March 2011 - Haskell-Cafe

Parallel Haskell Digest 1
by Eric Y. Kow 31 Mar '11

31 Mar '11

Parallel Haskell Digest ======================= Edition 1 2011-03-31 http://www.well-typed.com/blog/52 Hello Haskellers! If you're in the mood for a HWN chaser, I'd like to introduce the first Parallel Haskell Digest, a newsletter aiming to show off all the work that's going on using parallelism and concurrency in the Haskell community. The digest is a bit of an experiment. For the community in general, I hope to help you catch up with a monthly recap of news, interesting blog posts and discussions about parallelism in Haskell. For people (like me) who are new to parallelism and concurrency in Haskell, or maybe just have a passing interest, I hope to give you little nibbles, with regular features like the Word of Month, Featured Code and Parallel Puzzlers. Finally, as a bit of disclosure, I'm writing this digest as part of my work to promote the Parallel GHC Project. So don't be surprised if I give it a little special emphasis :-) Word of the Month ----------------- This very first edition of the PH digest is brought to you by the word /spark/. You may have heard of sparks and threads in the same sentence. What's the difference? A Haskell thread is a thread of execution for IO code. Multiple Haskell threads can execute IO code concurrently and they can communicate using shared mutable variables and channels. Sparks are specific to parallel Haskell. Abstractly, a spark is a pure computation which may be evaluated in parallel. Sparks are introduced with the par combinator; the expression (x `par` y) "sparks off" x, telling the runtime that it may evaluate the value of x in parallel to other work. Whether or not a spark is evaluated in parallel with other computations, or other Haskell IO threads, depends on what your hardware supports and on how your program is written. Sparks are put in a work queue and when a CPU core is idle, it can execute a spark by taking one from the work queue and evaluating it. On a multi-core machine, both threads and sparks can be used to achieve parallelism. Threads give you concurrent, non-deterministic parallelism, while sparks give you pure deterministic parallelism. Haskell threads are ideal for applications like network servers where you need to do lots of I/O and using concurrency fits the nature of the problem. Sparks are ideal for speeding up pure calculations where adding non-deterministic concurrency would just make things more complicated. Parallel GHC project news ---------------------------------------------------------------------- The Parallel GHC Project is an MSR-funded project to push the real-world use of parallel Haskell. Part of this project involves effort by Well-Typed to provide tools for use by the general community: Work shall soon begin working on making the "Modified Additive Lagged Fibonacci" and perhaps other random number generators from the SPRNG library available in Haskell. Current plans are to implement the algorithms directly in Haskell, and to expose them as instances of System.Random.RandomGen. These generators are attractive for use in Monte Carlo simulations because they are splittable and have good statistical quality, while providing high performance. Work is underway on extending the GHC EventLog and associated tools (ghc-events, ThreadScope). The aim is to support profiling of multi-process or distributed Haskell systems such as client/server or MPI programs. This involves incorporate some changes made in related projects (Eden, EdenTV). This work may have some benefits even for profiling single-process programs. It should also allow comparative profiling where multiple runs of the same program (e.g. different inputs or slightly different code) are viewed on the same timeline. For more information on the Parallel GHC project, see http://www.haskell.org/haskellwiki/Parallel_GHC_Project Featured Code ---------------------------------------------------------------------- The feature code for this month is hulk, an IRC server in 1250 lines of code. Hulk was coded up by Chris Done in one evening, and it was used the next morning. (Chris has done a bit of cleaning up since). Here's what Chris had to say about his experience writing Hulk: Haskell's lightweight threads make it natural (and guilt-free!) to choose the design of one-thread-per-client. With a single MVar containing a pure value for the whole server-state, and the help of a monad stack of ReaderT(connection information), WriterT (replies) and StateT (user details), it was trivial to make the bulk of the code completely pure. LineBuffering on the (Handle-wrapped) sockets tripped me up; as opposed to NoBuffering, this did not behave as expected: *many* threads would block when only *one* should have. Overall, it was textbook Haskell; keep the main code pure, and only the truly impure in the outer shell. It's up on hackage so you can install it with a quick cabal install hulk You can also fork his code on GitHub https://github.com/chrisdone/hulk Got a cool use of Parallel Haskell or an interesting problem? Nominate it for the PH digest featured code/parallel puzzler! Blog Posts ---------------------------------------------------------------------- 1. Parallelism is not Concurrency - Bob Harper https://existentialtype.wordpress.com/2011/03/17/parallelism-is-not-concurr… 2. Petri Net Concurrency - Edward Z. Yang http://blog.ezyang.com/2011/03/petri-net-concurrency Parallel-Haskell and Haskell Cafe ---------------------------------------------------------------------- 1. How does the Yesod web framework deal with concurrency? Ertugrul Soeylemez is using concurrent Haskell to serve up two related sites and spawn off some background workers would pose any problems with Yesod and WAI. No problem, according to Michael Snoyman http://www.haskell.org/pipermail/haskell-cafe/2011-January/088754.html 2. Concurrency best practices? Wren Ng Thorton is using STM to "run a lot of things in parallel without the headaches of locks." It works great, but then what if you want IO? How would you enforce atomic IO operations, eg. printing log messages in two threads without getting garbage on screen? Some suggestions were the twighlight-stm package, and using STM.TChan with potential performance penalty. http://www.haskell.org/pipermail/haskell-cafe/2011-February/088987.html 3. Possible bug in Control.Concurrent Krzysztof Skrzętnicki encountered an unexpected deadlock using Control.Concurrent. Was it a normal consequence of repeatedly reading from channel with only a single element put into it, or a bug? - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089135.html - http://hackage.haskell.org/trac/ghc/ticket/4154 4. Faster timeout but is it correct? Bas van Dijk proposed increasingly faster potential replacements for System.Timeout, including some that use the new GHC event manager. After much scrutiny, it turns out that one version is not faster and the events based one had a bug. Maybe next time! - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089360.html - http://hackage.haskell.org/trac/ghc/ticket/4963 5. Unable to get parallelism using `par`. SMP parallelism gains inferior than expected. C K Kashyap followed /A tutorial on Parallel and Concurrent programming in Haskell/ but couldn't get a speed-up. Others had better luck with different sets of inputs José Pedro Magalhães also tried using SMP parallelism without getting performance gains he was hoping for. Simon Marlow suggested analysing with Threadscope, checking for too many/few sparks, and checking to see if a lot of time was being spent in GC. - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089419.html - http://research.microsoft.com/en-us/um/people/.../parallel/AFP08-notes.pdf - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089635.html 6. Auto elimination of MVars using a monad or monad transformer. Chris Dew asked if GHC eliminates MVars during compilation, or if there was a way to eliminate them automatically. Ryan Ingram suggested looking into the Adaptive package and Functional Reactive Programming. http://www.haskell.org/pipermail/haskell-cafe/2011-February/089660.html Stack Overflow ---------------------------------------------------------------------- 1. Parallelism on divide & conquer algorithm (7 Feb) http://stackoverflow.com/questions/4920077/parallelism-on-divide-conquer-al… 2. How to best synchronize game engine and network server in Haskell? (11 Feb) http://stackoverflow.com/questions/4973401/how-to-best-synchronize-game-eng… 3. Haskell concurrency and Handles (27 Feb) http://stackoverflow.com/questions/5136375/haskell-concurrency-and-handles 4. Strict evaluation techniques for concurrent channels in Haskell (6 Mar) http://stackoverflow.com/questions/5211827/strict-evaluation-techniques-for… Help and feedback ---------------------------------------------------------------------- Got something for the digest? Send me an email at eric(a)well-typed.com. I'm particularly interested in short parallelism/concurrency puzzles, cool projects for featured code. Other comments and feedback (criticism and corrections especially!) would be welcome too. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

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Re: [Haskell-cafe] ANNOUNCE: enumerator 0.4.8
by John Lato 31 Mar '11

31 Mar '11

> > Message: 7 > Date: Tue, 29 Mar 2011 22:39:12 -0400 > From: wren ng thornton <wren(a)freegeek.org> > Subject: Re: [Haskell-cafe] ANNOUNCE: enumerator 0.4.8 > To: haskell-cafe(a)haskell.org > Message-ID: <4D9297D0.7060405(a)freegeek.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 3/29/11 4:40 AM, oleg(a)okmij.org wrote: > > Wren Thornton wrote: > >> This is often conflated with the iteratee throwing an error/exception, > >> which is wrong because we should distinguish between bad program > >> states and argument passing. > > > > I guess this is a matter of different points of view on exceptions. > > The problem is not so much the exceptions per se (one goto is about as > good as any other), it has more to do with the fact that important > things are being left out of the types. > > One of the great things about Haskell is that you can lean so heavily on > the type system to protect yourself when refactoring, designing by > contract, etc. However, if there's an unspoken code of communication > between specific enumerators and iteratees, it's very easy to break > things. This is why the communication should be captured in the types, > regardless of the control-flow mechanism used to implement that > communication. I'd like the static guarantee that whatever special > requests my iteratee could make, its enumerator is in a position to > fulfill those requests (or die trying). Allowing for the iteratee to be > paired with an enumerator which is incapable of handling its requests is > a type error and should be treated as such. > This has long been a goal of mine for iteratee (since before the 0.4 release), although I haven't really done any work on it. Maybe it's time to see if I can get a more satisfactory implementation. > > Wren Thornton wrote: > >> In an ideal framework the producers, transformers, and consumers of > >> stream data would have a type parameter indicating the up-stream > >> communication they support or require (in addition to the type > >> parameters for stream type, result type, and side-effect type). > > > > Very true. Currently the design of Iteratees quite resembles that of > > Control.Exception: everything can throw SomeException. Ideally one > > would like to be more precise, and specify what exceptions or sorts of > > exceptions could be thrown -- by Iteratees, and by ordinary Haskell > > functions. The design of a good effect system is still the topic of > > active research, although there are some encouraging results. > > Yeah, I'm not a big fan of extensible exceptions either. Don't get me > wrong, it's an awesome hack and it's far cleaner than the Java approach; > but it still goes against my sensibilities. > I've come around to the view that exceptions are a bad idea (in Haskell), and just making everything explicit (via Maybe, ErrorT, explicit-exceptions, or otherwise) is the best approach at present. John L.

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Re: [Haskell-cafe] Iteratee, ghc 6.12/7.0 strange behaviour - epollControl: permission denied (Operation not permitted)
by John Lato 31 Mar '11

31 Mar '11

> > > Message: 15 > Date: Mon, 28 Mar 2011 21:31:39 +0400 > From: Michael A Baikov <pacak(a)bk.ru> > Subject: [Haskell-cafe] Iteratee, ghc 6.12/7.0 strange behaviour - > epollControl: permission denied (Operation not permitted) > To: haskell-cafe(a)haskell.org > Message-ID: <E1Q4GHz-00077E-00.pacak-bk-ru(a)f146.mail.ru> > Content-Type: text/plain; charset=koi8-r > > > > I am still playing with lastest iteratee and i think i found something > strange. > > > let's suppose we have a file test.hs like this: > > import Data.Iteratee > import Data.Iteratee.IO > import Data.Iteratee.Char > > main = fileDriver printLines "/etc/passwd" > > It works fine when executed via runhaskell / ghci in ghc 6.12. Compiled > version in ghc 7 > also works, but when i am trying to execute it via runhaskell / ghci i am > getting this error: > > iter.hs: epollControl: permission denied (Operation not permitted) > > Any ideas? > Apologies, I won't have time to look at this for a few days. However, could you try a test using the functions from Data.Iteratee.IO.Posix directly? That would help narrow down the issue. I suspect it's either a bug in ghc's IO, or the semantics of some IO functions changed without my noticing. John L.

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cabal advice: library paths
by Michael Snoyman 31 Mar '11

31 Mar '11

Hi all, I have a package[1] which uses some system libraries (Qt to be precise). On Linux (and I'm hoping Mac), it's able to use pkg-config to determine which libraries are necessary and where to find all the files. On Windows, I've been less than successful using pkg-config. However, I *was* able to hardcode in the paths[2], and the resulting executables ran without a hitch. I still have two problems: * Obviously hardcoding paths in less than ideal. * Hackage won't allow me to upload this package anymore. It complains: 'extra-lib-dirs: c:\qt\2010.05\qt\lib' directory does not exist. Can anyone recommend a more elegant solution to the problem? Thanks, Michael [1] http://hackage.haskell.org/package/wai-handler-webkit [2] https://github.com/snoyberg/hack-handler-webkit/blob/master/wai-handler-web…

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object oriented technique
by Tad Doxsee 31 Mar '11

31 Mar '11

I've been trying to learn Haskell for a while now, and recently wanted to do something that's very common in the object oriented world, subtype polymorphism with a heterogeneous collection. It took me a while, but I found a solution that meets my needs. It's a combination of solutions that I saw on the web, but I've never seen it presented in a way that combines both in a short note. (I'm sure it's out there somewhere, but it's off the beaten path that I've been struggling along.) The related solutions are 1. section 3.6 of http://homepages.cwi.nl/~ralf/OOHaskell/paper.pdf 2. The GADT comment at the end of section 4 of http://www.haskell.org/haskellwiki/Heterogenous_collections I'm looking for comments on the practicality of the solution, and references to better explanations of, extensions to, or simpler alternatives for what I'm trying to achieve. Using the standard example, here's the code: data Rectangle = Rectangle { rx, ry, rw, rh :: Double } deriving (Eq, Show) drawRect :: Rectangle -> String drawRect r = "Rect (" ++ show (rx r) ++ ", " ++ show (ry r) ++ ") -- " ++ show (rw r) ++ " x " ++ show (rh r) data Circle = Circle {cx, cy, cr :: Double} deriving (Eq, Show) drawCirc :: Circle -> String drawCirc c = "Circ (" ++ show (cx c) ++ ", " ++ show (cy c)++ ") -- " ++ show (cr c) r1 = Rectangle 0 0 3 2 r2 = Rectangle 1 1 4 5 c1 = Circle 0 0 5 c2 = Circle 2 0 7 rs = [r1, r2] cs = [c1, c2] rDrawing = map drawRect rs cDrawing = map drawCirc cs -- shapes = rs ++ cs Of course, the last line won't compile because the standard Haskell list may contain only homogeneous types. What I wanted to do is create a list of circles and rectangles, put them in a list, and draw them. It was easy for me to find on the web and in books how to do that if I controlled all of the code. What wasn't immediately obvious to me was how to do that in a library that could be extended by others. The references noted previously suggest this solution: class ShapeC s where draw :: s -> String copyTo :: s -> Double -> Double -> s -- needs {-# LANGUAGE GADTs #-} data ShapeD where ShapeD :: ShapeC s => s -> ShapeD instance ShapeC ShapeD where draw (ShapeD s) = draw s copyTo (ShapeD s) x y = ShapeD (copyTo s x y) mkShape :: ShapeC s => s -> ShapeD mkShape s = ShapeD s instance ShapeC Rectangle where draw = drawRect copyTo (Rectangle _ _ rw rh) x y = Rectangle x y rw rh instance ShapeC Circle where draw = drawCirc copyTo (Circle _ _ r) x y = Circle x y r r1s = ShapeD r1 r2s = ShapeD r2 c1s = ShapeD c1 c2s = ShapeD c2 shapes1 = [r1s, r2s, c1s, c2s] drawing1 = map draw shapes1 shapes2 = map mkShape rs ++ map mkShape cs drawing2 = map draw shapes2 -- copy the shapes to the origin then draw them shapes3 = map (\s -> copyTo s 0 0) shapes2 drawing3 = map draw shapes3 Another user could create a list of shapes that included triangles by creating a ShapeC instance for his triangle and using mkShape to add it to a list of ShapeDs. Is the above the standard method in Haskell for creating an extensible heterogeneous list of "objects" that share a common interface? Are there better approaches? (I ran into a possible limitation to this approach that I plan to ask about later if I can't figure it out myself.) - Tad

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Haskell Weekly News: Issue 175
by Daniel Santa Cruz 30 Mar '11

30 Mar '11

Welcome to issue 175 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 20 - 26. You can find the HTML version at: http://bit.ly/gs9Rr8 Announcements Wren Ng Thornton [2]announced the release of unix-bytestring: Unix/Posix-specific functions for ByteStrengs. John Lato [3]announced an incremental update to the ListLike package. "This version adds newtype wrappers for strict and lazy ByteString, CharString, and CharStringLazy..." Mario Blazevic [4]announced the release of incremental-parser, which provides the usual set of Applicative and monad combinators with a few twists that make it unique. Duncan Coutts [5]announced the release of version 0.16.3 of c2hs, the FFI preprocessor tool. Jeff Wheeler [6]announced a new release of Yi, the text editor written in and extensible in Haskell. Christiaan Baaij [7]announced an incremental upgrade to 0.1.3.0 to CLaSH. Mario Blazevic [8]announced updates to monad-parallel, monad-coroutine, and SCC (Streaming Component Combinators). John Millikin [9]announced an update to enumerator to version 0.4.8. Quotes of the Week * Squarism: wo utilizing any tricks, i want 20 miljon rabbits to mate with each other once.. how many mate event will there be * mm_freak: using combinators instead of explicit recursion can be justified with about the same reasoning as using 'while' instead of explicit 'goto' in imperative programming * monochrom: <$> is pronounced Jacquelin von Brionée * geheimdienst: what does a pirate say when starting a sentence, then suddenly being hit in the nadgers? arr- ow! * monochrom: 1985 is way more than a decade ago * copumpkin: matlab is horrible as far as language design goes[.] it's fairly convenient for doing stuff in but I need to shower after every time I use it Top Reddit Stories * Debugging trick From (self.haskell), scored 60 with 11 comments. Read on [10]reddit. * Your friendly Haskell JSON library's encoding performance just got 4x faster From (serpentine.com) scored 45 with 2 comments. Read on [12]reddit. Read the [13]original post. * A monad for deterministic parallelism (ICFP'2011 submission) From (community.haskell.org) scored 41 with 8 comments. Read on [14]reddit. Read the [15]original post. * Haskell servers benchmarked against node/ruby/python/php/java. From (docs.yesodweb.com) scored 36 with 12 comments. Read on [16]reddit. Read the [17]original post. * The Yesod Framework and the Case of the Eight Level Monad Transformer Stack From (docs.yesodweb.com) scored 35 with 16 comments. Read on [18]reddit. Read the [19]original post. * Improving Persistent Performance (Yesod Blog) From (docs.yesodweb.com) scored 29 with 1 comments. Read on [20]reddit. Read the [21]original post. * Fast, clean Haskell bindings to Google's Snappy library From (hackage.haskell.org) scored 22 with 8 comments. Read on [22]reddit. Read the [23]original post. * Open Quark on Github (Haskell like Lang on the JVM) From (github.com) scored 15 with 3 comments. Read on [24]reddit. Read the [25]original post. * EclipseFP 2.0.4 released: supports GHC 7 From (jpmoresmau.blogspot.com) scored 15 with 7 comments. Read on [26]reddit. Read the [27]original post. * Demonstrating a Time Leak in Arrowized FRP From (blog.edwardamsden.com) scored 14 with 12 comments. Read on [28]reddit. Read the [29]original post. Top StackOverflow Answers * [30]Reading large file in haskell ? votes: 19 The construct "seq x x" is always useless. If y = seq x x and I force y then this forces x then returns x. This is equivalent to y=x and forcing y. Thus "seq forceEval forceEval" does nothing more than "forceEval". The error with your use of a fold is a common one. You are using a fold to perform a count of the bytes in the input. You should be using ... * [31]What to call a function that splits lists? votes: 12 I believe the function you're describing is breakBefore from the list-grouping package. Data.List.Grouping: http://hackage.haskell.org/packages/archive/list-grouping/0.1.1/doc/html/Da… st-Grouping.html ghci:: breakBefore even [3,1,4,1,5,9,2,6,5,3,5,8,9,7,9,3,2,3,8,4,6,2,6] [[3,1],[4,1,5,9],[2],[6,5,3,5],[8,9,7,9,3],[2,3],[8],[4],[6],[2],[6]] Top StackOverflow Questions * [32]Haskell record syntax votes: 16, answers: 1 * [33]Reading large file in haskell? votes: 12, answers: 1 * [34]Haskell laziness - how do I force the IO to happen sooner? votes: 9, answers: 3 * [35]case on monadic value votes: 7, answers: 2 * [36]Simplest way to find which data types are most garbage collected? votes: 7, answers: 2 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15484 3. http://article.gmane.org/gmane.comp.lang.haskell.general/18590 4. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15492 5. http://article.gmane.org/gmane.comp.programming.c2hs/247 6. http://article.gmane.org/gmane.editors.yi.devel/5346 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18592 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87540 9. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15506 10. http://www.reddit.com/r/haskell/comments/gbd7w/debugging_trick/ 12. http://www.reddit.com/r/haskell/comments/g8s0t/your_friendly_haskell_json_l… 13. http://www.serpentine.com/blog/2011/03/22/a-new-week-a-new-json-performance… 14. http://www.reddit.com/r/haskell/comments/gainp/a_monad_for_deterministic_pa… 15. http://community.haskell.org/~simonmar/papers/monad-par.pdf 16. http://www.reddit.com/r/haskell/comments/g7uia/haskell_servers_benchmarked_… 17. http://docs.yesodweb.com/blog/preliminary-warp-cross-language-benchmarks 18. http://www.reddit.com/r/haskell/comments/ga40w/the_yesod_framework_and_the_… 19. http://docs.yesodweb.com/blog/hamlet-lucius-widgets 20. http://www.reddit.com/r/haskell/comments/gbz43/improving_persistent_perform… 21. http://docs.yesodweb.com/blog/improving-persistent-performance 22. http://www.reddit.com/r/haskell/comments/ga080/fast_clean_haskell_bindings_… 23. http://hackage.haskell.org/package/snappy 24. http://www.reddit.com/r/haskell/comments/g7vcj/open_quark_on_github_haskell… 25. https://github.com/levans/Open-Quark 26. http://www.reddit.com/r/haskell/comments/g8du0/eclipsefp_204_released_suppo… 27. http://jpmoresmau.blogspot.com/2011/03/eclipsefp-204-released-supports-ghc-… 28. http://www.reddit.com/r/haskell/comments/g943x/demonstrating_a_time_leak_in… 29. http://blog.edwardamsden.com/2011/03/demonstrating-time-leak-in-arrowized.h… 30. http://stackoverflow.com/questions/5410777/reading-large-file-in-haskell/54… 31. http://stackoverflow.com/questions/5438558/what-to-call-a-function-that-spl… 32. http://stackoverflow.com/questions/5367167/haskell-record-syntax 33. http://stackoverflow.com/questions/5410777/reading-large-file-in-haskell 34. http://stackoverflow.com/questions/5373883/haskell-laziness-how-do-i-force-… 35. http://stackoverflow.com/questions/5395517/case-on-monadic-value 36. http://stackoverflow.com/questions/5410345/simplest-way-to-find-which-data-…

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[ghci] unknown symbol in base-unicode-symbols-0.2.1.2 ...
by John Obbele 30 Mar '11

30 Mar '11

Hi everyone ! I'm having a problem in GHCi when loading modules relying on the base-unicode-symbols package. My prompt gives me the following message: >> ghci $ GHCi, version 7.0.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m +System.USB Prelude System.USB> newCtx Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package deepseq-1.1.0.2 ... linking ... done. Loading package bytestring-0.9.1.10 ... linking ... done. Loading package text-0.11.0.5 ... linking ... done. Loading package base-unicode-symbols-0.2.1.2 ... linking ... <interactive>: /home/john/.cabal/lib/base-unicode-symbols-0.2.1.2/ghc-7.0.2/HSbase-unicode-symbols-0.2.1.2.o: unknown symbol `__stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziCategoryziUnicode_' ghc: unable to load package `base-unicode-symbols-0.2.1.2' Prelude System.USB> I thought this was due to me screwing around cabal/ghc-pkg caches but after a fresh install of Fedora15-alpha / cabal update / cabal install usb I'm still getting the same error. Moreover, a quick `nm HSbase-unicode-symbols-*.o | grep ControlziCategoryziUnicode` give me the following output. >> nm HSbase-unicode-symbols-0.2.1.2.o | grep -C 4 'ControlziCategoryziUnicode_' 0000000000000070 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziApplicativeziUnicode 0000000000000040 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziApplicativeziUnicode_ 0000000000000120 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziArrowziUnicode 00000000000000d8 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziArrowziUnicode_ U __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziCategoryziUnicode_ 00000000000001b8 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziMonadziUnicode 0000000000000188 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziMonadziUnicode_ 0000000000000250 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_DataziBoolziUnicode 0000000000000220 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_DataziBoolziUnicode_ Could someone please explain to me what's happening ? regards, /John

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Accessing to a Haskell type in C
by Yves Parès 30 Mar '11

30 Mar '11

Hello, To access a haskell type from C code, do we have to make this type instance of Storable (and so to also define an homomorph structure on the C side), or does FFI specifies some C function to enable C code to access the fields, or even some GHC-specific functions? (Even if I'd prefer a portable way) I mean, if I have the type: data Character = Character { pos :: (Float, Float), size :: Float, sprite :: Image } Do I compulsorily have to double its definition on C side: stuct CCharacter { float x; float y; float size; Image sprite; } and then make Character instance of Storable, which will make the program *copy* Characters to CCharacters each time that I must call a C function to, say, draw a character?

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Re: [Haskell-cafe] ArrowLoop and streamprocessors
by Patai Gergely 30 Mar '11

30 Mar '11

> So loop really doesn't seem to help here, but I couldn't find another > way either to feed outputs back into the system. > What I need is: > Either A B ~> Either C B -> A ~> C > > Does such a thing exist? At this point you don't really have enough structure to define such a feedback loop. Since you have streams of different rates, you need to find some common ground to merge them. The path taken by Reactive is to assign a timestamp to events, and ensure that merging preserves time ordering. Of course you'll immediately run into the problem of simultaneous occurrences, and it depends on the context how you want to resolve them. Also, what you're trying to achieve looks similar to Peakachu, so you might want to check it out, especially the Program abstraction [1]. It even has a feedback combinator with a slightly different signature. Gergely [1] http://hackage.haskell.org/packages/archive/peakachu/0.3.0/doc/html/FRP-Pea… -- http://www.fastmail.fm - mmm... Fastmail...

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CURL and upload a file
by Evgeny Dzhurinsky 30 Mar '11

30 Mar '11

Hello! I am trying to find out the way of uploading a file using HTTP multipart/form-data. I created simple application aimed to upload an image to imgpaste.com. The source is below: ========================================================= module ImgPaste.Upload where import Control.Monad import Network.Curl import Data.ByteString import Text.Regex.PCRE import Text.Regex.PCRE.ByteString type LocalCtx = ( ByteString, ByteString, ByteString, [ByteString] ) type UploadResult = Either UploadError ByteString data UploadError = UploadError { message :: String, response :: ByteString } deriving (Show) extractResponse :: CurlResponse_ [(String,String)] ByteString -> ByteString extractResponse = respBody uploadFile :: String -> IO (ByteString) uploadFile fileName = initialize >>= withCurlDo . (flip uploadFileWithCurl fileName) uploadFileWithCurl :: Curl -> String -> IO ( ByteString ) uploadFileWithCurl curl fileName = do liftM extractResponse $ do_curl_ curl "http://imgpaste.com/" [CurlPost True, CurlHttpPost postData, CurlVerbose True ] where postData = [ HttpPost "upfile" Nothing ( ContentFile fileName ) [] Nothing, makeFormPost "submit" "Upload", makeFormPost "keep" "a" ] makeFormPost name value = HttpPost name Nothing (ContentString value) [] Nothing -- <input type="text" name="copyfield" size="31" value="http://imgpaste.com/tmp/123456.png" /> extractUrl :: ByteString -> UploadResult extractUrl src = match (src =~ "<input type=\"text\" name=\"copyfield\" size=\"31\" value=\"([^\"]+?)\" />" :: LocalCtx) where match (_,_,_,[x]) = Right x match _ = Left $ UploadError "Can not parse content." src pasteImage :: String -> IO ( UploadResult ) pasteImage = liftM extractUrl . uploadFile ========================================================= however when invoking the function: pasteImage "image.png" no data is posted to the server. The protocol dump looks like: ========================================================= POST / HTTP/1.1 Host: imgpaste.com Accept: */* Content-Length: 241 Expect: 100-continue Content-Type: multipart/form-data; boundary=----------------------------630d154aaf3b ========================================================= so the request data is not sent. Can somebody please advice, what am I doing in a wrong way? Thank you in advance! -- regards Eugene Dzhurinsky

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