April 2011 - Beginners

Anyone have problems with cabal and SDL?
by Tom Murphy 17 Jul '11

17 Jul '11

When I use Cabal to get any program which depends on the SDL library, this is the error I get. " checking for sdl-config... no checking for sdl11-config... no configure: error: *** SDL not found! Get SDL from www.libsdl.org. If you already installed it, check it's in the path. If problem remains, please send a mail to the address that appears in ./configure --version indicating your platform, the version of configure script and the problem. cabal: Error: some packages failed to install: Raincat-1.1.1.2 depends on SDL-0.6.2 which failed to install. SDL-0.6.2 failed during the configure step. The exception was: exit: ExitFailure 1 SDL-image-0.6.1 depends on SDL-0.6.2 which failed to install. SDL-mixer-0.6.1 depends on SDL-0.6.2 which failed to install. " I have SDL 1.2.14 (correctly) installed on my system. I'd really appreciate any help with this. Tom

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haskell for FPGAS
by David Blubaugh 09 May '11

09 May '11

To All, Has anyone developed HASKELL FOR FPGA development ?? Thank You, Daid Blubaugh

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All about monads, exercise 4
by Nathan Hüsken 01 May '11

01 May '11

Hey, I am currently reading "All about monads". Unfortanlty I have only a pdf without the solutions to the exercises. in the previous exercises I defined: data Sheep = Sheep (Maybe Sheep) (Maybe Sheep) father :: Sheep -> Maybe Sheep father (Sheep f m) = f mother :: Sheep -> Maybe Sheep mother (Sheep f m) = m parent :: Sheep -> Maybe Sheep parent s = (mother s) `mplus` (father s) parentList :: Sheep -> [Sheep] parentList s = (maybeToList (mother s)) `mplus` (maybeToList (father s)) Now I am supposed to combine "parent" and "parentList" to a function parent :: MonadPlus m => Sheep -> m Sheep Problem, my parent function relies on "mother" and "father", and they give a Maybe monad. So I need: mother :: MonadPlus m => Sheep -> m Sheep do I not? But I have no Idea of how the body of this function would look. Anyone can give me a hint? Thanks! Nathan but what I really need, is a function

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Longest common prefix of a list of lists
by philipp siegmantel 29 Apr '11

29 Apr '11

Hi, For one of my toy project I had to find the longest common prefix of a list of strings. I figured, that this function should only work on list-structure, meaning it should have type > longestCommonPrefix :: [[a]] -> [a] Sadly the best I could come up with was using explicit recursion. It works as far as I tested it, but I have the feeling that there's a better way to do it. Here is the code: > longestCommonPrefix :: Eq a => [[a]] -> [a] > longestCommonPrefix [] = [] > longestCommonPrefix xss | any null xss = [] > | otherwise = loop xss [] > where loop ::Eq b => [[b]] -> [b] -> [b] > loop xss acc = > let xs = concatMap (take 1) xss > in if any (\x -> x /= head xs) (tail xs) > then reverse acc > else loop (map tail xss) (head xs : acc)

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is there a better way to ask for a hex formatted number?
by Sean Perry 29 Apr '11

29 Apr '11

I am using this currently: makeHex n = "0x" ++ map toUpper (showHex n "") -- gives me 0xFF for example I know i can use printf but I am not sending this to output when I am doing the formatting.

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Looking for more information about Pattern Matching
by Graham Hopper 29 Apr '11

29 Apr '11

Hi, I am very much a newbie to Haskell and currently working through http://learnyouahaskell.com and trying some of the http://www.haskell.org/haskellwiki/99_questions. I am struggling to get my head around pattern matching. I think I can understand the individual patterns presented in LYAH, but can't seem to find any information about generalising the formats. Is there a good resource that describes the patterns that are available or provides more details about how to create them. For example the format of [x] and (x:xs) seem very different and I cant quite get a handle on the syntax(?) behind them. I can see how patterns work when shown an example, but wouldn't know how to create one from scratch. I hope you can understand what I am trying to say - its a little difficult to explain. Thanks Graham

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Antw.: Question about CFloat and Float
by ch.gosch＠googlemail.com 29 Apr '11

29 Apr '11

Hi Alberto, thanks for the hint and the link. Do you have any information on whether that will continue to work in the future? I somehow have a bad feeling about it because the C* versions of the types would then be unnecessary. Thanks again, Christian ----- Reply message ----- Von: "Alberto Ruiz" <aruiz(a)um.es> An: "C Gosch" <ch.gosch(a)googlemail.com> Cc: <beginners(a)haskell.org> Betreff: [Haskell-beginners] Question about CFloat and Float Datum: Do., Apr. 28, 2011 15:27 Hi Christian, On 04/28/2011 02:27 PM, C Gosch wrote: > Dear list members, > I have the following problem: I have wrapped a C library using the FFI > and c2hs. > The C functions all take a bunch of pointers to C arrays of floats, > doubles, or ints as arguments. > Naturally, the Haskell representation of these is e.g. > Ptr CFloat > or, for single values, CFloat. > My question is: if I have an array, e.g. allocated with mallocArray, to > Haskell Float values (Ptr Float), > is the representation of Float exactly the same as for CFloat? Can I > safely use Ptr Float where the C function > expects the C type float* ? > I assume that I can not, and in that case I would like to find a way to > have Float on the Haskell side and CFloat on the C side. > Any hint on how I would do that without copying data and without too > large performance penalties? > > Thanks a lot, > Christian > I think that in ghc you can assume the same representation for CFloat and CDouble, they are newtypes of Float and Double: http://www.haskell.org/ghc/docs/7.0.3/html/libraries/base-4.3.1.0/Foreign-C… (I directly use arrays of Float and Double with C libraries without any problem). But Int may be different from CInt depending on the architecture, so we need explicit conversions. Alberto

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how to save the GHC errormessages in a file on Windows
by Kees Bleijenberg 28 Apr '11

28 Apr '11

When I try in Windows in cmd: ghc --make myprogram.hs > test.txt I only get the first lines in test.txt. The errormessages itself are on the screen, not in test.txt. In test.txt: Resolving dependencies... Configuring ftphs-1.0.7... Preprocessing library ftphs-1.0.7... Preprocessing executables for ftphs-1.0.7... Building ftphs-1.0.7... The errormessages: Could not find module 'Network.FTP.Client'.... and a lot more. How do I 'intercept' these messages? Kees

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Question about CFloat and Float
by C Gosch 28 Apr '11

28 Apr '11

Dear list members, I have the following problem: I have wrapped a C library using the FFI and c2hs. The C functions all take a bunch of pointers to C arrays of floats, doubles, or ints as arguments. Naturally, the Haskell representation of these is e.g. Ptr CFloat or, for single values, CFloat. My question is: if I have an array, e.g. allocated with mallocArray, to Haskell Float values (Ptr Float), is the representation of Float exactly the same as for CFloat? Can I safely use Ptr Float where the C function expects the C type float* ? I assume that I can not, and in that case I would like to find a way to have Float on the Haskell side and CFloat on the C side. Any hint on how I would do that without copying data and without too large performance penalties? Thanks a lot, Christian

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mSys and shadowing packages
by Kees Bleijenberg 28 Apr '11

28 Apr '11

After a lot of struggling I finally managed to install the packages network and ftpsh on a Windows machine. After installing msys and adjusting the path for mingW, the command "cabal install network" finally worked. At least I thought it was. But when I put "import Network.FTP.Client" in myprogram.hs, GHC can't find the package. After a while I found how to get more info: ghc --make -v myprogram.hs -package ftphs. There was a long list of warnings, missing packages, wrong versionnumbers... arghh... Ghc also complained about shadowed packages. i.e. When I do: ghc-dpg list, I get 2 versions of Win32-2.2.0.2. They have equal version numbers but the package-id's are different. One package is stored in \program files\Haskell Platform/2010.2.0.0/lib/package.conf.d and the other is stored at \Docs&Settings\Kees\Application Data\ghc\i386-mingw32-6.12.3\package.conf.d The packages from Docs&Settings map were creates using msys. The other were create before installing network, ftpsh etc (normal cmd command line). I wonder whether the 2 directories are okay and if not, how do I change the cabal-directory for msys/mingw? As a beginner I love the language. Using Haskell as a platform (at least for Win32 development) with all it's version dependencies is in my opinion difficult (compared to Java, C#, Delphi). Can somebody help? Kees

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