct LaTeX, I will be able to handle your file. Please do not use HTML or even DOC. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or ``historic'' overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep entries up. There is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports over distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write ``wishlist'' items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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