- Haskell-Cafe - Haskell.org

Theorems for free!
by Patrick Browne 27 Feb '12

27 Feb '12

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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 27 Feb '12

27 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 27 Feb '12

27 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 27 Feb '12

27 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 26 Feb '12

26 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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How far away is a release of cabal-install that works with ghc 7.4?
by Magnus Therning 25 Feb '12

25 Feb '12

It looks like HEAD on the dev branch of cabal (cabal/cabal-install) compiles fine with 7.4.1, so any thoughts on when we see an update posted on Hackage? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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Monadic Design Patterns for the Web Chapter Review
by Greg Meredith 25 Feb '12

25 Feb '12

*Dear Supporters, Friends and Colleagues, i’m writing to you to say that i will be going over the material in Chapters 3 through 9 of the book, MDP4tW in a Google+ hangout, 1 Chapter / week. If any would like to participate in reviewing the material presented, please let me know. i will limit participation in these reviews to 5 people / session with preference give to people who contributed to the Kickstarter campaign. Please RSVP to me at lgreg.meredith(a)gmail.com and i will let you know the dates and times.* Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Call for Journal Papers: STVR Special Issue on Tests and Proofs
by Achim D. Brucker 25 Feb '12

25 Feb '12

Apologies for duplicates. CALL FOR PAPERS STVR Special Issue on Tests and Proofs http://lifc.univ-fcomte.fr/tap2012/stvr/ The Software Testing, Verification & Reliability (STVR) journal (http://www3.interscience.wiley.com/journal/13635/home) invites authors to submit papers to a Special Issue on Tests and Proofs. Background ========== The increasing use of software and the growing system complexity make focused software testing a challenging task. Recent years have seen an increasing industrial and academic interest in the use of static and dynamic analysis techniques together. Success has been reported combining different test techniques such as model-based testing, structural testing, or concolic testing with static techniques such as program slicing, dependencies analysis, model-checking, abstract interpretation, predicate abstraction, or verification. This special issue serves as a platform for researchers and practitioners to present theory, results, experience and advances in Tests and Proofs (TAP). Topics ====== This special issue focuses on all topics relevant to TAP. In particular, the topics of interest include, but are not limited to: * Program proving with the aid of testing techniques * New challenges in automated reasoning emerging from specificities of test generation * Verification and testing techniques combining proofs and tests * Generation of test data, oracles, or preambles by deductive techniques such as: theorem proving, model checking, symbolic execution, constraint logic programming, SAT and SMT solving * Model-based testing and verification * Automatic bug finding * Debugging of programs combining static and dynamic analysis * Transfer of concepts from testing to proving (e.g., coverage criteria) and from proving to testing * Formal frameworks for test and proof * Tool descriptions, experience reports and evaluation of test and proof * Case studies combining tests and proofs * Applying combination of test and proof techniques to new application domains such as validating security procotols or vulnerability detection of programs * The processes, techniques, and tools that support test and proof Submission Information ====================== The deadline for submissions is 17th December, 2012. Notification of decisions will be given by April 15th, 2013. All submissions must contain original unpublished work not being considered for publication elsewhere. Original extensions to conference papers - identifing clearly additional contributions - are also encouraged unless prohibited by copyright. Submissions will be refereed according to standard procedures for Software Testing, Verification and Reliability. Please submit your paper electronically using the Software Testing, Verification & Reliability manuscript submission site. Select "Special Issue Paper" and enter "Tests and Proofs" as title. Important Dates: ================ * Paper submission: December 17, 2012 * Notification: April 15, 2013 Guest Editors ============= * Achim D. Brucker, SAP Research, Germany http://www.brucker.ch/ * Wolfgang Grieskamp, Google, U.S.A. http://www.linkedin.com/in/wgrieskamp * Jacques Julliand, University of Franche-Comté, France http://lifc.univ-fcomte.fr/page_personnelle/accueil/8 -- Dr. Achim D. Brucker, SAP Research, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch

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ANN: generic-deepseq 1.0.0.0
by Maxime Henrion 25 Feb '12

25 Feb '12

Hello all, Yesterday I uploaded a new package on Hackage called generic-deepseq. It implements the 'deepseq' function generically using the new GHC.Generics framework as found in GHC >= 7.2. It can be used as a replacement for the deepseq package. Given that hackage is currently down, here is an URL from a mirror where the package description & documentation can be found: http://hackage.factisresearch.com/package/generic-deepseq-1.0.0.0 Any suggestions are welcome. Cheers, Maxime Henrion

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SURVEY: hledger, shelltestrunner, rss2irc, FunGEn usage 2012
by Simon Michael 24 Feb '12

24 Feb '12

Hey all, I'm gathering usage data on my main FOSS projects, to help me prioritize and steer them. I've prepared a short survey, 10 optional questions that should take 1-5 minutes per project. If you use any of these projects and/or would like them to continue, you can help a lot by adding your experience to: hledger: https://docs.google.com/spreadsheet/viewform?formkey=dHVGUHBXRUhRNWF3Y2RsZG… shelltestrunner: https://docs.google.com/spreadsheet/viewform?formkey=dGpZSzdhWHlCUkJpR2hjX1… rss2irc (& hackagebot): https://docs.google.com/spreadsheet/viewform?formkey=dFhhLWZrZ3l4elFYa3M1Zm… FunGEn: https://docs.google.com/spreadsheet/viewform?formkey=dFN5dllFTGFFU3ZhclcxWT… Don't hold back! Honest critique welcome. You'll see a summary report after submitting your answers, and before long I'll follow up with the urls for full results. Note that contact info will be public, except that I'll keep email addresses private. It's optional but at least a little is quite useful in several ways. I made the survey generic, so feel free to re-use it. Best, -Simon

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Re: [Haskell-cafe] Streaming to JuicyPixels
by Vincent Berthoux 24 Feb '12

24 Feb '12

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 >> >> >

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Hackage documentation down?
by wren ng thornton 24 Feb '12

24 Feb '12

Hello all, I posted a minor update to unix-bytestring a few days ago in order to deal with the Num not implying Eq thing in GHC 7.4, but it looks like the documentation still hasn't been made (nor the built-on field). Are the Hackage builders down intentionally since the recent crash, or accidentally? -- Live well, ~wren

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Re: [Haskell-cafe] network-2.3.0.10 compiled for ghc 7.4.1 windows
by Holger Reinhardt 24 Feb '12

24 Feb '12

I just use the version of MSys that is included with Git [1]. This puts a "Git bash" icon on your desktop which you can then use to build the network library. [1] http://code.google.com/p/msysgit/ 2012/2/7 Alberto G. Corona <agocorona(a)gmail.com> > Nothing bur a long history of failures. The problem is the > configuration and versioning of MinGW and MSys. This is a nighmare. > > 2012/2/7 Holger Reinhardt <hreinhardt(a)gmail.com>: > > Oh you are using Cygwin. I'm using MSys so this is why I cannot reproduce > > your problem. Is there anything preventing you from using MSys? > > > > > > 2012/2/7 Alberto G. Corona <agocorona(a)gmail.com> > >> > >> The "problem" this time is in "Configure" : > >> > >> case "$host" in > >> *-mingw32) > >> EXTRA_SRCS="cbits/initWinSock.c, cbits/winSockErr.c, > >> cbits/asyncAccept.c" > >> EXTRA_LIBS=ws2_32 > >> CALLCONV=stdcall ;; > >> *-solaris2*) > >> EXTRA_SRCS="cbits/ancilData.c" > >> EXTRA_LIBS="nsl, socket" > >> CALLCONV=ccall ;; > >> *) > >> EXTRA_SRCS="cbits/ancilData.c" > >> EXTRA_LIBS= > >> CALLCONV=ccall ;; > >> esac > >> > >> > >> > >> Since I´m cross-compiling with cygwin, the variable Host does not > >> contain ¨*-muingw32" but "i686-pc-cygwin" > >> > >> changing the case , the library incorporates the lost C coded files. > >> > >> Now the library links fine win imported, but there is a runtime error: > >> > >> NetworkSocket.hsc:(948,3)-(1007,23): Non-exhaustive patterns in case > >> > >> maybe it is due to some other preprocessor directive mismatch > >> > >> > >> 2012/2/7 Holger Reinhardt <hreinhardt(a)gmail.com>: > >> > Did you also change the files in the /cbits/ folder? Because they also > >> > check > >> > for HAVE_WINSOCK_H. > >> > > >> > > >> > 2012/2/7 Alberto G. Corona <agocorona(a)gmail.com> > >> >> > >> >> The code is evolving and none of the versions match exactily with the > >> >> patch, but substituting HAVE_WINSOCK by HAVE WINSOCK2 in these files > >> >> solves the compilation problem at least in the network 2.3.0.10 > >> >> version from hackage. > >> >> > >> >> However it produces the same undefined references when this library > is > >> >> imported in my application. It seems that some object code is not > >> >> included in the final library. I verified that at least some of > these > >> >> undefined references correspond with C code in the source, but > >> >> somehow this is not included in the object library.... > >> >> > >> >> 2012/2/7 Johan Tibell <johan.tibell(a)gmail.com>: > >> >> > Note that there are two branches on github, master and stable. You > >> >> > want > >> >> > the > >> >> > latter. > >> >> > > >> >> > On Feb 7, 2012 8:23 AM, "Alberto G. Corona" <agocorona(a)gmail.com> > >> >> > wrote: > >> >> >> > >> >> >> This is quite different. > >> >> >> I don´t know how but I was looking at some other older patch > around > >> >> >> the same issue and I supposed that it was the one refered by Yohan > >> >> >> Tibell. > >> >> >> > >> >> >> I´ll try your patch. > >> >> >> > >> >> >> Thanks!. > >> >> >> > >> >> >> 2012/2/7 Holger Reinhardt <hreinhardt(a)gmail.com>: > >> >> >> > Hi, > >> >> >> > > >> >> >> > (I submitted the patch that Johan linked to) > >> >> >> > Network/Socket/Internal.hsc has the following code: > >> >> >> > > >> >> >> > #if defined(WITH_WINSOCK) || defined(cygwin32_HOST_OS) > >> >> >> > type CSaFamily = (#type unsigned short) > >> >> >> > #elif defined(darwin_HOST_OS) > >> >> >> > type CSaFamily = (#type u_char) > >> >> >> > #else > >> >> >> > type CSaFamily = (#type sa_family_t) > >> >> >> > #endif > >> >> >> > > >> >> >> > You have patched this part to always use 'unsigned short'. But > the > >> >> >> > real > >> >> >> > issue is that WITH_WINSOCK is not defined, even though it should > >> >> >> > be. The > >> >> >> > reason for this lies in include/HsNet.h: > >> >> >> > > >> >> >> > #if defined(HAVE_WINSOCK_H) && !defined(cygwin32_HOST_OS) > >> >> >> > # define WITH_WINSOCK 1 > >> >> >> > #endif > >> >> >> > > >> >> >> > The problem here is that it checks for HAVE_WINSOCK_H, but the > >> >> >> > configure > >> >> >> > script never defines this variable. Instead it > >> >> >> > defines HAVE_WINSOCK2_H. > >> >> >> > It > >> >> >> > seems that the network library used Winsock1 in the past and in > >> >> >> > the > >> >> >> > transition to Winsock2 someone forgot to change a few of the > >> >> >> > #ifdefs. > >> >> >> > > >> >> >> > My patch just changes all occurences of HAVE_WINSOCK_H > >> >> >> > to HAVE_WINSOCK2_H. > >> >> >> > You might want to try that and report back if it works for you. > >> >> >> > > >> >> >> > 2012/2/7 Alberto G. Corona <agocorona(a)gmail.com> > >> >> >> >> > >> >> >> >> Hi Johan, > >> >> >> >> The patch is not for the current version of network and the > code > >> >> >> >> is > >> >> >> >> quite different. Basically it is necesary to define this > >> >> >> >> variable > >> >> >> >> as > >> >> >> >> "unsigned short" that is the thing intended in the patch. > however > >> >> >> >> I > >> >> >> >> put it by brute force, without regard of the prerpocessor > >> >> >> >> directives. > >> >> >> >> With this change the code compiles well with: > >> >> >> >> > >> >> >> >> > >> >> >> >> > >> >> >> >> > >> >> >> >> > >> >> >> >> > http://neilmitchell.blogspot.com/2010/12/installing-haskell-network-library… > >> >> >> >> > >> >> >> >> However my compiled library lack the methods defined as > foreign. > >> >> >> >> I´ll > >> >> >> >> keep trying. > >> >> >> >> > >> >> >> >> 2012/2/6 Johan Tibell <johan.tibell(a)gmail.com>: > >> >> >> >> > Hi, > >> >> >> >> > > >> >> >> >> > Someone recently contributed a fix that should make network > >> >> >> >> > build > >> >> >> >> > with > >> >> >> >> > 7.4: https://github.com/haskell/network/pull/25 > >> >> >> >> > > >> >> >> >> > Can you see if that works for you? I haven't yet had time to > >> >> >> >> > merge > >> >> >> >> > and > >> >> >> >> > release that fix (I'm on vacation.) > >> >> >> >> > > >> >> >> >> > -- Johan > >> >> >> >> > > >> >> >> >> > >> >> >> >> _______________________________________________ > >> >> >> >> Haskell-Cafe mailing list > >> >> >> >> Haskell-Cafe(a)haskell.org > >> >> >> >> http://www.haskell.org/mailman/listinfo/haskell-cafe > >> >> >> > > >> >> >> > > >> > > >> > > > > > >

6 15

Haskell compilers targeting VMs
by Carlton Mills 24 Feb '12

24 Feb '12

LLVM has a .net and JVM code generator. I don't think it is production ready yet. GHC can generate code via LLVM. So in theory one could run on either VM. However, see the "Why isn't GHC available ..." link below. The GC and library integration would require lot's of work. Good senior project for someone who wishes to spend 3 years as a senior. -Carlton Daniel Peebles pumpkingod at gmail.com A more subtle issue is that there's some sort of memory leak that arises when you can't instruct the GC to follow projection functions of datatypes. I believe the GHC heap representation has a built-in notion of theseforwarding closures and the GC follows them when possible, but most VM GCsare just black boxes. It might still be possible to create an artificialfield in a representation of your closure that the GC can follow, but itcould be tricky.On Thu, Feb 23, 2012 at 11:59 AM, Tom Murphy <amindfv at gmail.com> wrote: >There are some substantial technical challenges: >>http://www.haskell.org/haskellwiki/GHC:FAQ#Why_isn.27t_GHC_available_for_.N… >>Not that it can't be done, but there's nothing ready yet. >>Tom (IRC: amindfv) >>> >> Does any one knows of an Haskell compiler targeting the JVM ? And of one >> targeting the .Net virtual machine ? >> >> Regards, >> ARJANEN Loïc

1 0

ANN: network-socket-options 0.1
by Joey Adams 24 Feb '12

24 Feb '12

I added a new package containing wrappers for getsockopt and setsockopt: http://hackage.haskell.org/package/network-socket-options The network package already has getSocketOption and setSocketOption. The problem is, these don't work for socket options that aren't represented by integers, such as SO_LINGER: http://trac.haskell.org/network/ticket/23 Another, less serious problem is that getSocketOption and setSocketOption don't leverage the type system as well as they could. Many options are boolean values; it'd be better to get and set them with 'Bool's instead of 'Int's. network-socket-options implements options using getter and setter functions, e.g.: getLinger :: HasSocket sock => sock -> IO (Maybe Seconds) setLinger :: HasSocket sock => sock -> Maybe Seconds -> IO () type Seconds = Int The HasSocket type class is defined to overload the getters and setters to work on raw file descriptors, not just Socket objects. This functionality should probably go in the network package itself. However, I decided to release it as a separate package so I could start using it sooner. If people like it enough, perhaps the network package can absorb it, as was done with network-bytestring. -Joey Adams

3 8

Haskell compilers targeting VMs
by ARJANEN Loïc Jean David 23 Feb '12

23 Feb '12

Hello, Does any one knows of an Haskell compiler targeting the JVM ? And of one targeting the .Net virtual machine ? Regards, ARJANEN Loïc

4 3

ANN: cabal-file-th-0.2.2
by Simon Michael 23 Feb '12

23 Feb '12

I have uploaded a new version of cabal-file-th which bumps the Cabal upper bound to avoid difficulties with GHC 7.4. Best - Simon

1 0

getAddrInfo: does not exist
by Alexander Vasiliev 23 Feb '12

23 Feb '12

Hi. I use FreeBsd and get the following error during execution. "mvz_server: getAddrInfo: does not exist (servname not supported for ai_socktype)" System: FreeBsd 7.1; amd64. Haskell: ghc-7.0.3; network-2.3.0.5. The programm works well on Ubuntu and on Windows XP. The piece of code is: bindit :: String -> ServData -> IO() bindit port sd@(mvzs, clnts) = withSocketsDo $ do addrinfos <- getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just port) I googled that this error was solved in ghc 6.8. But i use ghc 7.0.3. Can anyone help me? Best Regards, Alexander

4 4

Re: [Haskell-cafe] Clarifying a mis-understanding about regions (and iteratees)
by John Lato 23 Feb '12

23 Feb '12

ResourceT really addresses a different problem. Specifically, the issue of how to guarantee that finalizers will run in the context of short-circuiting monadic effects. It does this by providing a single region (not nested regions as Oleg describes) as the base of the monad stack, ensuring that any short-circuiting effects still need to go through the ResourceT layer's finalization steps. Same implementation strategy, different goal. It doesn't provide nested regions, although you may be able to stack multiple ResourceT's on top of each other (I have no idea if this works or not) to do so. The "regions" package (http://hackage.haskell.org/package/regions) is another implementation predating ResourceT. I don't know why it never caught on, except perhaps that at announcement time it was a solution without a problem. John L. > From: Yves Par?s <yves.pares(a)gmail.com> > > Hi, so there are different regions libraries? > Is there a shootout comparing them, possibly also with ResourceT from > conduit (which has also been implemented in a stand-alone package > http://hackage.haskell.org/package/resource-simple-0.1 by Robin Banks), for > I take it it tries to respond to the same problem? > > 2012/2/23 <oleg(a)okmij.org> > >> >> I have just come across the reddit discussion about regions and >> iteratees. >> >> http://www.reddit.com/r/haskell/comments/orh4f/combining_regions_and_iterat… >> >> There is an unfortunate misunderstanding about regions which I'd like to >> correct. I'm posting in Cafe in hope that some of the participants of >> that discussion read Haskell-Cafe (I'm not a redditor). >> >> The user Faucelme wrote: >> > Would it be possible to implement a region-based iteratee which opened >> some >> > file "A" and wrote to it, then opened some file "B" and wrote to it, >> while >> > ensuring that file A is closed before file B is opened? >> >> To which the user tibbe replied >> > You can typically explicitly close the files as well. >> >> and the user dobryak commented >> >> > Regions that Oleg refers to started out with region-based memory >> allocation, >> > which is effectively a stack allocation strategy, in which resource >> > deallocation is in LIFO order. So I think your assumption is correct. >> >> Regretfully, these comments are incorrect. First of all, memory >> regions were invented to get around the stack allocation, LIFO >> strategy. If the stack allocation sufficed, why do we need heap? We >> have heap specifically because the memory allocation patterns are >> complex: a function may allocate memory that outlives it. Regions >> let the programmer create arbitrarily many nested regions; everything >> in a parent region is available to a child. Crucially, a child may >> request any of its ancestors to allocate memory in their >> regions. Therefore, although regions are nested, memory does not have >> to be allocated and freed in LIFO order. >> >> The Lightweight monadic regions implement all these patterns for files >> or other resources (plus the dynamic bequest). >> http://okmij.org/ftp/Computation/resource-aware-prog/region-io.pdf >> >> The running example of the Haskell Symposium 08 paper was the >> following (see sec 1.1) >> >> 1. open two files for reading, one of them a configuration file; >> 2. read the name of an output file (such as the log file) from the >> configuration file; >> 3. open the output file and zip the contents of both input files into >> the output file; >> 4. close the configuration file; >> 5. copy the rest, if any, of the other input file to the output file. >> >> As you can see, the pattern of opening and closing is non-LIFO: the >> output file has to be opened after the configuration file and is >> closed also after the configuration file. Therefore, the user Faucelme >> can find the solution to his question in the code accompanying the >> Monadic Regions paper. >> >> Section 5 of the paper describes even more complex example: >> >> 1. open a configuration file; >> 2. read the names of two log files from the configuration file; >> 3. open the two log files and read a dated entry from each; >> 4. close the configuration file and the newer log file; >> 5. continue processing the older log file; >> 6. close the older log file. >> >> where the pattern of opening and closing is not statically known: >> it is decided on values read from the files. >> >> So, Faucelme's question can be answered in affirmative using the >> existing RegionIO library (which, as has been shown, well integrates >> with Iteratees). There is already a region library with the desired >> functionality. >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

1 0

Haskell Meeting in Munich
by Heinrich Hördegen 23 Feb '12

23 Feb '12

Dear all, it's time again for our monthly Haskell meeting in Munich. If you plan to join us on Tuesday, 28th of February at Cafe Puck in Munich at 19h30, please go to: http://www.haskell-munich.de/dates and hit the button. This will help us to reserve tables. With hope to meet many of you, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

1 0

Clarifying a mis-understanding about regions (and iteratees)
by oleg＠okmij.org 23 Feb '12

23 Feb '12

I have just come across the reddit discussion about regions and iteratees. http://www.reddit.com/r/haskell/comments/orh4f/combining_regions_and_iterat… There is an unfortunate misunderstanding about regions which I'd like to correct. I'm posting in Cafe in hope that some of the participants of that discussion read Haskell-Cafe (I'm not a redditor). The user Faucelme wrote: > Would it be possible to implement a region-based iteratee which opened some > file "A" and wrote to it, then opened some file "B" and wrote to it, while > ensuring that file A is closed before file B is opened? To which the user tibbe replied > You can typically explicitly close the files as well. and the user dobryak commented > Regions that Oleg refers to started out with region-based memory allocation, > which is effectively a stack allocation strategy, in which resource > deallocation is in LIFO order. So I think your assumption is correct. Regretfully, these comments are incorrect. First of all, memory regions were invented to get around the stack allocation, LIFO strategy. If the stack allocation sufficed, why do we need heap? We have heap specifically because the memory allocation patterns are complex: a function may allocate memory that outlives it. Regions let the programmer create arbitrarily many nested regions; everything in a parent region is available to a child. Crucially, a child may request any of its ancestors to allocate memory in their regions. Therefore, although regions are nested, memory does not have to be allocated and freed in LIFO order. The Lightweight monadic regions implement all these patterns for files or other resources (plus the dynamic bequest). http://okmij.org/ftp/Computation/resource-aware-prog/region-io.pdf The running example of the Haskell Symposium 08 paper was the following (see sec 1.1) 1. open two files for reading, one of them a configuration file; 2. read the name of an output file (such as the log file) from the configuration file; 3. open the output file and zip the contents of both input files into the output file; 4. close the configuration file; 5. copy the rest, if any, of the other input file to the output file. As you can see, the pattern of opening and closing is non-LIFO: the output file has to be opened after the configuration file and is closed also after the configuration file. Therefore, the user Faucelme can find the solution to his question in the code accompanying the Monadic Regions paper. Section 5 of the paper describes even more complex example: 1. open a configuration file; 2. read the names of two log files from the configuration file; 3. open the two log files and read a dated entry from each; 4. close the configuration file and the newer log file; 5. continue processing the older log file; 6. close the older log file. where the pattern of opening and closing is not statically known: it is decided on values read from the files. So, Faucelme's question can be answered in affirmative using the existing RegionIO library (which, as has been shown, well integrates with Iteratees). There is already a region library with the desired functionality.

2 1

Where to get kansas-lava version 2.5
by dilawar rajput 23 Feb '12

23 Feb '12

Hello Cafe, Greetings from Mumbai. Does anyone know where to get kansas-lava version 2.5? On their github repository (as well as on Hackage), they have not posteed version 2.5. However their kansas-lava-cores requires version kansas-lava 2.5. Currently I am working with kansas-lava 2.4 but I can not use their latest kansas-lava-core with it. -- Dilawar

2 2

Preventing leaked open file descriptors when catching exceptions
by Ryan Newton 23 Feb '12

23 Feb '12

Hi all, I'm trying to run a loop that repeatedly attempts to open a file until it succeeds. The file is a named pipe in nonblocking mode, so the writer can only connect after the reader has connected. (Perhaps there is some way to determine this by stat'ing the pipe, but I don't know it yet.) Thus I do something like the following: tryUntilNoIOErr $ do performGC -- The reader must connect first, the writer here spins with backoff. PIO.openFd filename PIO.WriteOnly Nothing fileFlags I'm running GC between iterations to try to make sure I get rid of open files. Also, in the "tryUntilNoIOErr" code below I have some debugging messages which indicate that ioeGetHandle reports no handles associated with the exceptions I'm getting back. (If there were handles provided I could close them explicitly.) In spite of these attempted precautions I'm seeing "too many open files" exceptions in simple benchmarks that should only have a maximum of ONE file open. Any hints / pointers? Thanks, -Ryan mkBackoff :: IO (IO ()) mkBackoff = do tref <- newIORef 1 return$ do t <- readIORef tref writeIORef tref (min maxwait (2 * t)) threadDelay t where maxwait = 50 * 1000 tryUntilNoIOErr :: IO a -> IO a tryUntilNoIOErr action = mkBackoff >>= loop where loop bkoff = handle (\ (e :: IOException) -> do bkoff BSS.hPutStr stderr$ BSS.pack$ " got IO err: " ++ show e case ioeGetHandle e of Nothing -> BSS.hPutStrLn stderr$ BSS.pack$ " no hndl io err." Just x -> BSS.hPutStrLn stderr$ BSS.pack$ " HNDL on io err!" ++ show x loop bkoff) $ action

4 5

Re: [Haskell-cafe] Data.Array.Accelerate initialization timings
by Paul Sujkov 23 Feb '12

23 Feb '12

Ah, it seems that I see now what's going wrong way. I'm not using the 'run' function from the CUDA backend, and so by default I guess the code is interpreted (the test backend used for semantics check). However, it's not perfectly clear how to use CUDA backend explicitly. If you have any suggestions, it would be a great help! On 20 February 2012 16:06, Alex Gremm <algremm(a)googlemail.com> wrote: > Hi Paul, > > even though I just started reading about Accelerate, it seems to me that > you didn't use the "use" method which according to [1] initiates > asynchronous data transfer from host to GPU. > > > Cheers, > Alex > > [1]: http://www.cse.unsw.edu.au/%7Echak/papers/acc-cuda.pdf > On 20/02/12 14:46, Paul Sujkov wrote: > > Hi everyone, > > > > since accelerate mail list seems to be defunct, I'm trying to ask > > specific questions here. The problem is: array initialization in > > Data.Array.Accelerate takes a 10x amount of time in contrast to both > > Data.Array and bare C++ CUDA array initialization. This can be due to > > Data.Array.Accelerate having two backends (however, it's own tests show > > that my nVidia card is CUDA-capable), but I'm not aware of how can I > > profile GPU to check whether it is used or not. Anyway, here's code: > > > > http://hpaste.org/64036 > > > > both generateArray (DIM3) and generateArray1 (DIM1) take the same amount > > of time to initialize array. I'd say the problem is in GPU memory > > copying time, but here's bare C++ code: > > > > http://hpaste.org/64037 > > > > which does exactly the same, but 10 times faster. I'm wandering what am > > I doing wrong and how to check if I really am. Thanks in advance if > > anyone can point me on my mistakes! > > > > -- > > Regards, Paul Sujkov > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Regards, Paul Sujkov

3 2

Haskell Weekly News: Issue 216
by Daniel Santa Cruz 23 Feb '12

23 Feb '12

Welcome to issue 216 of the HWN, a newsletter covering crowdsourced links for the Haskell community. This release covers the week of February 12 to 18, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-216.h… Quotes of the Week * monochrom: the selfish program's motive is to fool the programmer * WilliamByrd: Oleg is an emissary from some advanced alien race. Working with him is both exhausting and enlightening. * CodeWeaverX: do-block, or do-not-block. There is no try-catch-block. Top Reddit Stories * Where is Hackage 2.0? Domain: self.haskell, Score: 66, Comments: 42 On Reddit: [1] http://goo.gl/uXq10 Original: [2] http://goo.gl/uXq10 * Haskell Performance Patters [HTML slideshow] Domain: blog.johantibell.com, Score: 52, Comments: 13 On Reddit: [3] http://goo.gl/Y5tCb Original: [4] http://goo.gl/T9zZI * Concatenative, Row-Polymorphic Programming in Haskell (re: Why Concatenative Programming Matters) Domain: github.com, Score: 41, Comments: 21 On Reddit: [5] http://goo.gl/0w83b Original: [6] http://goo.gl/aw22p * Neil Mitchell - Shake: A Better Make Domain: neilmitchell.blogspot.com, Score: 37, Comments: 7 On Reddit: [7] http://goo.gl/MeFQ1 Original: [8] http://goo.gl/4HM0d * Why concatenative programming matters. Domain: evincarofautumn.blogspot.com, Score: 37, Comments: 23 On Reddit: [9] http://goo.gl/Udpdt Original: [10] http://goo.gl/VvUvt * GSOC 2012 Project Proposal - GHCi in the web browser Domain: apfelmus.nfshost.com, Score: 36, Comments: 34 On Reddit: [11] http://goo.gl/KS6d8 Original: [12] http://goo.gl/epgTZ * Yesod Blog :: Call for GSoC: expand the reach of Haskell with interactive websites and good persistence Domain: yesodweb.com, Score: 31, Comments: 20 On Reddit: [13] http://goo.gl/1HQ9d Original: [14] http://goo.gl/bxKIC * hxournal - A pen notetaking program written in Haskell Domain: ianwookim.org, Score: 25, Comments: 16 On Reddit: [15] http://goo.gl/4UG5E Original: [16] http://goo.gl/kQOMh * Type checking by theorem proving in IDRIS: Brady [pdf, 29 slides] Domain: cs.st-andrews.ac.uk, Score: 25, Comments: 33 On Reddit: [17] http://goo.gl/qAD5L Original: [18] http://goo.gl/mhdlC * Using Lawvere theories to combine effects Domain: blog.sigfpe.com, Score: 24, Comments: 0 On Reddit: [19] http://goo.gl/4IHq7 Original: [20] http://goo.gl/QS1aO * BayHac '12 Domain: haskell.org, Score: 23, Comments: 0 On Reddit: [21] http://goo.gl/nuxUm Original: [22] http://goo.gl/NCQGy * Hope for Haskell: making Hackage and cabal-install more awesome Domain: github.com, Score: 22, Comments: 23 On Reddit: [23] http://goo.gl/E9MmZ Original: [24] http://goo.gl/sLpkp * Thoughts on Haskell Development Domain: self.haskell, Score: 21, Comments: 77 On Reddit: [25] http://goo.gl/Qdt4b Original: [26] http://goo.gl/Qdt4b * Relating Category Theory to Programming Language Theory - MathOverflow Domain: mathoverflow.net, Score: 21, Comments: 1 On Reddit: [27] http://goo.gl/wA9E2 Original: [28] http://goo.gl/6Ox5n * The Other Prelude — any activity here? It seems like a worthwhile project. Domain: haskell.org, Score: 18, Comments: 13 On Reddit: [29] http://goo.gl/j94fj Original: [30] http://goo.gl/kwGCm Top StackOverflow Questions * Covariance / Contravariance in Haskell? votes: 24, answers: 2 Read on SO: [31] http://goo.gl/wBjaP * Haskell for someone who already knows some category theory votes: 17, answers: 1 Read on SO: [32] http://goo.gl/ms4dE * Haskell libraries overview and their quality [closed] votes: 16, answers: 1 Read on SO: [33] http://goo.gl/H3Rwg * Haskell existential quantification in detail votes: 12, answers: 1 Read on SO: [34] http://goo.gl/uiJZG * Derivative Towers and how to use the vector-space package (haskell) votes: 11, answers: 1 Read on SO: [35] http://goo.gl/lkPEl * Why doesn't Scala have an IO Monad? votes: 10, answers: 1 Read on SO: [36] http://goo.gl/dyTte * How to let a function [a] -> [a] operate on [(a,Int)]? votes: 9, answers: 2 Read on SO: [37] http://goo.gl/Js3As * Speed up runhaskell votes: 9, answers: 4 Read on SO: [38] http://goo.gl/yc3qo * Unnecessary type juggling to define Data.Void (package “void” on Hackage)? votes: 8, answers: 1 Read on SO: [39] http://goo.gl/FCmyv * Lazy vs eager evaluation and double linked list building votes: 8, answers: 3 Read on SO: [40] http://goo.gl/Z55Og Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/pn2oh/where_is_hackage_20/ 2. http://www.reddit.com/r/haskell/comments/pn2oh/where_is_hackage_20/ 3. http://blog.johantibell.com/2012/02/slides-from-haskell-performance.html 4. http://www.reddit.com/r/haskell/comments/pry5h/haskell_performance_patters_… 5. https://github.com/leonidas/codeblog/blob/master/2012/2012-02-17-concatenat… 6. http://www.reddit.com/r/haskell/comments/ptji8/concatenative_rowpolymorphic… 7. http://neilmitchell.blogspot.com/2012/02/shake-better-make.html 8. http://www.reddit.com/r/haskell/comments/plv2b/neil_mitchell_shake_a_better… 9. http://evincarofautumn.blogspot.com/2012/02/why-concatenative-programming-m… 10. http://www.reddit.com/r/haskell/comments/pmcav/why_concatenative_programmin… 11. http://apfelmus.nfshost.com/blog/2012/02/14-summer-of-code-proposal.html 12. http://www.reddit.com/r/haskell/comments/ppdgq/gsoc_2012_project_proposal_g… 13. http://www.yesodweb.com/blog/2012/02/call-for-gsoc-2012 14. http://www.reddit.com/r/haskell/comments/pqudh/yesod_blog_call_for_gsoc_exp… 15. http://ianwookim.org/hxournal 16. http://www.reddit.com/r/haskell/comments/por66/hxournal_a_pen_notetaking_pr… 17. http://www.cs.st-andrews.ac.uk/~eb/writings/idris-stp.pdf 18. http://www.reddit.com/r/haskell/comments/pp6i3/type_checking_by_theorem_pro… 19. http://blog.sigfpe.com/2012/02/using-lawvere-theories-to-combine.html 20. http://www.reddit.com/r/haskell/comments/pm46y/using_lawvere_theories_to_co… 21. http://www.haskell.org/haskellwiki/BayHac2012 22. http://www.reddit.com/r/haskell/comments/puzxx/bayhac_12/ 23. https://github.com/DanBurton/Blog/blob/master/Markdown/Hope.md 24. http://www.reddit.com/r/haskell/comments/pswbp/hope_for_haskell_making_hack… 25. http://www.reddit.com/r/haskell/comments/ppf7p/thoughts_on_haskell_developm… 26. http://www.reddit.com/r/haskell/comments/ppf7p/thoughts_on_haskell_developm… 27. http://mathoverflow.net/questions/4235/relating-category-theory-to-programm… 28. http://www.reddit.com/r/haskell/comments/put1g/relating_category_theory_to_… 29. http://www.haskell.org/haskellwiki/The_Other_Prelude 30. http://www.reddit.com/r/haskell/comments/ppwst/the_other_prelude_any_activi… 31. http://stackoverflow.com/questions/9302739/covariance-contravariance-in-has… 32. http://stackoverflow.com/questions/9252488/haskell-for-someone-who-already-… 33. http://stackoverflow.com/questions/9286799/haskell-libraries-overview-and-t… 34. http://stackoverflow.com/questions/9259921/haskell-existential-quantificati… 35. http://stackoverflow.com/questions/9313994/derivative-towers-and-how-to-use… 36. http://stackoverflow.com/questions/9345008/why-doesnt-scala-have-an-io-monad 37. http://stackoverflow.com/questions/9324862/how-to-let-a-function-a-a-operat… 38. http://stackoverflow.com/questions/9326097/speed-up-runhaskell 39. http://stackoverflow.com/questions/9261490/unnecessary-type-juggling-to-def… 40. http://stackoverflow.com/questions/9277176/lazy-vs-eager-evaluation-and-dou…

1 0

Haskell development in Mac OS X after Gatekeeper
by Tom Murphy 23 Feb '12

23 Feb '12

Does anyone know what this will mean for the future of Haskell development in OS X?: http://www.apple.com/macosx/mountain-lion/security.html I'm particularly interested in what it'll mean for 0) Distributing non-Cocoa-built apps, even if you're approved by Apple 1) Writing software for widespread use (a security setting is to only run software from the App Store, and I'd like to have my software function on users' computers.) 1.0) Aren't you unable to put software under the GPL or certain other open-source licenses on the App Store? amindfv / Tom

8 19

Toronto Haskell Users Meetup
by Christopher Olah 22 Feb '12

22 Feb '12

Hey All, There's going to be a Toronto Haskell Users Meetup at hacklab on March 7th starting at 7pm. Come by and meet your fellow Haskellers! http://thug1.eventbrite.com/ This will be the first one. I'm really looking forward to meeting other Haskellers in person! Haskell Users of Toronto, unite! Chris http://colah.ca PS. Thoughts about where else I should mention this are appreciated.

1 0

Monad laws in presence of bottoms
by Roman Cheplyaka 22 Feb '12

22 Feb '12

I just realised that many common monads do not obey the monad laws when it comes to bottoms. E.g. for the Reader monad: undefined >>= return /= undefined return () >>= undefined /= undefined return () >>= const undefined /= undefined return undefined >>= \x -> case x of () -> return () /= undefined Not a long time ago David Barbour argued on this list that bottoms should be ignored when checking the monad laws. Is that the commonly accepted interpretation of the laws? Is there any other interpretation in which the Reader monad obeys the laws? -- Roman I. Cheplyaka :: http://ro-che.info/

6 15

Re: [Haskell-cafe] Streaming to JuicyPixels
by Vincent Berthoux 22 Feb '12

22 Feb '12

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 >>> >> >> >

2 1

Re: [Haskell-cafe] ANN: network-socket-options 0.1
by Joey Adams 22 Feb '12

22 Feb '12

On Wed, Feb 22, 2012 at 5:56 AM, L Corbijn <aspergesoepje(a)gmail.com> wrote: > On Wed, Feb 22, 2012 at 5:58 AM, Joey Adams <joeyadams3.14159(a)gmail.com> wrote: >> I released network-socket-options 0.2, adding setSocketTimeouts and >> setHandleTimeouts. I'll post an announcement in a separate thread >> once the Haddock documentation is generated. >> >> -Joey >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > Hi Joey, > > Seeing the large number of getters and setters in this package I was > wondering if it might be improved by using 'StateVar's from the > StateVar package. > http://hackage.haskell.org/package/StateVar > > Lars > > P.S. It does mention OpenGL (where it's actively used), but replacing > that with 'socket' makes the description seem to fit rather well. Copying to the mailing list. Thanks for pointing out StateVar. However, setSocketOption and getSocketOption (or variants taking raw Fds or GHC FDs) will not have a class for socket options to abstract over. It'd be nicer to see a list of settable options under the SetSockOpt class, and a list of gettable options under the GetSockOpt class. -Joey

1 0

how to add monad-stack to network-conduit
by Alexander V Vershilov 22 Feb '12

22 Feb '12

Hello. I have got next problem: I want to have database connection pool in server based on network-conduit. So I wanted to be able to do > runTCPServer options action > where > action src snk = > pool <- ask > .... > In this situation I can make pool `action's parameter, but in more difficult situations (many methods inside) is would be painfull. Also I wanted to have some kind of State for each thread, e.g. > action src snk = src $= act1 $= act2 $= act3 $$ snk in act1, act2, act3 I want to read and change state. Is it possible? And how to do it if it is? -- Alexander Vershilov

2 2

ANNOUNCE: HaskellDB 2.0: Scrap your SQL strings
by Justin Bailey 22 Feb '12

22 Feb '12

What is it? ======== The HaskellDB library lets you generate SQL queries without writing any actual SQL. Unlike other query generating libraries, you choose the abstraction level. Queries can be built out of independent fragments, just like your programs. Leave hand-written, string-based, SQL libraries behind and check out HaskellDB. Where to get it? ============ You can get it from Hackage (http://hackage.haskell.org/package/haskelldb) or by using cabal (cabal install haskelldb). You also need drivers for your specific database. HaskellDB currently supports the HDBC family of drivers. First, download haskelldb-hdbc (http://hackage.haskell.org/package/haskelldb-hdbc) then get the appropriate package for your database: * ODBC: haskelldb-hdbc-odbc (http://hackage.haskell.org/package/haskelldb-hdbc-odbc) * Postgres: haskelldb-hdbc-postgresql (http://hackage.haskell.org/package/haskelldb-hdbc-postgresql) * Sqllite: haskelldb-hdbc-sqlite3 (http://hackage.haskell.org/package/haskelldb-hdbc-sqlite3) Documentation, Mailing List, Source, etc ======================= Go to http://trac.haskell.org/haskelldb for detailed information. What's New? ========= * New query capabilities: stored procedures, CASTs, query parameters, constants, and more. * Postgres SQL improvements. * Bug fixes around AND and OR associativity. * Query optimization improvements around aggregate expressions. * More tests. Details ===== Back in June 2009, Bjorn Bringert passed maintainership of HaskellDB to me. I've been working this summer to get a release out, which included: * Adding an examples/ directory to the source distribution. * Archiving and consolidating information to http://trac.haskell.org/haskelldb. * Accepting patches for various issues. I have used a lot of ORM & SQL-wrapper libraries during my career and HaskellDB is the first that I've found which does not require you to eventually break the abstraction. Learning to use the the library was a fun challenge but not one that many people are willing to take, when they can easily write SQL by hand. If the library was easier to get started with, I think it would be much more widely used. Help me get HaskellDB better documentation! If you are using HaskellDB and want to help: go to http://trac.haskell.org/haskelldb and write an article, write a blog post, get in touch with me, etc. When I saw that Rails 3.0 uses the "ARel" gem (based on the relational algebra) I groaned - Haskell has had that forever! We need to get the word out! Why 2.0? -------------- Last year Brian Bloniarz contributed an awesome patch which replaced HaskellDB's home-grown record system with the HList library by Oleg Kiselyov (and colleagues). I intended to work with the library for awhile and get a 1.0 release out. Unfortunately, HList proved too daunting for GHC and for me. Compile times went way up, and I could not isolate it enough to give GHC HQ a repro case. Even worse, HList required too many Oleg units for me to comprehend. I could usually fix compile errors, but found myself struggling to extend HaskellDB's functionality. In the end I felt I needed to back the patch out. I really hated to lose some of the functionality but in the end maintainability won out. At that point I had pushed a few patches with a v1.0 cabal file, so I thought it best to make this release 2.0. Who am I? --------------- I started using HaskellDB in early 2009. I use it more as a query-generating library than for executing SQL statements. I have written a largish system for generating a data-access layer in PHP from query descriptions in HaskellDB. Yes, I'm using Haskell to write PHP ... dirty! Unmaintained & Unrelated Packages ----------------------------------------------------- A couple of haskelldb related packages on Hackage are not maintained by me: * haskelldb-hdbc-mysql - This project was created and uploaded independent of me. Contact the author for specifics. However, the haskelldb-hdbc-odbc library is maintained by me and can be used to talk to MySQL. * HSQL (haskelldb-hsql, etc) - I cannot get this backend to compile on my platform. HSQL seems unmaintained so I have let the drivers decay. You'll notice I have not uploaded new packages or changed their version dependencies. * haskelldb-flat and haskelldb-dynamic - Both of these packages have suffered serious bitrot and are not maintained any longer.

5 7

Inject cabal version or VCS version as a CPP macro
by Eugene Kirpichov 22 Feb '12

22 Feb '12

Hi, I'd like my program to print something like "this is $program 1.0.4 git 45fea6b" when invoked with --version, or at least just the 1.0.4 part. Can Cabal expose the version as a preprocessor macro by default, or do I have to use Build-Type: Custom and add a preprocessing step of my own? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

4 8

ANN: hp2html, a tool for viewing GHC heap-profiles
by Iavor Diatchki 22 Feb '12

22 Feb '12

Hello, I am happy to announce the availability of a little tool that I wrote while I was doing some Haskell profiling. It converts GHC's heap-profiles into HTML, and renders them nicely using the flot library. Its functionality is similar to `hp2ps`. I wrote it because I find the HTML output easier to work with and, also, because it can cope with partial profiles, so one can refresh the profile while the program is running. The tool is a very short Haskell program, so it should be quite easy to modify and improve (and there is a lot that can be improved in it! :-). You can use the tool like this: hp2html my_profile.hp This will generate a self-contained HTML file called `my_profile.html`. I am attaching some screen-shots so that you can get an idea of what it looks like. In the generated HTML, hovering over data points shows their labels in the top-right corner. You can zoom into a region by selecting it. To go back to the zoomed-out view, double-click anywhere on the picture. Note that, by default, the tool does not stack data points on top-of each other like `hp2ps`. This makes it easier to see the individual values for the different entries, but it makes it harder to see the total amount for the program. To get the stacked view, use the radio button in the top left corner. `hp2html` is available on Hackage: http://hackage.haskell.org/package/hp2html-0.1 If you are interesting in improving the tool (please do!), the source repo is here: git clone git://github.com/yav/hp2html.git I hope that you find it useful, -Iavor

2 5

Need Help.
by Manoj Chaudhari 22 Feb '12

22 Feb '12

Hi, We are looking for senior technical resources with skills in Haskell/Functional programming. Experience : 6 to 20 years, Job Location : Pune (India). Regards! Manoj

2 1

Second Monad.Reader #20 call for copy
by Edward Z. Yang 21 Feb '12

21 Feb '12

Second Call for Copy: The Monad.Reader - Issue 20 ------------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 20 will be: **Monday, March 5** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (ezyang(a)mit.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

1 0

podcast?
by serialhex 21 Feb '12

21 Feb '12

Does anybody know of any good haskell/fp podcasts out there? i dont know if my googling skillz are just failing me, but i can't seem to find anything. thanks all! hex -- * my blog is cooler than yours: http://serialhex.github.com * The wise man said: "Never argue with an idiot. They bring you down to their level and beat you with experience." * As a programmer, it is your job to put yourself out of business. What you do today can be automated tomorrow. ~Doug McIlroy No snowflake in an avalanche ever feels responsible. --- CFO: “What happens if we train people and they leave?” CTO: “What if we don’t and they stay?”

4 4

types and number of evaluation steps
by Heinrich Hördegen 21 Feb '12

21 Feb '12

Dear all, I have a question about evaluation with respect to types and currying. Consider this programm: import Debug.Trace -- add :: Integer -> Integer -> Integer add :: Int -> Int -> Int add x y = x + y f a b c = trace "b" (add x c) where x = trace "a" (add a b) main :: IO () main = do print (f 1 2 3) print (f 1 2 4) Compiled with ghc-7.0.3: $ ghc --make Main.hs -o main -O2 The function add has to types. When we use type Int -> Int -> Int, the programm produces "b a 6 b a 7" as output which shows that the x from the where clause in f is evaluated twice. However, when we use type Integer -> Integer -> Integer, this will give "b a 6 b 7" which shows that x is evaluated only once. This was rather unexpected to me. Why does the number of evaluation steps depend on a type? Can anybody explain this or give a hint? Thank you very much, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

6 11

(no subject)
by M. R 21 Feb '12

21 Feb '12

1 0

Conduit Error Output: Control.Monad.Trans.Resource.stateCleanup
by Lyndon Maydwell 21 Feb '12

21 Feb '12

Hi Michael, Café. I'm writing some code using the conduit library and am encountering the following error output (while the program appears to function correctly) when using Data.Conduit.Lazy. The error given is: > profile_simple_test_data: Control.Monad.Trans.Resource.stateCleanup: There is a bug in the implementation. The mutable state is being accessed after cleanup. Please contact the maintainers. A reduced code snippet that generates this error is (also attached): > import Control.Monad > import System.Environment > import Control.Monad.IO.Class (liftIO) > import System.IO > import Data.Conduit.Lazy > import Data.List (sort) > > import Data.Conduit > > import Prelude hiding (map) > > main = getArgs >>= process > > process args = mapM_ sorted args > > sorted x = runResourceT (lazyConsume $ sourceFeed x) >>= (mapM_ print . id) > > sourceFeed :: ResourceIO m => FilePath -> Source m String > sourceFeed file = sourceIO > (openFile file ReadMode) > hClose > (\h -> liftIO $ do > eof <- hIsEOF h > if eof > then return IOClosed > else fmap IOOpen $ hGetLine h) when run over any text file. I may be doing something inconsistent with the correct use of sourceIO or lazyConsume, however, I tried to follow the example at http://www.yesodweb.com/home/snoyberg/blogs/conduit/conduit/source/source.d… as closely as possible. Is this a bug, or simply an incorrect use of Conduit?

2 5

Streaming to JuicyPixels
by Myles C. Maxfield 21 Feb '12

21 Feb '12

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

2 2

Re: [Haskell-cafe] Some thoughts on Type-Directed Name Resolution
by Anthony Clayden 21 Feb '12

21 Feb '12

> On 1/02/2012, at 11:38 AM, AntC wrote: > > As soon as you decide to make 'virtual record selectors' > > just ordinary functions (so they select but not update) > > , then you can see that field names are also just > ordinary functions (for selection purposes). So the > > semantics for field 'selection' (whether or not you use > > dot notation) is just function application. So > Type-Directed Name resolution is just instance resolution. > > So it all gets much easier. > > > Richard O'Keefe wrote: > ... Making f x > and x.f the same is pretty appealing, but it is imaginable > that the former might require importing the name of f from > a module and the latter might not. That is to say, it lets > f and .f have completely different meanings. Oh the joy! > Oh the improved readability! -- on some other planet, > maybe. > Hi Richard, I'm not sure I understand what you're saying. I'm proposing x.f is _exactly_ f x. That is, the x.f gets desugared at an early phase in compilation. If the one needs importing some name from a module, than so does the other. A 'one-sided dot doesn't mean anything. (Also, I feel vaguely nauseous even seeing it written down.) Under my proposal, the only thing .f could mean is: \z -> z.f which desugars to \z -> f z which means (by eta-reduction) f And to complete the story: the only thing (x.) could mean is: \g -> x.g So a use like: (x.) f -- or z f, where z = (x.) would desugar to f x which is the same as x.f A use like (x.)f (no spaces around the parens) would amount to the same thing. This is all so weird I'm inclined to say that one-sided dot is probably a syntax error, and reject it. It's too dangerously ambiguous between the syntax for 'proper' dot notation and function composition. Or is there something I'm not understanding? [Good to see another NZ'er on the list, by the way.] AntC

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Simple GUI library to view JSON?
by dokondr 20 Feb '12

20 Feb '12

Please advise on a simple GUI library to display JSON data. A library that is easy to build both on Win, Linux and OsX. I need a scrollable view to show a list of JSON objects. Every object may contain other objects (recursively). List may have thousands of objects. Fields may have very long text values, so the view must also be scrollable in horizontal dimension. JSON object view should be click-able and look like on this example: { hey: "guy", anumber: 243, - anobject: { whoa: "nuts", - anarray: [ 1, 2, "thr<h1>ee" ], more: "stuff" }, awesome: true, bogus: false, meaning: null, link: "http://jsonview.com", } Where '-' before the field object indicates that object was expanded and '+' means collapsed object. Clicking on expanded fields should collapse them and vice verse. So for this example, clicking on 'anobject' should result in: { hey: "guy", anumber: 243, anobject: { ... } awesome: true, bogus: false, meaning: null, link: "http://jsonview.com", } In short I need a view similar to the one provided by JSONView plugin for Firefox: https://addons.mozilla.org/en-US/firefox/addon/jsonview/ Thanks a lot for any info, comments and ideas about this project!

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[ANNOUNCE] HaTeX 3.2
by Daniel Díaz Casanueva 20 Feb '12

20 Feb '12

Hello, A new version (3.2) of HaTeX has been released today to Hackage: http://hackage.haskell.org/package/HaTeX-3.2 HaTeX is the LaTeX language syntax implementation done in Haskell. Key changes in this version are: + LaTeX parser implemented (so new dependency on parsec). + Greek alphabet added to AMSMath module. + New LaTeX package implemented: graphicx. + Function 'documentclass' changed. Further information on this release: http://deltadiaz.blogspot.com/2012/02/hatex-chapter-32.html Library homepage: http://dhelta.net/hprojects/HaTeX Thanks for reading, Daniel Díaz.

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