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Re: [Haskell-cafe] Generating random enums
by michael rice 02 May '09

02 May '09

I intuited that that's what the problem was. Thanks, Michael --- On Sat, 5/2/09, Rahul Kapoor <rk(a)trie.org> wrote: From: Rahul Kapoor <rk(a)trie.org> Subject: Re: [Haskell-cafe] Generating random enums To: "michael rice" <nowgate(a)yahoo.com> Cc: "John Van Enk" <vanenkj(a)gmail.com>, haskell-cafe(a)haskell.org Date: Saturday, May 2, 2009, 2:00 PM > OK, I think what you're saying is to work with (random) integers and use > fromEnum and toEnum to get corresponding DayOfWeek. But I get this when I > try to use toEnum: > > *Main> toEnum 2 ghci does not know what type of enum you want to create from the number 2. Try: toEnum 2 :: DayOfWeek That said, I would expect "toEnum 2" to give an error like: 'Ambiguous type variable `a'....'. So I am not sure why your error message says: '** Exception: Prelude.Enum.().toEnum: bad argument'

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Haskell Weekly News: Issue 116 - May 2, 2009
by Brent Yorgey 02 May '09

02 May '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090502 Issue 116 - May 02, 2009 --------------------------------------------------------------------------- Welcome to issue 116 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements GHC 6.10.3 prerelease. Ian Lynagh [2]announced a [3]prerelease [4]version of GHC 6.10.3. There have been [5]very few changes relative to 6.10.2. Unless any major problems are uncovered, the final release is expected to be built in a couple of days. graphviz-2009.5.1. Ivan Lazar Miljenovic [6]announced version 2009.5.1 of the [7]graphviz library, which provides a Haskell interface to the [8]GraphViz program. Major changes include support for polyparse >= 1.1, dependency on GHC 6.10.*, functions from the [9]Graphalyze library, and more. priority-sync-0.1.0.1: Cooperative task prioritization.. Christopher Lane Hinson [10]announced the release of the [11]priority-sync package for cooperative task prioritization. HaL4: Local Haskell meeting, Halle/Saale, Germany, June 12. Henning Thielemann [12]requested proposals for talks for [13]HaL4, a local Haskell meeting in Halle, Germany on June 12. TraverseAccum: an effectful accumulating map.. Florent Balestrieri [14]posted some code implementing an effectful accumulating map. LondonHUG talk: Engineering Large Projects in Haskell. Don Stewart [15]posted [16]slides from last week's [17]London HUG talk, which attempts to document some of the tips and tricks Galois has accumulated using Haskell commercially for the past 10 years. atom-0.0.2. Tom Hawkins [18]announced the release of [19]atom, a Haskell DSL for designed hard realtime embedded programs. Eaton is using it to control hydraulic hybrid refuse trucks and shuttle buses. Bamse-0.9.4, a Windows Installer generator. Sigbjorn Finne [20]announced a new version of [21]Bamse, a package and application for letting you quickly put together Windows Installers for your software projects/products from within the comforts of Haskell. New in this release is the support for generating MSIs from your Cabal projects, having them either be built from source or just have them be installed and registered at install-time. Dutch Haskell Users' Group (first meeting: May 6th). Chris Eidhof [22]announced the first meeting of the newly formed [23]Dutch Haskell Users' Group (DHUG), on May 6th at 19:30 in Utrecht. Haskell Symposium Submission site now open. Stephanie Weirich [24]announced that [25]submission to the Haskell Symposium is now open. The submission deadline is May 8. OpenGL, GLUT, OpenAL, and ALUT updates. Sven Panne [26]announced [27]new [28]bugfix [29]releases for the [30]OpenGL, [31]GLUT, [32]OpenAL, and [33]ALUT packages. control-monad-exception-0.1: Explicitly typed exceptions. Pepe Iborra [34]announced the [35]control-monad-exception package, which provides explicitly typed exceptions for Haskell. The type of a computation in the EM monad carries a list of the exceptions that the computation may throw. A exception is raised with 'throw', which in addition adds it to the type, and captured with 'catch', which correspondingly removes it from the type. Only safe computations (all exceptions handled) can escape from the monad. Haskell File Manager. Michael Dever [36]announced the first release of [37]Haskell File Manager, a program for viewing/managing the files on your computer. It has all the common functionality you would expect from your current file manager, copying, moving, deleting, renaming, opening and searching. uu-parsinglib-2.0.0. S. Doaitse Swierstra [38]announced the release of [39]uu-parsinglib, the first version of the new parsing combinator library package from Utrecht University. Features include online result construction, much simpler internals than the combinators in the uulib package, error correction, parsing ambiguous grammars, a monadic interface, and more. Takusen 0.8.4. Alistair Bayley [40]announced version 0.8.4 of [41]Takusen, a database package; this is mostly a "get it working with ghc-6.10" release. Discussion Google SoC: Space profiling reloaded. Patai Gergely [42]asked for ideas on his Google Summer of Code project to [43]improve the Haskell space profiling experience. Blog noise [44]Haskell news from the [45]blogosphere. * Bryan O'Sullivan: [46]Slides from my Erlang Factory talk this morning. * Thomas Hartman: [47]What to do when cabal install works in one environment, but not another.. * Christopher Lane Hinson: [48]ANN: priority-sync. * Well-Typed.Com: [49]"Hello world" now only 11k using GHC with shared libs. * The GHC Team: [50]The new GHC build system is here!. * Well-Typed.Com: [51]First round of Industrial Haskell Group development work. * London Haskell Users Group: [52]Don's slides. * Galois, Inc: [53]Engineering Large Projects in Haskell: A Decade of FP at Galois. * JP Moresmau: [54]Haskell RPG Game uploaded on Hackage!. * Holumbus: [55]Distributed data structures. * Niklas Broberg: [56]GSoC09 => haskell-src-exts -> haskell-src. * >>> Remco Niemeijer: [57]Forcing evaluation in Haskell. * Chris Eidhof: [58]Building commercial Haskell applications. * >>> Lab49: [59]Differentiating Types in Haskell. Quotes of the Week * MonadState: Do not try to change the state; that's impossible. Instead only try to realize the truth: There is no state. * Baughn: Those who would give up essential laziness for a little ephemeral performance, deserve neither laziness nor performance. * Axman6: what's @flush do? saves stuff to dick? * bos: Crummy languages give static types a bad name. About the Haskell Weekly News New editions are posted to [60]the Haskell mailing list as well as to [61]the Haskell Sequence and [62]Planet Haskell. [63]RSS is also available, and headlines appear on [64]haskell.org. To help create new editions of this newsletter, please see the information on [65]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [66]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/16852 3. http://www.haskell.org/ghc/dist/stable/dist/ghc-6.10.2.20090430-src.tar.bz2 4. http://www.haskell.org/ghc/dist/stable/dist/ghc-6.10.2.20090430-src-extrali… 5. http://www.haskell.org/ghc/dist/stable/docs/users_guide/release-6-10-3.html 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57826 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphviz 8. http://graphviz.org/ 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Graphalyze 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57776 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/priority-sync 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57764 13. http://www.iba-cg.de/hal4.html 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57730 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57719 16. http://www.galois.com/blog/2009/04/27/engineering-large-projects-in-haskell… 17. http://www.londonhug.net/ 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57676 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/atom 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57640 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bamse 22. http://article.gmane.org/gmane.comp.lang.haskell.general/17142 23. http://haskell.org/haskellwiki/Dutch_HUG 24. http://article.gmane.org/gmane.comp.lang.haskell.general/17140 25. http://www.easychair.org/conferences/?conf=haskell09 26. http://article.gmane.org/gmane.comp.lang.haskell.general/17132 27. http://article.gmane.org/gmane.comp.lang.haskell.general/17133 28. http://article.gmane.org/gmane.comp.lang.haskell.general/17134 29. http://article.gmane.org/gmane.comp.lang.haskell.general/17135 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OpenGL 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GLUT 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OpenAL 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ALUT 34. http://article.gmane.org/gmane.comp.lang.haskell.general/17120 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control%2Dmonad%… 36. http://www.haskell.org//pipermail/haskell/2009-April/021271.html 37. http://code.haskell.org/haskellfm 38. http://www.haskell.org//pipermail/haskell/2009-April/021284.html 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uu%2Dparsinglib 40. http://www.haskell.org//pipermail/haskell/2009-May/021293.html 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Takusen 42. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/57810 43. http://socghop.appspot.com/student_project/show/google/gsoc2009/haskell/t12… 44. http://planet.haskell.org/ 45. http://haskell.org/haskellwiki/Blog_articles 46. http://www.serpentine.com/blog/2009/05/01/slides-from-my-erlang-factory-tal… 47. http://blog.patch-tag.com/2009/05/01/what-to-do-when-cabal-install-works-in… 48. http://blog.downstairspeople.org/2009/04/29/ann-priority-sync/ 49. http://blog.well-typed.com/2009/04/hello-world-now-only-11k-using-ghc-with-… 50. http://ghcmutterings.wordpress.com/2009/04/28/the-new-ghc-build-system-is-h… 51. http://blog.well-typed.com/2009/04/first-round-of-ihg-development-work/ 52. http://www.londonhug.net/2009/04/27/dons-slides/ 53. http://www.galois.com/blog/2009/04/27/engineering-large-projects-in-haskell… 54. http://jpmoresmau.blogspot.com/2009/04/haskell-rpg-game-uploaded-on-hackage… 55. http://holumbus.fh-wedel.de/blog/?p=22 56. http://nibrofun.blogspot.com/2009/04/gsoc09-haskell-src-exts-haskell-src.ht… 57. http://bonsaicode.wordpress.com/2009/04/27/forcing-evaluation-in-haskell/ 58. http://blog.tupil.com/building-commercial-haskell-applications/ 59. http://blog.lab49.com/archives/3027 60. http://www.haskell.org/mailman/listinfo/haskell 61. http://sequence.complete.org/ 62. http://planet.haskell.org/ 63. http://sequence.complete.org/node/feed 64. http://haskell.org/ 65. http://haskell.org/haskellwiki/HWN 66. http://code.haskell.org/~byorgey/code/hwn/

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fromInteger for Lists
by Paul Keir 02 May '09

02 May '09

There's nothing better than making a data type an instance of Num. In particular, fromInteger is a joy. But how about lists? For example, if I have data Foo a = F [a] I can create a fromInteger such as fromInteger i = F [fromInteger i] and then a 19::(Foo Int), could become F [19]. Is it possible to do something similar for lists? So could [1,2,3]::(Foo Int) become something slightly different, say, F [1,2,3] Paul

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[tryReadAdvChan :: AdvChan a -> IO (Maybe a)] problems
by Belka 02 May '09

02 May '09

Hi! I need this function with requirement of heavy reads, *possibly under DDoS attack*. Was trying to write such function, but discovered some serious problems of ** possible racings, ** possible starvations ** unbalance: readAdvChan users may get better service than ones of tryReadAdvChan These are totally unacceptible for my case of DDoS risk. Actually, am I wrong thinking, that it can't be helped - and the degradation from cute concurency synchronization model of Chan is unavoidable? My (untested) code: ------------------------------------------- ------------------------------------------- module AdvChan ( AdvChan , newAdvChan , readAdvChan , writeAdvChan , writeList2AdvChan , advChan2StrictList , withResourceFromAdvChan , tryReadAdvChan , isEmptyAdvChan ) where import Control.Concurrent.Chan import Control.Concurrent.MVar data AdvChan a = AdvChan { acInst :: MVar Chan a , acWrite :: a -> IO () , acIsEmpty :: IO Bool } newAdvChan :: IO AdvChan a newAdvChan = do ch <- newChan mv_ch <- newMVar ch return AdvChan { acInst = mv_ch , acWrite = writeChan ch , acIsEmpty = isEmptyChan ch } readAdvChan :: AdvChan a -> IO a readAdvChan ach = modifyMVar (acInst ach) (\ ch -> do a <- readChan ch return (ch, a) ) writeAdvChan :: AdvChan a -> a -> IO () writeAdvChan = acWrite writeList2AdvChan :: AdvChan a -> [a] -> IO () writeList2AdvChan ach [] = return () writeList2AdvChan ach (h:t) = writeAdvChan ach h >> writeList2AdvChan ach t advChan2StrictList :: AdvChan a -> IO [a] advChan2StrictList ach = modifyMVar (acInst ach) (\ ch -> let readLoop = do emp <- isEmptyChan ch case emp of True -> return [] False -> do _head <- readChan ch _rest <- readLoop return (_head : _rest) in liftTuple (return ch, readLoop) ) withResourceFromAdvChan :: AdvChan a -> (\ a -> IO (a, b)) -> IO b withResourceFromAdvChan ach f = do res <- readAdvChan ach (res_processed, result) <- f res writeAdvChan ach res_processed return result isEmptyAdvChan :: AdvChan a -> IO Bool isEmptyAdvChan = acIsEmpty microDelta = 50 tryReadAdvChan :: AdvChan a -> IO (Maybe a) tryReadAdvChan ach = emp2Maybeness $ do mb_inst <- tryTakeMVar (acInst ach) case mb_inst of Nothing -> emp2Maybeness (threadDelay microDelta >> tryReadAdvChan ach) Just chan -> do emp <- isEmptyChan ch result <- case emp of True -> return Nothing False -> Just `liftM` readChan ch putMVar (acInst ach) chan return result where emp2Maybeness f = do emp <- isEmptyAdvChan ach case emp of True -> return Nothing False -> f ------------------------------------------- ------------------------------------------- Later after writing my own code, and understanding the problem I checked Hackage. Found "synchronous-channels" package there (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/synchronous-chan…) but it isn't any further in solving my the unbalacedness problems. Any suggestions on the fresh matter are welcome. Belka. -- View this message in context: http://www.nabble.com/-tryReadAdvChan-%3A%3A-AdvChan-a--%3E-IO-%28Maybe-a%2… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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swish - semantic web in Haskell
by Vasili I. Galchin 02 May '09

02 May '09

Hello, http://www.ninebynine.org/Software/swish-0.2.1.html .... I am trying to get permission from the author of Swish to move it into Hackage. I will write "cabalize" it get up to current Haskell standards. Before I go to this effort is anybody attempting to do this? Kind regards, Vasili

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The Haskell Reddit is 1 year old
by Don Stewart 02 May '09

02 May '09

Just a reminder, the Haskell Reddit is live and active: http://www.reddit.com/r/haskell/ It is a a place for fast, daily, comprehensive news about what's going on in the Haskell community, combining blogs, mail, irc, ghc's patches, the freaking types@ mailing list! It's all here, with comments. Enjoy. Contribute. -- Don

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ANN: Takusen 0.8.4
by Alistair Bayley 01 May '09

01 May '09

ANN: Takusen 0.8.4 This is mainly a "get it working with ghc-6.10" release. Changes since 0.8.3: - ODBC support: some bug fixes, and basic support for Out-parameters. There is at least one major outstanding bug with Out-parameters, where marshalling a String output bind-variable doesn't work correctly (if it was modified, you still get the String you passed in back, rather than the new String). - Sqlite: bug fix for resource leak: if a command (like rollback or commit) raised an error, the prepared-statement handle was not closed. - cabal: requires cabal 1.6 (or later). With ghc-6.6 on Windows you will need a later cabal than 1.6.0.1, as there is a bug which prevents building (setup configure fails with "setup: fdGetMode: invalid argument (Invalid argument)"). Eric Mertens patch for ODBC on OS x. Greg Leclerq patch for cabal 1.6. Thanks also to Austin Siepp and Brian Callendar for their patches. - ghc-6.10: will build with ghc's 6.10, 6.8, 6.6. Note that Takusen won't work with ghc-6.10.2 out-of-the-box, because the time package is missing. If you install time manually then it should be OK (but this is untested). We use the new extensible exceptions, but we have done so by including it within our own source tree. This will make it easier to use Takusen with older ghc's, and reduces the number of external package dependencies. We have dropped support for 6.4. That's not to say Takusen isn't usable with 6.4, just that we no longer test that configuration. The release bundle: http://hackage.haskell.org/packages/archive/Takusen/0.8.4/Takusen-0.8.4.tar… The latest code: darcs get http://darcs.haskell.org/takusen Docs: http://darcs.haskell.org/takusen/doc/html/index.html If you have cabal-install, then this command should work: cabal install Takusen --flags="sqlite odbc oracle postgres" ... but it doesn't (at least for the version of cabal-install that I have) because the .tar.gz file I uploaded has a bogus checksum. A comprehensive description of API usage can be found in the documentation for module Database.Enumerator (look for the Usage section): http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html Note that the haddock docs won't build with cabal-1.6.0.2 or earlier. This is because our literate-haskell source files need to be properly preprocessed before being passed to haddock. At the time of writing, you'll need the cabal HEAD to build the docs. Future plans: - Output bind-parameters and multiple-result sets for ODBC - support for Blobs and Clobs - FreeTDS backend (Sybase and MS Sql Server)... maybe For those of you unfamiliar with Takusen, here is our HCAR blurb: Takusen is a DBMS access library. Like HSQL and HDBC, we support arbitrary SQL statements (currently strings, extensible to anything that can be converted to a string). Takusen's 'unique-selling-point' is safety and efficiency. We statically ensure all acquired database resources - such as cursors, connection and statement handles - are released, exactly once, at predictable times. Takusen can avoid loading the whole result set in memory, and so can handle queries returning millions of rows in constant space. Takusen also supports automatic marshalling and unmarshalling of results and query parameters. These benefits come from the design of query result processing around a left-fold enumerator. Currently we fully support ODBC, Oracle, Sqlite, and PostgreSQL.

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What to do when cabal dependencies are not your friend.
by Thomas Hartman 01 May '09

01 May '09

I did a little write-up on an annoyance I frequently have when developing patch-tag, a happs application that has a lot of dependencies. Basically, on a virgin linux environment with the same cabal and ghc version as before, a cabal install that had previously worked, didn't work anymore. I assume this is because of changes on hackage that didn't affect my previous version because I had not done cabal update for a while. Luckly I had a working environment on a test box elsewehre so... http://blog.patch-tag.com/2009/05/01/what-to-do-when-cabal-install-works-in… Cheers, thomas.

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Thread priority?
by Christopher Lane Hinson 01 May '09

01 May '09

Is there any interest or movement in developing thread priority or any other realtime support in Haskell? Right now, if I have tasks that need to be responsive in real time, even if the realtime needs are very soft, it seems that the only option is to try to ensure that at least one hardware thread is kept clear of any other activity. To be very useful to me, thread priority would not need to come with very strict guarantees, but there would need to be a way to make sure that `par` sparks and DPH inherit the priority of the sparking thread. In part I ask because I'm working on a small library to support a degree of cooperative task prioritization. Friendly, --Lane

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Generating random enums
by michael rice 01 May '09

01 May '09

I'm using the code below to generate random days of the week [Monday..Sunday]. Is there a better/shorter way to do this? Michael ============== [michael@localhost ~]$ ghci dow GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( dow.hs, interpreted ) Ok, modules loaded: Main. *Main> random (mkStdGen 100) :: (DayOfWeek, StdGen) Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. (Friday,4041414 40692) *Main> random (mkStdGen 123) :: (DayOfWeek, StdGen) (Tuesday,4961736 40692) *Main> ============== import System.Random data DayOfWeek = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday deriving (Show, Read, Eq, Enum, Ord, Bounded) instance Random DayOfWeek where randomR (a,b) g = case (randomIvalInteger (toInteger (dow2Int a), toInteger (dow2Int b)) g) of (x, g) -> (int2Dow x, g) where dow2Int Monday = 0 dow2Int Tuesday = 1 dow2Int Wednesday = 2 dow2Int Thursday = 3 dow2Int Friday = 4 dow2Int Saturday = 5 dow2Int Sunday = 6 int2Dow 0 = Monday int2Dow 1 = Tuesday int2Dow 2 = Wednesday int2Dow 3 = Thursday int2Dow 4 = Friday int2Dow 5 = Saturday int2Dow 6 = Sunday random g = randomR (minBound,maxBound) g randomIvalInteger :: (RandomGen g, Num a) => (Integer, Integer) -> g -> (a, g) randomIvalInteger (l,h) rng | l > h = randomIvalInteger (h,l) rng | otherwise = case (f n 1 rng) of (v, rng') -> (fromInteger (l + v `mod` k), rng') where k = h - l + 1 b = 2147483561 n = iLogBase b k f 0 acc g = (acc, g) f n acc g = let (x,g') = next g in f (n-1) (fromIntegral x + acc * b) g' iLogBase :: Integer -> Integer -> Integer iLogBase b i = if i < b then 1 else 1 + iLogBase b (i `div` b)

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