August 2009 - Haskell-Cafe

Converting typeset mathematics into Haskell ?
by Mark Wassell 25 Aug '09

25 Aug '09

Does anyone know of any papers or projects, either for Haskell or any other language, that relate to what I am going to attempt to describe in the following: Go to any paper or book that includes some amount of calculation type mathematics. An example is the formula in the Description section of http://en.wikipedia.org/wiki/K-means_clustering. Think about how you would convert this into Haskell. You might then find yourself wondering why you have to convert it into Haskell at all. Given that most mathematics online is typeset using something like latex or mathml, why can't we parse this typesetting and convert it into Haskell code? In all likelyhood the conversion might be ambiguous so some rules might be needed to guide the conversion. Furthermore, if there are editors that allow you to edit these sorts of equations in a wysiwyg style, why not leverage these to edit the Haskell+Math code also in a wysiwyg style. I think this goes to the larger question of why are most programming languages (or to be more exact the way we write programs in those languages) is still based around a linear sequence of tokens rather than something that is more 2D (there is still some linearity as even in math formula there is still a top down and left to right flow). Mark

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Generics for constructing Rows
by Max Desyatov 25 Aug '09

25 Aug '09

Hi, all. I've come into trouble defining function `gmap` which will work on these data types: > data Row = Row > (E Name) > (E Salary) > (E Department) > type E a = Either (Maybe RowIndex) (Maybe a) > type RowIndex = Int `RowIndex`, `Name`, `Salary`, `Department` have kind * pseudocode: > gmap :: (E a -> E a) -> Row -> Row > readRow :: [String] -> Row -> Row > readRow l = gmap (\(Left (Just ri)) -> Right $ l `atMay` c >>= readMay) `atMay` and `readMay` are defined in module `Safe` from package `safe` > atMay :: [a] -> Int -> Maybe a > readMay :: (Read a) => String -> Maybe a Basically we have optional Row indices and try to read raw row (list of Strings) into the same Row type if index is present. At this moment I just have separate data type for row which has been read and just a list of row indices, but it is definitely flawed when I need to add fields to Row from time to time, as it is too easy to introduce bugs while positioning in list of row indices, not mentioning all those boilerplate code flowing around. I've tried to define gmap using libraries for generic programming but failed each time while type checking for different reasons. With `Data.Generics` and `gmapT` it fails because gmapT doesn't allow traversion function to have `Read a` constraint (Data and Typeable instances for Row and inner data types of course were derived). EMGM's map demands traversion function to be non-polymorphic, i.e. type-checker fails with the message, complaining it cannot match `E a` against `E Name`, against `E Salary` etc. For each of this I've spent smth about five hours just reading API docs, papers, and trying to beat type-checker. Maybe I'll look into Smash, RepLib and others, but I hope someone was also trying to define such gmap and succeeded. So, generic programming folks, is it even possible to define such function? I don't really care about using GHC extensions, I don't care about code being portable, I just want to remove boilerplate and prevent introducing bugs. Thanks in advance, Max.

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ANNOUNCE: graphtype — A simple tool to illustrate dependencies between Haskell types
by Max Desyatov 25 Aug '09

25 Aug '09

While developing applications which deal with complex data it is crucial to know how exactly you manipulate this data. Haskell provides excellent tools for expressing a data scheme you work with: ADTs, `type` and `newtype` declarations, type classes and much more is hidden in rich Haskell's type system. Obviously, when types of data in your domain you work with grow — all declarations grow, and it becomes hard to grasp all dependencies, to change them and to remove them deliberately. graphtype was developed to visualise type declarations in you Haskell source files. It produces .dot-file for subsequent processing with graphviz. Results for example file bundled with graphtype: http://i.piccy.info/i4/00/90/bfa07290012c2d3b455696bdaa86.png To play with it, you can use hackage: http://hackage.haskell.org/package/graphtype or hack some code: http://github.com/explicitcall/graphtype Visualisation of dependencies in complex type class hierarchies is still on the way. It isn't obvious how do to this nicely, as in most cases type class declarations are imported from other libraries, and you don't always have source files for them. Anyway, graphtype is fairly usable. Leave here your questions, suggestions and have fun looking at type dependencies in your code. WBR, Max

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Linking problem with GHCi
by Maurício CA 24 Aug '09

24 Aug '09

With a specific package (bindings-common, with the latest version in hackage) if I ask GHCi to read it I get: ghc-6.8.2: /home/mauricio/lib/bindings-common-0.2.5/ghc-6.8.2/HSbindings-common-0.2.5.o: unknown symbol `atexit' If I build a library or executable cabal package depending on it, I have no problems. Also, if I remove that 'atexit' wrap from 'bindings-common' I don't get this ghci error message anymore. Why would just this 'atexit' function be a problem? It's wrap does work well, and I can use it from a Haskell program through bindings-common (although not in ghci, of course). This happens at least in ghc-6.8.2 and 6.10.3. Thanks, Maurício

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rotate image
by h 24 Aug '09

24 Aug '09

Hello, I already studied the references of gtk2hs for hours, as well as searched in the web, but didn't found any working solution. The problem is to load an image, a png with alpha channel, rotate it e.g. 10 degree, and show it on the screen, basically as when it would be shown with imageNewFromFile with transparency. Thanks in advance for your help. -- Best regards H.

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ANN: rss2irc 0.3 released
by Simon Michael 24 Aug '09

24 Aug '09

rss2irc is an irc bot created by Don Stewart to watch rss feeds and announce new items on irc. I have been tweaking and testing it for a while, and have taken up the maintainer reins. I'm happy to announce release 0.3, with: - reliable http networking - irc flood protection - better error handling & reporting - extensive debugging output - Atom support - more useful defaults (enter channel silently, hide email addresses etc.) - precise control of irc output, including regexp rewrites - installable on mac osx Feedback and patches welcome. I am currently using it to run a number of announce-bots on freenode, including hackagebot and darcscommitbot. (A tip: if you experiment, avoid joining the irc server too frequently; once per minute may be ok.) home: http://hackage.haskell.org/package/rss2irc darcs repo: http://joyful.com/darcsweb/darcsweb.cgi?r=rss2irc -Simon

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Problem building encoding-0.6.0: missing Data.CharMap.Builder
by João Rafael Nicola 24 Aug '09

24 Aug '09

Hi, I am trying to install the encoding-0.6.0 package, but it complains that that package "Data.CharMap.Builder" is missing. I could not find any reference to that package anywhere on the web. Does anyone know where I can find it or a workaround? Thanks, João Rafael > cabal install encoding Resolving dependencies... /tmp/encoding-0.6.08632/encoding-0.6.0/Data/Encoding/Preprocessor/XMLMappingBuilder.hs:11:7: Could not find module `Data.CharMap.Builder': Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: encoding-0.6.0 failed during the configure step. The exception was: exit: ExitFailure 1

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Gtk2Hs drag and drop
by frantisek kocun 24 Aug '09

24 Aug '09

Hi, have somebody used drag and drop feature of Gtk2hs? I can't find any tutorial or demo. I'm using only the documentation but I think I'm missing something. Can anyone give me an example? Thanks in forward! Fero

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[ann] formlets 0.6
by Chris Eidhof 24 Aug '09

24 Aug '09

Hey everyone, I wanted to let you know that the formlets team has released a new version of the formlets [1] on hackage, a library to build type-safe, composable web forms. Most notably, Mightybyte and I worked on the massInput functionality, which is now ready for use! Mightybyte has an excellent article [2] on how to use the massInput to build dynamic forms on the client side. If you're building web apps with Haskell, make sure you check it out. -chris [1]: http://hackage.haskell.org/package/formlets [2]: http://softwaresimply.blogspot.com/2009/08/dynamic-list-formlets-in-haskell…

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Functional Dependencies and multiparameter typeclasses
by Harald ROTTER 24 Aug '09

24 Aug '09

Dear Haskellers, I am using multi parameter typeclasses to represent images consisting of pixels (e.g. bitmap images). {-# OPTIONS_GHC -XMultiParamTypeClasses #-} module Bitmap where -- | a pixel could be a Word8 (e.g. a graysclale image) -- | or a 3-tuple for RGB images class Pixel p where .... <pixel related functions go here> -- | an image could be a UArray or a list of lists of pixels class Pixel p => Image a p where width :: a -> Int height :: a -> Int dims :: a -> (Int, Int) dims img = (height img, width img) If I try to load this module into ghci I get: Could not deduce (Image a p) from the context (Image a p2) arising from a use of 'height' at ..... and Could not deduce (Image a p1) from the context (Image a p2) arising from a use of 'width' at ...... where both errors originate from the 'dims' function. Eventually I figured out that I could remedy the situattion by using functional dependencies like this: class Pixel p => Image a p | a -> p where ... However, I do not really understand the cause of the original problem. Why do I need the functional dependency to make this work ? Any help is appreciated. Thanks Harald. " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."

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