September 2009 - Haskell-Cafe

ANNOUNCE: A Levenberg-Marquardt implementation
by Bas van Dijk 14 Sep '09

14 Sep '09

Dear all, We like to announce the release of a Haskell binding to Manolis Lourakis's C levmar library at: http://www.ics.forth.gr/~lourakis/levmar/ This library implements the Levenberg-Marquardt algorithm which is an iterative technique that finds a local minimum of a function that is expressed as the sum of squares of nonlinear functions. It has become a standard technique for nonlinear least-squares problems and can be thought of as a combination of steepest descent and the Gauss-Newton method. When the current solution is far from the correct one, the algorithm behaves like a steepest descent method: slow, but guaranteed to converge. When the current solution is close to the correct solution, it becomes a Gauss-Newton method. Our binding consists of three packages: * http://hackage.haskell.org/package/bindings-levmar-0.1 A low-level wrapper around the C library. Note that the C library is lightly patched so that the functions can be safely called inside unsafePerformIO. The patched C library is bundled with this package. * http://hackage.haskell.org/package/levmar-0.1 A high-level wrapper around bindings-levmar. It provides a more familiar Haskell interface. For example, instead of passing a 'Ptr r' to the levmar function you can pass a [r]. levmar also provides some higher-level modules that use some type-level programming to add more type safety. * http://hackage.haskell.org/package/levmar-chart-0.1 Finally levmar-chart is a small package for quickly viewing the output of levmar in a chart. Unfortunately the documentation of these libraries is not available from hackage because bindings-levmar won't configure because of a missing dependency (lapack) on the hackage server. Instead I put the documentation at the following places: http://code.haskell.org/bindings-levmar/bindings-levmar-0.1/doc/html/bindin… http://code.haskell.org/levmar/levmar-0.1/doc/html/levmar/index.html Here follows a quick example: Suppose we have the following model functions: constant :: Num r => Model N1 r r linear :: Num r => Model N2 r r quadratic :: Num r => Model N3 r r cubic :: Num r => Model N4 r r constant a _ = a linear b a x = b * x + constant a x quadratic c b a x = c * x*x + linear b a x cubic d c b a x = d * x*x*x + quadratic c b a x And the jacobians: constantJacob :: Num r => Jacobian N1 r r linearJacob :: Num r => Jacobian N2 r r quadraticJacob :: Num r => Jacobian N3 r r cubicJacob :: Num r => Jacobian N4 r r constantJacob _ _ = 1 ::: Nil linearJacob _ a x = x ::: constantJacob a x quadraticJacob _ b a x = x*x ::: linearJacob b a x cubicJacob _ c b a x = x*x*x ::: quadraticJacob c b a x Now assume we have some sample data. If you call levmar like this: levmar cubic (Just cubicJacob) (-0.05 ::: 0.5 ::: -12 ::: 10 ::: Nil) samples 1000 defaultOpts Nothing Nothing noLinearConstraints Nothing You get the following fit (using levmar-chart): http://code.haskell.org/~roelvandijk/code/levmar-chart/cubicFit.png Note that levmar contains a demo with a lot more examples: http://code.haskell.org/levmar/Demo.hs Happy fitting, Roel and Bas van Dijk

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Program with ByteStrings leads to memory exhaust.
by Grigory Sarnitskiy 14 Sep '09

14 Sep '09

I have a simple program that first generates a large (~ 500 mb) file of random numbers and then reads the numbers back to find their sum. It uses Data.Binary and Data.ByteString.Lazy. The problem is when the program tries to read the data back it quickly (really quickly) consumes all memory. The source: http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=3607#a3607 or: module Main where import Data.Binary import Data.Int import System.Random import qualified Data.ByteString.Lazy as BL encodeFileAp f = BL.appendFile f . encode path = "Results.data" n = 20*1024*1024 :: Int getBlockSize :: BL.ByteString -> Int64 getBlockSize bs = round $ (fromIntegral $ BL.length bs) / (fromIntegral n) fillFile :: StdGen -> Int -> IO () fillFile _ 0 =return () fillFile gen i = do let (x, gen') = random gen :: (Double, StdGen) encodeFileAp path x fillFile gen' (i-1) processFile :: BL.ByteString -> Int64 -> Int -> Double -> Double processFile bs blockSize 0 sum = sum processFile bs blockSize i sum = let tmpTuple = BL.splitAt blockSize bs x = decode $ fst $! tmpTuple in processFile (snd tmpTuple) blockSize (i-1) $! sum + x main = do fillFile (mkStdGen 42) n results <- BL.readFile path putStrLn $ show $ processFile results (getBlockSize results) n 0

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why these two are not equivalent?
by Diego Souza 14 Sep '09

14 Sep '09

Hi, I was trying to solve a simple problem in SPOJ, however, after two weeks trying almost everything I could think of, I was still getting WrongAnswer. Then I decided to do the same thing in C++ and I really got puzzled when I got ACcepted. I tried to understand what was different without success. I hope someone can tell me why spoj says the Haskell version is wrong: http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=3583 [Haskell,WA] http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=3582 [C++,AC] The problem I'm talking about is this one: https://www.spoj.pl/problems/SBANK/ Thanks, -- ~dsouza yahoo!im: paravinicius gpg key fingerprint: 71B8 CE21 3A6E F894 5B1B 9ECE F88E 067F E891 651E

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Plotting parametric functions in Haskell
by Rafael Cunha de Almeida 14 Sep '09

14 Sep '09

Hello, I think it would be interesting to plot and visualize parametric (and other kind of functions) using the haskell language to define them [functions]. Does anyone know about some software or API that does just that? I started writing a plotter to do that using hopengl. But my computer graphics skills are rather bad and I couldn't produce anything really useful. []'s Rafael

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Community.haskell.org is down
by Neil Mitchell 14 Sep '09

14 Sep '09

Hi, http://community.haskell.org/ seems to be down for me. In general, who should this be reported to? Thanks Neil

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HECC License (was: [Haskell-cafe] Haskell Weekly News: Issue 130 - September 12, 2009)
by Thomas DuBuisson 14 Sep '09

14 Sep '09

> hecc-0.1. Marcel Fourn√© [5]announced the first release of hecc, the > Elliptic Curve Cryptography Library for Haskell. Implemented are > affine, projective, jacobian and modified jacobian point formats with > the basic operations. Included as an Example is a basic ECDH as well as > a basic speed test. This is great news - I hope it finds its way into the crypto library. One question - is this license a derivative of something I would have heard about? IANAL, but have just read the license it sounds fairly permissive in that it doesn't mandate distribution of source code (BSDish). Was this the intent? Did I miss something? Thomas

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RE: Darcs and NFS Resolution
by Lewis-Sandy, Darrell 13 Sep '09

13 Sep '09

Thanks to everyone who pointed me in the right direction on this problem, I have been able to find a work around that allows me to push to an archive on a network file share. After some digging, I discovered the root of the problem. Briefly, Darcs uses the standard library System.Directory to perform file manipulations. This library make use of compiler pragmas to conditionally compile either the Windows or Linux specific functions based on the host operating system. This approach assumes that when your operating system is a Linux variant, all your mounted volumes will natively support POSIX. When you have mounted CIFS volumes, this will result in errors when calling those library functions to create, rename, copy and delete files or folders! Given the flexibility of the Linux operating system, A more versatile implementation for System.Directory might be able to detect the file system of each volume at runtime and choose the appropriate functions to call. But I digress... To workaround the inability to push to a CIFS volume in darcs, I modified the workaround.hs module to shell out to the OS's rename function rather than using the Haskell library's rename function. Specifically: I added the following code after the #else near line 80: renameFile :: FilePath -> FilePath -> IO () renameFile old new = do ecode <- System.Cmd.rawSystem "mv" [old,new] case ecode of ExitSuccess -> return () ExitFailure n -> putStrLn ("renameFile failed with exit code" ++ show n) which ensures that when the operating system is not WIN32, that renaming of files will be performed by the OS shell. I then added the System.Cmd module to the list of imports by inserting the following code near line 21 import qualified System.Cmd(rawSystem) after a recompile I could push to a CIFS volume, for example: sudo darcs push /mnt/cifsvol this is an obvious hack, and does not address the inability to put to a CIFS share (put depends upon copyFile and would need to be hacked to shell-out as well). Archives on the CIFS share have to be created by navigating to that folder and initializing. Shelling out is clearly a poor long term solution; a longer term solution would ideally introduce into the Haskell System.Directory library the ability to apply the correct functions transparently in accordance with the file system of the volume. Cheers -Darrell Lewis-Sandy

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ANNOUNCE: arbtt-0.1
by Joachim Breitner 13 Sep '09

13 Sep '09

Hi, I just uploaded the Automatic Rule-Based Time Tracking tool on hackage. It is written in Haskell (duh :-)) and might be interesting for some of you to either use or hack on (or both). I have put an introduction on https://www.joachim-breitner.de/blog/archives/336-The-Automatic-Rule-Based-… As always, comments are very welcome. Enjoy, Joachim PS: If you don’t like to use the DSL I created for the rule, but rather want to use Haskell, it might also be possible to do the xmonad approach (maybe using dyre). If so, I’m curious to hear about it :-) -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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yhc compiled hbc
by Fernan Bolando 13 Sep '09

13 Sep '09

Hi all Are there any yhc users here? Can anybody send me a precompiled yhc cross-platform hbc file? regards fernan -- http://www.fernski.com

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[ANNOUNCE] A statistics library
by Bryan O'Sullivan 13 Sep '09

13 Sep '09

I'm pleased to announce a new Haskell statistics library, imaginatively named statistics <http://hackage.haskell.org/package/statistics>: http://hackage.haskell.org/package/statistics - Support for common discrete and continuous probability distributions (binomial, gamma, exponential, geometric, hypergeometric, normal, and Poisson) - Kernel density estimation - Autocorrelation analysis - Functions over sample data - Quantile estimation - Resampling techniques: jackknife and bootstrap estimation The statistics library certainly isn't yet comprehensive, but it has some features that I think make it very attractive as a base for further work: - It's very fast, building on some of the fantastic software that's available on Hackage these days. I make heavy use of Don Stewart's uvector library <http://hackage.haskell.org/package/uvector> (itself a port of Roman Leshchinskiy's vector library), which means that many functions allocate no memory and execute tight loops using only machine registers. I use Dan Doel's uvector-algorithms library<http://hackage.haskell.org/package/uvector-algorithms>to perform fast partial sorts. I also use Don's mersenne-random library <http://hackage.haskell.org/package/mersenne-random> for fast random number generation when doing bootstrap analysis. - I've put a fair amount of effort into finding and using algorithms that are numerically stable (trying to avoid problems like catastrophic cancellation). Whenever possible, I indicate which methods are used in the documentation. (For more information on numerical stability, see What Every Scientist Should Know About Floating-Point Arithmetic<http://docs.sun.com/app/docs/doc/800-7895> ). If you want to contribute, please get the source code and hack away: darcs get http://darcs.serpentine.com/statistics For more details, see http://bit.ly/ykOeK

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