July 2012 - Beginners

Can't square zero as a complex number?
by Scott N. Walck 26 Jul '12

26 Jul '12

Hi, Is there a reason why I can't use ** to square zero as a complex number? Prelude> :m Data.Complex Prelude Data.Complex> (0:+0) ** 2 NaN :+ NaN Prelude Data.Complex> (0 :: Complex Double) ** 2 NaN :+ NaN Using GHC 7.4.1. Thanks, Scott

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Very little (golfing) question...
by Corentin Dupont 26 Jul '12

26 Jul '12

Hi, I always write functions like this: addPlayer :: PlayerInfo -> State Game Bool addPlayer pi@(PlayerInfo {playerNumber = pn}) = do pls <- gets players case find (\(PlayerInfo {playerNumber = myPn}) -> pn == myPn) pls of Nothing -> do modify (\game -> game { players = pi : pls}) return True otherwise -> return False It simply adds a new PlayerInfo to a list contained in Game, with the condition that it doesn't exists already. Do you see a more elegant way to do this? Best, Corentin

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A Question about withFile
by Bin Shi 25 Jul '12

25 Jul '12

Hello, I wrote a simple test program as main = do withFile "a.txt" ReadMode (\h -> do c <- hGetContents h putStrLn c) then I got my expected results: I'm a.txt but if I changed to main = do c <- withFile "a.txt" ReadMode hGetContents putStrLn c I got just a empty line. Where am I wrong? Thanks.

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simple sound playback (Gnu/Linux)
by Christopher Howard 22 Jul '12

22 Jul '12

Hi. I've been playing around with gloss and its game modeling module. However, I am wondering what package to use to get simple sound playback. (For example, playing an explosion sound when a bomb lands.) There seem to be lots of modules in the sound category at hackage but I'm not sure what would be appropriate and simplest to use. (Only Gnu/Linux compatibility matters, if that helps.) -- frigidcode.com indicium.us

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Pb with toggleActionNew
by Pierre Michallet 22 Jul '12

22 Jul '12

I am trying to become acquainted with both haskell and Gtk thru the writing of small pieces of code exercising some Gtk functions. Having had problems with the ToggleEntry function (unable to get the checked/non checked status - refer to chapter 7.2 of the Gtk2HS tutorial) I have tried to understand the toggleAction thing. So I have written this : import Graphics.UI.Gtk.ActionMenuToolbar.ToggleAction ... togls::[ToggleAction] togls = let stone = toggleActionNew "STON" "Stones number" Nothing Nothing (myTog stone) deste = toggleActionNew "DEST" "Destination" Nothing Nothing (myTog deste) state = toggleActionNew "STAT" "Board status" Nothing Nothing (myTog state) in [stone,deste,state] myTog::ToggleAction->IO() myTog ta = putStrLn ("The name of the action is " ++ (toggleActionName ta)) The compiler is not happy with this and sends back the following: ------------------------------------------------------------------------------------------------------------------------------------------- The function 'toggleActionNew' is applied to five arguments but its type 'String ->String ->Maybe String -> Maybe stockId ->IO ToggleAction has only four. ------------------------------------------------------------------------------------------------------------------------------------------- To me, stone has 4 parameters ("STON", "Stones number", Nothing,Nothing) and a result (myTog stone) as in the definition given by the compiler. I am using ghc-7.0.3 gthk+-bundle_2.16.2-20090601_win32 gtk-0.12.3.1 documentation I am certainly missing something obvious to everybody but I could not solve it , so I would appreciate any help on this topic. Thanks in advance p.michallet(a)free.fr

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Where do you put your tests?
by Lorenzo Bolla 22 Jul '12

22 Jul '12

Hi all, I'm writing some library code and I have hunit-tests and quickcheck-tests. I'm facing the problem of where to put these tests. These are the options I can think of: - Put the tests for a function in the same file where the function is defined. This is good because tests are close to the code being tested and work as a sort of documentation for it. But it's bad because the file requires HUnit, QuickCheck, Test.Framework, etc. to compile. - Same as above, but use some sort of preprocessing flag to ignore the Test suite when compiling the library (hence no dependencies on test libraries), but leave the code in place. No dependencies, but ugly. - Put the tests in completely separate files. Good because library dependencies are reduced, but bad because tests are somewhat "disconnected" from the functions they are testing. What do you expert suggest? How should I organize my test suites? (The same conundrum applies to benchmarking code.) Thanks, Lorenzo

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automatically clean up Haskell code?
by Christopher Howard 20 Jul '12

20 Jul '12

Is there a program that can automatically clean up your Haskell code? That is, shrink to 80 columns, make tidy indentation, remove unnecessary whitespace, and all that sort of stuff...? -- frigidcode.com indicium.us

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How do I marshall a pointer over SendMessage LPARAM or WPARAM?
by Simon Peter Nicholls 18 Jul '12

18 Jul '12

I'm new to Haskell, and have had some good success with FFI so far, but using Win32's sendMessage to send a pointer in LPARAM or WPARAM is resulting in access violations at the other end. Is there some issue with my pointer conversions? Am I hitting some restriction, or missing some compiler options? It's driving me pretty crazy, after a very nice start to using Haskell. Some "sending" code: Foreign.C.String.withCWString "frustrator" $ \s -> do let wParam = System.Win32.Types.castPtrToUINT s :: System.Win32.Types.WPARAM Graphics.Win32.sendMessage wnd Graphics.Win32.wM_APP wParam 0 wndProc "receiving" code: | wmsg == Graphics.Win32.wM_APP = do s <- peekCWString $ System.Win32.Types.castUINTToPtr wParam putStrLn s return 0 The string will not be seen. Some extra notes: I can get wndProc messages and integral data generally. The pointer values match textually at both ends when "shown" to stdout. At the sending side I can pass the CWString to a regular FFI function call just fine, and castUINTToPtr will give me back a functioning Ptr for that call. I have also tried sending to & receiving from a working C++ program, without success. Access violations are reported when receiving, though again the address matches up. Silence from Haskell as before, when C++ is sending. I found someone else having an issue here: http://osdir.com/ml/haskell-cafe@haskell.org/2009-11/msg00731.html but no solution unfortunately.

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Re: [Haskell-beginners] haskell for system administration
by Erik Johnson 18 Jul '12

18 Jul '12

Hi Mike, I'm a little late to this thread but wanted to let you know it looks like Google Code changed the location of your project (URL). I've pasted the correct location inline. Thanks for this info., it looks helpful. By the way, I like the play on the name (ie. "Eddie" + "Haskell"). Erik Date: Mon, 21 May 2012 10:28:45 -0400 From: Mike Meyer <mwm(a)mired.org> Subject: Re: [Haskell-beginners] haskell for system administration To: Ben <benjo11111(a)gmail.com> Cc: beginners(a)haskell.org Message-ID: <20120521102845.6dc796c2(a)bhuda.mired.org> Content-Type: text/plain; charset=US-ASCII On Mon, 21 May 2012 19:43:47 +0530 Ben <benjo11111(a)gmail.com> wrote: >>> I am new to HASKELL.Currently I am using shell scripting for >>> Administration tasks and all.Kindly suggest me , does HASKELL usefull >>> for Linux system administration and reporting purpose.? >>Yes, it is. I wrote "eddie" to let me leverage Haskell list processing >> from the command line. The standard Unix tools generally cover this >> kind of thing fairly well, so it's only marginally useful. Windows >> users seem to find more use for it. http://code.google.com/eddie. http://code.google.com/p/eddie/ >>> Do we use HASKELL for network programming and all? >> Yup. You can find the code I wrote to extract statistical information >> on NCAA FBS seasons from the web at >> http://blog.mired.org/2012/01/analysis-of-fbs-controversies-part-i.html. >> While doing the fetch is a bit more complicated than using fetch (or >> curl, or whatever), the ease of analysis in Haskell and having the >> data already loaded when you want to start analysis makes up for that. <mike >> -- >> Mike Meyer <mwm(a)mired.org> http://www.mired.org/ >> Independent Software developer/SCM consultant, email for more information. >> O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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Review request
by C K Kashyap 17 Jul '12

17 Jul '12

Hi, I've written a small haskell program to extract a section from a file between start and end markers. For example, if I have a file such as below - a b c <bug> d e f </bug> g h i I'd like to extract the contents between <bug> and </bug> (including the markers). startTag = "<bug>"endTag = "</bug>" process = unlines . specialTakeWhile (f endTag) . dropWhile (f startTag) . lines where f t x = not (x =~ t) specialTakeWhile :: (a -> Bool) -> [a] -> [a] specialTakeWhile ff [] = [] specialTakeWhile ff (x:xs) = if ff x then x:(specialTakeWhile ff xs) else [x] It'll be great if I could get some feedback on this. Regards, Kashyap

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