April 2013 - Haskell-Cafe

Monad fold
by Christopher Howard 16 Apr '13

16 Apr '13

So, I'm doing something like this foldl (>>=) someA list :: Monad m => m a where list :: Monad m => [a -> m a], someA :: Monad m => m a Is there a more concise way to write this? I don't think foldM is what I want -- or is it? -- frigidcode.com

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primitive operations in StateT
by Clark Gaebel 16 Apr '13

16 Apr '13

Hi list! I want to use MVectors in a StateT monad transformer. How do I do that? StateT isn't a member of 'PrimMonad', and I have no idea how to make it one. Regards, - Clark

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Google Summer of Code
by Marcos Pividori 16 Apr '13

16 Apr '13

Hi, I am a Computer Science student from Argentina. I am interested in working this summer on some project related to Haskell for the Google Summer of Code. I have learned a lot about this, because in my career, we have a special approach in functional programming. So, I would like to know about recent projects and who could mentor me. I found a ticket about improving Parsec. http://hackage.haskell.org/trac/summer-of-code/ticket/1556 Is anyone working with this? Thanks, Marcos Pividori

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How to design an network client with user program.
by Alexander V Vershilov 15 Apr '13

15 Apr '13

Hello. I have next problem: I have a network client that connects to server, listens for messages and generate responces. So the control flow can be represended as: server -- input -> {generate output} -> output Output can be generated using default implementation or can overriden by user. The main difficulty appeares when I need to add a user program on the top of this logic, i.e. from user-side I want to have dsl:smth like withClient $ do x <- send message waitFor x timeout 5000000 forever $ sendRandomMessage i.e. an ability to send messages, waiting for some event (message to come), waiting for timeout. The question is how to define such logic without a big overhead. I see a solution using conduit, it's possible to create 3 processes: listener, user, sender. +----> user ----+ | | -input -> listener +----------------->+---- sender -> and use TQueue or TChan to send messages between them, however there can be another possible solutions, that uses less resources, or another design. -- Alexander

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Re: [Haskell-cafe] GSoC proposal: Data Visualization
by Ernesto Rodriguez 15 Apr '13

15 Apr '13

Dear All, Data visualization for hMatrix is basically my main objective, but I would like to be flexible in the following ways: * Have generic approach so other data types can be visualized with my library as long as the necessary instances are present. * Define a generic standard to represent values in multiple types of plots. This way graphics output can be easily extended for more back ends (gtk, qt, gnuplot). I am looking for alternatives. I will check GuiTV definitely, but currently I am considering the possibility of plotting data via javascript as suggested by Dough and using Haskell just to generate the values. I am using ghclive (https://github.com/shapr/ghclive) as fronted since the project looks nice and might be worth continuing it. Do you think there would be big objections if I continue working with that project since it was built for last GSoC? Best regards, Ernesto Rodriguez > > On Mon, Apr 15, 2013 at 2:10 PM, Michal J. Gajda <miga(a)nmr.mpibpc.mpg.de>wrote: > >> Dear Conal, Ernesto, Heinrich, >> >> >> On 04/15/2013 10:32 AM, Heinrich Apfelmus wrote: >> >> Ernesto Rodriguez wrote: >> >> >> For me it would already be a huge advantage if I >> could edit and re-evaluate expressions interactively (in a comfortable >> GUI, >> not ghci). Also a plot widget with sliders would also help. I was >> wondering >> if you know any reason the project has not been worked on for various >> months (as I see in the repo). Is there anyone working in this project >> and >> has a later version? I mean these are features that are even available in >> free math packages such as Sage. >> >> >> I was actually the initial mentor for this project. I'm not particularly >> happy about the result. As you can see, it hasn't been picked up by anyone >> else, including me, and I think that's because it missed the modularity >> goals I had in mind. >> >> Indeed interactive notebook functionality is missed by many :-). >> >> Is it a good time to ask about current status of GuiTV, which is possibly >> second closest to what Ernesto had in mind? >> http://www.haskell.org/haskellwiki/GuiTV >> Would be possible to ask Conal Elliot whether he is interested in >> mentoring a project that would add widget(s) for hmatrix visualisation? >> >> PS I believe Johan Malmström <http://homepage.mac.com/jmalmstrom/>'s >> HaXcel project, which may have tried to implement a similar functionality >> with a spreadsheet interface is dead: >> http://www.mrtc.mdh.se/projects/Haxcel/ >> -- >> Best regards >> Michal >> > > > > -- > Ernesto Rodriguez > > Bachelor of Computer Science - Class of 2013 > Jacobs University Bremen > > > > -- Ernesto Rodriguez Bachelor of Computer Science - Class of 2013 Jacobs University Bremen

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Re: [Haskell-cafe] NaN as Integer value
by wren ng thornton 15 Apr '13

15 Apr '13

On 4/13/13 1:18 PM, Jerzy Karczmarczuk wrote: > This is not a Haskell problem. For Ints, ALL representations are valid > numbers, a NaN is a specific float object, unless I'm mistaken, so the > introduction of such an abnormal number would require some serious > modifications of the representation. Also, the necessity of NaN for floats comes from the fact that floats include values for Infinity and -Infinity, which leads to various equations which cannot be resolved sensibly. This is different than mere undefinedness. Undefined values are underspecified, but that can often be resolved by choosing some arbitrary specification (often chosen via arguing from limits or combinatorial convenience). Whereas the problematic values due to infinities are overspecified, so no matter which answer you pick it's guaranteed to be the wrong answer half the time. Part of this whole problem comes from the fact that floats *do* decide to give a meaning to 1/0 (namely Infinity). That's the gateway drug,... -- Live well, ~wren

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GSoC proposal: Data Visualization
by Ernesto Rodriguez 15 Apr '13

15 Apr '13

Dear Haskell Community, During the last months I used Haskell for machine learning, particularly in the field of Echo State Neural Networks. The main drawback I encountered is that its difficult to visualize and plot data in Haskell in spite the fact there are a couple of plotting libraries. Data visualization is very important in the field of machine learning research (not so much in machine learning implementation) since humans are very efficient to analyze graphical input to figure out what is going on in order to determine possible adjustments. I was wondering if other members of the community have experienced this drawback and would be interested in improved data visualization for Haskell, especially if there is interest to use Haskell for machine learning research. I collected my ideas in the following page: https://github.com/netogallo/Visualizer . Please provide me with feedback because if the proposal is interesting for the community I would start working with it, even if it doesn't make it to this GSoC, but a project like this will need a lot of collaboration for it to be successful. Thank you very much, Best Regards, Ernesto -- Ernesto Rodriguez Bachelor of Computer Science - Class of 2013 Jacobs University Bremen

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yi-editor for emacer
by Junior White 15 Apr '13

15 Apr '13

Hi Cafe, I'm glad to announce my fork of yi-editor. As a emacer, I made yi-editor more emacs like, including the following change: 1. ido-mode like file find and buffer find 2. support more color in vty 3. automatic search tags file in parents's directory 4. change some keymap The following step to have a try: 1. clone : git clone https://github.com/onlyu/yi.git 2. build : ./yi/yi.sh --rebuild 3. run : ./yi/yi.sh Thanks for have a test, and thanks yi-editor, now I combine my favourite editor (emacs) and language (haskell) together. Apologize for my noisy!

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Optimizing queries in Haskell
by Jan Stolarek 15 Apr '13

15 Apr '13

Dear list, list comprehensions and SQL-like generalized comprehensions can be used to write queries in Haskell. Does anyone know of any work focused on optimizing such queries? A quick google didn't show up anything meaningful. Janek

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[Announcement] Sparse matrix Conjugate Gradient Solver
by Levent Erkok 15 Apr '13

15 Apr '13

I'm happy to announce the first release of conjugateGradient, implementing the Conjugate Gradient algorithm for solving linear systems of equations over sparse matrices: http://hackage.haskell.org/package/conjugateGradient The conjugate gradient algorithm only applies to the cases when the input matrix is symmetric and positive definite. See: http://en.wikipedia.org/wiki/Conjugate_gradient_method The advantage of this method is that it can handle really large sparse systems quite well, when other direct methods (such as LU decomposition) are just not practical due to algorithmic complexity. Such large and sparse systems naturally arise in engineering applications, such as in ASIC placement algorithms and when solving partial differential equations. Bug reports, feedback, and improvements are always welcome. -Levent.

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