August 2009 - Haskell-Cafe

Tips for deployment?
by Günther Schmidt 20 Aug '09

20 Aug '09

Hi all, my haskell app is getting closer to shipping and what I need to do now is to give product protection some thought. The product is for hospitals and therefore a very limited set of clients. I do not expect anyone to try to "hack" my application but I need a basic protection mechanism in place, if only to check for a valid license key. As I've never done this before I'd be grateful for some suggestions here. Günther

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Right way to implement setPixel function
by CK Kashyap 20 Aug '09

20 Aug '09

Hi, I had posted a note on line drawing algo with Haskell some time back. Now, I am trying to write a PNM image. import qualified Data.ByteString as B width = 256 height = 256 bytesInImage = width * height * 3 blankImage = B.pack $ take bytesInImage (repeat 0) type Color = (Int,Int,Int) setPixel :: B.ByteString -> Int -> Int -> Color -> B.ByteString setPixel image x y (r,g,b) = B.concat [beforePixel, pixel, afterPixel] where beforePixel = B.take before image afterPixel = B.drop (before+3) image pixel=B.pack [(fromIntegral r),(fromIntegral g),(fromIntegral b)] -- number of bytes before the 3 bytes of -- the pixel at x y before = (y * width * 3) + (x * 3) - 3 main = do putStrLn "P6" putStrLn ( (show width) ++ " " ++ (show height) ) putStrLn "255" -- Set a red pixel at 100 100 B.putStr (setPixel blankImage 100 100 (255,0,0)) Can I please have some review comments on the code above? Would recreating the entire ByteString for each setPixel be an overhead? Also, I am barely beginning to grasp the Monad concept....I was wondering if there could be a monadic style of implementation of this - that could potentially have a series of setPixels inside a do block? Regards, Kashyap

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Haddock: Failed to create dependency graph (when adding sections with * or a module heading)
by Jared Updike 20 Aug '09

20 Aug '09

I compiled and installed haddock-2.4.2 from the tarball source. Adding a few simple comments to the code here: https://dl.getdropbox.com/u/143480/doc/DualMap.hs and running haddock $ haddock -h -o doc Data/DualMap.hs Warning: Data.DualMap: could not find link destinations for: Data.Typeable.Typeable2 GHC.Base.Eq GHC.Show.Show GHC.Base.Ord GHC.Base.Bool Data.Set.Set yields: https://dl.getdropbox.com/u/143480/doc/Data-DualMap.html Things look good. (Note that this module only depends on libs that ship with GHC and no other source modules.) However, when I try to add sections (a la http://www.haskell.org/haddock/doc/html/ch03s04.html#id289234 ) in the comments with "-- * test" I get: $ haddock -h -o doc Data/DualMap.hs Data/DualMap.hs:20:0: parse error on input `-- * test' haddock: Failed to create dependency graph I have no idea where to begin getting this to work since this error message only tells me that Haddock.Interface.depanal returned Nothing (according to a grep of the haddock sources) but not how to stop the dependency analysis from failing. Perhaps I need some more command line arguments or references to missing link destinations in GHC/base/containers documentation or some haddock config file? Searching Google yielded plenty of cabal build errors of the same ilk for packages on hackage but nothing about how to fix them. Jared.

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Can't derive Binary for StdGen
by Grigory Sarnitskiy 20 Aug '09

20 Aug '09

Hello! I'm trying to derive Binary for StdGen with DrIFT: module Main where import System.Random import Data.Binary {-!for StdGen derive : Binary !-} data Foo = Foo StdGen StdGen deriving (Show) {-! derive : Binary !-} but I got error "DrIFT: can't find module System/Random" What shall I do?

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generalize RecordPuns and RecordWildCards to work with qualified names?
by Evan Laforge 20 Aug '09

20 Aug '09

Record punning is not all that useful with qualified module names. If I write '(M.Record { M.rec_x })' it says " Qualified variable in pattern" and if I write '(M.Record { rec_x })' it says 'Not in scope: `rec_x''. Could it be this extension be further extended slightly so that 'f (M.Record { M.rec_x })' will desugar to 'f (M.Record { M.rec_x = rec_x })'? Similarly, RecordWildCards could support this too. It seems simple and useful to me... am I missing anything fatally problematic about this? Would anyone else use it?

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gbp sign showing as unknown character by GHC
by Iain Barnett 20 Aug '09

20 Aug '09

Quick question: I've tested this in a couple of different terminals (roxterm and xterm), so I'm fairly sure it's GHC that's the problem. Have I missed a setting? GHCi, version 6.10.4 Prelude> putStrLn "£" � Hugs98 200609-3 Hugs> putStrLn "£" £ I get the same character output from a password generator I've writtern, after compilation with GHC [iainb]$ ./makepass2 50 2 >> testfile.txt [iainb]$ cat testfile.txt H(xW!:maNyxZ;h,IW=Uu4G$ztc>k@Q[g6?y:�TbG&5Nd")+"5+ Iain

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ANNOUNCE: ministg-0.2, an interpreter for STG operational semantics
by Bernie Pope 20 Aug '09

20 Aug '09

I'm pleased to announce the first public release of Ministg. Ministg is an interpreter for a high-level, small-step, operational semantics for the STG machine. The STG machine is the abstract machine at the core of GHC. The operational semantics used in Ministg is taken from the paper "Making a fast curry: push/enter vs. eval/apply for higher-order languages" by Simon Marlow and Simon Peyton Jones. Ministg implements both sets of evaluation rules from the paper. One of the main features of Ministg is the ability to record a trace of the execution steps as a sequence of html files. And example trace can be viewed here: http://www.cs.mu.oz.au/~bjpop/trace/step0.html The example shows the execution of a program which sums a list of three integers, using the well-known space-leaky version of sum. Follow the "next" and "previous" links to step forwards and backwards through the trace. The main reason I wrote Ministg is to explore various extensions to the STG machine. The current release features a simple extension in the form of call-stack tracing, which is strongly influenced by the cost-centre stacks of GHC. I expect it will also be a useful tool for people who are interested in learning more about the STG machine. More detailed information is available from the haskellwiki page: http://www.haskell.org/haskellwiki/Ministg You can download it from hackagedb: http://hackage.haskell.org/package/ministg Or you can get the latest code from patch-tag: darcs get http://patch-tag.com/r/ministg/pullrepo ministg Cheers, Bernie.

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Generics for constructing Rows
by Max Desyatov 20 Aug '09

20 Aug '09

Sorry for a type and probably confusing you, it must be: > readRow l = gmap (\(Left (Just ri)) -> Right $ l `atMay` ri >>= readMay) instead of > l `atMay` c

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Planning for a website
by Colin Paul Adams 20 Aug '09

20 Aug '09

I'm intending to replace my current website - which uses Drupal, with a hand-written-in-Haskell version this autumn, for a number of reasons (a principal one is the lack of an upgrade path in Drupal). So I'm currently looking into the libraries available to see how much I'll have to write myself. One problem will be to get GHC ported to DragonFly BSD, but that can wait until I have a test version of the site working on Linux. The next major challange is to implement something like Drupal's Image and Image Gallery modules. That doesn't seem to be a great problem, as the exif and GD libraries seem to cover everything I'll need. So my major decision is what framework and html-generating libraries to use. There is such a wide choice on the Haskell Wiki. But I guess some are more maintained than others. For instance, WASH attracts me, with it's guarantee of valid generated pages, but it isn't clear to me that it's actively maintained (last date I can see on the web pages is 2006). HappStack is obviously currently maintained, and since it seems to have a blogging module in development, that is attractive. HASP is maintained too, I think. Has anyone written a comparison of the various libraries, or gone through the same decision process recently? -- Colin Adams Preston Lancashire

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Control.Parallel on 6.10.4 - Mac OS X version
by Kevin Smith 20 Aug '09

20 Aug '09

I'm using the Control.Parallel package on ghc version 6.10.4 and I don't seem to be getting any parallel threads or sparks created at all with the "-threaded" compilation option using runtime flags "+RTS -N2". I'm using simple examples from the paper "A Tutorial on Parallel and Concurrent Programming in Haskell" by Jones and Singh. I'm running on an Intel Mac core 2 duo. Does anyone know how I can check if the parallel functionality in ghc is working on my 6.10.4 ghc package installed on this platform ? I installed it from the "supported" runtime pkg for OS X from the ghc website. thanks

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