November 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

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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>.

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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

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PEPM'09 -- Call for Participation
by G. Puebla and G. Vidal 30 Nov '08

30 Nov '08

ACM SIGPLAN Workshop on Partial Evaluation and Program Manipulation (PEPM'09) January 19-20, 2009 Savannah, Georgia, USA http://clip.dia.fi.upm.es/Conferences/PEPM09 CALL FOR PARTICIPATION IMPORTANT DATES * Hotel reservation deadline: December 18, 2008 * Early registration deadline: December 19, 2008 VENUE PEPM'09 and all POPL'09 affiliated events will take place at the Hyatt Regency Savannah hotel. SCOPE The PEPM Symposium/Workshop series aims at bringing together researchers and practitioners working in the areas of program manipulation, partial evaluation, and program generation. PEPM focuses on techniques, theory, tools, and applications of analysis and manipulation of programs. PEPM is classified as category A in the CORE ranking of ICT conferences. INVITED TALKS: * Umut Acar. Toyota Technological Institute and Univ. of Chicago. Title: Self-Adjusting Computation * Cristina Cifuentes. Sun Microsystems Laboratories. Title: Program Analysis for Bug Detection using Parfait PROGRAM CHAIRS: German Puebla, Technical University of Madrid, Spain German Vidal, Technical University of Valencia, Spain PROGRAM COMMITTEE: David Binkley, Loyola College, USA Radhia Cousot, CNRS, France Silvia Crafa, University of Padova, Italy Stephen A. Edwards, Columbia University, USA Lidia Fuentes, University of Malaga, Spain John P. Gallagher, Roskilde University, Denmark Thomas Jensen, IRISA, France Yukiyoshi Kameyama, University of Tsukuba, Japan Siau Cheng Khoo, National University of Singapore, Singapore Julia Lawall, University of Copenhagen (DIKU), Denmark Shin-Cheng Mu, Academia Sinica, Taiwan Naoki Nishida, Nagoya University, Japan Maurizio Proietti, CNR, Italy Armin Rigo, University of Dusseldorf, Germany Simon Thompson, Kent University, UK Tarmo Uustalu, Tallinn University of Technology, Estonia Wim Vanhoof, Namur University, Belgium Joost Visser, Software Improvement Group, The Netherlands Janis Voigtlander, TU Dresden, Germany PRELIMINARY PROGRAM * MONDAY, January 19, 2009 ---------------------- ** Invited talk 9:00-10:00 Umut Acar. Toyota Technological Institute and University of Chicago. Title: Self-adjusting computation ---------------------- ** Static Analysis 10:30-12:30 *** Linear Logical Approximations Robert Simmons and Frank Pfenning *** Guided model checking for programs with polymorphism Neha Rungta and Eric Mercer *** Program Interpolation Andrew Moss and Dan Page *** Translation and Optimization for a Core Calculus with Exceptions Cristina David, Cristian Gherghina and Wei-Ngan Chin ---------------------- ** Partial Evaluation and Specialization 13:30-15:00 *** Is there a Fourth Futamura projection? Robert Glueck *** Type-based Specialization of XML Transformations Kazutaka Matsuda, Zhenjiang Hu and Masato Takeichi *** Partially evauated sensor networks Leon Evers and Jan Kuper ---------------------- ** Program Transformation I 15:30-17:00 *** Shortcut fusion rules for the derivation of circular and higher-order monadic programs Alberto Pardo, Joao Fernandes and Joao Saraiva *** A Flexible Framework for Programming with Non-deterministic Functions Francisco Javier Lopez-Fraguas, Juan Rodriguez-Hortala and Jaime Sanchez-Hernandez *** Program Transformation for Numerical Precision Matthieu Martel ---------------------- * TUESDAY, January 20, 2009 ---------------------- ** Invited talk 9:00-10:00 Cristina Cifuentes. Sun Microsystems Laboratories. Title: Program Analysis for Bug Detection using Parfait ---------------------- ** Types 10:30-12:30 *** Shifting the Stage: Staging with Delimited Control Yukiyoshi Kameyama, Oleg Kiselyov and Chung-chieh Shan *** Static Consistency Checking for Verilog Wire Interconnects Cherif Salama, Gregory Malecha, Walid Taha, Jim Grundy and John O'Leary *** Improving Type Error Messages for Generic Java Nabil El Boustani and Jurriaan Hage *** Bidirectional data-flow analyses, type-systematically Maria Joao Frade, Ando Saabas and Tarmo Uustalu ---------------------- ** Slicing and Profiling 13:30-15:00 *** CCCP - Complete Calling Context Profiling in Virtual Execution Environments Philippe Moret, Walter Binder and Alex Villazon *** CProf: Customizable Calling Context Cross-Profiling for Embedded Java Processors Philippe Moret, Walter Binder and Alex Villazon *** SOC: a Slicer for CSP Specifications Marisa Llorens, Javier Oliver, Josep Silva, Salvador Tamarit and Michael Leuschel ---------------------- ** Program Transformation II 15:30-17:00 *** Clone Detection and Removal for Erlang/OTP within a Refactoring Environment Huiqing Li and Simon Thompson *** From Spreadsheets to Relational Databases and Back Jacome Cunha, Joao Saraiva and Joost Visser *** Designing Aspects for Side-Effect Localization Kung Chen, Jia-Yin Lin, Shu-Chun Weng and Siau-Cheng Khoo ----------------------

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ANNOUNCE: Haskell Communities and Activities Report (15th ed., November 2008)
by Janis Voigtlaender 29 Nov '08

29 Nov '08

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (15th edition, November 2008) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2009: target deadline for contributions to the May 2009 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Re: [Haskell] Re: Help : A problem with IO
by abdullah abdul Khadir 27 Nov '08

27 Nov '08

Thanks all, I got it working finally. What did i learn ? a) I need to put a do after else for more than one instruction (?) b) All similar type of questions are to be redirected to haskell-beginner and haskell-cafe Points noted. Thank you once again, Abdullah Abdul Khadir On Wed, Nov 26, 2008 at 10:56 PM, wman <666wman(a)gmail.com> wrote: > if it should really read whole line, it must try to somehow chain the > values into the string / list of chars (using (:) ) > > getMyLine :: IO [Char] > getMyLine = do > c <- getChar > if (c == '\n') > then return "" -- imho [] looks nicer ;-) > else do > rest <- getMyLine > return $ c : rest > > -- or > > getMyLine :: IO [Char] > getMyLine = do > c <- getChar > if (c == '\n') > then return "" -- imho [] looks nicer ;-) > else getMyLine >>= return . (c:) > > -- or even shorter and still equivalent ;-) > > getMyLine :: IO [Char] > getMyLine = getChar >>= (\c -> if (c == '\n') then return [] else fmap > (c:) getMyLine) > > > > 2008/11/26 abdullah abdul Khadir <abdullah.ak2002(a)gmail.com> > >> Hi, >> >> The function getMyLine written by me is intended for getting a >> complete string from the standard input. >> >> import IO >> >> getMyLine :: IO [Char] >> getMyLine = do >> c <- getChar >> if(c == '\n') >> then return "" >> else cs <- getMyLine >> return [c] >> >> However I keep getting the following error: >> >> io.hs:14:2: Parse error in pattern >> Failed, modules loaded: none. >> >> I fail to understand what the error is. I tried out various things >> such as changing the alignment and so on to no avail. The following program, >> however, compiled successfully though it has the same structure as the >> previous program. >> >> checkd :: IO Bool >> checkd = do >> c <- getChar >> if(c=='d') >> then return True >> else return False >> >> Prelude> :load ./io.hs >> [1 of 1] Compiling Main ( io.hs, interpreted ) >> Ok, modules loaded: Main. >> *Main> checkd >> d >> True >> >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell >> >> >

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Wait for *either* MVar to be set
by Peter Verswyvelen 27 Nov '08

27 Nov '08

Maybe this is not the correct mailing list to ask this question, but as a similar question was asked seemingly unanswered by Duncan Coutts in 2002 (http://markmail.org/message/kftpnulks7mbz2ij) and as this most likely has to do with the GHC runtime, I might have more luck to get an answer here... Sorry for the spam if this is inappropriate; please say so and I will resubscribe to the Haskell Cafe mailing list, not polluting this main list anymore... So here's my question. GHC provides MVar for nice lightweight concurrency synchronization. However, preemptive multitasking operating systems offer support for waiting for multiple "MVars", until *either* one of them returns (or timeouts). I see no way of achieving this with GHC's MVar, unless using Conal Elliott's unamb package on Hackage that emulates this by spawning and killing two threads (which might be an expensive operation, I'm not sure) Am I wrong in this? If so, is this something that might be considered as a future enhancement in the GHC libraries and runtime? Thanks very much, Peter Verswyvelen CTO - Anygma

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ANNOUNCE: Turbinado V0.2 "Still Ugly"
by Alson Kemp 26 Nov '08

26 Nov '08

Progress continues on Turbinado. Turbinado can be found at: http://www.turbinado.org The source can be found at: http://github.com/alsonkemp/turbinado/tree/master (see the /App directory for the code for www.turbinado.org) New in V0.2: * A better "Environment" type (rather than using Dynamics) (-> a 50% speed boost? That's unpossible!); * A functional early version of an ORM for PostgreSQL (note: still needs to handle updates; hdbc-postgresql has a bug with sensing nullable columns); * A prettier website; * Licensing -> BSD. Expect V0.3 in the next week or so with: * More view helpers; * An ORM which handles INSERT/UPDATE...; * A little CMS built using the ORM. Future release: * Separate the website out from the framework. For now, they're evolving together, so live together.

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ANN: "Real World Haskell", now shipping
by Bryan O'Sullivan 26 Nov '08

26 Nov '08

Good evening - John Goerzen, Don Stewart and I are delighted to announce the availability of our book, "Real World Haskell". It is 710 pages long, and published by O'Reilly Media. This is the first book to comprehensively cover modern Haskell programming. From an introduction to functional programming, it focuses on teaching through many worked examples. We discuss the "awkward squad" of I/O, concurrency, and exceptions. We cover network programming, databases, and system hacking. We motivate and work with monoids, applicative functors, monads, and monad transformers. We show you how to debug code, and how to ship well-tested software. Better yet, the book is available under a Creative Commons license, so you can read as much of it as you please before you buy: http://book.realworldhaskell.org/ We developed this book with the enthusiastic and voluble support of the Haskell community, and we are proud to share our work in a fashion that will help newcomers to our field. And best of all, if you order now (at least in North America), you can have a copy of the book in your hands in a matter of days. Thank you from all of us to our friends in the Haskell world who have been so generous with their feedback and kind words!

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Help : A problem with IO
by abdullah abdul Khadir 26 Nov '08

26 Nov '08

Hi, getMyLine :: IO [Char] getMyLine = do c <- getChar if(c == '\n') then return "" else cs <- getMyLine return [c]

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