November 2008 - Haskell-Cafe

Strictness problems with Control.Event?
by Brian Troutwine 05 Nov '08

05 Nov '08

I'm attempting to use Control.Event to limit HTTP requests made by a dippy little scraper I'm constructing to once per second but I think, maybe, that the Events are not being evaluated. First, some imports. > import qualified Data.ByteString as B > import qualified Data.ByteString.Char8 as C > import Network.Curl.Download > import Network.Curl.Opts > import System.Exit > import System.Environment > import System.Time > import Control.Event > import Control.Monad > import Data.Char The Event function of this program is append. It takes a local path and a URL, retrieves the contents pointed to by the URL and appends them to the local path. The function download, below, performs the retrieval. > append :: FilePath -> C.ByteString -> IO () > append f u = > B.appendFile f . addNew . C.filter (not . isAscii) =<< download u > where addNew = C.append (C.pack "\n") > download :: B.ByteString -> IO B.ByteString > download url = do > res <- openURIWithOpts [CurlEncoding "gzip", CurlUserAgent "aule-0.0.2"] $ C.unpack url > case res of > Left _ -> exitFailure > Right cont -> B.putStrLn cont >> return cont The function sched adds a list of Events to the evtSys system, a fixed time delay between each. > sched :: EventSystem -> ClockTime -> Integer -> Integer -> (t -> IO ()) -> [t] -> IO () > sched _ _ _ _ _ [] = return () > sched evtSys (TOD sec _) delay mul action (x:xs) = do > addEvent evtSys (TOD (sec + (delay*mul)) 0) (action x) > sched evtSys (TOD sec 0) delay (mul + 1) action xs main is, as usual, pretty boring. The program compiles and runs, but no output file is made. This being one of my first appreciable Haskell programs, I rather imagine I've run into a strictness problem, maybe. Would someone be so kind as to steer me in the right direction? > main :: IO () > main = do > [i, o] <- getArgs > eS <- initEventSystem > t <- getClockTime > inp <- B.readFile i > sched eS t 1 1 (append o) (C.lines inp) Thanks, Brian

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two problems with Data.Binary and Data.ByteString
by Tim Newsham 05 Nov '08

05 Nov '08

I have a program that read in and populated a large data structure and then saved it out with Data.Binary and Data.ByteString.Lazy.Char8: saveState db = B.writeFile stateFile =<< encode <$> atomically (readTVar db) when I go to read this in later I get a stack overflow: loadState db = do d <- decode <$> B.readFile stateFile atomically $ writeTVar db d Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. or from ghci: d <- liftM decode (Data.ByteString.Lazy.Char8.readFile "savedState.bin") :: IO InstrsDb fromList *** Exception: stack overflow The data type I'm storing is a Map (of maps): type DailyDb = M.Map Date Daily type InstrsDb = M.Map String DailyDb What's going on here? Why is the system capable of building and saving the data but not in reading and umarhsalling it? What is the proper way to track down where the exception is happening? Any debugging tips? I also noticed another issue while testing. If my program loads the data at startup by calling loadState then all later calls to saveState give an error: Log: savedState.bin: openFile: resource busy (file is locked) this does not occur if the program wasnt loaded. My best guess here is that B.readFile isnt completing and closing the file for some reason. Is there a good way to force this? Tim Newsham http://www.thenewsh.com/~newsham/

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HOpenGL wiki link to documentation is broken
by Jefferson Heard 05 Nov '08

05 Nov '08

I suspect this has to do with the latest GHC not including it by default, but the HOpenGL wiki's documentation links are broken. -- Jeff

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Re: [Haskell-cafe] Efficient parallel regular expressions
by roger peppe 05 Nov '08

05 Nov '08

On Tue, Nov 4, 2008 at 6:44 PM, i wrote: > i'm sorry if this is obviously wrong (i haven't used Text.Regex), but > can't you do this with submatches? rights or wrongs of regexps aside, i just checked that the above approach *is* feasible with Text.Regex here's some code: >module Multimatch(multimatch) where > import Text.Regex > import qualified Data.List as DL > import qualified Data.Maybe as DM > > brcount :: String -> Int > brcount ('\\' : _ : s) = > brcount s > brcount ('(' : s) = > 1 + brcount s > brcount (_ : s) = > brcount s > brcount [] = > 0 > > -- given a list of strings representing regular expressions, > -- each associated with a tag, match against a string > -- and return the match, along with the associated tag, > -- or Nothing if there's no match. > multimatch :: [(tag, String)] -> (String -> Maybe (tag, String)) > multimatch rs = > let re = mkRegex $ DL.intercalate "|" $ map ((\s -> "(x(" ++ s ++ "))") . snd) rs in > let tags = submatches rs in > (\ s -> > do > ms <- matchRegex re ("x" ++ s) > (tag, m) <- DL.find (\(_, m) -> not (null m)) (zip tags ms) > return (DM.fromJust tag, tail m)) > submatches [] = > [] > submatches ((tag, r) : rs) = > (Just tag : take (brcount r + 1) (repeat Nothing)) ++ submatches rs i'm sure there's a more compact implementation in there somewhere: i'm just a haskell newbie. cheers, rog.

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Re: [Haskell-cafe] Memory efficiency questions for real-time graphics
by T Willingham 05 Nov '08

05 Nov '08

On Sat, Nov 1, 2008 at 2:11 PM, Sebastian Sylvan <sebastian.sylvan(a)gmail.com> wrote: > On Sat, Nov 1, 2008 at 6:57 PM, T Willingham <t.r.willingham(a)gmail.com> >> >> The per-vertex computation is a quite complex time-dependent function >> applied to the given domain on each update. Yet even if it were >> simple, I would still first implement the formulas in Haskell and >> leave the optimization for later, if at all. > > I'd be very surprised to see a vertex transform that would be faster to > implement on the CPU than the GPU. It appears you misunderstood "complex time-dependent function". This is quite a bit of Haskell code involving a significant amount of octonion algebra. It could, in principle, be put on the GPU, however that should be the very last step after everything else works. > There are various ways of writing out your vertex data too, so if it doesn't > change too often you can still do the transformation on the GPU, As I previously stated, it changes every frame. Take a highly complicated function and apply it to N vertices. Now increase N until the framerate is affected. That is where I am. It is obvious that any N-sized allocations will cause the framerate to stutter. This is not just theoretical: I *see* it as I increase N while it's running. This is the point of my initial email, the subject of which is memory efficiency.

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Making 'community' server our social network
by Maurício 05 Nov '08

05 Nov '08

Hi, I've beeing doing something with darcs that is so great that, although I'm sure a lot of people are already doing the same, I think it would be nice to share with you. I did 'cd ~' and then: darcs initialize darcs add .emacs darcs add .xmonad/xmonad.hs darcs add .inputrc ... etc. So I'm using darcs to keep track of all my configuration files, and now I don't need to care about reinstalling the OS, changing machines from office to home, changing configurations and then realizing it was a mistake etc. Then I thought community.haskell.org could offer a default darcs repositories for all users named after their owners. For instance, if you want to check my personal files you would do: darcs get http://code.haskell.org/MauricioAntunes That would allow us not only to share our configuration, but also share all those small files that are not "professional" enough to became projects for their own, but that are nevertheless interesting since we put our good ideas there: scripts to start our favorite software or do system maintenance; small Haskell utilities to do some cool math or science trick, stress some language feature or download our standard music collection from the web; a list of favorite sites and a related completion script so we can list then in 'dmenu' completion; etc. etc. etc. The next bonus step would be to get a list at all users default repository main page of all other users they have granted write access to some of their projects. Then we would be able to navigate throw linked lists of people with related interests and needs. I think we need no more to get what will be MySpace, Facebook or something else for text-driven people. And then, of course, world domination. Although I don't really imagine any business model around that :) Best, Maurício

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Re: [Haskell-cafe] HXT/Practical/Weather.hs difficulties
by Brian Troutwine 04 Nov '08

04 Nov '08

Oh my, how silly of me. The page has moved to http://www.weather.gov/xml/current_obs/KAGC.xml ! Firefox simply follows the redirect. Thanks for the putStrLn suggestion, George. On Tue, Nov 4, 2008 at 5:28 PM, George Pollard <porges(a)porg.es> wrote: > On Tue, 2008-11-04 at 17:04 -0800, Brian Troutwine wrote: >> > blt@doritos:~ >> > $ ./weather >> > >> > error: ""<!DOCTYPE HTML PUBLIC \"-//IETF//DTD HTML 2.0/..."" (line 1, column 16): >> > unexpected "P" >> > expecting white space, "[" or ">" >> > >> > Unable to parse weather data. >> >> Woops. Would someone be so kind as to explain why this is failing? I'm >> at rather a loss. > > The HXT parser is stopping because it's expecting XML not HTML, which is > what you're receiving for some reason. > > I just loaded up the weather URL in Firefox > [http://www.weather.gov/xml/current_obs/KAGC.xml] and it is XML... Is it > possible that you are behind a proxy that is blocking the page? (I can't > run the code at the moment; cleaning out Haskell packages to upgrade to > 6.10.) > > You could try printing out the contents of the page as well (putStrLn > doc) so you can see the rest of the HTML that `weather` is receiving. > -- Brian

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HXT/Practical/Weather.hs difficulties
by Brian Troutwine 04 Nov '08

04 Nov '08

Hello all, I'm working through the practical HXT examples and have gotten stumped with the weather example[1]. I'm using GHC 6.8.3 and HXT 8.1.0. I compile the example like so... > blt@doritos:~ > $ ghc --make weather.hs > [1 of 1] Compiling Main ( weather.hs, weather.o ) > Linking weather ... And execute it... > blt@doritos:~ > $ ./weather > > error: ""<!DOCTYPE HTML PUBLIC \"-//IETF//DTD HTML 2.0/..."" (line 1, column 16): > unexpected "P" > expecting white space, "[" or ">" > > Unable to parse weather data. Woops. Would someone be so kind as to explain why this is failing? I'm at rather a loss. [1] http://www.haskell.org/haskellwiki/HXT/Practical/Weather1 --Brian P.S. Is this question better suited to the haskell-beginner list?

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announce [("InfixApplicative", 1.0), ("OpenGLCheck", 1.0), ("obj", 0.1)]
by Thomas Davie 04 Nov '08

04 Nov '08

Dear Haskellers, I've just uploaded a few packages to Hackage which I have produced while working at Anygma. I thought that people might be interested in knowing about these: obj-0.1: A library for loading and writing obj 3D models. This is still an early version and rather limited, but it's a starting point. Features: • Load models, complete with normals, texture coordinates and materials • Compute normals where smoothing groups are specified • An example program to load an obj model and render it spinning on the screen. • Faster loading that Maya itself! Bugs: • Memory usage is rather large • The exposed API is rather limited To Dos: • Add support for loading groups • Support for smooth surfaces OpenGLCheck-1.0: A micro-package containing instances of Arbitrary for the data structures provided in Graphics.Rendering.OpenGL. InfixApplicative-1.0: A second micro-package containing a pair of functions -- (<^) and (^>) which can be used to provide an infix liftA2 thus: Suppose we wanted to calculate liftA2 (+) [1,2] [2,3], but are unhappy with the fact that (+) is no longer infix, we may now use [1,2] <^(+)^> [2,3] Thanks -- any comments are greatly appreciated! Tom Davie

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Syntax question regarding 'import'
by Maurício 04 Nov '08

04 Nov '08

Hi, About this teste code: module Main (main) where { import Foreign hiding (unsafePerformIO,Foreign.Ptr,Data.Bits,) ; blo = xor 10 10 :: Int ; main = return () } Data.Bits is in the 'hiding' list. According to the syntax reference, this seems not to be allowed. GHC allows it, but do allow following code to use 'xor' from Data.Bits. I think this is correct, since 'xor' was exported by Foreign unqualified, but does 'Data.Bits' in that list mean something or it's just ignored? Haskell syntax allows a comma at the end of names to be imported or exported, like in the second line. What does that mean? Thanks, Maurício

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