April 2006 - Haskell-Cafe

Control.Exceptions and MonadIO
by oleg＠pobox.com 24 Apr '06

24 Apr '06

Robert Dockins wrote: > One additional (very unfortunate) point is that higher-order IO monad > combinators will not work on your monad, eg, the ones in > Control.Exception. Although that is true in general, for many useful and interesting cases (including ReaderT, the state transformer, and the newtype wrapping of IO) one _can_ use catch, bracket etc. constructs in MonadIO. Please see this message and the follow-up discussion: http://www.haskell.org/pipermail/haskell/2006-February/017547.html

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How to get Haddock to generate docs for a directory tree of modules
by Brian Hulley 24 Apr '06

24 Apr '06

Hi - I've been looking at the docs for Haddock at http://haskell.org/haddock/haddock-html-0.7/index.html but I can't seem to find any option to recursively traverse a directory generating hyperlinked docs for all modules anywhere in the directory or any sub directory etc. Is there a simple undocumented flag I'm missing? Thanks, Brian.

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Advice needed on best way to simulate an STL vector
by Brian Hulley 24 Apr '06

24 Apr '06

Hi - In C++, STL provides a vector class which behaves as an array except you can insert/delete elements from it. I'm wondering what is the best Haskell data structure to use to simulate this, either mutable or immutable. I've looked at the Array interface, but although there is the // operation to replace some elements, there does not appear to be a simple way to delete/insert elements. Ideally I'd like functions like: -- Insert new elements starting at the specified index, moving the others up to make room insert:: Array i e -> i -> [e] -> Array i e -- Delete a range of elements, moving later elements back down delete:: Array i e -> i -> i -> Array e -- Append a new element to the end of an array push_back :: Array i e -> e -> Array i e Is there an efficient way of implementing these operations in Haskell, based on arrays, or should I be using some other data structure altogether eg a list? Also, for large arrays, am I right in thinking that it is still more efficient to use immutable arrays rather than mutable arrays (because it is easier for gc to always just deal with immutable values)? Thanks, Brian.

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Independent component analysis and RDF
by Joel Reymont 23 Apr '06

23 Apr '06

Howdy folks! Does anyone have sample code for independent component analysis (ICA), singular value decomposition (SVD) aka spectral graph partitioning, or semidiscrete decomposition (SDD)? I'm trying to learn this rocket science and apply it to RDF graph analysis. Thanks, Joel -- http://wagerlabs.com/

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Re: Predicates in HaXml
by luby liao 23 Apr '06

23 Apr '06

For this purpose, try h1 [keep /> dfilter (parseXtract "/greetings/greeting[1]/-")] cheers, Luby Liao > I am just trying out a few things with the Xtract tool. I have a > simple XML document: > > <greetings> > <greeting>hello</greeting> > <greeting>world</greeting> > </greetings> > > I am using the Xtract query /greetings/greeting[1]/- hoping to get the > text "world" > e.g. > mkElem "h1" [applypred (keep) (parseXtract "/greetings/greeting[1]/-")] > > but this just returns the entire xml document. Have i got something > fundamentally wrong here? > > Thanks > Tom Spencer > >

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Re: [Haskell] Haskell.org and Google Summer of Code 2006
by Creighton Hogg 22 Apr '06

22 Apr '06

Well, now that we've started this discussion I presume it's okay if I ask a few things. I don't know if these are good ideas, but I thought I'd throw them out. Basically, I am a student and would be eligible for the SoC. I'd like to work on something related to numerics support or performance in Haskell. Either building on existing gsl bindings, making something out of whole cloth, or spending some intensive work on compiler optimizations. My dream would be to build up enough monte carlo support in haskell that things like high energy physics event simulators could be coded in the language without sacrificing too much performance and having a hundred times the flexibility and *maintainability*. Creighton Hogg On 4/22/06, Paolo Martini <paolo(a)nemail.it> wrote: > 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> > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Google SoC
by Pepe Iborra 21 Apr '06

21 Apr '06

http://code.google.com/soc/ This is not news. SoC was presented a few days ago, and by now there are a lot of projects available, yet none(?) Haskell related :( This is a plea for Haskell FOSS project managers to apply as mentor organizations, so that we students can have a choice. Surely there are a lot of contributions that can be tackled by a motivated student in three months time. GHC related stuff, work in Visual Haskell, tasks in a given Haskell framework, Darcs stuff, you name it. Deadline for mentor organizations is May 1, but they might close it earlier if they get their hands full. pep

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matrix computations based on the GSL
by Alberto Ruiz 21 Apr '06

21 Apr '06

Hello all! This is my first message to the list. In my work I often need linear algebra algorithms and other numeric computations. An option is using scientific computing systems like Matlab, Mathematica, Maple, etc. In Haskell there are several modules and bindings to matrix libraries; many of them are excellent. But I think that Haskell would be even more attractive if the basic matrix computations (what we find, for example, in the GNU Octave Quick Reference) were easily available in a standard library. We have all we need: an interactive environment, excellent graphics (HOpenGL), FFI and ForeignPtr with automatic garbage collection, QuickCheck, etc. Computations can be implemented by the GSL (GNU Scientific Library), a well designed numerical library written in C. Well... I know that I am trying to reinvent the wheel: I am working on a user friendly functional interface to matrix computations based on the GSL. I think that many numeric problems that can be solved using Octave or a similar system can be also solved more elegantly using Haskell. The library is very preliminary and incomplete but some simple problems can already be solved. The sources, haddock documentation and the draft of a tutorial can be found at: http://dis.um.es/~alberto/hmatrix/matrix.html I am not a Haskell expert, I only use the most basic programming techniques, so any suggestion will be greatly appreciated... Alberto

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C++ parser in Haskell?
by Ravi Nanavati 20 Apr '06

20 Apr '06

It turns out we might find such a beast useful at Bluespec. If anyone has written a C++ parser in Haskell (or knows of one), we'd like to hear about it. Thanks, - Ravi Nanavati Bluespec, Inc.

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Spawning a terminal window?
by ihope 20 Apr '06

20 Apr '06

Does Haskell have a handy way to spawn a new terminal window and give you a handle to access it with? --ihope

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