May 2009 - Beginners - Haskell.org

Gtk2Hs + Sourceview on Windows
by mwinter＠brocku.ca 13 May '09

13 May '09

Hi, I have ghc 6.10.1 and gtk2hs 0.10.0 installed on my windows vista computer. Both were installed using the installer on the webpages. I am able to use gtk, glade etc but not sourceview or cairo. In a forum I found that those have to be enabled using ./configure --enable-sourceview (similar for cairo). But my windows installation does not seem to have a script configure. What am I supposed to do? Thanks, Michael

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International Summer School on Advances in Programming Languages (precedes ICFP'09)
by Janis Voigtlaender 13 May '09

13 May '09

Potential participants of the below summer school should pre-register their interest now. The organizers need that information to go ahead with the planning. ===================================================================== International Summer School on Advances in Programming Languages 25th-28th August, 2009 Heriot-Watt University, Edinburgh, Scotland http://www.macs.hw.ac.uk/~greg/ISS-AiPL Overview ~~~~~~~~ This four-day residential International Summer School on Advances in Programming Languages has a major theme of Concurrency, Distribution, and Multicore. Intended primarily for postgraduate research students, the School offers lectures and practical sessions on an engaging blend of cutting edge theoretical and practical techniques from international experts. The Summer School is supported by the Scottish Informatics and Computer Science Alliance (http://www.sicsa.ac.uk/) a Scottish Funding Council Research Pool. Participants from SICSA member institutions may attend at no cost. Confirmed Topics/Speakers * Static and dynamic languages, Prof Philip Wadler, University of Edinburgh * Compiler technology for data-parallel languages, Dr Sven-Bodo Scholz, University of Hertfordshire * New applications of parametricity, Dr Janis Voigtlaender, Technical University of Dresden * Automatic vectorising compilation, Dr Paul Cockshott, University of Glasgow * Foundational aspects of size analysis, Prof Marko van Eekelen / Dr Olha Shakaravska, Radboud University Nijmegen * Context oriented programming, Dr Pascal Costanza, Vrije Universiteit Brussels * Multi-core programming, Dr Phil Trinder, Heriot-Watt University * Multi-core compilation, Dr Alastair Donaldson, Codeplay Software Ltd * Principles and Applications of Refinement Types, Dr Andrew D. Gordon, Microsoft Research, Cambridge * Resource aware programming in Hume, Prof Greg Michaelson, Heriot-Watt University / Prof Kevin Hammond, University of St Andrews * Haskell concurrency & parallelism, Dr Satnam Singh, Microsoft Research, Cambridge Location ~~~~~~~~ The Summer School is at Heriot-Watt University's Riccarton campus, set in pleasant parkland to the west of Edinburgh, with easy access to the airport, city and central Scotland (http://www.hw.ac.uk/welcome/directions.htm) The Summer School immediately precedes the 2009 International Conference on Functional Programming (http://www.cs.nott.ac.uk/~gmh/icfp09.html) and takes place during the Edinburgh International Festival (http://www.eif.co.uk/) , and the associated Edinburgh Festival Fringe (http://www.edfringe.com/) and Edinburgh International Book Festival (http://www.edbookfest.co.uk/) Steering Committee ~~~~~~~~~~~~~~~~~~ Prof Prof Greg Michaelson, Heriot-Watt University (Convenor), <G.Michaelson(a)hw.ac.uk> Prof Kevin Hammond, University of St Andrews Dr Patricia Johann, University of Strathclyde Prof Philip Wadler, University of Edinburgh Fee ~~~ Full rate: £400; (free for SICSA students) Includes: four nights single room, en-suite accommodation with breakfast, lunch and dinner, plus coffee breaks and session materials. Day rate: £200; (free for SICSA students) Includes: lunch, coffee breaks, session materials Registration of Interest ~~~~~~~~~~~~~~~~~~~~~~~~ If you are interested in attending the International Summer School, please complete the form available from (http://www.macs.hw.ac.uk/~greg/ISS-AiPL/ISS-AiPL%20register.doc) or below, and return it to: <ISS-AiPL-register(a)macs.hw.ac.uk> ********** International Summer School on Advances in Programming Languages 25th-28th August, 2009 Heriot-Watt University, Edinburgh, Scotland Registration of Interest Name: Address: Email: Phone: SICSA Uni: Yes / No Rate: Full / Day Accessibility requirements: Dietary requirements: Return to: ISS-AiPL-register(a)macs.hw.ac.uk **********

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The main thread and waiting for other threads
by Thomas Friedrich 13 May '09

13 May '09

Hi everyone, I have a problem with the following example in the Real World Haskell book, which aims to develop a module for controlling different threads. See, http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… in the chapter "The main thread and waiting for other threads". -- file: ch24/NiceFork.hs import Control.Concurrent import Control.Exception (Exception, try) import qualified Data.Map as M data ThreadStatus = Running | Finished -- terminated normally | Threw Exception -- killed by uncaught exception deriving (Eq, Show) -- | Create a new thread manager. newManager :: IO ThreadManager -- | Create a new managed thread. forkManaged :: ThreadManager -> IO () -> IO ThreadId -- | Immediately return the status of a managed thread. getStatus :: ThreadManager -> ThreadId -> IO (Maybe ThreadStatus) -- | Block until a specific managed thread terminates. waitFor :: ThreadManager -> ThreadId -> IO (Maybe ThreadStatus) -- | Block until all managed threads terminate. waitAll :: ThreadManager -> IO () When I run this through ghci I get the following failure: [1 of 1] Compiling NiceFork ( NiceFork.hs, interpreted ) NiceFork.hs:17:26: Class `Exception' used as a type In the type `Exception' In the data type declaration for `ThreadStatus' Failed, modules loaded: none. Any idea on how to solve this? Exception is a class not a type, so what to put there instead? Cheers, Thomas

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defining 'init' in terms of 'foldr'
by Michael Mossey 12 May '09

12 May '09

In S. Thompson's book, problem 9.13 asks us to define 'init' in terms of foldr. I was baffled at first because I didn't see a natural way to do this. It would look something like init xs = foldr f initialValue xs where f would cons on each character except the rightmost. f <when passed rightmost char> b = [] f <when passed any other char a> b = a : b How does f "know" when it is passed the first character? initialValue has to signal this somehow. On #haskell, one person suggested doing it with some post-processing: init xs = snd $ foldr f (True,[]) xs where f _ (True,_) = (False,[]) f a (False,b) = (False,a:b) I had an idea. If the initial value is the entire list, then its length can function as the "signal" that we are dealing with the rightmost char. This requires no post-processing: init xs = foldr f xs xs where f a b | length b == length xs = [] | otherwise = a:b These seem contrived. I wonder if there is a more natural solution that Thompson had in mind. Any comments? -Mike

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A Quantity Type - Integer without the Negative #'s
by aditya siram 11 May '09

11 May '09

Hi all, Is there a datatype in Haskell that can be used to represent only quantities >= 0? I got bitten by a bug because I forgot to reject an amount that was below zero after applying a decrementing operator. A simple unit test would have caught this, but I was wondering if there was some way of getting the type system to ensure this. thanks ... -deech

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Performance of function defined in a 'where' clause
by Kevin Haines 11 May '09

11 May '09

Hi All, I'm trying to write a bit of code that maps each byte in a block of Word8's to 3xWord8 using an array; i.e. mapping from 8 bit to 24 bit colour (this is an OpenGL application, and I'm using textures). I should point out that this is experimental code, and I'm still learning Haskell (and *loving* it, by the way!), so it probably looks a little unpolished. First, some data: data Palette = Palette { palRed :: Word8, palGrn :: Word8, palBlu :: Word8 } palette = listArray (0,49) paletteList paletteList = [ Palette 0 0 0, Palette 0 0 0, Palette 0 0 0, ..... Then, my first implementation, which took 57% time under profiling, was: loadTile :: Int -> Int -> IO (Ptr Word8) loadTile lat lon = do terrainBytes <- readTile lat lon -- implementation #1 mapM_ (paletteMapper terrainBytes rgbBytes) [0..tileSize^2-1] free terrainBytes return rgbBytes where tileSize = 128 paletteMapper :: Ptr Word8 -> Ptr Word8 -> Int -> IO () paletteMapper tb rgb idx = do v <- peekElemOff tb idx pokeByteOff rgb (idx*3) (palRed (palette!v)) pokeByteOff rgb (idx*3+1) (palGrn (palette!v)) pokeByteOff rgb (idx*3+2) (palBlu (palette!v)) I tried moving paletterMapper out of the 'where' clause and into the top level, which then took only 26% of time - i.e. half the time: paletteMapper :: Ptr Word8 -> Ptr Word8 -> Int -> IO () paletteMapper tb rgb idx = do v <- peekElemOff tb idx pokeByteOff rgb (idx*3) (palRed (palette!v)) pokeByteOff rgb (idx*3+1) (palGrn (palette!v)) pokeByteOff rgb (idx*3+2) (palBlu (palette!v)) loadTile :: Int -> Int -> IO (Ptr Word8) loadTile lat lon = do terrainBytes <- readTile lat lon -- implementation #1 mapM_ (paletteMapper terrainBytes rgbBytes) [0..tileSize^2-1] free terrainBytes return rgbBytes where tileSize = 128 I don't understand why - the functions are the same, except for the scope they're in. Can anyone elaborate on what's happening? Incidentally, I now realise a faster way (14%) is: loadTile :: Int -> Int -> IO (Ptr Word8) loadTile lat lon = do terrainBytes <- readTile lat lon rgbBytes <- mallocBytes (3*(tileSize^2)) mapM_ (\x -> do v <- peekElemOff terrainBytes x pokeByteOff rgbBytes (x*3) (palRed (palette!v)) pokeByteOff rgbBytes (x*3+1) (palGrn (palette!v)) pokeByteOff rgbBytes (x*3+2) (palBlu (palette!v)) ) [0..tileSize^2-1] free terrainBytes return rgbBytes where tileSize = 128 (There may be faster/better ways still, I'm all ears :-) Cheers Kevin

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SQL Lexer
by Patrick LeBoutillier 11 May '09

11 May '09

Hi all, In the process of writing an SQL parser I started by writing a lexer. The code can be found here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=4736#a4736 You can run it like this in ghci: Prelude SQL.Lexer> runLexer "select * from TABLE order by FIELD" [Token Reserved "select",Token Space " ",Token Operator "*",Token Space " ",Token Reserved "from",Token Space " ",Token Identifier "TABLE",Token Space " ",Token Reserved "order",Token Space " ",Token Reserved "by",Token Space " ",Token Identifier "FIELD"] Since this is pretty much my first Haskell project over 10 lines long, I'm looking for some feedback of any kind. Ultimately I would like to use this lexer to build a functional SQL parser using Parsec. Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Pros and Cons of Haskell libsndfile packages?
by Charles Turner 08 May '09

08 May '09

Hi all- I noticed that there were two different packages that provided a Haskell interface to the libsndfile library: HSoundFile: <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HSoundFile > hsoundfile: <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsndfile > Does anyone have any comments of the relative merits of either of these? Thanks! Charles

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How to wait till a process is finished before invoking the next one?
by Thomas Friedrich 07 May '09

07 May '09

Hi everyone, I have the following problem, and I hope that someone of yours might be able to help me. The Haskell program I am writing has the following setup: writeData :: [String] -> IO () writeData cs = ... runProgram:: [String] -> IO () runProgram cs = ... writeFeatures :: [String] -> IO () writeFeatures cs = ... runTestOnFeatures :: IO () runTestOnFeatures = ... main :: IO () main = do cs <- getArgs writeData cs runProgram cs writeFeatures cs runTestOnFeatures Each of the above function take a list of filenames, run certain command-line programs on them, which I invoke by runCommand, and each of them produce multiple output-files. Each function in main needs a couple of those output-files that are produced by the function directly above it. How do I get Haskell to wait, till all the data is written to the disk, before invoking the next command. The way the program is currently written, Haskell doesn't see that the input of one function depends on the output of another, and tries to run them all at the same time. Any ideas? Thanks everyone for your help. Cheers, Thomas

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Real World Haskell Chapter 5
by PJ Fitzpatrick 07 May '09

07 May '09

Hi, I have compiled the SimpleJSON.hs file from chapter 5 and from another file attempted to import it. I am getting a could not find module error and the prelude tells me that the locations searched were SimpleJSON.hs and SimpleJSON.lhs. I am running Vista, GHC 6.10.2 Any ideas? tks, PJ

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