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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.

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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/

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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 >>> >> >> >

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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

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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

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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.

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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/

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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

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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

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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.

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