- Haskell-Cafe - Haskell.org

Re: Type families not as useful over functions
by oleg＠okmij.org 13 Feb '09

13 Feb '09

John Ky wrote: > Is there a way to define type r to be all types except functions? Perhaps the following article How to write an instance for not-a-function http://okmij.org/ftp/Haskell/typecast.html#is-function-type answers your question. It shows several complete examples.

1 0

Is this related to continuations somehow?
by Andrew Wagner 13 Feb '09

13 Feb '09

So, I was reading a bit about continuations the other day, and, since I've been thinking about good ways of expressing chess strategies in Haskell, I thought that I'd play around a bit with something like continuations for game-playing strategies. The idea is that you have combinators that allow you full access to the strategies which remain to be applied. In this way, strategies can "activate" and "de-activate" other strategies. Here's a simple little toy app for Tic-Tac-Toe (Naughts and Crosses): http://codepad.org/nN9JsxFK You can run main on 'example', and see that it searches every line and fails. And, as you can see, it aborts after finding a win in example2. This would be easily extensible to say things like "if you've seen a blocking move, and you don't have a win, then play the blocking move", and of course the other deep intricacies of the game. My question is, is this, in fact, related to continuations somehow? Could continuations simplify it? Or am I doing something completely different?

1 0

Re: Google Summer of Code 2009
by Matthew Elder 13 Feb '09

13 Feb '09

> So in general, think hard about missing capabilities in Haskell: > > * tools > * libraries > * infrastructure > > that benefit the broadest number of Haskell users or developers. > > Another route is to identify a clear niche where Haskell could leap > ahead of the competition, with a small investment. > > -- Don > One capability I would love to see as a SoC project is the ability to generate javascript (a small subset) as an additional output target from ghc. Something like this was already done with yhc here<http://www.haskell.org/haskellwiki/Yhc/Javascript>but if it could be ported to GHC and cabalized, this would really be a boon to the web dev community; javascript is quickly becoming one of the most predominant languages used for GUI creation (mainly via the web). -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

1 0

Re: [Haskell-cafe] Google Summer of Code 2009
by Andrzej Jaworski 13 Feb '09

13 Feb '09

> If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing Here is an idea that if done right might bootstrap Haskell real world applications with the help of greed and adrenaline:-) The ignition: (0) Bind Haskell to an automatic trading platform [API] (1) write real-time streamer and stock scanner. [PType] should offer more than was demonstrated in [F#] (2) Join [apps] from any angle (3) Consider a [DSL] for data analysis or write an [EasyLanguage] [API] http://www.interactivebrokers.com/en/p.php?f=programInterface&ib_entity=llc [F#] http://channel9.msdn.com/pdc2008/TL11/ [PType] http://research.nii.ac.jp/~hu/pub/aplas04-xu.pdf [apps] http://www.interactivebrokers.com/en/general/poll/ibconsultants.php?accept_… [DSL] http://www.cc.gatech.edu/~saswat/research/one.pdf [EasyLanguage] https://www.tradestation.com/support/books/pdf/EL_Essentials.pdf https://www.tradestation.com/support/books/pdf/EL_FunctionsAndReservedWords…

1 0

Re: [Haskell] Google Summer of Code 2009
by Andrzej Jaworski 13 Feb '09

13 Feb '09

> If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing Here is an idea that if done right might bootstrap Haskell real world applications with the help of greed and adrenaline:-) The ignition: (0) Bind Haskell to an automatic trading platform [API] (1) write real-time streamer and stock scanner. [PType] should offer more than was demonstrated in [F#] (2) Join [apps] from any angle (3) Consider a [DSL] for data analysis or write an [EasyLanguage] [API] http://www.interactivebrokers.com/en/p.php?f=programInterface&ib_entity=llc [F#] http://channel9.msdn.com/pdc2008/TL11/ [PType] http://research.nii.ac.jp/~hu/pub/aplas04-xu.pdf [apps] http://www.interactivebrokers.com/en/general/poll/ibconsultants.php?accept_… [DSL] http://www.cc.gatech.edu/~saswat/research/one.pdf [EasyLanguage] https://www.tradestation.com/support/books/pdf/EL_Essentials.pdf https://www.tradestation.com/support/books/pdf/EL_FunctionsAndReservedWords…

1 0

can't figure out a type
by John Lato 12 Feb '09

12 Feb '09

Hello, I'm working on some code like the following: > class Chunkable c el | c -> el where > cLength :: c -> Int > cHead :: c -> Maybe el I want to be able to map over this type, like this: > cMap :: Chunkable c' el' => (el -> el') -> c -> c' but this isn't quite right. c' shouldn't be any instance of Chunkable, it should be the same instance except parameterized over a different type. Another approach would be something like: class (Functor c) => Chunkable c el ... except that's not right either. I think c has the wrong kind to be a Functor instance. I expect there's something very basic I'm missing. Could anyone point in the proper direction of how to do this? Can this be expressed with associated types, perhaps? Thanks, John Lato

3 6

Re: Graph library, was: Haskell.org GSoC
by Louis Wasserman 12 Feb '09

12 Feb '09

fgl uses pretty much the most beautiful way of abstracting graphs I've seen; a brief review: type Context a b -- a collected representation of a vertex's number, label, and all information on edges going into and out of that vertex match :: Graph gr => Node -> gr a b -> (Maybe (Context a b), gr a b) -- if a vertex is in the graph, extracts its adjacency information in a Context and returns the graph without that vertex (&) :: DynGraph gr => Context a b -> gr a b -> gr a b -- melds a vertex and its adjacency information into the graph I've been doing a huge amount of messing around with graph algorithms recently, and this inductive style of graph querying and modification make several algorithms far more intuitive than most imperative implementations I've seen. In addition, both of these lend themselves well to use in a State monad. Take the following code to extract the connected component of a vertex in an undirected graph: extractComponentM :: DynGraph gr => gr a b -> Node -> State (gr a b) (gr a b) extractComponentM partialComponent v = State (match v) >>= maybe (return partialComponent) expandContext where expandContext cxt = liftM (cxt &) (foldM extractComponentM partialComponent (neighbors' cxt)) extractComponent :: DynGraph gr => gr a b -- the initial graph -> Node -- the node whose component to find -> (gr a b, gr a b) -- the original graph without the component, and the component extractComponent g v = runState (extractComponentM empty v) g That's far more compact -- and intuitive to someone familiar with both fgl and the State monad -- than a standard connected-component finder in an imperative language, speaking as someone who wrote code along these lines imperatively for several years. (Fun fact: I've been working on a nice encapsulation of priority queues as monad transformers to make other common graph algorithms both efficient and make them this pretty to read, too. Any thoughts on that would be appreciated.) I strongly feel that fgl, though it could be improved in some areas, makes a better truly general graph abstraction than anything else I've seen. Areas that could specifically be improved include the stuff that uses LRTrees, and other stuff that isn't especially intuitive, elegant, or even efficient in fgl's implementation. Louis Wasserman wasserman.louis(a)gmail.com

1 0

Re: Google Summer of Code 2009
by John Lato 12 Feb '09

12 Feb '09

Johan Tibell wrote: > On Thu, Feb 12, 2009 at 2:12 AM, Felipe Lessa <felipe.lessa(a)gmail.com> wrote: >> Do we already have enough information to turn >> http://okmij.org/ftp/Haskell/Iteratee/ into a nice, generic, cabalized >> package? I think Iteratees may prove themselves as useful as >> ByteStrings. > > I still haven't figured out what the "correct" definition of Iteratee > would look like. The Iteratee code that Oleg wrote seems to have the > structure of some kind of "two level" monad. I think that's the reason > for the frequent occurrences of >>== and liftI in the code. There > seems to be some things we yet have to discover about Iteratees. > I concur. I've recently been involved with several discussions on this topic, and there are some issues that remain. The "two level monad" part doesn't bother me, but I think the type should be slightly more abstract and I'm not sure of the best way to do so. IMO liftI is used more because of Oleg's particular style of coding than anything else. I don't think it need be common in user code, although it may be more efficient. I think that, if a GSOC project were to focus on Iteratees, it would need to look at issues like these. I can't judge as to whether this is an appropriate amount of work for GSOC, however simply packaging and cabal-izing Oleg's Iteratee work (or Johan's, or my own) is likely of too small a scope. John Lato

3 2

CFP: 5th Haskell Hackathon, April 17-19, Utrecht
by Sean Leather 12 Feb '09

12 Feb '09

Call for participation: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The 5th Haskell Hackathon April 17 - 19, 2009 Utrecht, The Netherlands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The Haskell Hackathon is a collaborative coding festival with a simple focus: build and improve Haskell libraries, tools, and infrastructure. Of course, we have fun doing it, too! All hackers are welcome to visit and work on various Haskell-related projects. You decide what you want to work on! Lists of projects and concerted efforts are collected on the Hac5 website. == General Information == Information about the projects as well as practicalities such as traveling, accommodation, etc. is available from the Hac5 website: http://haskell.org/haskellwiki/Hac5 == Registration == Please register by following the instructions on the registration page: http://haskell.org/haskellwiki/Hac5/Register Once you've registered, please add your info to the attendees self-organizing page: http://haskell.org/haskellwiki/Hac5/Attendees If you are looking to share costs or want to meet up prior to the Hackathon with other attendees, feel free to organize via the Attendees wiki page or join the Hackathon IRC channel on freenode: #haskell-hac5 == Organizers == Andres Löh, Utrecht University (UU) José Pedro Magalhães, UU Sean Leather, UU Eelco Lempsink, UU + Tupil Chris Eidhof, UU + Tupil ... and more ...

1 0

TASE 2009 - CALL FOR PAPERS
by CRACIUN F. 12 Feb '09

12 Feb '09

TASE 2009 - Final CALL FOR PAPERS ****************************************** * 3rd IEEE International Symposium on * Theoretical Aspects of Software Engineering * (TASE 2009) * 29-31 July 2009, Tianjin, China * http://www.dur.ac.uk/ieee.tase2009 * * For more information email: IEEE.TASE2009(a)durham.ac.uk ********************************************************** Large scale software systems and the Internet are of growing concern to academia and industry. This poses new challenges to the various aspects of software engineering, for instance, the reliability of software development, web-oriented software architecture and aspect and object-orientation techniques. As a result, new concepts and methodologies are required to enhance the development of software engineering from theoretical aspects. TASE 2009 is a forum for researchers from academia, industry and government to present ideas, results, and ongoing research on theoretical advances in software engineering. TASE 2009 is the third in a series of conference, sponsored by IEEE CS and IFIP. The first TASE conference was held in Shanghai, China, in June 2007. The second TASE conference was held in Nanjing, China, in June 2008. Topics of Interest: Authors are invited to submit high quality technical papers describing original and unpublished work in all theoretical aspects of software engineering. Topics of interest include, but are not limited to: * Requirements Engineering * Specification and Verification * Program Analysis * Software Testing * Model-Driven Engineering * Software Architectures and Design * Aspect and Object Orientation * Embedded and Real-Time Systems * Software Processes and Workflows * Component-Based Software Engineering * Software Safety, Security and Reliability * Reverse Engineering and Software Maintenance * Service-Oriented Computing * Semantic Web and Web Services * Type System and Theory * Program Logics and Calculus * Dependable Concurrency * Software Model Checking Program Co-Chairs ----------------- Wei-Ngan Chin (National Univ. of Singapore, Singapore) Shengchao Qin (Durham University, UK) Program Committee ----------------- Bernhard Aichernig (Graz University of Technology, Austria) Stefan Andrei (Lamar University, USA) Keijiro Araki (Kyushu University, Japan) Farhad Arbab (CWI and Leiden University, Netherlands) Jonathan Bowen (King's College London, UK) Michael Butler (University of Southampton, UK) Juan Chen (Microsoft Research, USA) Tyng-Ruey Chuang (Academica Sinica, Taiwan) Jim Davies (University of Oxford, UK) Zhenhua Duan (Xidian University, China) Xinyu Feng (Toyota Technological Inst. at Chicago, USA) Dieter Gollmann (Hamburg University of Technology, Germany) Tetsuo Ida (University of Tsukuba, Japan) Radu Iosif (Verimag, CNRS, France) Xuandong Li (Nanjing University, China) Kung-Kiu Lau (University of Manchester, UK) Shaoying Liu (Hosei University, Japan) Dorel Lucanu (University of Iasi, Romania) Tom Maibaum (McMaster University, Canada) Darko Marinov (Univ. of Illinois at Urbana-Champaign, USA) Hong Mei (Peking University, China) Huaikou Miao (Shanghai University, China) Peter Mueller (ETH Zurich, Switzerland) Viet Ha Nguyen (Vietnam National University, Vietnam) Sungwoo Park (Pohang Univ. of Science and Technology, Korea) Corneliu Popeea (MPI-SWS, Germany) Geguang Pu (East China Normal University, China) Zongyan Qiu (Peking University, China) Volker Stolz (UNU/IIST, Macau) Jing Sun (University of Auckland, New Zealand) Jun Sun (National Univ. of Singapore, Singapore) Kenji Taguchi (National Institute of Informatics, Japan) Yih-Kuen Tsay (National Taiwan University, Taiwan) Elizabeth Vidal (San Agustin National University, Peru) Ji Wang (National University of Defense Technology, China) Linzhang Wang (Nanjing University, China) Xianbing Wang (Wuhan University, China) Wang Yi (Uppsala University, Sweden) Jim Woodcock (University of York, UK) Hongyu Zhang (Tsinghua University, China) Jian Zhang (Chinese Academy of Sciences, China) Jianjun Zhao (Shanghai Jiao Tong University, China) Hong Zhu (Oxford Brookes University, UK) Huibiao Zhu (East China Normal University, China) Important Dates: February 20, 2009: Title and abstract submission deadline February 27, 2009: Paper submission deadline April 20, 2009: Acceptance/rejection notification May 11, 2009: Camera-ready version due July 29 - 31, 2009: TASE 2009

1 0