- Beginners - Haskell.org

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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Comments on Map/Reduce Code
by Thomas Bach 17 Jul '12

17 Jul '12

Hello there, I'm new to Haskell and want to learn it a bit while experimenting with Hadoop's Map/Reduce programming model. So, I wanted to implement the standard ‘word counter’ problem in Haskell. The problem is as follows: We have several texts with words separated by white-space. We want to count the occurrences of all words in all the texts (where ‘but’ and ‘but,’ can be seen as two different words). This is done in two phases. In the Map phase a program gets a part of the text from stdout and has to produce a "KEY\tVALUE" pair (that is, the key separated with a tab to the value), which has to be passed to stdin. In our case we simply produce "WordX\t1" for every word WordX. This list is sorted by the key and later on fed as stdin to the reducer (the second phase). The reducer now has to sum up all the occurrences we trivially counted in the Map phase and put it as "WordX\tNumber" to stdout. So, here is an example: vince@roku:~/tmp echo "foo foo bar bar foo bar zoo bar foo" | runhaskell mapper.hs foo 1 foo 1 bar 1 bar 1 foo 1 bar 1 zoo 1 bar 1 foo 1 vince@roku:~/tmp echo "foo foo bar bar foo bar zoo bar foo" | runhaskell mapper.hs | sort | runhaskell reducer.hs bar 4 foo 4 zoo 1 And here is the code I've come up with: vince@roku:~/tmp cat mapper.hs import qualified Data.ByteString.Lazy.Char8 as C postFix :: C.ByteString postFix = C.pack "\t1" formatter :: C.ByteString -> C.ByteString formatter x = C.append x postFix main :: IO () main = do contents <- fmap C.words C.getContents C.putStr . C.unlines $ map formatter contents vince@roku:~/tmp cat reducer.hs import qualified Data.ByteString.Lazy.Char8 as C import qualified Data.List as L tuppleize :: String -> (String, Int) tuppleize line = (\xs -> (head xs, read (last xs))) $ words line group :: Eq a => [(a, b)] -> [[(a, b)]] group = L.groupBy (\x y -> fst x == fst y) summation :: Num b => [(a, b)] -> (a, b) summation (x:[]) = x summation (x:xs) = (fst x, (snd x) + (snd (summation xs))) formatter :: (String, Int) -> String formatter = (\w -> (fst w ++ "\t" ++ show (snd w))) main = do contents <- C.getContents putStr . unlines $ map formatter $ map summation $ group $ map tuppleize $ lines $ C.unpack contents As already said, I'm a Haskell beginner. Could you provide some comments on the code? Thanks in advance, Thomas.

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I can see lists "working" for matrix algorithms if the access patterns are mostly from the start or end of rows and columns.
by KC 15 Jul '12

15 Jul '12

Are there any such matrix algorithms that operate mainly from the start or end or rows and columns? Note: If the access pattern starts from the end of a row then one stores the row in reverse order in a list. -- -- Regards, KC

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Hackage down?
by Lorenzo Bolla 14 Jul '12

14 Jul '12

Is hackage.haskell.org down? cabal update times out here... How do you guys work around this? It seems to happen pretty often. L.

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Graph Coloring Library?
by Alec Story 14 Jul '12

14 Jul '12

I have a problem where I need to find the smallest k-coloring for a graph that represents conflicts between objects. Is there a Haskell library that will do this for me? I'm not particularly concerned about speed, and it's unlikely that I'll generate really bad edge cases, but I'd prefer to do something other than write the really bad try-every-case algorithm. -- Alec Story Cornell University Biological Sciences, Computer Science 2012

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Concurrent vs GHC
by Mauricio Hernandes 12 Jul '12

12 Jul '12

Hello, I'm having a problem with GHC. When I compile the code bellow it does nothing, but If I try to use ghci it works normally. it seems a simple problem, but I can't understand. Thanks for the help Mauricio import System.IO import Control.Concurrent import Data.List main = do input <- newMVar [1..30000] ia <- newEmptyMVar ib <- newEmptyMVar ic <- newEmptyMVar forkIO $ do x <- readMVar input putMVar ia x forkIO $ do a <- readMVar ia putMVar ib ( sum a ) forkIO $ do a <- readMVar ia putMVar ic ( reverse a ) forkIO $ do b <- readMVar ib c <- readMVar ic print b print c

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Trouble compiling haskell platfrom
by Jeff Lasslett 12 Jul '12

12 Jul '12

Greetings, I'm trying to recompile & reinstall the haskell platform 12.2.0.0. I have installed it successfully on a prior occasion but I decided to reinstall as I think I may have messed up my installed packages a bit while playing around with yesod. I intend to use cabal-dev or virtualenv for that in future. So I thought I'd do a clean install. I blew away my haskell platform and ghc, both of which were installed in a dir under my home dir. I also blew away my ~/.cabal/ dir. I then reinstalled by ghc 7.4.1 into ~/prg/haskell-platform-12.2.0.0. I then unpacked the sources of haskell-platform-12.2.0.0 and configured it with the prefix set to where ghc is. configure finished without error so I ran 'make'. It quits with what follows. How do I fix this?? With thanks, Jeff * Now do "make" followed by "make install" ************************************************** jeff@chunks:~/src/haskell-platform-2012.2.0.0$ make scripts/build.sh ************************************************** Scanning system for any installed Haskell Platform components... Found: HUnit-1.2.4.2 OpenGL-2.2.3.1 GLUT-2.1.2.1 html-1.0.1.2 parallel-3.2.0.2 random-1.0.1.1 QuickCheck-2.4.2 stm-2.3 syb-0.3.6.1 haskell-src-1.0.1.5 text-0.11.2.0 transformers-0.3.0.0 mtl-2.1.1 fgl-5.4.2.4 parsec-3.1.2 network-2.3.0.13 HTTP-4000.2.3 regex-base-0.93.2 regex-posix-0.95.1 regex-compat-0.95.1 xhtml-3000.2.1 cgi-3001.1.7.4 zlib-0.5.3.3 haskell-platform-2012.2.0.0 New packages to install: None! All done. ************************************************** Building alex-3.0.1 "/home/jeff/prg/haskell-platform-12.2.0.0/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.14.0" Linking Setup ... "./Setup" "configure" "--package-db=../../packages/package.conf.inplace" "--prefix=/home/jeff/prg/haskell-platform-12.2.0.0" "--with-compiler=/home/jeff/prg/haskell-platform-12.2.0.0/bin/ghc" "--with-hc-pkg=/home/jeff/prg/haskell-platform-12.2.0.0/bin/ghc-pkg" "--with-hsc2hs=/home/jeff/prg/haskell-platform-12.2.0.0/bin/hsc2hs" "--enable-library-profiling" "--ghc-pkg-option=--package-conf=../../packages/package.conf.inplace" "--user" Warning: defaultUserHooks in Setup script is deprecated. Configuring alex-3.0.1... "./Setup" "build" Building alex-3.0.1... Preprocessing executable 'alex' for alex-3.0.1... src/Data/Ranged/RangedSet.hs:64:8: Could not find module `Test.QuickCheck' There are files missing in the `QuickCheck-2.4.2' package, try running 'ghc-pkg check'. Use -v to see a list of the files searched for. Error: Building the alex-3.0.1 package failed make: *** [build.stamp] Error 2

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