April 2011 - Beginners

rizwanhudda@gmail.com is inviting you to freecharge.in
by rizwanhudda＠gmail.com 18 Apr '11

18 Apr '11

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Nested Iterations with Takusen
by Pepe Barbe 18 Apr '11

18 Apr '11

Hello everyone, I am new to Haskell and I am trying to use the Takusen library to query an SQL database. My desire is two perform a two level query query where the first query will iterate through some values that will be used in the second level to get the data of interest. I am having issues with the types and I haven't been able to figure out what am I doing wrong. Below is some sample code that shows the problem I am dealing with: -- sample.hs import Database.PostgreSQL.Enumerator import Database.Enumerator db_host = CAhost "host" db_user = CAuser "user" db_password = CApassword "pass" db_name = CAdbname "name" db_auth_tokens = [db_user, db_password, db_host, db_name] connection = connect db_auth_tokens secondLevelQuery :: IO Integer secondLevelQuery = do let query = sql "SELECT count(*) from alarms" iter :: Monad m => Integer -> IterAct m Integer iter count _ = result' count withSession connection (doQuery query iter 0) firstLevelQuery :: IO (Integer, Integer) firstLevelQuery = do let query = sql "SELECT count(*) from alarms" iter :: Monad m => Integer -> IterAct m (Integer, Integer) iter count _ = do count' <- secondLevelQuery result' (count', count) withSession connection (doQuery query iter (0,0)) main = firstLevelQuery -- end sample.hs And the output from ghc is: sample.hs:23:31: Could not deduce (m ~ IO) from the context (Monad m) bound by the type signature for iter :: Monad m => Integer -> IterAct m (Integer, Integer) at sample.hs:(22,7)-(24,43) `m' is a rigid type variable bound by the type signature for iter :: Monad m => Integer -> IterAct m (Integer, Integer) at sample.hs:22:7 Expected type: m Integer Actual type: IO Integer In a stmt of a 'do' expression: count' <- secondLevelQuery In the expression: do { count' <- secondLevelQuery; result' (count', count) } In an equation for `iter': iter count _ = do { count' <- secondLevelQuery; result' (count', count) } I would appreciate any help in figuring out what am I doing wrong. Thanks, Pepe

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Help with type declaration using MArray
by Perry James 17 Apr '11

17 Apr '11

Hi all, I'm working through Brown & Cook's "Monadic Memoization Mixins" and have come across some code that I can't get to compile with GHC. The error I get for arrayDict :: (*MArray *arr (*Maybe *b) m, *Ix *a, *Ord *a) => a -> arr a (*Maybe *b) -> *Dict *a b m is Non type-variable argument in the constraint: MArray arr (Maybe b) m (Use -XFlexibleContexts to permit this) In the type signature for `arrayDict': arrayDict :: (MArray arr (Maybe b) m, Ix a, Ord a) => a -> arr a (Maybe b) -> Dict a b m Enabling FlexibleContexts causes other errors. What could be going wrong? In case this isn't enough context, I've posted the full source code for this section of the paper at smipple<http://www.smipple.net/snippet/Perry%20James/Monadic%20Memoization%20Mixins> . Thanks in advance for any help, Perry

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Fwd: ANNOUNCE : leksah-0.10.0 rpms available for Fedora 14
by lakshminaras2002＠gmail.com 17 Apr '11

17 Apr '11

leksah-0.10.0 rpms available for Fedora 14 x86: Download this script install_x86.sh<http://narasim.fedorapeople.org/leksah/f14/i686/install_x86.sh> Open command line, cd into the folder where the script is downloaded. Type in the following command ]$ chmod u+x install_x86.sh Become root, ]$ su password: ]$ ./install_x86.sh x86_64: Download this script install_x86_64.sh<http://narasim.fedorapeople.org/leksah/f14/x86_64/install_x86_64.sh> Open command line, cd into the folder where the script is downloaded. Type in the following command ]$ chmod u+x install_x86_64.sh Become root, ]$ su password: ]$ ./install_x86_64.sh The rpms can be found at this location, http://narasim.fedorapeople.org/leksah/f14/. The script runs yum internally. - Lakshmi Narasimhan T V

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Calling a foreign function: superlinear comlexity
by Serguei Son 16 Apr '11

16 Apr '11

I call GSL's gsl_ran_ugaussian function in the following way (using bindings-gsl): module Main where import Bindings.Gsl.RandomNumberGeneration import Bindings.Gsl.RandomNumberDistributions import Foreign import Control.Monad import Data.List main = do let n = 100000 p <- peek p'gsl_rng_mt19937 rng <- c'gsl_rng_alloc p lst <- replicateM n $ c'gsl_rng_uniform rng print $ sum lst As I increase n from 10^4 to 10^5 to 10^6 execution time grows superlinearly. To forestall the answer that the reason is the overhead of List, this code scales approximately linearly: module Main where import Foreign import Control.Monad import Data.List main = do let n = 100000 let lst = map sin [1..n] print $ sum lst Another interesting observation: when I wrap the sin function of math.h with signature CDouble -> IO CDouble calling it repeatedly scales superlinearly, whereas when I wrap it as a pure function calling it repeatedly scales linearly. What is the reason for this performance and how can I make the first code scale linearly in execution time?

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ghc process gets killed on a 512MB machine
by "Ionuț G. Stan" 16 Apr '11

16 Apr '11

Hi, I'm trying to install the highlighting-kate package[1] on a small Linode 512MB instance. Throughout the installation process, top reports increasing memory usage of GHC, and ends up with the process being killed (ExitFailure 9, confirmed by the dmesg command). I'm using GHC 7.0.3, and this problem didn't appear with GHC 6.12.3 and a previous version of highlighting-kate (don't know which for sure). I mean, I had it installed successfully at some point. Is there any workaround for this problem? Other than fork the package repo and reduce the number of supported syntax files. Thanks! [1]: http://hackage.haskell.org/package/highlighting-kate -- Ionuț G. Stan | http://igstan.ro/

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parsec, return remaining input?
by Rob Nikander 15 Apr '11

15 Apr '11

Hi, Is there a way to return the remaining input, so that parsec can be used to parse one piece at a time. So something like: chunk = many (noneOf ",:") parse chunk "abc,123:xyz" Would return ("abc", ",123:last") or similar? thanks, Rob

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homepage for the Gtk2Hs project?
by MAN 15 Apr '11

15 Apr '11

Is there a new location for the homepage of gtk2hs? haskell.org/gtk2hs displays a 404 error, and mighty google hasn't been able to find any other.

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Using QuickCheck to verify that an expression can always be evaluated
by Amy de Buitléir 15 Apr '11

15 Apr '11

Occasionally I write a QuickCheck test where I want to evaluate something just to make sure it doesn't crash, but I don't care about the actual value. For example, in the test below, I just want to make sure that the expression "foldl' apply newWorkbench gs" doesn't crash. I don't really care whether or not "devotion w /= -1", that's just an expression (which is always true) that I wrote to force evaluation of w. But I assume there's a better way to do this? prop_all_gene_sequences_are_legal :: [Gene] -> Property prop_all_gene_sequences_are_legal gs = property $ devotion w /= -1 where w = foldl' apply newWorkbench gs Thank you in advance for any suggestions.

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please critique my first stab at systems programming
by Sean Perry 14 Apr '11

14 Apr '11

The idea is to walk the disk looking for a signature, say NTFS or EXT. Since we do not know where the block containing this identifier is, we read the blocks in one at a time. Long term I would like to support command line arguments for the name of the file and the offset to start looking. Comments on style, idioms, etc. welcomed. I am particularly interested in the hIsEOF check and if there is a better way to handle that. import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as BC import IO import System.IO chunkSize = 512 searchForPattern handle pat = searchForPattern' 0 handle pat searchForPattern' index handle pat = do eof <- hIsEOF handle if eof then return Nothing else do bytes <- B.hGet handle chunkSize case BC.breakSubstring pat bytes of (x, y) | BC.null y -> searchForPattern' (index + 1) handle pat | otherwise -> return (Just index) main = do fromHandle <- openBinaryFile "blocks" ReadMode result <- searchForPattern fromHandle (BC.pack "PART") case result of Nothing -> putStr "Not Found.\n" Just n -> putStr $ "Found at " ++ show n ++ ".\n" hClose fromHandle putStr "Done.\n"

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