August 2008 - Haskell

Announcing Djinn, version 2004-12-11, a coding wizard
by Lennart Augustsson 07 Apr '11

07 Apr '11

Howdy, y'all! I've written a small program that takes a (Haskell) type and gives you back a function of that type if one exists. It's kind of fun, so I thought I'd share it. It's probably best explained with a sample session. calvin% djinn Welcome to Djinn version 2005-12-11. Type :h to get help. # Djinn is interactive if not given any arguments. # Let's see if it can find the identity function. Djinn> f ? a->a f :: a -> a f x1 = x1 # Yes, that was easy. Let's try some tuple munging. Djinn> sel ? ((a,b),(c,d)) -> (b,c) sel :: ((a, b), (c, d)) -> (b, c) sel ((_, v5), (v6, _)) = (v5, v6) # We can ask for the impossible, but then we get what we # deserve. Djinn> cast ? a->b -- cast cannot be realized. # OK, let's be bold and try some functions that are tricky to write: # return, bind, and callCC in the continuation monad Djinn> type C a = (a -> r) -> r Djinn> returnC ? a -> C a returnC :: a -> C a returnC x1 x2 = x2 x1 Djinn> bindC ? C a -> (a -> C b) -> C b bindC :: C a -> (a -> C b) -> C b bindC x1 x2 x3 = x1 (\ c15 -> x2 c15 (\ c17 -> x3 c17)) Djinn> callCC ? ((a -> C b) -> C a) -> C a callCC :: ((a -> C b) -> C a) -> C a callCC x1 x2 = x1 (\ c15 _ -> x2 c15) (\ c11 -> x2 c11) # Well, poor Djinn has a sweaty brow after deducing the code # for callCC so we had better quit. Djinn> :q Bye. To play with Djinn do a darcs get http://darcs.augustsson.net/Darcs/Djinn or get http://darcs.augustsson.net/Darcs/Djinn/Djinn.tar.gz Then just type make. (You need a Haskell 98 implementation and some libraries.) And then start djinn. For the curious, Djinn uses a decision procedure for intuitionistic propositional calculus due to Roy Dyckhoff. It's a variation of Gentzen's LJ system. This means that (in theory) Djinn will always find a function if one exists, and if one doesn't exist Djinn will terminate telling you so. This is the very first version, so expect bugs. :) Share and enjoy. -- Lennart

8 13

Haskell-mode 2.0
by Stefan Monnier 02 Feb '09

02 Feb '09

I have recently taken over maintainership of Haskell-mode, and after making a bunch of changes, I figured it would be a good idea to make a new release. You can find this new release at: http://www-perso.iro.umontreal.ca/~monnier/elisp/ If you think it should also be found at http://www.haskell.org/haskell-mode/, then please tell John Peterson to get back to me (my email to him seem to go straight to the bitbucket. John?). This release has several changes that have all been only lightly tested, especially w.r.t compatibility with various Emacsen. See appended the NEWS file that describes some of the notewirthy changes. Stefan Changes since 1.45: * keybindings C-c <char> have been replaced by C-c C-<char> so as not to collide with minor modes. * The following modules are now automatically activated without having to add anything to haskell-mode-hook: haskell-font-lock (just turn on global-font-lock-mode). haskell-decl-scan (just bind `imenu' to some key). * In recent Emacsen, haskell-doc hooks into eldoc-mode. * haskell-hugs and haskell-ghci are superceded by inf-haskell. * Indentation rules have been improved when using layout inside parens/braces. * Symbols like -> and \ can be displayed as actual arrows and lambdas. See haskell-font-lock-symbols. * Tweaks to the font-lock settings. Among other things paren-matching with things like \(x,y) should work correctly now. * New maintainer <monnier(a)gnu.org>.

3 6

ANNOUNCE: cabal-install 0.5
by Duncan Coutts 05 Dec '08

05 Dec '08

cabal-install 0.5 ================= cabal-install version 0.5 is out: http://haskell.org/cabal/download.html or get it from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-install If you are already using a cabal-install pre-release then you can just: $ cabal update $ cabal install cabal-install New features in cabal-install ============================= Command line improvements ------------------------- The most immediately noticeable thing is that the command line interface now has all the commands that runhaskell Setup.hs has. Of course it still has the features to download and install packages from hackage. It also gained an upload command. So it now provides a command line interface to the whole Cabal/Hackage system. There’s no need to use runhaskell Setup.hs ever again. There is also bash command line completion support included which I find is a great time saver. Installing and upgrading ------------------------ The next big thing is that it includes a new package dependency resolution system that finds correct and sensible solutions more of the time and has better default behaviour. The new behaviour should be similar to other package managers that people are used to. For example, suppose you’ve got xmonad-0.5 installed and version 0.7 is the latest on hackage, then $ cabal install xmonad will install xmonad-0.7. The older version, xmonad-0.5, will remain installed. The behaviour of install is to upgrade as little as possible to satisfy your request, but sometimes you want to upgrade all the dependencies too. Supposing now that we have xmonad-0.7 installed, but we’re still using X11-1.4.1 and the latest version on hackage is X11-1.4.2, then $ cabal upgrade xmonad will install X11-1.4.2 and *re-install* xmonad-0.7, this time built against the newer X11-1.4.2. So in general, the install command will install the latest version of things but will try and use any existing installed versions of dependencies while the upgrade command will also try to use the latest versions of all dependencies. As a special case, cabal upgrade on its own will try to upgrade all the packages that you have installed. For both command there is a --dry-run flag so you can see what would be installed without actually doing it. Hugs ---- Yes, it even works with hugs. That is, cabal-install built by ghc can manage the installation of packages for hugs. In principle cabal-install should be able to be run by hugs but currently the zlib binding is using a function that hugs does not support. Note that for hugs, Cabal does not know what packages are already installed because there is no equivalent of the package database that ghc has. So that means cabal-install cannot do very sensible installation planning. It should work ok so long as all the dependencies are already installed. Windows ------- Yes, it even works on Windows. The one caveat is that cabal-install cannot currently upgrade itself because Windows makes it hard for a process to overwrite its own executable file. It needs more complex trickery with the Win32 API. In the meantime the workaround is to rename the cabal.exe file first eg to cabal-foo.exe, then run cabal-foo install cabal-install. Build reporting --------------- One feature that made it into this release is build reporting. cabal-install keeps logs of all packages that you install (at least packages from hackage, not local ones). It records a bit of information about each package, in particular whether the outcome was successful or not. You can see these build reports in ~/.cabal/packages/$server/build-reports.log. For example, there is one from my machine for xmonad: package: xmonad-0.7 os: linux arch: x86_64 compiler: ghc-6.8.2 client: cabal-install-0.5.1 flags: -testing small_base dependencies: X11-1.4.2 base-3.0.1.0 containers-0.1.0.1 directory-1.0.0.0 mtl-1.1.0.0 process-1.0.0.0 unix-2.3.0.0 install-outcome: InstallOk docs-outcome: NotTried tests-outcome: NotTried The plan in the longer term is to let people upload these build reports to hackage so we can get a wider range of testing data about the packages on hackage. Duncan

2 1

The real Monad Transformer
by Henning Thielemann 13 Nov '08

13 Nov '08

It was argued that people avoid Haskell because of terms from Category theory like 'Monad'. This problem can now be solved by a wrapper which presents all the WWW without monads! Start e.g. at http://saxophone.jpberlin.de/MonadTransformer?source=http%3A%2F%2Fwww%2Ehas… Of course the tool is written in Haskell, that is, Haskell helps solving problems which only exist because of Haskell. Bug reports and feature requests can be tracked at https://sourceforge.net/projects/parallelweb

8 10

Compiler Construction course using Haskell?
by Johannes Waldmann 31 Aug '08

31 Aug '08

Hello. I plan to give a course in compiler construction, using Haskell as the implementation language (not as source or target language). Something along these lines: 1. combinator parsers (Parsec), 2. simple interpreter (arithmetical expressions) 3. add algebraic data types, functions 4. type checker 5. code generator. Ideally, 2..5 would be using the very same tree traversal code and just change the monad for evaluation. Any comments appreciated. Have you given such a course? Taken? If I really decide to do it, then slides (in German) will be made available. Best regards, J.W.

5 5

Haskell Weekly News: Issue 83 - August 30, 2008
by Brent Yorgey 30 Aug '08

30 Aug '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080830 Issue 83 - August 30, 2008 --------------------------------------------------------------------------- Welcome to issue 83 of HWN, a newsletter covering developments in the [1]Haskell community. This is the "better late than never" edition. As an excuse I could tell you that my home internet service has been horrible (now fixed) and I was away from home for a few days with my wife celebrating our third wedding anniversary. But instead, I give you a link to the [2]Uncyclopedia entry on Haskell. If you haven't already seen it, you should give it a read, being sure not to drink any milk at the same time, or at least pointing your nose away from the keyboard if you insist on drinking milk. Community News If Dell sends John Goerzen (CosmicRay) one more catalog, it will [3]actually be a federal crime. Announcements LogFloat 0.9. wren ng thornton [4]announced a new official release of the [5]logfloat package for manipulating log-domain floating numbers. This release is mainly for those who are playing with Transfinite rather than LogFloat, but the interface changes warrant a minor version change. validating xml lib - need some guidance. Marc Weber [6]asked for help developing an xml generating library validating the result against a given DTD. gsl-random 0.1 and monte-carlo-0.1. Patrick Perry [7]announced that he has started on [8]bindings for the random number generators and random distributions provided by the gsl. He has also written a [9]monad and transformer for doing monte carlo computations that uses gsl-random internally. For a quick tutorial in the latter package, see [10]his blog. Wired 0.1.1. Emil Axelsson [11]announced the first release of the hardware description library [12]Wired. Wired can be seen as an extension to Lava that targets (not exclusively) semi-custom VLSI design. A particular aim of Wired is to give the designer more control over the routing wires' effects on performance. darcs weekly news #1. Eric Kow [13]sent out the first edition of the new Darcs Weekly News! zip-archive 0.0. John MacFarlane [14]announced the release of the [15]zip-archive library for dealing with zip archives. The Monad.Reader - Issue 11. Wouter Swierstra [16]announced a new issue of [17]The Monad.Reader, with articles by David Place, Kenn Knowles, and Doug Auclair. First Monad Tutorial of the Season. Hans van Thiel [18]announced a new monad tutorial, [19]The Greenhorn's Guide to becoming a Monad Cowboy. "Real World Haskell" hits a milestone. Bryan O'Sullivan [20]proudly announced that the draft manuscript of Real World Haskell is [21]complete! It is now available online in its entirety. The authors expect the final book to be published around the beginning of November, and to weigh in at about 700 pages. Mueval 0.5.1, 0.6, 0.6.1, 0.6.2, 0.6.3, 0.6.4. Gwern Branwen [22]announced a number of new releases of [23]Mueval. Lambdabot now uses mueval for all its dynamic Haskell evaluation needs. Hoogle Database Generation. Neil Mitchell (ndm) [24]announced that a new release of the [25]Hoogle command line is out, including bug fixes and additional features. Upgrading is recommended.Two interesting features of Hoogle 4 are working with multiple function databases (from multiple packages), and running your own web server. Blog noise [26]Haskell news from the [27]blogosphere. * Real-World Haskell: [28]Source handed over to production. * Douglas M. Auclair (geophf): [29]Earning \bot-Trophies. * Douglas M. Auclair (geophf): [30]Scanner-parsers II: State Monad Transformers. * Douglas M. Auclair (geophf): [31]Scanner-parsers I: lifting functions. * >>> software engineering radio: [32]Episode 108: Simon Peyton Jones on Functional Programming and Haskell. A podcast interview with Simon Peyton Jones. * Neil Mitchell: [33]Running your own Hoogle on a Web Server. * Braden Shepherdson: [34]Announcing xmonad-light. Braden is rolling out a new configuration framework for xmonad, providing an easier learning curve for those not wanting to learn Haskell right away, and an easy transition to a more powerful Haskell configuration when they want it. * Gabor Grief: [35]Category. Gabor is excited that base-3.0 will include Control.Category. * Douglas M. Auclair (geophf): [36]Ten = 1+2+3+4. Solving an arithmetic puzzle with Haskell, Prolog-style. * Paul R Brown: [37]perpubplat now on github. * Douglas M. Auclair (geophf): [38]"Lucky you!"?. Doug shares some secrets of his success in getting Haskell/Dylan/Mercury/Prolog jobs. * Neil Mitchell: [39]Hoogle Database Generation. Neil releases a new command-line version of Hoogle, including bug fixes and additional features -- multiple function databases and the ability to run your own Hoogle server. * Patrick Perry: [40]A Monte Carlo Monad for Haskell. * Mads Lindstroem: [41]Proposal: Adding composability to WxHaskell. * Tom Moertel: [42]PXSL Tools now on Hackage and GitHub. * Gabor Greif: [43]Uploaded. Gabor uploads his first contribution to Hackage, [44]thrist-0.1. * Muad`Dib (vixey): [45]Tail Call Optimization doesn't exist in Haskell. * >>> Hans van Thiel: [46]The Greenhorn's Guide to becoming a Monad Cowboy. YAMT * Dan Piponi (sigfpe): [47]Untangling with Continued Fractions: Part 2. Dan continues his excellent series on rational tangles, this time showing the connection between tangles and rational numbers. * >>> Praki Prakash: [48]Learning Haskell Redux. * Gabor Greif: [49]Ca(ni)balized. Gabor creates his first cabal package! * Real-World Haskell: [50]Our writing is now complete!. * Tom Moertel: [51]Thinking algebraically: a functional-programming dividend that pays during your imperative-programming day job. * Jason Dagit: [52]Darcs 2 Real-World Push Performance Evaluation. * Well-Typed.Com: [53]What's wrong with make?. A lot, apparently: it's too static, and makes it easy to write incorrect rules. Duncan (dcoutts) provides an analysis. * Dougal Stanton: [54]Lord of the Flies as a window onto monadic IO. * Real-World Haskell: [55]A tighter page count estimate. Real World Haskell is going to be about 700 pages!? * Real-World Haskell: [56]Real World Haskell tutorial next month at DEFUN 2008. Quotes of the Week * shepheb: #haskell isn't so much on-topic when discussing Haskell, but off-off-topic. * chrisdone: it's neat how you learn haskell because you are drawn in by the purely functional paradigm, and then you find loads more things like algebraic data types, monad abstractions, arrows and applicative, lack of objects... so that when people say "well, it's not haskell, but at least X is functional", it's just not the same at all * lambdabot: [tristes_tigres] @vixen unsafe [lambdabot] you're turning me on :) * b\6: sometimes i make variables f and ck and find some reason to multiply them like f*ck if i'm having a bad day. * waynemokane: wow... thanks everyone - it looks like I have a full day of reading type signatures ahead of me. * mauke: call/cthulhu * seydar: monads are just like saran wrap. * b\6: keep this info private, but you can actually overclock your brain. the technique i use is to loop mplayer playing oggs but increase the speed like -speed 1.25 for 125% normal. your brain's speed increases accordingly, allowing you to solve problems much more easily. About the Haskell Weekly News New editions are posted to [57]the Haskell mailing list as well as to [58]the Haskell Sequence and [59]Planet Haskell. [60]RSS is also available, and headlines appear on [61]haskell.org. To help create new editions of this newsletter, please see the information on [62]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [63]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://uncyclopedia.org/wiki/Haskell 3. http://changelog.complete.org/posts/741-From-Dell,-a-Uniquely-Terrible-Expe… 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43829 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43825 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43764 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gsl-random 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monte-carlo 10. http://quantile95.com/2008/08/27/a-monte-carlo-monad-for-haskell/ 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43658 12. http://www.cs.chalmers.se/~emax/wired/index.html 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43626 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43539 15. http://johnmacfarlane.net/zip-archive/ 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43491 17. http://www.haskell.org/haskellwiki/The_Monad.Reader 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43403 19. http://www.muitovar.com/monad/moncow.xhtml 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43401 21. http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43372 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mueval 24. http://neilmitchell.blogspot.com/2008/08/hoogle-database-generation.html 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hoogle 26. http://planet.haskell.org/ 27. http://haskell.org/haskellwiki/Blog_articles 28. http://www.realworldhaskell.org/blog/2008/08/30/source-handed-over-to-produ… 29. http://logicaltypes.blogspot.com/2008/08/earning.html 30. http://logicaltypes.blogspot.com/2008/08/scanner-parsers-ii-state-monad.html 31. http://logicaltypes.blogspot.com/2008/08/scanner-parsers-i-lifting-function… 32. http://www.se-radio.net/podcast/2008-08/episode-108-simon-peyton-jones-func… 33. http://neilmitchell.blogspot.com/2008/08/running-your-own-hoogle-on-web-ser… 34. http://braincrater.wordpress.com/2008/08/28/announcing-xmonad-light/ 35. http://heisenbug.blogspot.com/2008/08/category.html 36. http://logicaltypes.blogspot.com/2008/08/ten-1234.html 37. http://mult.ifario.us/p/perpubplat-now-on-github 38. http://logicaltypes.blogspot.com/2008/08/lucky-you.html 39. http://neilmitchell.blogspot.com/2008/08/hoogle-database-generation.html 40. http://quantile95.com/2008/08/27/a-monte-carlo-monad-for-haskell/ 41. http://lindstroem.wordpress.com/2008/03/16/proposal-adding-composability-to… 42. http://blog.moertel.com/articles/2008/08/24/pxsl-tools-now-on-hackage-and-g… 43. http://heisenbug.blogspot.com/2008/08/uploaded.html 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/thrist 45. http://muaddibspace.blogspot.com/2008/08/tail-call-optimization-doesnt-exis… 46. http://www.muitovar.com/monad/moncow.xhtml 47. http://sigfpe.blogspot.com/2008/08/untangling-with-continued-fractions_23.h… 48. http://monadict.blogspot.com/2008/08/learning-haskell-redux-when-you-are.ht… 49. http://heisenbug.blogspot.com/2008/08/canibalized.html 50. http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ 51. http://blog.moertel.com/articles/2008/08/20/thinking-algebraically-a-functi… 52. http://blog.codersbase.com/2008/08/21/darcs-2-real-world-push-performance-e… 53. http://blog.well-typed.com/2008/08/whats-wrong-with-make/ 54. http://www.dougalstanton.net/blog/index.php/2008/08/21/lord-of-the-flies-as… 55. http://www.realworldhaskell.org/blog/2008/08/20/a-tighter-page-count-estima… 56. http://www.realworldhaskell.org/blog/2008/08/20/real-world-haskell-tutorial… 57. http://www.haskell.org/mailman/listinfo/haskell 58. http://sequence.complete.org/ 59. http://planet.haskell.org/ 60. http://sequence.complete.org/node/feed 61. http://haskell.org/ 62. http://haskell.org/haskellwiki/HWN 63. http://code.haskell.org/~byorgey/code/hwn/

1 0

ANNOUNCE: Wired 0.1.1
by Emil Axelsson 27 Aug '08

27 Aug '08

Hello, This is to announce the first release of the hardware description library Wired. Wired can be seen as an extension to Lava that targets (not exclusively) semi-custom VLSI design. A particular aim of Wired is to give the designer more control over the routing wires' effects on performance. The only working backends in this version are simulation and verification (through Lava), and generation of postscript layouts. Soon, there will also be static timing analysis and DEF file generation. The DEF format is accepted by most tools for physical design. Wired is available from Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Wired or using Darcs: darcs get http://www.cs.chalmers.se/~emax/darcs/Wired/ There is also a homepage at: http://www.cs.chalmers.se/~emax/wired/index.html Comments, questions, patches are welcome. Cheers, / Emil

1 0

Final CFP and extended deadline IFL 2008
by Sven-Bodo Scholz 26 Aug '08

26 Aug '08

******************************************************************************** * * FINAL CALL FOR PAPERS -- EXTENDED DEADLINE!! * CALL FOR PARTICIPATION * * 20th International Symposium on the * Implementation and Application of Functional Languages * IFL 2008 * 10-12.Sept 2008, Hatfield UK * * http://events.sac-home.org/ifl2008/ * ******************************************************************************** UPDATED DEADLINES: * Submission for draft proceedings: NOW 1. Sept * Early Registration: NOW 2. Sept Please note, that the draft proceedings do NOT require full papers; extended abstracts will be considered too. ******************************************************************************** The aim of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. They provide an open forum for researchers who wish to present and discuss new ideas and concepts, work in progress, preliminary results, etc. related primarily but not exclusively to the implementation and application of functional languages. Formal proceedings are produced after the symposium, so that authors can incorporate the feedback from discussions at the symposium in their published papers. Topics ====== Topics of interest include, but are not limited to: * language concepts * type checking * compilation techniques * (abstract) interpretation * generic programming techniques * automatic program generation * array processing * concurrent/parallel programming * concurrent/parallel program execution * functional programming on embedded systems * functional programming on multi-cores/ many-cores * heap management * runtime profiling * performance measurements * debugging and tracing * (abstract) machine architectures * verification * formal aspects * tools and programming techniques Papers on applications or tools demonstrating the suitability of novel ideas in any of the above areas and contributions on related theoretical work are also welcomed. The change of the symposium name adding the term application, introduced in 2004, is to reflect the broader scope IFL has gained over the years. Paper Submissions ================= Prospective authors are encouraged to submit papers to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English, conform to the Springer-Verlag LNCS series format and not exceed 16 pages. The draft proceedings will appear as a Technical Report of the School of Computer Science of the University of Hertfordshire. Attendees of IFL 2008 will have the opportunity to submit a revised version of their paper for post-symposium reviewing. As in previous years, we hope that selected papers will be published by Springer Verlag in the Lecture Notes in Computer Science (LNCS) Series. The Peter Landin Prize ====================== Since 2002 every year the Peter Landin Prize of 150 GBP is awarded to the best paper presented at the symposium, as selected by the program committee. Important Dates =============== * Submission for draft proceedings: NOW 1. Sept. * Early Registration: NOW 2. Sept. * Symposium: 10-12. September * Submission for post-refereeing: 14. November * Notification of acceptance / rejection: 23. January 2009 * Submission of a camera ready version: 20. February 2009 Contact ======= For further details see <http://events.sac-home.org/ifl2008/> or contact us by email: events <at> sac-home.org

1 0

Top Level <-
by Ashley Yakeley 26 Aug '08

26 Aug '08

Is there any interest in implementing a top level "<-" to run monadic code? Currently this sort of thing is done with unsafePerformIO and switching off inlining with some pragma. Indeed, the 'atomically' haddock actually advises doing this to declare top-level TVars. The same trick is used in the source of Data.Unique and System.Random. This is bad Haskell. To avoid observation of effects, etc., we would need a new monad rather than IO. There would be equivalents of IO functions such as these: newIORef newMVar newTVarIO newUnique I can think of two uses: 1. Global mutable state. For instance, here's the count variable for Data.Unique rewritten: uniqSource :: MVar Integer uniqSource <- newMVarTL 0 Isn't that much nicer? <http://haskell.org/haskellwiki/Top_level_mutable_state> 2. Solving the expression problem using open witnesses, a recent hobby-horse of mine. For instance, here's a simple scheme for extensible exceptions using top-level "<-": -- declare exception carrying an Int myException :: Exn Int <- newExn -- throw with 5 foo = do ... throw myException 5 -- catch, print the Int bar = catch foo myException (\i -> putStrLn (show i)) I already have code for this, except with an unsafe hack to do the "<-" declaration: <http://hackage.haskell.org/packages/archive/open-witness/0.1.1/doc/html/Dat…> My open-witness package, which also shows how to do Typeable/Dynamic safely: <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/open-witness> -- Ashley Yakeley Seattle, WA

6 5

ANN: The Monad.Reader - Issue 11
by Wouter Swierstra 25 Aug '08

25 Aug '08

I am pleased to announce that a new issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 11 consists of the following three articles: * David F. Place How to Refold a Map * Kenneth Knowles First-Order Logic a la Carte * Douglas M. Auclair MonadPlus: What a Super Monad! The Monad.Reader is a quarterly magazine about functional programming. It is less formal than a journal, but a bit more enduring than a wiki page or blog post. If you'd like to write something for the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue just yet. Expect a deadline early 2009. Wouter

1 0