October 2009 - Haskell-Cafe

Reverse dependencies in Hackage
by Roel van Dijk 17 Oct '09

17 Oct '09

A few weeks ago I wrote a patch that adds reverse dependencies to hackage [1]. I have know hosted a small test hackage which demonstrates this feature. It can be found here: http://bifunctor.homelinux.net/~roel/hackage Browse to your favorite packages and find out how much other packages depend on them! I also noticed that work is underway to implement a new hackage-server [2] based on happstack. If people find this feature useful I could also write a patch against the hackage-server code base [3]. Regards, Roel van Dijk [1] - http://hackage.haskell.org/trac/hackage/ticket/576 [2] - http://sparky.haskell.org:8080/ [3] - http://code.haskell.org/hackage-server/

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Monadic correctness
by Andrew Coppin 17 Oct '09

17 Oct '09

Suppose we have newtype Foo x instance Monad Foo runFoo :: Foo x -> IO x What sort of things can I do to check that I actually implemented this correctly? I mean, ignoring what makes Foo special for a moment, how can I check that it works correctly as a monad.

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Genuine Need For Persistent Global State?
by Alan Carter 17 Oct '09

17 Oct '09

Hi, I've been looking at the patches given by Tom at Beware the Jabberwolk, for building Linux kernel modules in Haskell. Once I'd got Tom's stuff building, the next thing was to build a little driver which actually does something. Step by step I was making progress, and I've now got a little function which can see all the characters which are catted to the device file. That's when I got stuck. Trouble is, my function is (ultimately) being called from the C kernel stuff. It isn't on the bottom of a call graph coming from a Haskell main. A driver really needs to know where it's at. So I seem to need some kind of global, persistent state, and Control.Monad.State seems to be out because I can't pass a State around my call graph. I've thought about trying to create some State when I initialize, pass some kind of pointer back to the C shim which actually does the calling into Haskell, and then passing it back into Haskell on the writes and reads, but that seems dangerous because the Haskell garbage collector would (understandably) get the wrong idea and delete it. Also it's ugly - having to use C for something Haskell can't do. I've been looking at the Halfs Haskell file system, which surely must have solved its own version of this problem, but whatever it does in its FSState seems very complicated and is beyond my comprehension :-( So my questions are: 1) Am I right in thinking that I have a genuine need for global, persistent state? 2) Halfs works. Am I right in thinking it has (somehow) solved this problem? 3) Is there a simple way to maintain global persistent state that I can stash between calls into a function, and access without needing to pass the state in? If so, is there an example anywhere? Thanks in advance, Alan -- ... the PA system was moaning unctuously, like a lady hippopotamus reading A. E. Housman ..." -- James Blish, "They Shall Have Stars"

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There is no Text.Regex module
by zaxis 17 Oct '09

17 Oct '09

In Archlinux i can use Text.Regex as below: import Text.Regex let [y,m,d] = map (\x -> read x::Int) $ splitRegex (mkRegex "-") dateStr However, in Ubuntu 9.10 it doesnot work reporting no Text.Regex module. So i download and install regex-base and regex-posix from Hackage. But it still doesnot work ! $dpkg -l ghc6 名称版本简介 ================================================================================== ghc6 6.10.4-1ubuntu2 GHC - the Glasgow Haskell Compilation system SIncerely! -- View this message in context: http://www.nabble.com/There-is-no-Text.Regex-module-tp25935308p25935308.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Weekly News: Issue 136 - October 17, 2009
by jfredett＠gmail.com 17 Oct '09

17 Oct '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091017 Issue 136 - October 17, 2009 --------------------------------------------------------------------------- Welcome to issue 136 of HWN, a newsletter covering developments in the [1]Haskell community. Over the last week, fpisfun from reddit announced a [2]new subreddit for simple, direct applications of Haskell to common problems which normally might be solved by a perl script or bit of bash. There are a number of examples already there, and the numbers keep growing. It's not just command-line utilities either, there is also [3]this post on calculating the bend needed for a bay window curtainrod. There is plenty of great material if you're new to Haskell and looking for some basic examples of different simple projects, examples of using monads and functors appropriately, or just want to see some 'real' programs in Haskell, it's worth a look! Also, a small correction, due to some issues with the tools, unicode characters get replaced with their ASCII 'equivalents' so that an 'e' with an umlaut becomes an 'e' character. Thanks to Wolfram Kahl for pointing out that this change is not as innocent as I thought, apologies thusly to Guenther Schmidt (I'm told thats a better approximation of the correct spelling) for the error, I apologize in advance for any other similar errors. I'm working on improving the unicode support in the tools, but it's a nasty bug to catch. This was a long editorial, so I won't take any more of your time, Haskellers, the Haskell Weekly News! Announcements Call for Contributions - Haskell Communities and Activities Report, November 2009 edition. voigt [4]called for contributions to the November 2009 issue of the Haskell Communities and Activities Report. See post for details. Call for Participation: VSTTE 2009. Jean-Christophe Filliatre [5]announced a call for participation for the VSTTE Workshop. WFLP 2010 Call for papers. Pablo Nogueira [6]announced a call for papers for the 2010 WFLP. BAHUG Next meeting: October 21st at MIT (32G-882). Ravi Nanavati [7]announced the next meeting of the Boston Area Haskell User's Group. Which will be in the CSAIL Reading Room at MIT. This editor hopes to actually make it to this one, so maybe I'll see you there! fp-southwales, the South Wales Functional Programming User Group. Andy Gimblett [8]announced the formation of fp-southwales, a user group for anybody interested in functional programming in the area of south Wales, UK. The Monad.Reader (14) - Call for copy. Brent Yorgey [9]announced the call for copy for the next issue of `The Monad.Reader`, Issue #14 Extensions to Vec uploaded (useful for GPipe programs). Tobias Bexelius [10]announced two extensions to the Vec package: Vec-Transform and Vec-Boolean. Vec-Transform provides some 4x4 transform matrices such as perspective projection and rotation. Vec-Boolean provides Data.Boolan instances for the Vec data types. Bindings to FFMpeg library. Vasyl Pasternak [11]announced the next release of the hs-ffmpeg library. Downloadable from Hackage along with the ffmpeg-tutorials, which show capabilities of this library. The installation process is a bit tricky so Vasyl put up a [12]blog post which describes the installation process. Haskell Hackathon in Zurich, Switzerland. Johan Tibell [13]announced a Hackathon in Zurich, to be held in March in the Google office. No concrete date has been decided, but if your interested, make sure to add your name [14]here. Reverse Dependencies in Hackage (demo). Roel van Dijk [15]announced his patch to add reverse-dependencies to Hackage, see the linked post for information on where to see an example version of hackage. Discussion Relational Algebra. Guenther Schmidt [16]asked about whether or not a EDSL for relational algebra in Haskell. GHC devs. Andrew Coppin [17]asked about how many GHC Developers there are, and was somewhat surprised with the answer. Fuzzy Logic / Linguistic Variables. Neal Alexander [18]demonstrated an implementation of fuzzy logic taken from the book: 'Programming Game AI by Example' by Mat Buckland. In the book, the code was in C++, rewriting it in Haskell made for much more succint, readable code. Graph Library Using Associated Types. Lajos Nagy [19]asked about using Associated types in a Graph library. Is proof by testing possible? Muad'Dib [20]asked us Haskell Mentat's about whether it was possible to prove a function correct in a finite number of tests. The discussion brought forth not only answers but some /very cool/ results about the application of compactness (in the topological sense) to testing. Blog noise [21]Haskell news from the [22]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Don Stewart (dons): [23]LACSS 2009: Domain Specific Languages and Haskell. * FP Lunch: [24]Factorising folds for faster functions. * Mikael Vejdemo Johansson (Syzygy-): [25][MATH 198] Lecture 4 and a question for the community. More of Syzygy's lectures on category theory, Syzygy also asks about what it means to 'integrate' a datatype. * Neil Brown: [26]Emulating Shared Mutable Variables with Message-Passing Processes. * Galois, Inc: [27]Tech Talk: Writing Linux Kernel Modules with Haskell. * Galois, Inc: [28]Domain Specific Languages for Domain Specific Problems. * FP-Syd: [29]Sydney FP Group: FP-Syd #18.. * Neil Brown: [30]The octopus, the boids and GHC 6.12.1rc1. * Brent Yorgey: [31]Call for submissions: Monad.Reader issue 15. * Brent Yorgey: [32]diagrams 0.2.1, and future plans. * Don Stewart (dons): [33]Self-optimizing data structures: using types to make lists faster. * Martijn van Steenbergen: [34]Context Synonyms. * Joachim Breitner: [35]arbtt: Now with Documentation. * Dan Piponi (sigfpe): [36]Vectors, Invariance, and Math APIs. * Creighton Hogg: [37]How to teach Category Theory?. Quotes of the Week * sproingie: How do you beta-reduce a problem like Maria * Baughn: There's also the language, Quine, whose interpreter is implemented as a symlink to /bin/cat * benmachine: [on the best editor] opinions vary. from correct opinions i.e. mine to other ones. [editor's comment: Obviously, this question is settled, the best editor is *garbled*] * pozic: statistics. a wonderful tool to control people. * FunctorSalad: I think the 'asks', 'gets' etc family of names is sort of cute, like the program is talking about itself in the third person * mmorrow:: Right, that's just beggin for a State monad. About the Haskell Weekly News New editions are posted to [38]the Haskell mailing list as well as to [39]the Haskell Sequence and [40]Planet Haskell. [41]RSS is also available, and headlines appear on [42]haskell.org. To help create new editions of this newsletter, please see the information on [43]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [44]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.reddit.com/r/EnHaskell 3. http://www.reddit.com/r/EnHaskell/comments/9syi3/calculating_the_bend_for_a… 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17572 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17570 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64933 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64928 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64858 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64717 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64673 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64901 12. http://progandprog.blogspot.com/2009/10/video-processing-on-haskell-easy.ht… 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64947 14. http://haskell.org/haskellwiki/ZuriHac#Possible_Attendees 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64915 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64899 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64867 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64860 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64812 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64757 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://donsbot.wordpress.com/2009/10/16/lacss-2009-domain-specific-language… 24. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=267 25. http://blog.mikael.johanssons.org/archive/2009/10/math-198-lecture-4-and-a-… 26. http://chplib.wordpress.com/2009/10/14/emulating-shared-mutable-variables-w… 27. http://www.galois.com/blog/2009/10/13/haskellkernelmodules/ 28. http://www.galois.com/blog/2009/10/13/domain-specific-languages-for-domain-… 29. http://www.mega-nerd.com/erikd/Blog/FP-Syd/fp-syd-18.html 30. http://chplib.wordpress.com/2009/10/12/the-octopus-the-boids-and-ghc-6-12-1… 31. http://byorgey.wordpress.com/2009/10/12/call-for-submissions-monad-reader-i… 32. http://byorgey.wordpress.com/2009/09/24/diagrams-0-2-1-and-future-plans/ 33. http://donsbot.wordpress.com/2009/10/11/self-optimizing-data-structures-usi… 34. http://martijn.van.steenbergen.nl/journal/2009/10/11/context-synonyms/ 35. https://www.joachim-breitner.de/blog/archives/342-arbtt-Now-with-Documentat… 36. http://blog.sigfpe.com/2009/10/vectors-invariance-and-math-apis.html 37. http://abstractabsurd.blogspot.com/2009/10/how-to-teach-category-theory.html 38. http://www.haskell.org/mailman/listinfo/haskell 39. http://sequence.complete.org/ 40. http://planet.haskell.org/ 41. http://sequence.complete.org/node/feed 42. http://haskell.org/ 43. http://haskell.org/haskellwiki/HWN 44. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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ANNOUNCE IDynamic-0.1
by Alberto G. Corona 16 Oct '09

16 Oct '09

IDynamic is variant of Data.Dynamic that can be indexed, serialized., stored, transmitted trough communications etc. So it can be used in abstract data containers, persistence, communications etc. http://hackage.haskell.org/package/IDynamic I Just uploaded it, so the documentation has not been created by Hackage. However I uincluded an example in the pachage. {-# OPTIONS -XTypeSynonymInstances #-} module Main where import Data.IResource import Data.IDynamic import Data.Typeable instance IResource Int where keyResource x= "I" serialize = show deserialize = read defPath _= "saved/" instance IResource String where keyResource x= take 5 x serialize = show deserialize = read defPath _= "saved/" main= do putStrLn "see the code to know the meaning of he results" registerType :: IO Int -- register both datatypes (Int, and String) registerType :: IO String let x= 1 :: Int let list= [IDynamic x, IDynamic "hello, how are you"] --typical Dynamic usage let assoc= zip (map keyResource list) list print $ lookup (keyResource (5 ::Int)) assoc -- lookup a IDynamic object from a container and show it mapM writeResource list mds ← readResource $ IDynamic "hello" -- save both objects case mds of Nothing → error "must have been Just!" --why? Just ds → do putStrLn $ serialize ds -- serialize the dynamic object and print it let str= fromIDyn ds :: String -- casting putStrLn str -- print the string read let y= fromIDyn ds :: Int -- casting error print y

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Fwd: [Haskell-cafe] Re: Best Editor In Windows
by Alberto G. Corona 16 Oct '09

16 Oct '09

I use Leksah. It´s an IDE . It works fine in Windows. http://leksah.org/ http://leksah.org/Leksah-0.6.1.0.exe By the way, keep the good work Leksah people !. 2009/10/16 Stefan Monnier <monnier(a)iro.umontreal.ca> > The only thing I haven't figured out is how to do tab-completion of > > words in the ghci buffer. Do I need to use a different key > > combination? I couldn't find that in the documentation. > > I think it's just a missing feature. > > > Stefan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Generating random enums
by michael rice 16 Oct '09

16 Oct '09

What is the minimum I need to do to get this function to generate a three direction tuple? Michael ===================== import System.Random import Data.Ord data Dir = North | South | East | West deriving (Show, Read, Eq, Enum, Ord, Bounded) threeDirs :: StdGen -> (Dir,Dir,Dir) threeDirs gen = let (firstDir, newGen) = random gen (secondDir, newGen') = random newGen (thirdDir, newGen'') = random newGen' in (firstDir, secondDir, thirdDir) ===================== GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l dir.hs [1 of 1] Compiling Main ( dir.hs, interpreted ) dir.hs:15:29: No instance for (Random Dir) arising from a use of `random' at dir.hs:15:29-42 Possible fix: add an instance declaration for (Random Dir) In the expression: random newGen' In a pattern binding: (thirdDir, newGen'') = random newGen' In the expression: let (firstDir, newGen) = random gen (secondDir, newGen') = random newGen (thirdDir, newGen'') = random newGen' in (firstDir, secondDir, thirdDir) Failed, modules loaded: none. Prelude>

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Fwd: [Haskell-cafe] x -> String
by Alberto G. Corona 16 Oct '09

16 Oct '09

yep add Show to your abstrac container. , for example: data SDynamic= forall a.Show a => SDynamic a instance Show SDynamic where show (SDynamic a)= show a 2009/10/16 David Virebayre <dav.vire+haskell(a)gmail.com<dav.vire%2Bhaskell(a)gmail.com> > On Fri, Oct 16, 2009 at 8:19 PM, Andrew Coppin > <andrewcoppin(a)btinternet.com> wrote: > > Is there any way that you can turn an arbitrary Haskell value into a > string? > > > I rephrase: There *is* a way to turn arbitrary values into strings. I > know > > there is, because the GHCi debugger *does* it. The question is, does > anybody > > know of an /easy/ way to do this? > > Ghci only displays values with a Show instance. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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package name in TypeRep?
by Johannes Waldmann 16 Oct '09

16 Oct '09

Dear all, I want a function that produces the URL of the haddocumentation of a given type, generically. My solution is that the argument (type) is an instance of Data.Typeable and I am parsing the output of show of the TypeRep, cf. http://dfa.imn.htwk-leipzig.de/cgi-bin/cvsweb/common/Gateway/Help.hs?rev=1.… That stops working when I split my project into several packages. Is there a way to obtain the package name from the TypeRep? Because I seem to need it to procuce the correct URL. Or else, is there a way to merge all the per-package haddock outputs? Perhaps a pseudo-package that imports all and re-exports everything? Best - Johannes.

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