- Haskell-Cafe - Haskell.org

Inferring rewrite rules for higher order functions
by Joachim Breitner 20 Jan '13

20 Jan '13

Dear list, with rewrite rules I can tell the compiler to replace "map id" by "id", but I still have to write that rule by hand. Does anyone of you know about research into inferring such a rewrite rule automatically? I could imagine that for a function of type "f :: some -> args -> T a -> T b" and not-too-bad type constructors T it might be possible under what conditions on "some" and "args" infer when "f arg1 arg2 = id" holds, and if the conditions are fulfilled by functions like "id", generate a rewrite rule "f id id = id" automatically. Thanks, 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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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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