January 2013 - Haskell-Cafe

Why should we write "a `par` b `pseq` (f a b)" instead of "a `par` b `par` (f a b)"?
by Petr P 20 Jan '13

20 Jan '13

Dear Haskellers, I've been playing with par and pseq, and I wonder: Is there any reason to use a `par` b `pseq` (a + b) instead of a `par` b `par` (a + b) except that the second version sparks twice instead of just once (which probably degrades performance a bit)? It seems to me that the second variant would work as well: The main thread would block on one of the sparked computations, but the other one would still be evaluated in parallel. The second variant seems to have one additional advantage: If the function that combines 'a' and 'b' isn't strict (and perhaps we don't know that in advance), the main thread won't be blocked by evaluating the computation it doesn't need. Like a `par` b `pseq` (const a b) will block until both 'a' and 'b' are evaluated, but a `par` b `par` (const a b) will finish as soon as 'a' is evaluated. I found this link on SO: Why do we need 'seq' or 'pseq' with 'par' in Haskell? < http://stackoverflow.com/q/4576734/1333025> but it doesn't really solve this objection. Thanks for help, Petr Pudlak

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Mobile app development?
by Andrew Pennebaker 19 Jan '13

19 Jan '13

There are currently very few options, especially free and open source options, when it comes to developing cross-platform mobile applications. It's basically web apps with JavaScript, or C++. If Haskell supported app development on Android, iOS, and Windows RT, that alone would bring in more developers. Similarly, there are very few languages for mobile development that take advantage of multiple cores and multiple CPUs. Haskell's `parmap` is an amazing selling point. Can we please prioritize mobile support? I'd much rather write everything in ML than PhoneGap. -- Cheers, Andrew Pennebaker www.yellosoft.us

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Stackage mailing list
by Joachim Breitner 19 Jan '13

19 Jan '13

Dear Michael, I’m wondering if I missed something, but is there a mailing list for stackage? Or has one of the standard lists (-cafe, libraries) been designated for questions about stackage? What I want to know is if you plan to provide a website for stackage soon where the list of included packages and their dependencies, including the tested and approved version numbers can be seen. I’d like to take that and integrate it on http://people.debian.org/~nomeata/platform.html and http://people.debian.org/~nomeata/hackagevsdebian.html Also, once you do that, it should be possible to add Stackage to the list of Distributions listed on hackage, so that on hackage one already sees the package’s stackage status and whether it lags behind there. The integrations is rather simple, just provide a file with that format: http://people.debian.org/~nomeata/cabalDebianMap.txt and bug the hackage maintainers to add it. Greetings, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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Chordify, a new web startup using Haskell
by José Pedro Magalhães 19 Jan '13

19 Jan '13

Hi all, I'd like to introduce Chordify <http://chordify.net/> [1], an online music player that extracts chords from musical sources like Soundcloud, Youtube or your own files, and shows you which chord to play when. Here's an example song: http://chordify.net/chords/passenger-let-her-go-official-video-passengermus… The aim of Chordify is to make state-of-the-art music technology accessible to a broader audience. Behind the scenes, Chordify uses the HarmTrace Haskell package to compute chords from audio. I've been working on this project with a couple of colleagues for a while now, and recently we have made the website public, free to use for everyone. We do not use Haskell for any of the frontend/user interface, but the backend is entirely written in Haskell (and it uses pretty advanced features, such as GADTs and type families [3]). We're particularly interested in user feedback at this stage, so if you're interested in music and could use an automatic chord transcription service, please try Chordify! Cheers, Pedro [1] http://chordify.net/ [2] http://hackage.haskell.org/package/HarmTrace [3] José Pedro Magalhães and W. Bas de Haas. Functional Modelling of Musical Harmony: an Experience Report. In Proceedings of the 16th ACM SIGPLAN International Conference on Functional Programming (ICFP'11), pp. 156–162, ACM, 2011. http://dreixel.net/research/pdf/fmmh.pdf

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resources for learning Hindley-Milner type inference for undergraduate students
by oleg＠okmij.org 19 Jan '13

19 Jan '13

Petr Pudlak wrote: > could somebody recommend me study materials for learning Hindley-Milner > type inference algorithm I could recommend to undergraduate students? Perhaps you might like a two-lecture course for undergraduates, which uses Haskell throughout http://okmij.org/ftp/Haskell/AlgorithmsH.html#teval It explained HM type inference in a slightly different way, strongly emphasizing type inference as a non-standard interpretation. The second main idea was relating polymorphism and `inlining' (copying). Type schemas were then introduced as an optimization, to inlining. It becomes clear why it is unsound to infer a polymorphic type for ref []: expressions of polymorphic types are always inlined (conceptually, at least), which goes against the dynamic semantics of reference cells. The lectures also show how to infer not only the type but also the type environment. This inference helps to make the point that `tests' cannot replace typing. We can type check open expressions (and infer the minimal environment they make sense to use in). We cannot run (that is, dynamically check) open expressions.

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resources for learning Hindley-Milner type inference for undergraduate students
by Petr P 18 Jan '13

18 Jan '13

Dear Haskellers, could somebody recommend me study materials for learning Hindley-Milner type inference algorithm I could recommend to undergraduate students? The original paper is harder to understand, I'm looking for something more didactic. The students are familiar with the lambda calculus, natural deduction and System F. Thanks for help, Petr Pudlak

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ANNOUNCE: MFlow 0.2
by Alberto G. Corona 18 Jan '13

18 Jan '13

Hello. haskellers and, specially, web developpers. This is the second version of MFlow, a deep-first effort in the development of a platform for web applications at the higher level, by including as much haskell magic as possible. It is at the same time experimental and intended to be used in industry. I believe that haskell will not have its chance in the Web if it does not bring unique advantages and challenging paradigms beyond speed and type safety. Deep-first means that the effort is more in the addition higher level features rather than in being complete and bug free, with the conviction that feedback from real usage at the highest level is the best guide for development. Rather than to mimic other platforms in other languages, MFlow is as Haskell'ish as possible, and introduces new approaches like statefulness, event sourcing, back-execution, composable, active, self contained widgets and persistent STM. All of them collaborate to create a high level environment for web programming. This release adds bidings for WAI, blaze-html, stateful AJAX, active widgets, requirements, content management, multilanguage and URLs to pages inside stateful procedures. The package: [http://hackage.haskell.org/package/MFlow The entry in my blog, with the announcement and the philosophy behind http://haskell-web.blogspot.com.es/2013/01/announce-mflow-02.html The same, in other words: http://haskell-web.blogspot.com.es/2012/12/on-spirit-of-mflow-anatomy-of-wi… An online example, with some links that explain the new features. http://81.169.134.95:8081 Stateful, but virtually stateless, thanks to event sourcing http://haskell-web.blogspot.com/2013/01/stateful-but-stateless-at-last-than… Example of use of active widgets http://haskell-web.blogspot.com.es/2013/01/now-example-of-use-of-active-wid… Bug tracker https://github.com/agocorona/MFlow/issues Source repository git clone http://github.com/agocorona/MFlow Feedback is welcomed warmly Alberto.

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ANN: crypto-pubkey: all your public key crypto algorithms belong to us.
by Vincent Hanquez 18 Jan '13

18 Jan '13

Hi Cafe, I've recently released crypto-pubkey [1][2], which provide a comprehensive solution for public key cryptography. Most known RSA modes (PKCS15, OAEP, PSS) are supported, and there's also DSA and ElGamal signature support. Most of the code originally lived in cryptocipher, but have now been made better and safer, and support more modes. I've spend some good chunk of time adding KATs and tests, documentation, and making sure the performance was ahead of other haskell implementations. Enjoy, [1] http://hackage.haskell.org/package/crypto-pubkey-0.1.1 [2] https://github.com/vincenthz/hs-crypto-pubkey -- Vincent

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Feedback on FFI bindings for C++ library
by Nathan Hüsken 18 Jan '13

18 Jan '13

Hey, I would like to write FII bindings in haskell for cocos2d-x (http://www.cocos2d-x.org/) which is a C++ library. Since I have little experience with this, I would like some feedback before I discover that concept is bad half way. In cocos2d there is a base class with much functionality: CCNode Many classes derive from it (i.E. CCLayer) and use the virtual functions of CCNode (i.E. setPosition). How do I map this to haskell? The general Idea: I have a typeclass, in which everything that is derived from CCNode is an instance: class NodeDerived a where toNode :: a -> Node setPosition :: a -> (Double,Double) -> IO () setPosition a pos = setNodePosition (toNode a) pos instane NodeDerived Layer where toNode = layerToNode and layerToNode :: Layer -> Node would be implemented in C++ as: CCNode* layerToNode(CCLayer* l) { return dynamic_cast<CCNode*>(l); } This way I would have only to implement toNode for every class derived from CCNode and get all the functions CCNode defines. What do you think if this Idea? How would you do it? Thanks! Nathan

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TAP 2013: Final Call for Papers
by Achim D. Brucker 18 Jan '13

18 Jan '13

(Apologies if you receive this announcement multiple times.) !LNCS publication is confirmed! **************************************************** *** *** *** TAP 2013 *** *** *** *** Abstract submission: January 25, 2013 *** *** Paper submission: February 1, 2013 *** **************************************************** *** TAP 2013 solicits both full papers and *** *** (industrial) experience/tool papers *** *** in combining proofs and (security) testing *** **************************************************** CALL FOR PAPERS 7th INTERNATIONAL CONFERENCE ON TESTS AND PROOFS (TAP 2013) http://www.spacios.eu/TAP2013 Budapest, Hungary, June 18-19, 2013 The TAP conference is devoted to the synergy of proofs and tests, to the application of techniques from both sides and their combination for the advancement of software quality. Testing and proving seem to be contradictory techniques: once you have proved your program to be correct then additional testing seems pointless; on the other hand, when such a proof in not feasible, then testing the program seems to be the only option. This view has dominated the research community since the dawn of computer science, and has resulted in distinct communities pursuing the seemingly orthogonal research areas. However, the development of both approaches has lead to the discovery of common issues and to the realization of potential synergy. Perhaps, use of model checking in testing was one of the first signs that a counterexample to a proof may be interpreted as a test case. Recent breakthroughs in deductive techniques such as satisfiability modulo theories, abstract interpretation, and interactive theorem proving, have paved the way for new and practically effective methods of powering testing techniques. Moreover, since formal, proof-based verification is costly, testing invariants and background theories can be helpful to detect errors early and to improve cost effectiveness. Summing up, in the past few years an increasing number of research efforts have encountered the need for combining proofs and tests, dropping earlier dogmatic views of incompatibility and taking instead the best of what each of these software engineering domains has to offer. The TAP conference aims to bring together researchers and practitioners working in the converging fields of testing and proving, and will offer a generous allocation of papers, panels and informal discussions. Topics of interest cover theory definitions, tool constructions and experimentations, and include (other topics related to TAP are welcome): - Bridging the gap between concrete and symbolic techniques, e.g. using proof search in satisfiability modulo theories solvers to enhance various testing techniques - Transfer of concepts from testing to proving (e.g., coverage criteria) and from proving to testing - Program proving with the aid of testing techniques - New problematics in automated reasoning emerging from specificities of test generation - Verification and testing techniques combining proofs and tests - Generation of test data, oracles, or preambles by deductive techniques such as: theorem proving, model checking, symbolic execution, constraint logic programming - Model-based testing and verification - Generation of specifications by deduction - Automatic bug finding - Debugging of programs combining static and dynamic analysis - Formal frameworks - Tool descriptions and experience reports - Case studies combining tests and proofs - Domain specific applications of testing and proving to new application domains such as validating security protocols, vulnerability detection of programs, security Important Dates: ================ Abstract submission: January 25, 2013 Paper submission: February 1, 2013 Notification: March 3, 2013 Camera ready version: April 5, 2013 TAP conference: June 17-21, 2013 Program Chairs: =============== Margus Veanes (Microsoft Research, USA) Luca Vigano` (University of Verona, Italy) Program Committee: ================== Paul Ammann Dirk Beyer Achim D. Brucker Robert Clarisò Marco Comini Catherine Dubois Juhan Ernits Gordon Fraser Angelo Gargantini Christoph Gladisch Martin Gogolla Arnaud Gotlieb Wolfgang Grieskamp Reiner Hähnle Bart Jacobs Thiérry Jeron Jacques Julliand Gregory Kapfhammer Nikolai Kosmatov Victor Kuliamin Michael Leuschel Karl Meinke Alexandre Petrenko Holger Schlingloff T.H. Tse Margus Veanes (co-chair) Luca Viganò (co-chair) Burkhart Wolff Fatiha Zaidi Submission: =========== Please submit your papers via easychair https://www.easychair.org/conferences/?conf=tap2013 TAP 2013 will accept two types of papers: - Research papers: full papers with at most 16 pages in LNCS format (pdf), which have to be original, unpublished and not submitted elsewhere. - Short contributions: work in progress, (industrial) experience reports or tool demonstrations, position statements; an extended abstract with at most 6 pages in LNCS format (pdf) is expected. Accepted papers will be published in the Springer LNCS series and will be available at the conference. The contents of previous TAP proceedings is available at: http://www.informatik.uni-trier.de/~ley/db/conf/tap/ -- Dr. Achim D. Brucker, SAP AG, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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