- Beginners - Haskell.org

requested module name differs from name found in interface file
by Larry Evans 20 Oct '08

20 Oct '08

With a file containing: > module Main where > > import Array > import Control.Functor.Fix I get: > make > ghc -i/root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 -c > catamorphism.example.hs > > catamorphism.example.hs:19:0: > Bad interface file: > /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2/Control/Functor/Fix.hi > Something is amiss; requested module main:Control.Functor.Fix > differs from name found in the interface file > category-extras-0.53.5:Control.Functor.Fix > make: *** [all] Error 1 I used cabal to install category-extras: > /home/evansl/download/haskell/libs # cabal install category-extras > Resolving dependencies... > Downloading category-extras-0.53.5... > Configuring category-extras-0.53.5... > Preprocessing library category-extras-0.53.5... ... > /usr/bin/ar: creating dist/build/libHScategory-extras-0.53.5.a > Installing library in /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 > Registering category-extras-0.53.5... > Reading package info from "dist/installed-pkg-config" ... done. > Saving old package config file... done. > Writing new package config file... done. > /home/evansl/download/haskell/libs # What should I do to import the Functor.Fix as shown here: http://comonad.com/reader/2008/recursion-schemes/ TIA. -Larry

1 0

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.

3 2

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.

3 2

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

1 0

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.

2 3

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

3 3

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

3 7

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

4 4

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

2 1

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

3 2