May 2009 - Haskell-Cafe

Combining computations
by michael rice 03 May '09

03 May '09

If you look at this stuff long enough it almost begins to make sense. Maybe. ;-) I've been messing around with MonadPlus and I understand its usage with the Maybe and List monads. Since one use of Monads is combining computations, how can I combine a Maybe with a List? let m1 = Nothing let m2 = [1] let m3 = m1 `mplus` m2 ==> [1] --if the Maybe is Nothing, do nothing let m1 = Just 1 let m2 = [] let m3 = m1 `mplus` m2 ==> [1] --if the Maybe is not Nothing, add it to the list Or am I misunderstanding combining computations? Michael

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Re: [Haskell-cafe] Combining computations
by Kalman Noel 03 May '09

03 May '09

michael rice schrieb: > let m1 = Just 1 > let m2 = [] > let m3 = m1 `mplus` m2 ==> [1] --if the Maybe is not Nothing, add it to the list > > Or am I misunderstanding combining computations? You just got the type of mplus wrong: mplus :: (MonadPlus m) => m a -> m a -> m a Note that it takes two values of the same type (m a), but you're giving it values of different types. That is, combining computations of different types is not within the scope of MonadPlus. In this case, it makes sense to convert (Just 1) to [1] via Data.Maybe.maybeToList, thus: m1 = Just 1 m2 = [2,3] m3 = maybeToList m1 `mplus` m2 -- [1,2,3] Note also that, in this example, Monoid (mappend) instead of MonadPlus (mplus) would be sufficient. Actually MonadPlus becomes useful only when you are concerned about the extra properties that its instances are expected to satisfy. Kalman

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Dynamically altering sort order
by Denis Bueno 02 May '09

02 May '09

Hi all, Suppose I have the following interface to a sorting function: sort :: (Ord a) => [a] -> IO [a] -- sort large, on-disk array of records but I don't have a sortBy where you can simply pass a compare function. Wrapped around this is a command-line program which should allow the user to specify different orderings on Records. For example, if the Record has three fields, the user should be able to say "sort only on the first two". Is there an Ord instance that can be dynamically changed in this way? My first idea is something like this: data CompareRecord = CR{ rCompare :: Record -> Record -> Ordering, unCR :: Record } instance Ord CompareRecord where compare (CR cmp x) (CR _ y) = cmp x y where the rCompare field would be a function that is based on the flags passed to the command-line problem. But this has an ugly asymmetry. Does anyone have any other ideas? Thanks. Denis

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Runge-Kutta and architectural style
by Richard O'Keefe 02 May '09

02 May '09

I was interested to see a Runge-Kutta package posted to this list recently, particularly as I have a fairly simple-minded non-adaptive RK "generator": an AWK script that takes a table and some optional stuff and spits out C. The Haskell package is, of course, a lot prettier than my AWK program, as well as offering some adaptive methods, which is important. We can imagine a spectrum of RK packages: (1) Higher order function taking some runtime parameters. That's what we got. (2) The same specialised for a known table at compile time. Doable using the code that we were given plus SPECIALIZE pragmas. I don't how well that works across modules. (3) A generator that writes Haskell source. (4) Template Haskell. (5) A generator that generates native code to be called through FFI. The question is, how do you decide what's the appropriate approach?

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Decoupling OpenAL/ALUT packages from OpenGL
by Sven Panne 02 May '09

02 May '09

I'd like to get some feedback from the Haskell community about some packaging issues, so here is my problem: As a medium-term goal, I'd like to decouple the OpenAL/ALUT packages from the OpenGL package, because there are very sensible use cases where you might need some sound, but not OpenGL. The current coupling has mostly historic reasons. The OpenAL package depends on the OpenGL package in 3 areas: * OpenAL uses OpenGL's notion of StateVars all over the place. * OpenAL's Buffer and Source are instances of OpenGL's ObjectName class. * OpenAL's sources and listeners have some properties like velocity, orientation, direction and position which are modeled by OpenGL's Vector3 and Vertex3 data types. The ALUT package depends on the OpenGL package because of GettableStateVars. The packages are supposed to fit nicely together, so using the same types is a must, but the actual packaging is not nice. So the obvious idea is to introduce 3 new packages which lift out functionality from the OpenGL package: * a small "StateVar" package, consisting only of OpenGL's StateVar module (in a better place in the name hierarchy, of course, perhaps "Data.StateVar"?) * a tiny "ObjectName" package, consisting only of OpenGL's ObjectName class (In "Data.ObjectName"? I'm not very sure about a good place in the hierarchy here.) * a package containing most of the data types/newtypes in OpenGL's VertexSpec module (Vertex{2,3,4}, Color{3,4}, etc.) plus instances for the base classes like Eq, Ord, Show, etc. I really don't know a good name for this package and what a good place in the hierarchy would be (probably something like "Data.Foo", but what is Foo?) The point is: The first two package would be very small. Would this be OK? Does anybody see other alternatives? What would be good names for those packages and where in the naming hierarchy should they really go? Cheers, S.

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

02 May '09

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: [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> fromEnum Wednesday 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. 2 *Main> toEnum 2 *** Exception: Prelude.Enum.().toEnum: bad argument *Main> Michael --- On Fri, 5/1/09, John Van Enk <vanenkj(a)gmail.com> wrote: From: John Van Enk <vanenkj(a)gmail.com> Subject: Re: [Haskell-cafe] Generating random enums To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Friday, May 1, 2009, 12:36 PM When you derive Enum, you get fromEnum and toEnum for free. You don't need your dow2Int stuff or int2Dow. Replace those with fromEnum and toEnum respectively. /jve On Fri, May 1, 2009 at 12:26 PM, michael rice <nowgate(a)yahoo.com> wrote: 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) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- /jve

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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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