April 2006 - Haskell - Haskell.org

The 2006 GHC Hackathon
by Simon Marlow 26 Apr '06

26 Apr '06

Dearest GHC hackers, We're considering the possibility of organising a GHC Hackathon around ICFP this year. This wiki page has tentative details, and you can indicate whether you'd be interested in coming, and give us your suggestions: http://hackage.haskell.org/trac/ghc/wiki/Hackathon Cheers, Simon

1 0

Haskell.org and Google Summer of Code 2006
by Paolo Martini 26 Apr '06

26 Apr '06

Hello, It is now official. Haskell.org is a mentoring organization for this year's Summer of Code programme. You can see it listed on the official site <http://code.google.com/soc/> clicking on `Haskell.org (ideas)' you'll be directed to the trac site we are setting up. (It's 4 AM and I've just finished adding the projects, and cleaning things up a bit.) I am _very_ happy about seeing this project working out for real. I fear that I wouldn't be able to reach everybody even mailing all the important mailing list; I hope many students will be able to take advantage of this great occasion. Help me spread the word, tell everybody about the project :-) You can find the new site here <http://hackage.haskell.org/trac/summer-of-code/>, the StartPage will be updated with details during the program. Projects ideas and volunteers for mentoring are still very much welcome! students' application period starts on May 1. Thanks very much for the support, hope to hear thoughts and proposals, Paolo. --

1 0

parallel seq
by Scherrer, Chad 25 Apr '06

25 Apr '06

I was thinking about the dynamic behavior of par, and there's something that's a little confusing to me. Am I right in understanding that (x `par` y) evaluates to y, and may or may not evaluate x along the way? I think it would be easier to reason about if we knew that x would be evaluated, either by a newly-spawned thread or by the parent thread. If I'm understanding this correctly, this could be done by defining x `pSeq` y = x `par` y `seq` x `seq` y (seq has higher precedence than par) When (x `pSeq` y) is evaluated, x may or may not be evaluated by a new thread. The parent thread continues along evaluating y. Then the parent makes sure x was evaluated before finally returning y. I've not seen this approach used before -- Is there something I'm missing that makes using pSeq as above just silly? Thanks! Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

1 0

wxHaskell with active GHCI input loop?
by Simon Guest 25 Apr '06

25 Apr '06

Hello Haskell Hackers, I am trying to create a wxHaskell program which, while the GUI is active, enables the user to type arbitrary haskell program fragments to interact with the GUI. Initially I thought I would run the wxHaskell main loop in a different thread, and interact via the GHCI main loop, but I see that multiple threads are not supported by wxHaskell. I am wondering if hs-plugins is the way to go, arranging for string input to be taken in the GUI, and passed to the hs-plugins eval function. However I want the interactive Haskell to be able to manipulate objects displayed on the GUI, and am not sure if this can be done with such a subservient hs-plugins evaluator? Any suggestions? cheers, Simon

1 0

New monadic implementation of Control.Exception
by Brian Hulley 25 Apr '06

25 Apr '06

Hi - Following indications in http://www.haskell.org/pipermail/haskell/2006-February/017547.html I've wrapped all the functions in Control.Exception so that they can be used with any monad constructed using the common state transformers and based (ultimately) on IO. I'd suggest something like this could be incorporated into Haskell' perhaps as a replacement for Control.Exception. This also relates to http://hackage.haskell.org/trac/haskell-prime/ticket/110 but I don't know how to attach a file to a ticket therefore I've just enclosed it with this email. (In the discussion following Oleg's original post my experience implementing the generalised exception functions showed that getUnliftIO :: m(m a -> IO a) mentioned in a later post proved to be a red herring since StateT can't support this operation yet it *can* support all the exception ops in a sensible way.) So far, I've only tested catch, catchJust, throwM using a (StateT s IO) monad, so I can't be absolutely sure that I've got the semantics for the other monads correct, but at least they all compile.... :-) Regards, Brian.

1 1

Parallel list comprahensions
by Iavor Diatchki 25 Apr '06

25 Apr '06

Hello, Does the parallel list comprahension notation support guards that involve both variables that are being generated? I was trying to write something that is essentially: [ f x y | (x,y) <- zip xs ys, p x y ] and I thought that it would be nice to rewrite it as a parallel list comprahension, but I am stuck on the `p x y` part. I can define my own function to do that: pzip f p xs ys = concat (zipWith zipper xs ys) where zipper x y = if p x y then [f x y] else [] or use the normal list comprahension version so it is not a problem, but I thought I'd check if there already was a notation that I didn't know about. -Iavor

1 0

Haskell Summer School (Camp) for Teens
by John Peterson 25 Apr '06

25 Apr '06

The Computer Science department at Western State College is offering a very unique summer camp opportunity for high school Juniors / Seniors. Although our brochure and web page never mention Haskell, we're using Haskell on top of the Panda 3D game engine to allow students with no computer experience to do interesting things with computing, math, and physics. In addition, our program includes some recreational opportunities that those boring summer schools can't compete with! :-) If you know anyone that would be interested in this, please pass the message on. Our brochure is here: http://www.western.edu/computerscience/computer_camp.html And you don't have to tell them that this is a secret Haskell Indoctrination Camp. John Peterson Now at jpeterson(a)western.edu

1 0

Haskell.org and Google Summer of Code 2006
by Paolo Martini 24 Apr '06

24 Apr '06

Hello everybody, nice to meet you all. Last year I did work on an Haskell project during the first year of the Google Summer of Code programme[1]. I wrote the bindings to the Cairo Vector Graphics library and integrated them in Gtk2Hs. You can see last year's announcement on the website[2]. I had to take Google itself as my mentoring organization, in absence of a better-suited one, no wonder I was the only one with an Haskell project, eventually I got one of its 13 slots available -- Chris DiBona[3], the project leader, seem to like Haskell very much! They recently announced they would be running the programme again this year, and I thought we could surely do better! My idea is that the best way we are going to get more Haskell projects done is having Haskell.org as a mentoring organization this year. Let me briefly summarize how the project works; it starts with a number of mentoring organizations that support active open source projects. They are required to publish a list of projects for students to apply for, and some mentors which will take care of the pupils during their work. Part of the fun is of course that the students can propose their own projects too. Each successful applicant gets an initial $500, three months of coding time, a mid- term $2,000 of are doing well and a final $2,000 on successful completion of the project (The site[4] does explain it all.) I contacted many of the people who usually reside in the #haskell IRC channel asking for collaboration, and they promptly accepted, forming a quite big number of well organized people -- as now, those three people are in charge for the administrative work: - Isaac Jones <http://www.syntaxpolice.org/>, - Donald Bruce Steward <http://www.cse.unsw.edu.au/~dons/>, - David Himmelstrup <http://darcs.haskell.org/~lemmih/aboutMe.html>. Not surprisingly, since the people of the channel are known to be very kind and helpful, answering questions at every time of the day, I've got a good number of volunteers for mentoring too (which I'm very happy about): - David Himmelstrup, - John Goerzen <http://www.complete.org/>, - Cale Gibbard <http://www.haskell.org/hawiki/CaleGibbard>, - Einar Karttunen <http://www.cs.helsinki.fi/u/ekarttun/>, - Shae Matijs Erisson <http://www.ScannedInAvian.com/>, - Duncan Coutts <http://haskell.org/gtk2hs/development/>. - Andres L\¨oh <http://www.iai.uni-bonn.de/~loeh>, - Ian Lynagh <http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/>, And the list is getting bigger and bigger! Unsure about where to get an "authoritative answer" about things like this I have mailed Simon Peyton-Jones, he likes the idea (Thanks very much!) He explained to me that since the Haskell community is very decentralized I would better send a message to the Haskell mailing list to propose this project. I hope you will be as enthusisatic as I am. Hacking Haskell code during the summer, funded enough to keep up with the other things (and getting some pretty cool hardware lately), was a *great* experience. I sincerely hope that many students will apply for Haskell works. We need projects done! I'll be mailing you the url of the wikipage collecting projects and details. Thank you very much, Paolo. -- [1] <http://code.google.com/summerofcode.html> [2] <http://haskell.org/gtk2hs/?p=30> [3] <http://dibona.com>

4 6

Example Code Wiki area
by Paul Johnson 23 Apr '06

23 Apr '06

I've created a new area for Example Code on the Wiki. I'd like to call on all those who contributed code to the old "CodeOnTheWiki" area to bring it over to the new area. However you should be aware that doing so implicitly licenses the code under the Simple Permissive License. Paul.

1 0

How to avoid both duplicate instances and extraneous instance declarations?
by Robin Bate Boerop 22 Apr '06

22 Apr '06

I am having trouble making a type system for my EDSL in Haskell. Can someone help? A simplified description of my problem follows. I have three classes, A, B1, and B2. I want all instances of B1 to be instances of A. I want all instances of B2 to be instances of A. None of the classes have methods. The following does NOT work, because of a duplicate instance declaration for A: class A a class B1 b1 class B2 b2 instance B1 x => A x instance B2 x => A x -- duplicate instance, won't compile data T = T instance B1 T The following DOES work, but it requires that I explicitly give instance declarations of A for all instances of B1 and B2: class A a class A a => B1 a class A a => B2 a data T = T instance A T -- I don't want to have to specify this! instance B1 T Of course, this is a simplified example. In a complicated type class hierarchy, in a program with many instances of, say B1, having to provide instance declarations for A is laborious. Is there a way to avoid this? How can I tell Haskell "all instances of B1 are automatically instances of A". -- Robin Bate Boerop

4 10