August 2009 - Haskell-Cafe

Dutch HUG: meeting next week (September 11th) in Utrecht
by Tom Lokhorst 31 Aug '09

31 Aug '09

>From lovely Edinburgh, I'd like to invite functional programmers in The Netherlands to the Dutch Haskell User Group [1, 2] meeting next Friday at 19:00. For the first time, we'll have people giving talks! Thomas (noknok) will be talking about his system for doing propositional logic in Haskell. Pedro will give an introductory talk about generic programming, and Sean will talk about xformat, a library for extensible and type-safe formatting with scanf- and printf-like functions. If you're interested in giving a short 5-minute lighting talk about something related to Haskell or functional programming, we'd very much like to hear from you! The meeting will be in the Booth Hall [3] of the Utrecht University Library. There's free car parking [4] at the library, and it's easily accessible by bike and bus [5]. We have a poll [6] that shows all people who plan to attend. See you next Friday! - Tom Lokhorst [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug [3]: http://www.uu.nl/EN/library/contact/university_library/zaalverhuur/Pages/de… [4]: http://www.uu.nl/EN/library/contact/university_library/Parkeren/Pages/defau… [5]: http://www.uu.nl/EN/library/contact/university_library/plattegrondenrou/Pag… [6]: http://doodle.com/a7vx3a6imcue4ghr

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Get data from HTML pages
by José Romildo Malaquias 31 Aug '09

31 Aug '09

Hello. I am porting to Haskell a Java application I have written to manage collections of movies. Currently the application has an option to indirectly import movie data from web pages. For that first the user should access the page in a web browser. Then the user should copy the rendered text in the web browser into an import window in my application and click an "import" button. In response the application parses the given text and collects any relevant data it knows about, using regular expressions. For instance, to get the director information from a movie in the AllCenter web site I use the following regular expression: ^Direção:\s+(.+)$ I want to modify this scheme in order to eliminate the need to copy the rendered text from a web browser. Instead my application should download and parse the HTML page directly. Which libraries are available in Haskell that would make it easy to get content information from a HTML document, in the way described above? Regards, Romildo

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Takusen ODBC - MS Access
by Günther Schmidt 31 Aug '09

31 Aug '09

Hi all, recently I had problems getting Takusen ODBC to work with MS-Access on Win32. It seems, that the problems originated from Takusens UTF8 code, the path of the Access Database contains an Umlaut. This even causes problems when one does not specify a connection String but merely a DSN, no further problems occured when I moved the Access file directly to C:\. I also could not get takusen to work with MS Text / CSV ODBC driver, irregardless of Umlauts, the exception was about some optional feature not implemented (on MS's ODBC site). Could anyone suggest a work around? I notice that when you're a haskeller on Win32 *and* German (the Umlauts), things become a bit more difficult than for the English / linux haskellers. Günther

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Takusen - is anyone currently using it on Win32 - ODBC?
by Günther Schmidt 31 Aug '09

31 Aug '09

Hi all, is anyone currently using takusen with odbc on Win32? In particular with MS Access? I'm asking because I noticed that when database libraries are declared to work with ODBC no one seems to mean Win32 ODBC, but rather Unix ODBC. Günther

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Re: How to use dyre relaunch/restore State of Gtk2hs Object?
by Will Donnelly 31 Aug '09

31 Aug '09

Hi Andy, Your program didn't compile as given, so I cut out some of the event handling fanciness you were doing. Also, my modified version restarts whenever the 'r' key is pressed. I figure those changes aren't important to the problem at hand. Anyway, I don't think it's ever going to be possible to define a Binary instance for a TextView, since all of the functions which can yield information about it are impure. I would recommend extracting the relevant data beforehand, and then persisting it in a tuple. You could use some 'marshal<WIDGET>' and 'recreate<WIDGET>' functions if you need to persist many different widget types. Saving just the text can be done as follows: <code> module DyreExample where import Graphics.UI.Gtk hiding (get) import qualified Graphics.UI.Gtk.Gdk.Events as E import qualified Config.Dyre as Dyre import Config.Dyre.Relaunch import System.IO import Data.Binary data Config = Config { message :: String, errorMsg :: Maybe String } defaultConfig :: Config defaultConfig = Config "Dyre Example v0.1" Nothing showError :: Config -> String -> Config showError cfg msg = cfg { errorMsg = Just msg } realMain Config{message = message, errorMsg = errorMsg } = do initGUI textView <- textViewNew textBuffer <- textViewGetBuffer textView text <- restoreBinaryState "" putStrLn $ "Restored state: " ++ text textBufferSetText textBuffer text rootWindow <- windowNew rootWindow `onDestroy` mainQuit windowFullscreen rootWindow rootWindow `containerAdd` textView widgetShowAll rootWindow rootWindow `onKeyPress` (\event -> dyreKeyTest event textView) mainGUI dyreExample = Dyre.wrapMain $ Dyre.defaultParams { Dyre.projectName = "Main" , Dyre.realMain = realMain , Dyre.showError = showError } dyreKeyTest :: E.Event -> TextView -> IO Bool dyreKeyTest ev textView = do case E.eventKeyName ev of "r" -> do textBuffer <- textViewGetBuffer textView sI <- textBufferGetStartIter textBuffer eI <- textBufferGetEndIter textBuffer text <- textBufferGetText textBuffer sI eI True putStrLn $ "Relaunching with state: " ++ text relaunchWithBinaryState text Nothing return True _ -> return False </code> Other comments: 1. It isn't necessary to explicitly tell Dyre to do a custom compile. It will take care of figuring that out once you restart it. 2. Instead of manually setting paths in the code, you can use the '--dyre-debug' command-line flag, which will cause Dyre to look for configurations in the current directory, and store temporary files in a 'cache' subdirectory. 3. Giving a project name of 'Main' causes Dyre to see the file 'Main.hs' as a custom configuration. I'm not sure if that's what you intended, but it will make testing custom configurations harder than it needs to be. 4. Good luck with the rest of your project! - Will Donnelly

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Edinburgh Hackathon round-up
by Dougal Stanton 31 Aug '09

31 Aug '09

Hello Haskellers, On Saturday & Sunday (28th & 29th August) we held a Hackathon in Edinburgh, before the ICFP got fully underway. If you were there, thank you for coming. We'll be writing a summary of what happened soon, and we'd love to hear what you did. If you started a new project or fixed some insidious bug, identified a major bottleneck or just enriched the testing structure around your chosen project, it would be great to know. Just a line or two is all we need! I am hoping that everyone who attended will see this message, as they must have seen Eric's original advertising emails. But please pass this message along if you know there's someone we missed. Cheers, Dougal -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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specialised monad transformers
by Alberto G. Corona 31 Aug '09

31 Aug '09

Hi all I want to define a partial monad transformer i.e not defined for all kinds of data but the ones that have instances of certain classes. That is because in my lift instance code there are some hidden use of these classes. But because in the MonadTrans definition does not refers to the data, no specialised lift seems possible: class MonadTrans t where lift :: Monad m => m a -> t m a Is that true? A possible adition could be to define (note: I did not test it, it is just an idea): class SpecialisedMonadTrans (t m a) where lift :: Monad m => m a -> t m a liftMN :: Monad m => m a -> t a The whole problem has certain relation with effect control. This kind of definition permits a more fine control of effect handling when passing from a monad to other depending on the idiosyncrasy of the monad m and the kind of a.

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happstack-helpers fails to install on Mac OSX
by Colin Paul Adams 31 Aug '09

31 Aug '09

I've just installed the Haskell Platform (2009.2.0.2) on Mac OSX 10.5.6, including GHC 6.10.4. Then I started added some packahges with cabal install. But happstack-helpers-0.30 fails with: Happstack/Helpers/DirBrowse.hs:88:105 Couldn't match expected type `String' against inferred type `Bool' In the third argument of `hscolour', namely `False' In the second argument of `(.)', namely `BrowseHtmlString . (hscolour defaultColourPrefs False False f)' Is this a known error, and if so, is there a workaround? -- Colin Adams Preston Lancashire

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Data.Time is so weak ?!
by zaxis 31 Aug '09

31 Aug '09

today = fromGregorian 2009 8 31 then how to calculate the day passing 150 days ? today + 150 doesnot work ! -- View this message in context: http://www.nabble.com/Data.Time-is-so-weak--%21-tp25218462p25218462.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re[Haskell-cafe] duction Sequence of simple Fibonacci sequence implementation
by SimonK77 31 Aug '09

31 Aug '09

Hallo everyone, as Haskell uses the Lazy Evaluation reduction policy also known as Outermost Graph Reduction, I was wondering if the following simple implementation of the Fibonacci sequence would result in linear runtime behaviour. fib :: Integer -> Integer fib 0 = 0 fib 1 = 1 fib n = fib (n - 2) + fib (n - 1) Is the following reduction sequence realistic, or am I admitting to much intelligence to the Haskell Compiler? http://www.bilder-hochladen.net/files/bxlk-6-jpg.html http://www.bilder-hochladen.net/files/thumbs/bxlk-6.jpg I hope someone can help... -- View this message in context: http://www.nabble.com/Reduction-Sequence-of-simple-Fibonacci-sequence-imple… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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