February 2012 - Haskell-Cafe

Job Opportunity: Data Scientist at madvertise Mobile Advertising, Berlin
by Adam Drake 27 Feb '12

27 Feb '12

Hello Haskell-Cafe, We are a start-up company (currently approximately 70 people) and are looking to grow our research and technology group by hiring a new Data Scientist. This person should have a strong quantitative background and Haskell experience is a plus. Bonus points if you have experience building sizable tool chains and applications or your Haskell experience has involved analysis and machine learning techniques on large data sets. For more information, please visit the job listing on our website. http://madvertise.com/en/2012/02/14/data-scientist-fm/ If you have any questions, feel free to contact me directly. You can also email questions to Julian Fichter in HR: julian [dot] fichter (at) madvertise (dot) com. -- Best Regards, Adam Drake

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Reminder Haskell Munich
by Heinrich Hördegen 27 Feb '12

27 Feb '12

Dear all, Munich's Haskellers will meet tomorrow, 28. Feb 2012, at 19h30 at Cafe Puck: http://www.haskell-munich.de/dates See you, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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ghc-mod: Expanding Template Haskell
by Kazu Yamamoto 26 Feb '12

26 Feb '12

Hello, I have released ghc-mod v1.10.10. With this version, you can expnad Template Haskell by using C-cC-e on Emacs. http://mew.org/~kazu/proj/ghc-mod/en/ Enjoy! --Kazu

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Using multiplate to get free variables from a syntax tree
by Matt Brown 26 Feb '12

26 Feb '12

Hi all, I'm reading the haskellwiki article on multiplate. Is it possible to modify the getVariablesPlate example to return the free variables? One idea I had is to store an environment in a reader monad, and use local to update the environment at a let expression. Couldn't figure it out, though. Any ideas? Thanks! -matt http://www.haskell.org/haskellwiki/Multiplate

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ANN: shelltestrunner-1.2
by Simon Michael 26 Feb '12

26 Feb '12

I'm pleased to announce the release of shelltestrunner 1.2. Also, in case you missed it there's a survey where you can help steer the project (below). shelltestrunner tests command-line programs (or arbitrary shell commands.) It reads simple declarative tests specifying a command, some input, and the expected output, and can run them run in parallel, selectively, with a timeout, in color, and/or with differences highlighted. shelltestrunner has been tested on gnu/linux, mac and windows; projects using it include hledger, berp, cblrepo and eddie. shelltestrunner is free software released under GPLv3+. New in 1.2: * support latest cmdargs, test-framework, and GHC 7.4. * more readable non-quoted failure output by default; for quoted output, use `-p/--precise` * the `--all`, `--diff` and `--precise` options now interact well Install: $ cabal install shelltestrunner Home: http://joyful.com/shelltestrunner User survey: https://docs.google.com/spreadsheet/viewform?formkey=dGpZSzdhWHlCUkJpR2hjX1… One confession: I've shipped with failing unicode tests on GHC 7.4 as I can't see unicode on this terminal - patch welcome!

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FFI: Overhead of foreign unsafe imports
by Yves Parès 26 Feb '12

26 Feb '12

Hello, When I was using C code from Python, the overhead put on calling C code by Python was significant. To simplify, say I have on C-side two procedures f and g, I do all the stuff to call them in a row from Python, well I'm better off factorizing: adding on C side a wrapper h procedure that calls them both, and then call h from Python, because then I will have half as much overhead: Instead of SwitchToC -> call f -> SwitchToPython -> SwitchToC -> call g -> SwitchToPython, the factorization leads to SwitchToC -> call f -> call g -> SwitchToPython, which gives the same result yet is different performance-wise because each switching has a cost. This is painful, because if another time I have to call f and j (another function), then I have to make another wrapper. In Haskell world, now, given that my functions f and g would have been imported using *unsafe*: foreign import unsafe "f" f :: Thing -> Foo -> IO () foreign import unsafe "g" g :: Stuff -> Bar -> IO () foreign import unsafe "h" h :: Thing -> Foo -> Stuff -> Bar -> IO () Are doStuff = f x y >> g z w and doStuff = h x y z w equivalent, or is there an overhead (e.g. due to IO monad, or due to the way the FFI does the calls) when compiled (if need be with optimizations) with GHC?

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Re: [Haskell-cafe] Streaming to JuicyPixels
by Vincent Berthoux 26 Feb '12

26 Feb '12

Hi, I didn't thought it would be a full rewrite, when I finish Jpeg writing functionality, I will look at it in depth, which will not be immediately. Feel free to use any code snippet from JuicyPixels if it help your for your own library. Regards Vincent Berthoux Le 24 février 2012 06:42, Myles C. Maxfield <myles.maxfield(a)gmail.com> a écrit : So I started working on moving JuicyPixels to a streaming interface, and > have some observations. This is going to be a pretty major change, touching > pretty much every function, and the end result will end up looking very > close to the code that I have already written. I'm not nearly as close to > the code as the author is, and I've already made some mistakes due to not > understanding how the original code is structured and layed out. > > Because this is essentially turning out to be a rewrite, I think it makes > more sense for me to just work on my own library, and release it as a > streaming alternative to JuicyPixels. How do you feel about this, Vincent? > > Thanks, > Myles > > On Wed, Feb 22, 2012 at 12:30 PM, Vincent Berthoux < > vincent.berthoux(a)gmail.com> wrote: > >> Hi, >> Please go ahead, and github is the perfect medium for code sharing :) >> >> Regards >> >> Vincent Berthoux >> >> Le 22 février 2012 20:20, Myles C. Maxfield <myles.maxfield(a)gmail.com> a >> écrit : >> >> Let's put aside the issue of getting access to the pixels before the >>> stream is complete. >>> >>> How would you feel if I implemented use of the STT monad transformer on >>> top of Get in JuicyPixels, in order to get rid of the (remaining >> >>> getBytes) call, and then expose the underlying Get interface to callers? >>> This would allow use for streaming. >>> >>> Is this something that you feel that I should pursue? I can send you a >>> GitHub Pull Request when I'm done. >>> >>> Thanks, >>> Myles >>> >>> On Wed, Feb 22, 2012 at 5:01 AM, Vincent Berthoux < >>> vincent.berthoux(a)gmail.com> wrote: >>> >>>> Hi, >>>> >>>> I can understand your performance problems, I bumped into them before >>>> the first release of Juicy Pixels and took a long time to get 'correct' >>>> performance out of the box, and the IDCT is not the only 'hot point', I got >>>> problems with intermediate data structure as well. Any list has proven a >>>> bit problematic performance wise, so I rewrote some things which would have >>>> been easily implemented with forM_ or mapM_ with manual recursion. >>>> >>>> I didn't knew STT existed, so it open a new area of reflection for the >>>> streaming Jpeg decoder, instead of using the (remaining >> getBytes) combo, >>>> staying in the Get monad might help. The reason to expose the ST monad is >>>> that I use it internally to mutate the final image directly, and I'd prefer >>>> to avoid freezing/unfreezing the underlying array. So in order to give >>>> access to the intermediate representation, a type could be (STT s (StateT >>>> JpegDecodingState Get) (MutableImage s PixelYCbCr8)) (the Jpeg decoder only >>>> produce (Image PixelYCbCr8 internally)). This should allow a freeze then a >>>> color conversion. >>>> >>>> As it would induce performance loss, this version should exist >>>> alongside current implementation. This is not trivial, but it's far from >>>> impossible. For the IDCT implementation, I don't think a package make much >>>> sense, if you want to use it, just grab it and customize the interface to >>>> your needs :). >>>> >>>> Regards >>>> >>>> Vincent Berthoux >>>> >>>> Le 21 février 2012 06:16, Myles C. Maxfield <myles.maxfield(a)gmail.com>a écrit : >>>> >>>> Hello, and thanks for the quick reply. >>>>> >>>>> You're right that using (remaining >> getBytes) won't work for >>>>> streaming, as it would pull the rest of the stream into a buffer, thereby >>>>> delaying further processing until the image is completely downloaded. :-( >>>>> >>>>> I'm a little unclear about what you mean about the use of the ST >>>>> monad. There is an STT<http://hackage.haskell.org/packages/archive/STMonadTrans/0.2/doc/html/Contr…> monad >>>>> transformer, so you could wrap that around Get. Is that what you're >>>>> referring to? >>>>> >>>>> As an aside: I didn't realize that JuicyPixels existed, so I wrote a >>>>> JPEG decoder specifically designed for streaming - it doesn't request a >>>>> byte of input unless it has to, uses a StateT (wrapped around Attoparsec) >>>>> to keep track of which bit in the current byte is "next", and does the >>>>> Huffman decoding in-line. However, I didn't use ST for the IDCT, so my own >>>>> decoder has serious performance problems. This prompted me to start >>>>> searching around for a solution, and I came across JuicyPixels, which >>>>> already exists and is much faster than my own implementation. I'm hoping to >>>>> get rid of my own decoder and just use JuicyPixels. If you're curious, my >>>>> own code is here: https://github.com/litherum/jpeg. >>>>> >>>>> Is it reasonable to extend JuicyPixels to fit my use case? It sounds >>>>> like JuicyPixels wouldn't work so well as it stands. I'd be happy to do >>>>> whatever work is necessary to help out and get JuicyPixels usable for me. >>>>> However, if that would require a full (or near-full) rewrite, it might make >>>>> more sense for me to use my own implementation with your IDCT. Is there a >>>>> way we can share just the IDCT between our two repositories? Perhaps making >>>>> a new IDCT8 library that we can both depend on? >>>>> >>>>> As for what API I'd like to be able to use, just a "Get DynamicImage" >>>>> should suffice (assuming it has streaming semantics as described above). It >>>>> would be really nice if it was possible to get at the incomplete image >>>>> before the stream is completed (so the image could slowly update as more >>>>> data arrives from the network), but I'm not quite sure how to elegantly >>>>> express that. Do you have any ideas? >>>>> >>>>> I think that having 2 native jpeg decoders (Actually 3, because of this >>>>> package <http://hackage.haskell.org/package/jpeg>) is detrimental to >>>>> the Haskell community, and I would really like to use JuicyPixels :D >>>>> >>>>> Thanks, >>>>> Myles C. Maxfield >>>>> >>>>> >>>>> On Mon, Feb 20, 2012 at 3:01 PM, Vincent Berthoux < >>>>> vincent.berthoux(a)gmail.com> wrote: >>>>> >>>>>> Hi, >>>>>> >>>>>> I can expose the low level parsing, but you would only get the >>>>>> chunks/frames/sections of the image, Cereal is mainly used to parse the >>>>>> structure of the image, not to do the raw processing. For the raw >>>>>> processing, I rely on `remaining >> getBytes` to be able to manipulate data >>>>>> at bit level or to feed it to zlib, and the documentation clearly state >>>>>> that remaining doesn't work well with runGetPartial, so no read ahead, but >>>>>> even worse for streaming :). >>>>>> >>>>>> To be fair, I never thought of this use case, and exposing a >>>>>> partially decoded image would impose the use of the ST Monad somehow, and >>>>>> Serialize is not a monad transformer, making it a bit hard to implement. >>>>>> >>>>>> By curiosity what kind of API would you hope for this kind of >>>>>> functionality? >>>>>> >>>>>> Regards >>>>>> >>>>>> Vincent Berthoux >>>>>> >>>>>> Le 20 février 2012 22:08, Myles C. Maxfield <myles.maxfield(a)gmail.com >>>>>> > a écrit : >>>>>> >>>>>> Hello, >>>>>>> I am interested in the possibility of using JuicyPixels for >>>>>>> streaming images from the web. It doesn't appear to expose any of its >>>>>>> internally-used Serialize.Get functionality, which is problematic for >>>>>>> streaming - I would not like to have to stream the whole image into a >>>>>>> buffer before the decoder can start decoding. Are there any plans on >>>>>>> exposing this API, so I can use the runGetPartial function to facilitate >>>>>>> streaming? >>>>>>> >>>>>>> In addition, does the library do much readahead? There's no point in >>>>>>> exposing a Get interface if it's just going to wait until the stream is >>>>>>> done to start decoding anyway. >>>>>>> >>>>>>> Thanks, >>>>>>> Myles C. Maxfield >>>>>>> >>>>>> >>>>>> >>>>> >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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Finally, a pure iteratee library that deals with errors
by Daniel Waterworth 26 Feb '12

26 Feb '12

Hi all, There's seems to be a growing trend amongst the Haskell community for writing replacements of enumerator. It's shortcomings are well documented, but the solution is, IMO, undecided. This is my attempt: https://gist.github.com/1915841 I have largely copied pipes' concept. The major difference is that pipes are now able to push, pull, complete or throw an error. Errors are propagated up and down a pipeline, triggering all of the error handlers. The library is in a fairly unrefined state, but the core functionality is there. I look forward to your feedback, Daniel

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question in install gtk2hs's webkit hackage
by Canhua 26 Feb '12

26 Feb '12

hi, I am cabal-installing webkit hackage on windows, and get a error: pkg-config pckage webkit-1.0 version > 1.1.15 is required but could be not found What should I next? install webkit? How to? I find there is a webkitgtk+ project. Which one should I install. I have try to install both of them follow their docs, and failed. I just have no experience in build software on windows. Thanks.

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Data.IntMap union complexity
by Clark Gaebel 25 Feb '12

25 Feb '12

Looking at IntMap's left-biased 'union' function [1], I noticed that the complexity is O(n+m) where n is the size of the left map, and m is the size of the right map. Since insertion [2] is O(min(n, W)) [ where W is the number of bits in an Int ], wouldn't it be more efficient to just fold 'insert' over one of the lists for a complexity of O(m*min(n, W))? This would degrade into O(m) in the worst case, as opposed to the current O(n+m). Or am I just crazy? Regards, - clark [1] http://hackage.haskell.org/packages/archive/containers/0.4.2.1/doc/html/Dat… [2] http://hackage.haskell.org/packages/archive/containers/0.4.2.1/doc/html/Dat…

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