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Re: [Haskell-cafe] Combining computations
by michael rice 03 May '09

03 May '09

Thanks for all the help, everyone. I think this stuff is starting to come together. Michael --- On Sun, 5/3/09, Tillmann Rendel <rendel(a)cs.au.dk> wrote: From: Tillmann Rendel <rendel(a)cs.au.dk> Subject: Re: [Haskell-cafe] Combining computations To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Sunday, May 3, 2009, 7:33 AM Hi, normally, one uses monads to express and combine computations in the same monad. However, you can convert between some monads, e.g. from Maybe to List: import Data.Maybe (maybeToList) > let m1 = Nothing > let m2 = [1] > let m3 = maybeToList m1 `mplus` m2 > let m1 = Just 1 > let m2 = [] > let m3 = maybeToList m1 `mplus` m2 In fact, you can convert from Maybe to any MonadPlus. maybeToMonadPlus Nothing = mzero maybeToMonadPlus (Just x) = return x And you can convert from List to any MonadPlus: listToMonadPlus Nothing = [] listToMonadPlus (x : xs) = return x `mplus` listToMonadPlus xs Now you should be able to do: m1 = maybeToMonadPlus (Just 1) m2 = listtoMonadPlus [2, 3] m3 = m1 `mplus` m2 :: Just Int -- Just 1 m4 = m1 `mplus` m2 :: [Int] -- [1, 2, 3] The reason this is possible is that Maybe and List do not support additional effects beyond what is common to all MonadPlus instances. Another option is to never specify which monad your computations are in in the first place. Instead, only specify which computational effects the monad should support. m1 = mzero :: MonadPlus m => m a m2 = return 1 :: (Monad m, Num a) => m a m3 = m1 `mplus` m2 `mplus` Just 2 -- Just 1 m4 = m1 `mplus` m2 `mplus` [2, 3] -- [1, 2, 3] In this version, m1 and m2 are polymorphic computations, which can be used together with List computations, Maybe computations, or any other MonadPlus instances. m1 needs MonadPlus, while m2 is happy with any Monad instance. This fact is encoded in their type. Tillmann

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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 03 May '09

03 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 03 May '09

03 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 03 May '09

03 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 03 May '09

03 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 03 May '09

03 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 03 May '09

03 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 03 May '09

03 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 02 May '09

02 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.

2 1

What to do when cabal dependencies are not your friend.
by Thomas Hartman 02 May '09

02 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.

2 1

Thread priority?
by Christopher Lane Hinson 02 May '09

02 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

6 7

Generating random enums
by michael rice 02 May '09

02 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)

5 5

Re: [Haskell-cafe] chr/ord?
by michael rice 01 May '09

01 May '09

Thank you. I assumed when I saw this: "we could use Maybe's mplus to define a function, parent s = (mother s) `mplus` (father s)" that mplus was *already* present in Maybe. Michael --- On Fri, 5/1/09, Neil Mitchell <ndmitchell(a)gmail.com> wrote: From: Neil Mitchell <ndmitchell(a)gmail.com> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org>, "Anton van Straaten" <anton(a)appsolutions.com> Date: Friday, May 1, 2009, 6:32 AM Hi > But I get this when I try to use it: > > sheep.hs:30:22: Not in scope: `mplus' > [michael@localhost ~]$ You need to import Control.Monad. You can find answers to these sorts of questions with Hoogle: http://haskell.org/hoogle/?hoogle=mplus Thanks Neil

1 0

Re: [Haskell-cafe] chr/ord?
by michael rice 01 May '09

01 May '09

Understood. Thanks. A little further on in the tutorial they define a "parent" function. The mplus operator is used to combine monadic values from separate computations into a single monadic value. Within the context of our sheep-cloning example, we could use Maybe's mplus to define a function, parent s = (mother s) `mplus` (father s), which would return a parent if there is one, and Nothing is the sheep has no parents at all. For a sheep with both parents, the function would return one or the other, depending on the exact definition of mplus in the Maybe monad. But I get this when I try to use it: sheep.hs:30:22: Not in scope: `mplus' [michael@localhost ~]$ And here's the sheep.hs file, attempting to use "parent" near the bottom ========= {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Mon Nov 10 11:59:14 2003> License: GPL -} {- DESCRIPTION Example 1 - Our first monad Usage: Compile the code and execute the resulting program. It will print Dolly's maternal grandfather. -} -- everything you need to know about sheep data Sheep = Sheep {name::String, mother::Maybe Sheep, father::Maybe Sheep} -- we show sheep by name instance Show Sheep where show s = show (name s) -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x parent s = (mother s) `mplus` (father s) -- now we can use `comb` to build complicated sequences maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = (Just s) `comb` mother `comb` father fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandmother s = (Just s) `comb` father `comb` mother `comb` mother mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father -- this builds our sheep family tree breedSheep :: Sheep breedSheep = let adam = Sheep "Adam" Nothing Nothing eve = Sheep "Eve" Nothing Nothing uranus = Sheep "Uranus" Nothing Nothing gaea = Sheep "Gaea" Nothing Nothing kronos = Sheep "Kronos" (Just gaea) (Just uranus) holly = Sheep "Holly" (Just eve) (Just adam) roger = Sheep "Roger" (Just eve) (Just kronos) molly = Sheep "Molly" (Just holly) (Just roger) in Sheep "Dolly" (Just molly) Nothing -- print Dolly's maternal grandfather main :: IO () main = let dolly = breedSheep in do print (parent dolly) -- END OF FILE Michael --- On Wed, 4/29/09, Anton van Straaten <anton(a)appsolutions.com> wrote: From: Anton van Straaten <anton(a)appsolutions.com> Subject: Re: [Haskell-cafe] chr/ord? To: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 12:33 PM michael rice wrote: > Since I'm trying to learn Monads, let's look at this as a teaching moment. The example code (see below), which I pulled off YAMT (Yet Another Monad Tutorial ;-)), is the source of my 'comb' function. > > I understand the code as it now stands, and I understand that the Prelude (>>=) would replace the 'comb'. Adding whatever statements are needed, how would you "specialize" the (>>=) to Maybe and solve this particular problem. Saying that "comb is just (>>=) specialized to Maybe" just means that you can define comb like this: comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb = (>>=) Which also of course means that you can typically use (>>=) instead of comb. Although in some cases, being more specific about the type can be useful. You can do this sort of specialization for any polymorphic function, e.g.: -- id is predefined in Haskell, definition given as example id :: a -> a id x = x intID :: Int -> Int intId = id In that case, the compiler basically specializes the function for you, providing a version of it that's specific to Ints. However, (>>=) is defined by the Monad type class, and as it happens there's also already a definition for it that's specific to the Maybe type. You can see GHC's source for it here: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Maybe.html#… Not surprisingly, that definition is essentially identical to the definition of comb: (Just x) >>= k = k x Nothing >>= _ = Nothing So defining "comb = (>>=)" just uses that definition. Anton _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Few Alex questions
by Dimitry Golubovsky 01 May '09

01 May '09

Hi, Q1: I am trying to create an Alex equivalent of the following flex file: http://code.haskell.org/yc2js/es-idl/lexer.ll (lexer for Web IDL which is a dialect of OMG IDL) I haven't been able to find Alex equivalent for the following line: PoundSign ^{WhiteSpace}*# that is any amount of whitespace at the beginning of line followed by '#' I defined a @WhiteSpace macro some place earlier, but Alex gives me parser error on the line I created: @PoundSign = ^@WhiteSpace*# in the position 28 that is position of '@', next to caret, that is, caret is not recognized by Alex. Can beginning of line (caret) be recognized by Alex? Q2: The original line says: MultiLineComment \/\*(([^*])|(\*[^/]))*\*\/ basically same as comments in C or Java. I had to prefix star and slash even within square brackets with backslashes to make it compile. So I got: @MultiLineComment = \/\*(([^\*])|(\*[^\/]))*\*\/ Is this correct understanding that if we want to match any character except for an asterisk, then Alex would like to see [^\*] rather than [^*]? And [^\/] rather than [^/]? Or would it be better to use a hex code for the asterisk and slash? Thanks. -- Dimitry Golubovsky Anywhere on the Web

2 1

ANNOUNCE: graphviz-2009.5.1
by Ivan Lazar Miljenovic 01 May '09

01 May '09

I'd like to announce version 2009.5.1[1] of the graphviz[2] library, less than a week since Matthew Sackman passed maintainership onto me[3]. The graphviz library provides a Haskell interface to the GraphViz[4] program. [1] http://hackage.haskell.org/packages/archive/graphviz/2009.5.1/graphviz-2009… [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphviz [3] http://www.mail-archive.com/haskell@haskell.org/msg21994.html [4] http://graphviz.org Major changes with this release are: * Supports polyparse >= 1.1 (the previous version didn't work with 1.3). * Requires base == 4.* (i.e. GHC 6.10.*) due to the new exception handling. * Includes functions from my Graphalyze [5] library for actually running the various GraphViz commands, etc. * Slight API breakages: - dirCommand, undirCommand and commandFor no longer return String, but GraphvizCommand - Data.GraphViz.ParserCombinators is no longer exported by the Cabal file, since I see no need for anyone to require acccess to this. If this is a problem for you, please let me know and I'll re-export it. * A lot more Haddock documentation. * A darcs repository is now available [6]. Patches are welcome! [5] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Graphalyze [6] http://code.haskell.org/graphviz Plans for the future: * Add support for Data.Graph-style graphs (maybe even making it generic such that custom graph types can be used). * Allow the user to choose whether the graph is directed or undirected. * Include all attributes offered by the Dot language. * Remove (or at least minimise) usage of extensions to increase portability. Oh, and to pre-empt Don, this is already available in the Gentoo-Haskell overlay ;-) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

How to install HOpenGL to Windows?
by Alexandr N. Zamaraev 01 May '09

01 May '09

I get darcs source HOpenGL Try runghc Setup.hs configure I see: Configuring OpenGL-2.3... Warning: The 'build-type' is 'Configure' but there is no 'configure' script. Setup.hs: configure script not found. runghc Setup.hs build wrote: Preprocessing library OpenGL-2.3... Building OpenGL-2.3... Graphics\Rendering\OpenGL\GL\BasicTypes.hs:24:0: HsOpenGLConfig.h: No such file or directory How to build HOpenGL? Os Windows Vista Home Ru + sp ghc 6.10.1

7 13

Google SoC: Space profiling reloaded
by Patai Gergely 01 May '09

01 May '09

Hello all, I'm one of the lucky individuals accepted for this year's Summer of Code. Here's the abstract of my proposal: http://socghop.appspot.com/student_project/show/google/gsoc2009/haskell/t12… There's less than a month left before I'm supposed to jump into coding, and I'd love to hear about any little idea you think would make this project even better! I created a project page with a rough draft of what I intend to concentrate on, but the issue tracker is obviously begging for content, so feel encouraged to populate it. :) http://code.google.com/p/hp2any/ I also intend to expand the description later. Gergely -- http://www.fastmail.fm - The professional email service

2 1

Arrow preprocessor and *** combinator
by Peter Verswyvelen 01 May '09

01 May '09

After doing some pragmatic tests, it seems that neither the arrow preprocessor nor GHC's builtin one generate / optimize to the (***) combinator. For example, p = proc (x,y) -> do x' <- f <- x y' <- g <- y return (x',y') is equivalent to p = f *** g But I have the impression this kind of rewrites is not being done? Is it possible to provide rewrite rules for arrows? Since *** is a method of the arrow type class, it might have a more optimal implementation. I haven't looked at the arrow preprocessor or GHC's code yet, but comments are welcome, I might see things wrongly. Cheers, Peter

3 4

unsafeSTToIO and stToIO
by Xiao-Yong Jin 01 May '09

01 May '09

Hi, Between the following two functions stToIO :: ST RealWorld a -> IO a stToIO (ST m) = IO m unsafeSTToIO :: ST s a -> IO a unsafeSTToIO (ST m) = IO (unsafeCoerce# m) All I can see is that the safe one uses RealWorld instead of an arbitrary thread s used in the unsafe one. I really don't understand the difference between these two. Why is the one without RealWorld unsafe? I tried google, but couldn't find anything helpful. -- c/* __o/* <\ * (__ */\ <

5 5

Help from C libraries experts
by Maurício 01 May '09

01 May '09

Hi, Do you understand very well a C library and would like Haskell to have a binding for it? I've been working on this package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bindings My goal is to have a place where one can find reliable and comprehensive low-level bindings to foreign libraries, so that writing higher level bindings becomes an easier task. If you are willing to help me with questions about your favorite C library, like compile options I should be carefull about or differences I may find between systems, I'll be glad to write a low level binding to it. Best, Maurício

6 10

Getting started - help
by applebiz89 30 Apr '09

30 Apr '09

What I need to do is build a type of database. Firstly have a few questions, whether what I have done it right...and how to do something else. This is my algebraic type for the database (correct me if its wrong) data Film = Film String String Int String with this as the data. testDatabase :: [Film] testDatabase = [("Casino Royale", "Martin Campbell",2006, "Garry, Dave, Zoe")] the last set of strings are the fans to the film. I want to be able to add a new fan, but not sure how to specifically insert it to there this is what I have so far: becomeFan :: String -> String -> [Film] -> [Film] becomeFan = do putStr "Which film are you a fan of: " filmTitle <- getLine do putStr "What is your name: " fanName <- getLine (if filmTitle == filmName then fanName : [Film]) -- View this message in context: http://www.nabble.com/Getting-started---help-tp23314466p23314466.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 6

Why do I need a 'do'?
by michael rice 30 Apr '09

30 Apr '09

Hi, Why do I need a 'do' in the code below? Michael ================== import System.Random rollDice :: IO Int rollDice = getStdRandom (randomR (1,6)) rollNDice :: Int -> [IO Int] rollNDice 0 = [] rollNDice n = rollDice : rollNDice (n-1) ================= [michael@localhost ~]$ ghci rnd0 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 ( rnd0.hs, interpreted ) Ok, modules loaded: Main. *Main> rollDice 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. 6 *Main> rollDice 3 *Main> rollNDice 3 <interactive>:1:0: No instance for (Show (IO Int)) arising from a use of `print' at <interactive>:1:0-10 Possible fix: add an instance declaration for (Show (IO Int)) In a stmt of a 'do' expression: print it *Main>

4 4

default values in a record structure
by Vasili I. Galchin 30 Apr '09

30 Apr '09

Hello, Is there anyway when defining a dat type record struct to indicate default values for some of the fields? Regards, Vasili

4 3

A question about laziness and foreign resources (and libxml2)
by Graham Fawcett 30 Apr '09

30 Apr '09

Hi folks, I'm having trouble reasoning about laziness and FFI resources. I've written a little C wrapper function to augment Text.XML.LibXML; given a Ptr Node, it will return the node's tag-name: foreign import ccall unsafe _getName :: Ptr Node -> IO CString Given LibXML's 'withNode' function, I can write: withNode :: Node -> (Ptr Node -> IO a) -> IO a getName1 :: Node -> IO String getName1 n = withNode n $ \n' -> _getName n' >>= peekCString But I'm tempted to write this instead: getName2 n = withNode n return >>= _getName >>= peekCString In a strict language, I'd observe that the (Ptr Node) in getName2 was escaping the lexical scope of the withNode call, and so getName2 would be unsafe (since it might get GC'd upon leaving that scope). But in Haskell, I can't reason out whether that's the case. Is 'withNode n return' a risky proposition? Is lexical scope indeed the right test here? If not, how might you reason about the safety of this code? Thanks, Graham

1 0

INAP 2009: 1st CFP
by Pedro Salgueiro 30 Apr '09

30 Apr '09

[apologies for multiple posts; please distribute] ------------------------------------------------- First Call for Papers INAP 2009 18th International Conference on Applications of Declarative Programming and Knowledge Management November 5-7, 2009 Evora, Portugal http://www.di.uevora.pt/inap2009/ http://inap.dialogengines.com/ Organized by the Portuguese AI Society (APPIA), the INAP Committee and the Society of Logic Programming (GLP e.V.) == Overview == Declarative Programming is a family of advanced paradigms for the modeling and solving of complex problems. These specification and implementation methods have attracted more and more attention over the past years, e.g. in the domains of databases and natural language processing, for modeling and the processing of combinatorial problems, and for establishing systems for the web. == INAP 2009 == INAP is a communicative and dense forum for intensive discussion of applications of important technologies related to Prolog, Logic and Constraint Programming as well as closely related advanced software. It comprehensively covers the impact of programmable logic solvers in the Internet Society, its underlying technologies, and leading edge applications in industry, commerce, government, and societal services. INAP 2009 continues a tradition of successful workshops cast around the applications of declarative programming, which were held in Kobe (1997), Tokyo (1995, 1996, 1998 - 2001), Potsdam (2004), Fukuoka (2005) and Wuerzburg (2007). We invite the submission of high quality papers on the described topics, especially, but not exclusively, on different aspects of Declarative Programming, Constraint Processing and Knowledge Management as well as their use for Distributed Systems and the Web: - Knowledge Management, e.g. Data Mining, Decision Support, Deductive Databases - Distributed Systems and the Web, e.g. Agents and Concurrent Engineering, Semantic Web - Constraints, e.g. Constraint Systems, Extensions of Constraint (Logic) Programming - Theoretical Foundations, e.g. Deductive Databases, Nonmonotonic Reasoning - Systems and Tools for Academic and Industrial Use - Knowledge-based Web Services - Logic Solvers and Applications == Workshop Format == The technical program of the workshop will include invited presentations (to be announced), regular technical sessions with presentations of the accepted papers, system demonstrations and a panel discussion. == Conference Venue == The conference will be held at the University of Evora, Portugal in November 5-7, 2009. Evora is a nice and quiet historical city located in the south of Portugal that can be reached from Lisbon by train or coach in under 2 hours. It is a small city of 60.000 inhabitants, 120 km inland from Lisbon and classified by Unesco as World Heritage. The University of Evora was established in the 16th Century and is the 2nd oldest Portuguese University. The social program is promising since the region is very rich in historical sites (Stone Age, Roman, Medieval and Renaissance remains) and also offers a very special gastronomy. See http://en.wikipedia.org/wiki/%C3%89vora for more information. == Important Dates == Paper Submission Deadline: June 29, 2009 Notifications to Authors: August 17, 2009 Camera-ready Version Deadline: September 14, 2009 INAP 2009 Workshop: November 5-7, 2009 == Submission Guidelines == Participants should submit a paper (maximum 15 pages, PDF format), describing their work in topics relevant to the workshop. Accepted papers will be presented during the workshop. At least one author of an accepted contribution is expected to register for the workshop, and present the paper. All submissions should include the author's name(s), affiliation, complete mailing address, and email address. Authors are requested to prepare their submissions, following the LNCS/LNAI Springer format. Please see: http://www.springer.de/comp/lncs/authors.html for further details. The submission should be submitted through the electronic submission site: http://www.easychair.org/inap2009/ The deadline for receipt of submissions is June 29, 2009. Papers received after this date will not be reviewed. Eligible papers will be peer-reviewed by at least three members of the Program Committee. Authors will be notified via email of the results by August 17, 2009. Authors of accepted papers are expected to improve their paper based on reviewers' comments and to send a camera ready version of their manuscripts by September 14, 2009. Accepted papers will be included in the workshop proceedings, which will be distributed to the participants. As in previous editions, we plan to publish selected papers in a proceedings volume in the Springer Lecture Notes in Artificial Intelligence (LNAI) series. == Organizing Committee == Vitor Nogueira vbn AT di.uevora.pt Salvador Abreu spa AT di.uevora.pt Pedro Salgueiro pds AT di.uevora.pt Universidade de Evora Portugal == Program Committee == Salvador Abreu, University of Evora, Portugal (co-chair) Sergio Alvarez, Boston College, USA Philippe Codognet, CNRS/JFLI, Tokyo, Japan Vitor Santos Costa, University of Porto, Portugal Daniel Diaz, University of Paris I, France Ulrich Geske, University of Potsdam, Germany Gopal Gupta, UT Dallas, USA Petra Hofstedt, Technical University of Berlin, Germany Ulrich Neumerkel, Technical University of Vienna, Austria Vitor Nogueira, University of Evora, Portugal Enrico Pontelli, New Mexico State University, USA Irene Rodrigues, University of Evora, Portugal Carolina Ruiz, Worcester Polytechnic Institute, USA Dietmar Seipel, University of Wuerzburg, Germany (co-chair) Terrance Swift, CENTRIA, Portugal Hans Tompits, Technical University of Vienna, Austria Masanobu Umeda, Kyushu Institute of Technology, Japan Armin Wolf, Fraunhofer FIRST, Berlin, Germany Osamu Yoshie, Waseda University, Japan == Contact Information == inap2009 AT di.uevora.pt Universidade de Evora Departamento de Informatica Largo dos Colegiais, 2 7004-516 Evora - PORTUGAL

1 0

Cabal's default install location
by Thomas Davie 30 Apr '09

30 Apr '09

There seems to be an assumption amongst the community that a user's home directory is the most useful place for cabal to install to by default. A few people have challenged that. I wanted to find out which one most people do actually prefer, so please go and vote on this poll. http://noordering.wordpress.com/2009/04/21/cabals-default-install-location/ It's no more than a straw poll, I don't know how protected it is against ballot stuffing, but I'm pretty confident that Haskellers are trustworthy enough not to play with it that way. I'll tell you all the result next week. Bob

11 18

Re: [Haskell-cafe] chr/ord?
by michael rice 30 Apr '09

30 Apr '09

Hi Yitz, Thanks for your input. Since I'm trying to learn Monads, let's look at this as a teaching moment. The example code (see below), which I pulled off YAMT (Yet Another Monad Tutorial ;-)), is the source of my 'comb' function. I understand the code as it now stands, and I understand that the Prelude (>>=) would replace the 'comb'. Adding whatever statements are needed, how would you "specialize" the (>>=) to Maybe and solve this particular problem. Michael --- On Wed, 4/29/09, Yitzchak Gale <gale(a)sefer.org> wrote: From: Yitzchak Gale <gale(a)sefer.org> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 4:30 AM Michael Rice wrote: > -- comb is a combinator for sequencing operations that return Maybe > comb :: Maybe a -> (a -> Maybe b) -> Maybe b > comb Nothing _ = Nothing > comb (Just x) f = f x comb is essentially the same as something in the Prelude: it is just (>>=) specialized to Maybe. (>>=) :: Monad m => m a -> (a -> m b) -> m b > Now what am I misunderstanding in the code below? > lst = [('A',65),('B',66),('C',67),('D',68)] Brandon S. Allbery wrote: > ...it defaulted to [(Char,Integer)]. This is a manifestation > of the Monomorphism Restriction... While it may be debatable whether the Monomorphism Restriction is helpful in compiled code, it is unquestionably a major nuisance at the GHCi prompt, for this and other reasons. I highly recommend that you create a .ghci file in your home directory containing the line: :set -XNoMonomorphismRestriction In my opinion, MR should be off by default in GHCi. -Yitz

2 1

Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Oops, here's the code. Michael =========================== {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Mon Nov 10 11:59:14 2003> License: GPL -} {- DESCRIPTION Example 1 - Our first monad Usage: Compile the code and execute the resulting program. It will print Dolly's maternal grandfather. -} -- everything you need to know about sheep data Sheep = Sheep {name::String, mother::Maybe Sheep, father::Maybe Sheep} -- we show sheep by name instance Show Sheep where show s = show (name s) -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x -- now we can use `comb` to build complicated sequences maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = (Just s) `comb` mother `comb` father fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandmother s = (Just s) `comb` father `comb` mother `comb` mother mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father -- this builds our sheep family tree breedSheep :: Sheep breedSheep = let adam = Sheep "Adam" Nothing Nothing eve = Sheep "Eve" Nothing Nothing uranus = Sheep "Uranus" Nothing Nothing gaea = Sheep "Gaea" Nothing Nothing kronos = Sheep "Kronos" (Just gaea) (Just uranus) holly = Sheep "Holly" (Just eve) (Just adam) roger = Sheep "Roger" (Just eve) (Just kronos) molly = Sheep "Molly" (Just holly) (Just roger) in Sheep "Dolly" (Just molly) Nothing -- print Dolly's maternal grandfather main :: IO () main = let dolly = breedSheep in do print (father dolly) -- END OF FILE --- On Wed, 4/29/09, Yitzchak Gale <gale(a)sefer.org> wrote: From: Yitzchak Gale <gale(a)sefer.org> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 4:30 AM Michael Rice wrote: > -- comb is a combinator for sequencing operations that return Maybe > comb :: Maybe a -> (a -> Maybe b) -> Maybe b > comb Nothing _ = Nothing > comb (Just x) f = f x comb is essentially the same as something in the Prelude: it is just (>>=) specialized to Maybe. (>>=) :: Monad m => m a -> (a -> m b) -> m b > Now what am I misunderstanding in the code below? > lst = [('A',65),('B',66),('C',67),('D',68)] Brandon S. Allbery wrote: > ...it defaulted to [(Char,Integer)]. This is a manifestation > of the Monomorphism Restriction... While it may be debatable whether the Monomorphism Restriction is helpful in compiled code, it is unquestionably a major nuisance at the GHCi prompt, for this and other reasons. I highly recommend that you create a .ghci file in your home directory containing the line: :set -XNoMonomorphismRestriction In my opinion, MR should be off by default in GHCi. -Yitz

1 0

Use GHC API to parse Haskell source code
by Haoyu Bai 29 Apr '09

29 Apr '09

Hi, I'm trying to use GHC API to parse Haskell source code and get an AST but get a "Parse: noRebindableInfo" error. So far my program can parse the simplest code: module HelloWorld where main = do putStrLn $ show "hello" But when I change the show "hello" to show 0, eg, try to parse the following code: module HelloWorld where main = do putStrLn $ show 0 I get a "Parse: noRebindableInfo" error (or exception?) and nothing in the output file. I must did something wrong, but I can't figure out it. The following is my program (highlighted source code here: http://pastebin.com/f556455ff ). I compiled it with "ghc --make -package ghc Parse.hs". The GHC version I used is 6.10.2. After compiled, I run the following command to execute the program: "./Parse hello.hs test.out" where test.out is the output file for the AST. ------------------------------------------------------------------------------------------------------------- module Main where import qualified Parser import StringBuffer import Lexer import FastString import SrcLoc import DynFlags import StaticFlagParser import HscTypes import GHC import GHC.SYB.Utils import System import IO doParse infile dflags = do buf <- hGetStringBuffer infile let loc = mkSrcLoc (mkFastString infile) 1 0 return $ unP Parser.parseModule (mkPState buf loc dflags) parseFile infile = defaultErrorHandler defaultDynFlags $ {-defaultCleanupHandler $-} runGhc (Just "/usr/lib/ghc-6.10.2/") $ do hsc_env <- getSession let dflags0 = hsc_dflags hsc_env let dflags = dflags0{ verbosity = 99 } setSessionDynFlags dflags handleSourceError (\e -> do printExceptionAndWarnings e liftIO $ exitWith (ExitFailure 1)) $ do liftIO $ doParse infile dflags main :: IO () main = do args <- getArgs let [infile, outfile] = args hSetBuffering stdout NoBuffering res <- parseFile infile case res of POk _ x -> bracket (openFile outfile WriteMode) hClose (\h -> hPutStrLn h $ showData Parser 2 x) PFailed loc err -> do hPutStrLn stderr $ "Failure when parsing " ++ show infile ++ ": " -- ++ show err {-exitWith (ExitFailure (-1))-} ------------------------------------------------------------------------------------------------------------- The actual output of the program is: ------------------------------------------------------------------------------------------------------------- Using package config file: /usr/lib/ghc-6.10.2/package.conf hiding package base-3.0.3.1 to avoid conflict with later version base-4.1.0.0 wired-in package ghc-prim mapped to ghc-prim-0.1.0.0 wired-in package integer mapped to integer-0.1.0.1 wired-in package base mapped to base-4.1.0.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0.1.0 wired-in package syb mapped to syb-0.1.0.1 wired-in package template-haskell mapped to template-haskell-2.3.0.1 wired-in package dph-seq mapped to dph-seq-0.3 wired-in package dph-par mapped to dph-par-0.3 Parse: noRebindableInfo ------------------------------------------------------------------------------------------------------------- Thank you very much! -- Haoyu Bai

1 0

Non-atomic "atoms" for type-level programming
by Claus Reinke 29 Apr '09

29 Apr '09

The recent GHC trac ticket revision reminded me of the old open type-sharing problem with type tags and record labels: - if type-level tags (such as 'data TTrue'/'data TFalse') are declared repeatedly in separate modules, they represent separate types, preventing shared use (your type-level predicate doesn't return my version of 'TTrue'/'TFalse') - if record field labels (as used in your run-of-the-mill extensible record library) have to be declared, that is not only inconvenient, but prevents sharing of field labels over independent code bases (see the old Haskell' ticket #92 for discussion http://hackage.haskell.org/trac/haskell-prime/wiki/FirstClassLabels ; also http://hackage.haskell.org/trac/ghc/ticket/1872 for alternative extensible records libs) Since Haskell has no concept of type sharing, the only way to express this is to declare types in a common import. But there is no way for that common import to predict all possible future uses, and we can't just keep adding more labels/tags to it, forcing all dependencies to be updated and kept in sync. Using Template Haskell, and QuasiQuotes in particular, we can now at least work around this issue, by splitting the atoms:-) - 'undefined :: Tag' becomes 'undefined :: (TT :. Ta :. Tg)' - 'label :: Label' becomes '(Ll :< La :< Lb :< Le :< Ll) :: (Ll :< La :< Lb :< Le :< Ll)' - a common import, Data.Label, provides type letters and combinators: 'data La = La' and 'data a :< b = a :< b' 'data Ta' and 'data a :. b' .. - quasiquoting and Show instances, also provided by Data.Label, try to hide the internal structure of labels and tags, at least at expression level. Since there is no quasiquoting at type-level (why?), generating type synonyms seems the best we can do there.. - since record field labels are constructed from type letters, this would also provide a basis for - type-level numbers (type-level quasiquoting would help, though) - label ordering: http://hackage.haskell.org/trac/ghc/ticket/2104 http://hackage.haskell.org/trac/ghc/ticket/1894 In actual use, this is what tags and labels from Data.Label look like: ------------- -- the usual extensible-records-as-nested-pairs data label := value = label := value deriving Show data field :& record = field :& record deriving Show infixr :& -- no quasiquoting for types:-(, so generate type synonyms instead $(genTypeTag "TTrue") $(genTypeTag "TFalse") -- a type-level predicate, using shared tags TTrue/TFalse class HasField record label tbool | label record -> tbool instance HasField () label TFalse instance HasField ((label:=value):&record) label TTrue instance HasField record label tbool => HasField (field:&record) label tbool -- record field selection, driven by field label types class Select record label value where (!) :: record -> label -> value instance .. -- some example records -- no need to declare field labels, and they will be -- shared with other importers of Data.Label! a = ([$l|this|] := True) :&([$l|that|] := "hi") :&() b = ([$l|that|] := "there") :&([$l|x|] := 100) :&([$l|y|] := 200) :&() -- we don't even need label values for this, -- type tags are sufficient, as long as we don't -- evaluate the (undefined) values of tags c = ([$t|this|] := 'x') :&([$t|y|] := ()) :&() -- testing Show and record selection records = do print a print b print c print $ (a ! [$l|this|]) print $ (c ! [$t|this|]) print $ (a ! [$l|that|]) ++ ", " ++ (b ! [$l|that|]) ------------- *Main> [$l|label|] label *Main> :t [$l|label|] [$l|label|] :: Ll :< (La :< (Lb :< (Le :< Ll))) *Main> [$l|label|] `seq` () () *Main> [$t|label|] label *Main> :t [$t|label|] [$t|label|] :: Tl :. (Ta :. (Tb :. (Te :. Tl))) *Main> [$t|label|] `seq` () *** Exception: Prelude.undefined *Main> :t [$l|100|] [$l|100|] :: L1 :< (L0 :< L0) *Main> records (this := True) :& ((that := "hi") :& ()) (that := "there") :& ((x := 100) :& ((y := 200) :& ())) (this := 'x') :& ((y := ()) :& ()) True 'x' "hi, there" ------------- For example code, see http://community.haskell.org/~claus/misc/Data/Label.hs http://community.haskell.org/~claus/misc/Data/Label/TH.hs http://community.haskell.org/~claus/misc/labels.hs Claus

4 11

Make XNoMonomorphismRestriction the default in GHCi
by Yitzchak Gale 29 Apr '09

29 Apr '09

I hereby propose that XNoMonomorphismRestriction be the default in GHCi. I submitted a ticket: http://hackage.haskell.org/trac/ghc/ticket/3202 Note that I am NOT expressing any opinion about MR in general - just about what the default should be in interactive GHCi sessions only. Please read my comments in the ticket, and respond here or in the ticket. Thanks, Yitz

1 0

HaL4: Local Haskell meeting, Halle/Saale, Germany, June 12
by Henning Thielemann 29 Apr '09

29 Apr '09

Hi all, we like to remind the potential participants of our Local Haskell meeting HaL4 to submit proposals for talks. See the recent announcement in German: http://www.haskell.org/pipermail/haskell/2009-April/021220.html --------------------------------------------- HaL4 : Haskell - Tutorial + Workshop + Party am Freitag, dem 12. Juni 2009, in Halle/Saale --------------------------------------------- Das traditionsreiche HaL-Treffen bietet eine gute Mischung von Haskell-bezogenen Themen aus Forschung, Anwendung und Lehre mit vielen Möglichkeiten zu Diskussion und Unterhaltung bei der anschließenden Party. Der Workshop wird in diesem Jahr ergänzt durch Tutorien für Haskell-Ein- und Umsteiger. Diesmal findet das Treffen in Halle/Saale im Institut für Informatik der Martin-Luther-Universität Halle-Wittenberg statt. Wir planen: 10 - 13 Uhr: Tutorien 15 - 19 Uhr: Fachvorträge 19 - 22 Uhr: Grillparty Wir freuen uns auf rege Teilnahme sowie spannende Vorträge mit heißen Diskussionen und bitten um Vortragsanmeldungen bis zum 8. Mai. Gedacht sind 4 mal je 30 min Vortrag + 30 min Diskussion. Weitere Informationen auf http://www.iba-cg.de/hal4.html Beste Gruesse Henning Thielemann

1 0

chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Hi, My Prelude docs must be out of date because chr and ord don't seem to be there. How do I access these functions? Michael =============== [michael@localhost ~]$ ghci 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. Prelude> chr 65 <interactive>:1:0: Not in scope: `chr' Prelude> ord 'A' <interactive>:1:0: Not in scope: `ord' Prelude>

3 2

Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Yep, that fixed it.. Thanks, Brandon. Michael --- On Tue, 4/28/09, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: From: Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "Tim Wawrzynczak" <inforichland(a)gmail.com>, "Lennart Augustsson" <lennart.augustsson(a)gmail.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Tuesday, April 28, 2009, 11:44 PM On Apr 28, 2009, at 23:32 , michael rice wrote:Thank guys, Now what am I misunderstanding in the code below? lst = [('A',65),('B',66),('C',67),('D',68)] You didn't give a type for lst, so it defaulted to [(Char,Integer)]. This is a manifestation of the Monomorphism Restriction, invoked because lst doesn't take any arguments: it forces lst to have a definite type, so defaulting is used to fix the numeric part to Integer. If you add a type specification for lst, things will work as you expect. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.comsystem administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.eduelectrical and computer engineering, carnegie mellon university KF8NH

2 1

Typing efficient folds
by Keith Battocchi 29 Apr '09

29 Apr '09

I'm trying to write some code to do folds on nested datatypes as in http://web.comlab.ox.ac.uk/people/Jeremy.Gibbons/publications/efolds.pdf but running into trouble getting things to typecheck. Given the types data Nest a = Nil | Cons(a, (Nest (Pair a))) type Pair a = (a,a) and the following function to map over pairs: pair f (a,b) = (f a, f b) the paper indicates that an efficient fold over a nest is defined as follows efold :: forall n m b. (forall a. n a) -> (forall a . (m a, n (Pair a)) -> n a) -> (forall a. Pair (m a) -> m (Pair a)) -> (forall l z. (l b -> m (z b)) -> Nest (l b) -> n (z b)) efold e f g h Nil = e efold e f g h (Cons(x, xs)) = f(h x, efold e f g (g . pair h) xs) However, when I try to compile this, I get the error "Couldn't match expected type `l' against inferred type `z'". I'm new to Haskell so I'm probably missing something obvious, but this code looks to me like it ought to work. Any thoughts on what I'm doing wrong? Thanks, Keith

3 4

Compiling on windows - again
by Gü?nther Schmidt 29 Apr '09

29 Apr '09

Hi guys, I'm sorry, I asked this before ... What is the flag you have to pass during ghc --make .... in order to produce an exe on Windows that doesn't open a DOS window. Günther

2 1

Question about implementing an off-side rule in Parsec 2
by Bas van Gijzel 29 Apr '09

29 Apr '09

Hello everyone, I'm doing a bachelor project focused on comparing parsers. One of the parser libraries I'm using is Parsec (2) and I'm going to implement a very small subset of haskell with it, with as most important feature the off-side rule (indentation based parsing) used in function definitions and possibly in where clauses. But I'm still a bit stuck on how to implement this cleanly. I tried to search for some examples on blogs but I haven't found anything yet. As far as I can see the way to go would be using getState and updateState methods defined in Parsec.Prim and to use the methods in Parsec.Pos to compare the difference in indendation for tokens. But I haven't completely wrapped my head around any state monad yet and I don't understand Parsec enough yet to see how to use the methods Parsec.Pos and state easily. Some examples or pointers to something to read would really be helpful. Thanks in advance, Bas van Gijzel

7 7

Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Thank guys, Now what am I misunderstanding in the code below? I would think that *Main> comb (Just 65) foo and *Main> comb (lookup 'A' lst) foo would return the same result Just 'A' Michael ===========Haskell code============= import Data.Char lst = [('A',65),('B',66),('C',67),('D',68)] -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x foo :: Int -> Maybe Char foo n = Just (chr n) ================Ghci=========== [michael@localhost ~]$ ghci 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. Prelude> :load test1 [1 of 1] Compiling Main ( test1.hs, interpreted ) Ok, modules loaded: Main. *Main> comb (Just 65) foo Just 'A' *Main> comb Nothing foo Nothing *Main> lookup 'A' lst Just 65 *Main> (lookup 'A' lst) Just 65 *Main> comb (lookup 'A' lst) foo <interactive>:1:22: Couldn't match expected type `Integer' against inferred type `Int' In the second argument of `comb', namely `foo' In the expression: comb (lookup 'A' lst) foo In the definition of `it': it = comb (lookup 'A' lst) foo *Main> --- On Tue, 4/28/09, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: From: Lennart Augustsson <lennart.augustsson(a)gmail.com> Subject: Re: [Haskell-cafe] chr/ord? To: "Tim Wawrzynczak" <inforichland(a)gmail.com> Cc: "michael rice" <nowgate(a)yahoo.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Tuesday, April 28, 2009, 10:53 PM You can also use the more general toEnum and fromEnum. -- Lennart (iPhone) On Apr 29, 2009, at 2:13, Tim Wawrzynczak <inforichland(a)gmail.com> wrote: Michael, those functions are not in the Prelude, they're in Data.Char. On Tue, Apr 28, 2009 at 8:08 PM, michael rice <nowgate(a)yahoo.com> wrote: Hi, My Prelude docs must be out of date because chr and ord don't seem to be there. How do I access these functions? Michael =============== [michael@localhost ~]$ ghci 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. Prelude> chr 65 <interactive>:1:0: Not in scope: `chr' Prelude> ord 'A' <interactive>:1:0: Not in scope: `ord' Prelude> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

name for monad-like structure?
by Michael Vanier 29 Apr '09

29 Apr '09

I've stumbled upon a structure that is like a weaker version of a monad, one that supports return and >> but not >>=. Has anyone seen this before, and if so, does it have a standard name? Mike

6 8

breaking too long lines
by Christian Maeder 29 Apr '09

29 Apr '09

Hi, according the several style guides, lines shouldn't be too long (longer than 78 characters). http://www.cs.caltech.edu/courses/cs11/material/haskell/misc/haskell_style_… http://www.haskell.org/haskellwiki/Programming_guidelines However, I miss hints how to break lines best. Therefore I make some suggestions here and ask for comments. If a "one-liner" does not fit on one line I break the line after "=" rather than breaking the following infix- or prefix-expression. (I break in the same way after "->" or "<-" in case- or do- expressions) (Some people move "=" to the next line, but I only suggest this for proper infix operators, below.) If a "do" or a short "case ... of" or "let ... in" fits, I leave it at the end of the previous line behind "=" ("->" or "<-"). A line should be broken after "do" or "of" (from "case") in order to allow insertions without breaking the layout, provided there is preceding text at all. The following is fine, because there's no text before "do") c x = do y <-... z ... But this is fine, too, if not better: c x = do y <- ... z ... It's not necessary to put "do" (or "let") on a separate line (and care about the indentation of that keyword): c x = do y <-... z ... Because layout may easily break (if b is renamed) and the layout block starts already too far to the right, this is really bad: c b x = unless b $ do y <- ... z ... In many cases a "do" can be moved to the end of the previous line: c b x = unless b $ do y <- ... z What applies to "do" could be applied to "let" as well (and vice versa), but: f x = let y = x + x z = y + y in z does not look as nice as: f x = let y = x + x z = y + y in z And also a "let" (without "in") within a "do" should not be broken after "let". A long infix-expression should be broken _before_ an infix symbol to better indicate it's continuation: f ++ g ++ h f . g $ h x (One should also put spaces around infix operators and should not put unnecessary brackets around prefix applications.) Surely a long prefix expression can be broken anywhere, but I try to break expressions on the top-level and not within too deeply nested sub-expressions: f arg1 arg2 arg3 (longArg4 sa1 sa2 sa3 sa4) arg5 arg6 arg7 After a line break (following "=", "do", "<-", or "->") I try to stay as far to the left as layout permits (using 2 spaces as minimal indentation). I don't care if "=", <-", or "->" of one block line up (which surely is difficult for the top-level module block) and I may put parts of the rhs below parts of the lhs: let longPattern = case bla of Nothing -> "don't know" Just b -> "a longer expression" vN = ... The standard breaking of if-then-else (within "do") is: if ... then ... else ... but I think the following variations are also fine: ... <- if ... then ... else ... if ... then ... else ... By chance I rarely use guards and "where", therefore I give no examples for those expressions. Somewhat tricky I've found are 2 do-expressions connected by an infix-operator within an outer do-expression: do c <- letter do d <- digit return [c, d] <|> do u <- char '_' return [c, u] The line containing "<|> do" is critical to indentation. If "<|> do" is moved (2 columns) to the left it'll be wrong (by chance c will not be in scope). If "<|> do" is moved (3 columns) to the right it'll mean something different, namely parsing and returning a letter and a digit _or_ parsing a letter, a digit, and an underscore, but only return the letter and the underscore. In order to more clearly put the infix operator into the middle I've tried to insert one more space: do c <- letter do d <- digit return [c, d] <|> do u <- char '_' return [c, u] Also the indentation of data types is an issue, because I think, code shouldn't be indented due to long (constructor) names. I've nothing against long names, but one shouldn't try to put blocks to the right of them: data TName = LongConstructorName { selector1 :: C1 , ... } | LongSecondConstructor .... deriving ... Cheers Christian

31 72

Is curryfication practical with dynamic scope?
by Loup Vaillant 29 Apr '09

29 Apr '09

Hi, can this code (\a -> \b -> a) 1 2 eval to normal form in a strict language with dynamic scope? Thanks, Loup PS: some context: I am currently implementing a toy language. The AST is this one: data whnf = -- Weak Head Normal Form | Unit | Int of int | Bool of bool | Prim of (whnf -> whnf) -- curried | Fun of string * expr -- curried data expr = | Whnf of whnf | Var of string | App of expr * expr | If of expr * expr * expr Dynamic typing, dynamic scope, and, as you can see, functions have to be curried. I plan to implement lexical scope.

2 2

Deriving type family data
by Tuve Nordius 29 Apr '09

29 Apr '09

If I for some data type wants to derive, in this case Data and Typeable for use with syb code, but the problem is the same regardless what I want to derive. data family Something data Tree = Leaf Something | Fork Something Tree Tree deriving (Data, Typeable) The problem is I want to derive a class for a data type that depends on some non instantiated data type. I can of course rewrite Tree as: data (Data a) => Tree a = Leaf a | Fork a Tree Tree deriving (Data, Typeable) but then data family Something is redundant, I want to think of Something as a not yet instantiated abstract data type Is there anyway to express that either the instance of Something used in Tree should be a member of Data, or even better any instance of Something should be a member of Data or maybe even better that every instance should derive data. data family Something deriving Data //Tuve Nordius

2 1

TraverseAccum: an effectful accumulating map.
by Florent BALESTRIERI 28 Apr '09

28 Apr '09

Hello fellow haskellers. This message is a valid literate haskell file. I ran it with ghc 6.10.1 > module TraverseAccum where > import Control.Applicative > import Data.Traversable A year ago, I read "Why Attribute Grammars Matter". In it we found a function on lists wich combined three traversals into one to compute the differences to the average: > avg_diff_list :: [Double] -> [Double] > avg_diff_list xs = > let nil = const (0,0.0,[]) > cons x f mean = > let (n,s,ds) = f mean > in (n+1,s+x, (x - mean) : ds) > (n,s,ds) = foldr cons nil xs (s/n) > in ds Then I found "Applicative programming with effects" which abstracted the notions of traversal and folding. We can try and generalize avg_diff_list to work over any Traversable data structure. Here is a possible version with two traversals: > avg_diff' :: (Traversable t, Fractional f) > => t f -> t f > avg_diff' t = > let start = (0,0.0) > step (n,s) x = ((n+1, s+x), \mean -> x - mean) > ((n,s), diffs) = mapAccumL step start t > in sequenceA diffs (s/n) But we can do better with only a single traversal. The idea is to define a function traverseAccumL which does the work of mapAccumL but with an effectful map. (mapM) The definition of mapAccumL uses a StateL applicative functor whereas traverseAccumL uses a StateL monad transformer. First a few definitions. > -- left-to-right state transformer > newtype StateLT m s a = StateLT { runStateLT :: s -> m (s, a) } > instance Functor m => Functor (StateLT m s) where > fmap f (StateLT k) = StateLT $ \ s -> fmap f <$> k s > instance (Functor m, Monad m) => Applicative (StateLT m s) where > pure x = StateLT (\ s -> return (s, x)) > StateLT kf <*> StateLT kv = StateLT kfv > where kfv s = do (s', f) <- kf s > (s'', v) <- kv s' > return (s'', f v) Incidently, I always wondered why Monad wasn't a subclass of Functor. > traverseAccumL :: ( Traversable t, Functor m, Monad m) > => (a -> b -> m (a, c)) -> a -> t b -> m (a, t c) > traverseAccumL f s t = runStateLT (traverse (StateLT . flip f) t) s I could have done without StateLT but the signature would have been different: traverseAccumL :: (Traversable t, Monad m) => (a -> b -> m (c, a)) -> a -> t b -> m (t c, a) traverseAccumL f s t = runStateT (mapM (StateT . flip f) t) s Now the definition of avg_diff calls traverseAccumL with the reader monad > avg_diff :: (Traversable t, Fractional f) > => t f -> t f > avg_diff t = > let start = (0,0.0) > step (n,s) x mean = ((n+1, s+x), x - mean) > ((n,s), diffs) = traverseAccumL step start t (s/n) > in diffs The dual definition traverseAccumR is almost the same. The only difference lies in the definition of <*> StateRT kf <*> StateRT kv = StateRT kfv where kfv s = do (s', v) <- kv s (s'', f) <- kf s' return (s'', f v) > -- right-to-left state transformer > newtype StateRT m s a = StateRT { runStateRT :: s -> m (s, a) } > > instance Functor f => Functor (StateRT f s) where > fmap f (StateRT k) = StateRT $ \ s -> fmap f <$> k s > > instance (Functor m, Monad m) => Applicative (StateRT m s) where > pure x = StateRT (\ s -> return (s, x)) > StateRT kf <*> StateRT kv = StateRT kfv > where kfv s = do (s', v) <- kv s > (s'', f) <- kf s' > return (s'', f v) > > traverseAccumR :: ( Traversable t, Functor m, Monad m) > => (a -> b -> m (a, c)) -> a -> t b -> m (a, t c) > traverseAccumR f s t = runStateRT (traverse (StateRT . flip f) t) s -- Florent B.

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Can subclass override its super-class' default implementation of a function?
by siki 28 Apr '09

28 Apr '09

I'm not sure if this is possible at all. I'd like to do something like this: class A a where foo :: a -> Double foo a = 5.0 class (A a) => B a where foo a = 7.0 data Blah = Blah data Bar = Bar instance A Blah instance B Bar let blah = Blah bar = Bar foo blah -- should print out 5 foo bar -- should print out 7 Basically, I have a bunch of instances that have a common functionality but I'd like to be able to group those instances and give each group a different default implementation of that functionality. It's so easy to do this in Java, for example, but I have no idea how to do it in Haskell. The above example will not compile, btw. Thanks a lot -- View this message in context: http://www.nabble.com/Can-subclass-override-its-super-class%27-default-impl… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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