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data.time.zoneinfo
by Anatoly Yakovenko 21 Oct '10

21 Oct '10

I am trying to figure out how to use this library, seems like this should work Prelude Data.Time.ZoneInfo> ctx <- newContext Nothing Prelude Data.Time.ZoneInfo> name <- zoneInfoName (utcOlsonZone ctx) True Prelude Data.Time.ZoneInfo> newOlsonZone ctx name *** Exception: Data.Time.ZoneInfo.newOlsonZone: invalid argument (Invalid argument) but it doesn't, neither does Prelude Data.Time.ZoneInfo> newOlsonZone ctx "America/New_York" *** Exception: Data.Time.ZoneInfo.newOlsonZone: invalid argument (Invalid argument) and my zoneinfo seems to be kosher on my machine $ ls /usr/share/zoneinfo/America/New_York /usr/share/zoneinfo/America/New_York $ ls /usr/share/zoneinfo/GMT /usr/share/zoneinfo/GMT what am i missing?

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Announce: splot-0.1.1
by Eugene Kirpichov 21 Oct '10

21 Oct '10

Hi cafe, I wrote yet another tiny visualization tool for my purposes and uploaded it on hackage. splot is a tool for visualizing the lifecycle of many concurrent multi-stage processes. For example, I'm using it to find the performance bottlenecks in a distributed application: processes are tasks and tasks have stages like "task in queue", "validating", "running", "response in queue". http://www.haskell.org/haskellwiki/Splot The tool is very basic but sufficient for my purposes. I'll be happy if anyone else also finds it useful (or not useful, but tells me in which exact way). P.S. Remember, there's also http://www.haskell.org/haskellwiki/Timeplot - a tool with similar purpose but a different (and bigger) set of features. There's a new version coming in a few days. However, I wasn't able to quickly implement the functionality of splot in Timeplot, and I needed it quickly, so I wrote a separate tool. -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)
by caseyh＠istar.ca 21 Oct '10

21 Oct '10

Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)

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playing around with network.curl
by Michael Litchard 21 Oct '10

21 Oct '10

I'm using this tutorial as a guide http://flygdynamikern.blogspot.com/2009/03/extended-sessions-with-haskell-c… github has changed since this was posted, but I have managed a successful login. Now I am faced with dealing with a re-direct. I found this constructor CurlFollowLocation Bool on this page http://hackage.haskell.org/packages/archive/curl/1.3.5/doc/html/Network-Cur… It seems to do what I want, But I am not clear on how to use it. Could someone provide an example? Thanks. End goal is to snarf the cookie that establishes the session.

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hs-plugins and memory leaks
by Evan Laforge 21 Oct '10

21 Oct '10

I was happy to see the recent announcement about hs-plugins being updated to work with newer ghc. I have a project and had always been planning to use it. However, there are some questions I've had about it for a long time. The 'yi' paper mentions both 'yi' and 'lambdabot' as users of hs-plugins. However, both those projects have long since abandoned it. I can't find any documentation on why, or even any documentation at all for Yi wrt its dynamic code execution system, but from looking at the source it looks like it uses hint for dynamic code execution and dyre for configuration. Dyre in turn uses serialization to pass the old state to the reconfigured app. So we have retreated from the idea of hotswapping the application state. It seems to me that the advantages as put forth in the 'yi' paper still hold. Changing the configuration in yi is rather heavyweight. Relinking the entire editor takes a long time, and yi is still a relatively small program. Editors can keep most of their state on disk and can have very simple GUI state, so perhaps the serialization and deserialization isn't such a problem, but this doesn't hold for other programs. It seems to me the loss is significant: there's a big difference between being able to experiment with a command by editing and rerunning it immediately, and having to wait 10s or more for the app to recompile, relink, shut down the ui, serialize all state, and restart. And if you add hint, you are linking in large parts of ghc, with an even slower link time. So, yi is no longer a dynamically reconfigurable application, and is now merely a configurable application. The apparent loss of such a useful feature (you might even say a defining feature) would presumably only happen if keeping it was untenable. And of course that makes me reluctant to make any kind of design that relies on it without first knowing why all existing users jumped ship. I can think of one possible reason, and that's a memory leak. In ghc/rts/Linker.c:unloadObj there's a commented out line '// stgFree(oc->image);'. In a test program I wrote that behaves like 'plugs', every executed line increases the size of the program by 12-16k. I have to remove the resolveObjs call from plugs for it to work, but once I do it displays the same leak. So my questions are: Why did lambdabot and yi abandon plugins? Is unloadObj a guaranteed memory leak? As far as I can tell, it's never called within ghc itself. If the choices are between a memory leak no matter how you use it and dangerous but correct if you use it right, shouldn't we at least have the latter available as an option? E.g. a reallyUnloadObj function that also frees the image. If I uncomment that line will it fix the problem? Is it safe to do so if I first force all thunks that might contain unloaded code? Long shot, but are there any more principled ways to guarantee no pointers to a chunk of code exist? The only thing I can think of is to have the state be totally strict and consist only of types from the static core. Would it be possible to hand responsibility for the memory off to the garbage collector? GHC now supports dynamic libraries. Given that plugins may need to link large portions of the static core "library", can it be loaded as a dynamic library so both the core and the plugins can share the same code? I haven't been able to find many references to ghc's support for dynamic linking.

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How to make cabal pass flags to happy?
by Niklas Broberg 21 Oct '10

21 Oct '10

Hi all, I want to do something I thought would be quite simple, but try as I might I can't find neither information nor examples on how to achieve it. What I want specifically is to have happy produce a GLR parser from my .ly file, and I want this to happen during 'cabal install'. Which in turn means I want cabal to pass the "--glr" flag to happy during setup. My best guess is that I might want to use 'ppHappy' [1], or something in the vicinity, but there's no documentation for the combinator and it's far from obvious how to pass arguments to it. ... help? :-) Cheers, /Niklas

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libefence useful for debugging ghc+ffi programs?
by Johannes Waldmann 20 Oct '10

20 Oct '10

Hi. I have an application compiled with ghc and using C via FFI (and lots of Haskell libraries as well) and it keeps annoying me with segfaults and (worse) unpredictable behaviour. I guess a C programmer would now apply electric-fence or valgrind. Is this a reasonable approach for ghc+ffi as well? E.g., this: Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x4114f950 (LWP 8343)] 0x00007f5af4e273b5 in free () from /usr/lib/libefence.so.0 (gdb) where #0 0x00007f5af4e273b5 in free () from /usr/lib/libefence.so.0 #1 0x00007f5af4059519 in ?? () from /usr/lib/libcurl.so.4 #2 0x00007f5af4059748 in ?? () from /usr/lib/libcurl.so.4 #3 0x00007f5af4059abd in ?? () from /usr/lib/libcurl.so.4 #4 0x00007f5af405f080 in ?? () from /usr/lib/libcurl.so.4 #5 0x00000000007bef95 in ?? () #6 0x00007f5aefb66628 in ?? () #7 0x00007f5af0df9f00 in ?? () #8 0x0000000000000000 in ?? () Is this a reason to distrust libcurl (the C libs) or curl (the Haskell package)? Thanks - J.W.

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Return value of a (windows) process
by Arnaud Bailly 20 Oct '10

20 Oct '10

Hello, I have the following code (fragment) I use to wrap execution of various processes in Haskell, in the spirit of Don Stewart's slides about "Scripting in Haskell". instance MonadExec IO where exec proc args = do (exit, out, err) <- liftIO $ readProcessWithExitCode proc args "" case exit of ExitFailure e -> throwError $ userError ("Fail execution of " ++ program proc args ++ ": " ++ (show e) ++", " ++err) ExitSuccess -> return out Here is the abridged declaration of MonadExec: class (Monad m) => MonadExec m where exec :: String -> [String] -> m String The issue I am facing is that readProcessWithExitCode does not seem to return the expected failures. When I run some failing process within windows command shell, I have: D:\projets\psug-dojo\lags>"d:/Program Files/scala-2.8.0/bin/scalac.bat" -d target\test-classes -classpath target\classes;D:\projets\ psug-dojo\lags\test-lib\specs_2.8.0-1.6.5.jar;D:\projets\psug-dojo\lags\test-lib\scalacheck_2.8.0-1.7.jar;D:\projets\psug-dojo\lags\ test-lib\junit-4.7.jar -d target\test-classes src\test\scala\oqube\lags\LagsTest.scala src\test\scala\oqube\lags\LagsTest.scala:86: error: not found: value beS_== bid.sublists(List(Request(1,2,3))) must beS_==(List(Nil,List(Request(1,2,3)))) ^ one error found D:\projets\psug-dojo\lags>echo %ERRORLEVEL% 1 D:\projets\psug-dojo\lags> but wrapping this same process in my "shell", I always got an ExitSuccess. What am I doing wrong ? Thanks Arnaud

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ANN: fountain-0.0.0
by Tom Hawkins 20 Oct '10

20 Oct '10

This library [1] implements a fountain code [2]. Fountain codes are forward error correction codes for erasure channels [3]. A fountain code encodes a message into an infinite stream of packets -- transmitters generate message packets at random, on-the-fly. To reconstruct the message, receivers simply need to capture enough packets for the decoding process. As a rateless code, fountain codes automatically adapt to varying channel conditions. Some of the more interesting applications of fountain codes include unsynchronized data broadcast and distributed download. For example, a multiple number of devices can transmitting content to multiple receivers without any coordination. Because packets are generated at random, receivers increase their bandwidth simply by listening to more transmitters. Note that receivers can also start generating packets and forwarding the message on even before they have decoded the complete message. This library provides a packet generator and a decoder for one of the first known fountain codes: LT codes [4]. It also includes a test function to experiment with message lengths, and encoding degrees -- it runs a simulation to determine the number of packets needed to decode a message. -Tom [1] http://hackage.haskell.org/package/fountain [2] http://en.wikipedia.org/wiki/Fountain_code [3] http://en.wikipedia.org/wiki/Binary_erasure_channel [4] http://en.wikipedia.org/wiki/LT_codes

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Getting results of of runSTUArray
by Henry Laxen 20 Oct '10

20 Oct '10

1{- Dear Group, I am trying to do something that I think should be very simple, but I just can't seem to figure out how. The idea is that I want to use runSTUArray to build up an array, but I want to look at the array after each iteration and possiblely accumulate some elements. -} import Debug.Trace import Control.Monad import Control.Monad.ST import Data.Array.ST incArray arr index = do x <- readArray arr index writeArray arr index (x+1) checkArray = filter (\(i,e) -> e == 1) run = runSTUArray ( do a <- newArray (0,9) 0 :: ST s (STUArray s Int Int) all <- forM [1..10] (\i -> do let j = i `mod` 4 incArray a j b <- getAssocs a -- now run something like checkArray and build up a list of "wanted" elements -- which I would like to return from run, but I don't see how return b) -- Or, if I cound run (lazily) on "all" (the result of the forM) -- I could do my accumulation let theAnswerIWant = concatMap checkArray all return (trace (show theAnswerIWant) a)) -- I can't seem to return anything but a here {- result is: array [(1,1),(1,1),(2,1),(1,1),(2,1),(3,1),(0,1), (1,1),(2,1),(3,1),(0,1),(2,1),(3,1),(0,1),(3,1), (0,1)] (0,9) [(0,2),(1,3),(2,3),(3,2),(4,0),(5,0),(6,0),(7,0),(8,0),(9,0)] is there any way to get theAnswerIWant out of run? Thanks in advance for any insights. -}

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