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ANNOUNCE: swish 0.3.0.0
by Doug Burke 07 Apr '11

07 Apr '11

I am pleased to announce an update of the swish package to version 0.3.0.0 [1]. It is mainly meant to be a maintenance upgrade over the existing version on Hackage [2] by Vasili I. Galchin, but does add additional functionality in that it now supports the NTriples format and the N3 parser and formatter has been updated to better match the current specification. For those not aware of what Swish actually is, it is an experiment by Graham Klyne in writing a "Semantic Web" framework in Haskell [3]. I note that the wonderful machinery behind Hackage has already processed the documentation so you can read more about it at [1] as well as the very-sparsely-documented Wiki at [4]. [1] http://hackage.haskell.org/package/swish-0.3.0.0 [2] http://hackage.haskell.org/package/swish-0.2.1 [3] http://www.ninebynine.org/Software/swish-0.2.1.html [4] https://bitbucket.org/doug_burke/swish/wiki/Home Thanks for reading this far, Doug

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ANNOUNCE: DocTest-0.2.0
by Simon Hengel 07 Apr '11

07 Apr '11

I just uploaded a new version of DocTest[1] to Hackage. WHAT IS DocTest? ================ DocTest is a port of Python's doctest[2] to Haskell. It can be used to verify, that examples in Haddock comments[3] do still work. This also provides you with a simple mechanism to write unit test, without the burden of maintaining a dedicated test suite. A basic example of usage is at [4]. WHAT'S NEW IN THIS VERSION? =========================== Support for blank lines in the result of an expression (analogous to the <BLANKLINE>-feature of Python's doctest). Here is an example of usage: -- | -- Some example: -- -- >>> putStrLn "foo\n\nbar" -- foo -- <BLANKLINE> -- bar Currently this is implemented in DocTest, but we will move it to Haddock with the next release. Cheers, Simon [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/DocTest [2] http://docs.python.org/library/doctest.html [3] http://www.haskell.org/haddock/doc/html/ch03s08.html#id566093 [4] http://haskell.org/haskellwiki/DocTest _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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mtlx has a nice design but is slow
by Sean Leather 07 Apr '11

07 Apr '11

I just refactored my type and transform system prototype (introduced in [1] but changed since then) from using mtlx [2] (type-indexed monad transformers described in [3]) to mtl using RWST. mtlx allowed me to cleanly separate the various monadic components in a convenient way. Unfortunately, I found it to be too slow. The refactoring was an experiment to see how slow. I was rather surprised: Running time of a compiled main with a list of tests: mtlx (7 transformers): 2 min 52 sec mtl (RWST): 0 min 13 sec It's frustrating to see such a huge performance gap for a better design. Regards, Sean [1] http://splonderzoek.blogspot.com/2011/03/draft-type-changing-program-improv… [2] http://hackage.haskell.org/package/mtlx [3] http://www.ittc.ku.edu/~marks/cgi-bin/pubs/monadfactory.pdf

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Haskell Weekly News: Issue 176
by Daniel Santa Cruz 07 Apr '11

07 Apr '11

Welcome to issue 176 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 27 to April 02, 2011. You can find the HTML version at: http://bit.ly/fEdQka Announcements Wren Ng Thornton [2]announced the release of version 0.3.4 of unix-bytestring. Ian Lynagh and the GHC team [3]announced version 7.0.3 of GHC, containing a handful of bugfixes. Jose A. Ortega Ruiz [4]annouced the release of version 0.13 of xmobar. Might want to check it out specially if you are using xmonad! Jason Dagit [5]annouced release 0.2.0 of the nehe-tuts package, tutorials for using OpenGL. David Sorokin [6]annouced the release of his new library aivika: a multi-paradigm simulation library. Tom Hawkins [7]annouced version 0.3.1 of ImProve: an imperative DSL for hard realtime embedded applications. Eric Y. Kow [8]introduced the first issue of Parallel Haskell Digest, "a newsletter aiming to show off all the work that's going on using parallelism and concurrency in the Haskell community." Here is to hoping we get more of these! George Giorgidze [9]announced version 0.5.5 of DSH (Database Supported Haskell). Jasper Van der Jeugt [10]annouced the 7th Ghent Functional Programming Group meeting on Tuesday, April 26, in celebration of the group's first year of existance. The meetup will be at the Technicum building of Ghent University at 19:30. Quotes of the Week * lispy: [in response to "C++ is a huge language, the type system is complex"] real and imaginary components :) the imaginary bits are the type safety * gwern: amazing `catch` (how sweet the type), that saved an exception like me! I once was thrown, but now am caught; was impure, but now pure be * hpc: appEndo sounds like a harry potter spell * monochrom: yeah, get out of Turing tarpit, provable termination, only to get into Godel tarpit * xplat: to me, the great thing about pointfree programming is you don't have to let variable names get in the way of your logic. and the great thing about pointful programming is you don't have to let '(flip .) . (.) . (flip flip . join)' get in the way of your logic. it's nice to have multiple tools in your toolbox * lispy: I've never looked at the gtk2hs source. I'm not sure I can muster the intestinal fortitude * kmc: time to play "transfinite ordinal or asian emoticon" * killing-joke: rip oop. "Object-oriented programming is eliminated entirely from the introductory curriculum, because it is both anti-modular and anti-parallel by its very nature, and hence unsuitable for a modern CS curriculum." Top Reddit Stories * Promo code to get Learn You a Haskell 40% off + e- book for free + swag From (learnyouahaskell.com) scored 68 with 25 comments. Read on [11]reddit. Read the [12]original post. * Real Time Edge Detection in Parallel Haskell From (disciple-devel.blogspot.com) scored 39 with 9 comments. Read on [13]reddit. Read the [14]original post. * Modern OpenGL with Haskell From (arcadianvisions.com) scored 39 with 10 comments. Read on [15]reddit. Read the [16]original post. * GHC migration to Git complete From (haskell.org) scored 37 with 15 comments. Read on [17]reddit. Read the [18]original post. * Haskell is 21 years old today! From (twitter.com) scored 37 with 8 comments. Read on [19]reddit. Read the [20]original post. * Re-implementing XMonad's core in Coq : Wouter Swiestra :: PDF From (cs.ru.nl), scored 34 with 13 comments. Read on [21]reddit. Read the [22]original post. * GHC 7.0.3 released From (haskell.org) scored 33 with 9 comments. Read on [23]reddit. Read the [24]original post. * "Reactive-banana and the essence of FRP". I implemented a small FRP library that can be mixed freely with imperative code. From (apfelmus.nfshost.com) scored 30 with 7 comments. Read on [25]reddit. Read the [26]original post. * New release of OpenGL NeHe tutorials in Haskell From (self.haskell), scored 27 with 22 comments. Read on [27]reddit. * Do you read academic papers on a Kindle? From (dmwit.com) scored 26 with 27 comments. Read on [28]reddit. Read the [29]original post. Top StackOverflow Questions * [30]Implementing lazy functional languages votes: 19, answers: 3 * [31]Is it possible to roll your own syntax sugar (like do-notation, or arrow-notation) in Haskell? votes: 12, answers: 2 * [32]Are there any LL Parser Generators for Functional Languages such as Haskell or Scala? votes: 11, answers: 4 * [33]Programming language for functional parallelism: F# vs Haskell votes: 10, answers: 6 * [34]easy "undo" in functional data structures votes: 8, answers: 2 About the Haskell Weekly News If you have any comments/thoughts on the newsletter, don't hesitate to contact me at dstcruz(a)gmail.com. Happy coding! Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15521 3. http://article.gmane.org/gmane.comp.lang.haskell.general/18607 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87583 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87584 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18612 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87628 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87709 9. http://article.gmane.org/gmane.comp.lang.haskell.general/18635 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87752 11. http://www.reddit.com/r/haskell/comments/ge10y/promo_code_to_get_learn_you_… 12. http://learnyouahaskell.com/ 13. http://www.reddit.com/r/haskell/comments/gdudz/real_time_edge_detection_in_… 14. http://disciple-devel.blogspot.com/2011/03/real-time-edge-detection-in-hask… 15. http://www.reddit.com/r/haskell/comments/gepfm/modern_opengl_with_haskell/ 16. http://www.arcadianvisions.com/blog/?p=224 17. http://www.reddit.com/r/haskell/comments/ggu2r/ghc_migrati%C3%B6n_t%C3%B6_g… 18. http://www.haskell.org/pipermail/cvs-ghc/2011-April/060798.html 19. http://www.reddit.com/r/haskell/comments/ggwv3/haskell_is_21_years_old_toda… 20. http://twitter.com/donsbot/status/54026417721188352 21. http://www.reddit.com/r/haskell/comments/gf3sb/reimplementing_xmonads_core_… 22. http://www.cs.ru.nl/~wouters/Talks/BrouwerExtraction.pdf 23. http://www.reddit.com/r/haskell/comments/gcmsz/ghc_703_released/ 24. http://www.haskell.org/pipermail/haskell/2011-March/022673.html 25. http://www.reddit.com/r/haskell/comments/gd46d/reactivebanana_and_the_essen… 26. http://apfelmus.nfshost.com/blog/2011/03/28-essence-frp.html 27. http://www.reddit.com/r/haskell/comments/gcsi1/new_release_of_opengl_nehe_t… 28. http://www.reddit.com/r/haskell/comments/gfz5k/do_you_read_academic_papers_… 29. http://dmwit.com/pdfsplit 30. http://stackoverflow.com/questions/5451645/implementing-lazy-functional-lan… 31. http://stackoverflow.com/questions/5450003/is-it-possible-to-roll-your-own-… 32. http://stackoverflow.com/questions/5507665/are-there-any-ll-parser-generato… 33. http://stackoverflow.com/questions/5492930/programming-language-for-functio… 34. http://stackoverflow.com/questions/5447054/easy-undo-in-functional-data-str…

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Re: [Haskell-cafe] mtlx has a nice design but is slow
by Mark Snyder 07 Apr '11

07 Apr '11

I'm the author for mtlx, and admittedly I didn't do anything about efficiency when I implemented it. Is there any initial place I ought to look for tips on inlining? I'll start with the pragmas page (http://www.haskell.org/ghc/docs/7.0.3/html/users_guide/pragmas.html) I suppose. I'm happy for any suggestions--optimizing hasn't been one of my focuses, so it's time for me to learn I suppose! I did switch to newtype definitions in the most recent version (0.1.5), which probably helped a lot compared to data definitions, but there are no inline pragmas in use, so perhaps there are some good opportunities for optimization. ~Mark Snyder > >Is the package missing some obvious inlining in the instances? > >> I just refactored my type and transform system prototype (introduced in [1] >> but changed since then) from using mtlx [2] (type-indexed monad transformers >> described in [3]) to mtl using RWST. mtlx allowed me to cleanly separate the >> various monadic components in a convenient way. Unfortunately, I found it to >> be too slow. The refactoring was an experiment to see how slow. I was rather >> surprised: >> >> Running time of a compiled main with a list of tests: >> ? mtlx (7 transformers): 2 min 52 sec >> ? mtl (RWST): 0 min 13 sec >> >> It's frustrating to see such a huge performance gap for a better design. >> >> Regards, >> Sean >>

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Re: [Haskell-cafe] How to contact OpenGL package maintainer (where is Sven?)
by Jason Dagit 06 Apr '11

06 Apr '11

No response yet from Sven after about a month and no one seems to have heard from him in over a year. I'm going to take over for now under the assumption that Sven is missing. == My plans for moving forward == * Assemble an opengl taskforce, a few people have already mentioned an interest in being on the team * clean up the current cabal files (I already wrote patches for that) * put the repos on github to make team collaboration easier * add the RULES that Andy Gill suggested for realToFrac * look at adding instances for MArray so that GLfloat et al can be stored in IOUArrays * add support for opengl 4.x * look at adding deprecation pragmas for deprecated opengl calls * new hackage releases * anything else that comes up Thanks, Jason On Sun, Mar 27, 2011 at 2:11 PM, Jason Dagit <dagitj(a)gmail.com> wrote: > I sent the message below to Haskell-Cafe about a week ago. I got one > response saying that Sven has disappeared in the past but reappeared when > updates were necessary. I still haven't heard from Sven. Now I'm widening > my search. My original email to Sven was on March 11th. > > It looks like the OpenGL packages on hackage[1,2,3,4] have not been > updated in some time. No updates later than Oct 2009. I tried to email > Sven directly using the email address listed on hackage but after over two > weeks I still haven't heard from him. I sent some patches to the opengl list > about a week ago but I noticed that Sven hasn't posted on that list since > Oct 2009 when he released the current version of OpenGLRaw. None of the > public darcs repos have patches newer than Oct 2009. Also, the homepage url > listed in the packages is a 404: > http://www.haskell.org/HOpenGL/ > > My concern is that Sven has disappeared for some reason. I hope he's well. > He has always done top notch work in the past maintaining these libraries. > Perhaps he's simply busy or lost interest? > > Does anyone know if he was looking for a new maintainer? Perhaps you've > heard from him more recently than Oct 2009? > > If a new maintainer is needed, I would consider nominating myself :) > > Thanks, > Jason > > [1] http://hackage.haskell.org/package/OpenGLRaw > [2] http://hackage.haskell.org/package/OpenGL > [3] http://hackage.haskell.org/package/GLURaw > [4] http://hackage.haskell.org/package/GLUT >

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ANN: Hakyll 3.1
by Jasper Van der Jeugt 06 Apr '11

06 Apr '11

Hello all, I've just uploaded Hakyll 3.1.0.0 [1] to Hackage. It contains some backwards-incompatible changes (but they are really for the greater good). The most important change is the matching done in the rules DSL. What used to be written as: route "posts/*" someRoute compile "posts/*" someCompiler Is now written as: match "posts/*" $ do route someRoute compile someCompiler This new style is way more expressive, as you can define custom predicates such as: match (predicate (\i -> matches "foo/*" i && not (matches "foo/bar" i))) $ do ... For more information, see the Pattern documentation [2]. Another slight change is that the `Hakyll.Web.Tags` [3] module now uses an association list instead of a `Data.Map` -- in case you want to change the order in which tags are displayed. If you have any questions or trouble migrating, feel free to drop a mail to me or here [4] or visit us in #hakyll on Freenode. [1]: http://hackage.haskell.org/package/hakyll [2]: http://jaspervdj.be/hakyll/reference/Hakyll-Core-Identifier-Pattern.html [3]: http://jaspervdj.be/hakyll/reference/Hakyll-Web-Tags.html [4]: https://groups.google.com/forum/#!forum/hakyll Cheers, Jasper

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ncurses trouble
by Andrew Pennebaker 06 Apr '11

06 Apr '11

I'm having trouble using Haskell and ncurses Mac OS X (10.6.6 to be precise). GHC 6.12.3 gives me a lot of trouble, and GHC 7.0.2 even more. rogue.hs, is a text adventure game in the works. $ cat rogue.hs {-# LANGUAGE OverloadedStrings #-} import Data.Text import UI.NCurses main :: IO () main = runCurses $ do win <- defaultWindow updateWindow win $ do moveCursor 10 10 drawText "Hello world! Hit 'q' to exit" render waitForQ win waitForQ :: Window -> Curses () waitForQ win = do ev <- getEvent win Nothing case ev of Just (EventCharacter 'q') -> return () _ -> waitForQ win $ sudo port install ncurses ncursesw $ cabal install ncurses Resolving dependencies... Configuring ncurses-0.2... cabal: c2hs version >=0.15 is required but it could not be found. cabal: Error: some packages failed to install: ncurses-0.2 failed during the configure step. The exception was: ExitFailure 1 Hmm. Cabal couldn't find a solution to the c2hs dependency. So I install it manually. $ cabal install c2hs $ cabal install ncurses Resolving dependencies... Configuring ncurses-0.2... cabal: Missing dependency on a foreign library: * Missing C library: ncursesw This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: ncurses-0.2 failed during the configure step. The exception was: ExitFailure 1 MacPorts' ncurses isn't detected by Haskell, so I use Homebrew's. $ brew install ncursesw $ ghc --make -o rogue rogue.hs [2 of 2] Compiling Main ( rogue.hs, rogue.o ) Linking rogue ... ld: warning: in /usr/local/lib/libncursesw.dylib, file was built for unsupported file format which is not the architecture being linked (i386) It appears that the Homebrew ncurses library is x86, and GHC is x86_64, or vice-versa. $ sudo port install ncurses +universal $ sudo port install ncursesw +universal $ brew install --universal ncursesw These don't visibly improve compilation in any way. I thought updating Haskell would fix the problem. $ wget http://lambda.galois.com/hp-tmp/2011.2.0.0/Haskell%20Platform%202011.2.0.0-… $ sudo installer -pkg "Haskell Platform 2011.2.0.0-i386.pkg" -target / $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 $ cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library But it only made the problem worse. Now ncurses won't even build. $ cabal install ncurses Resolving dependencies... Configuring ncurses-0.2... Preprocessing library ncurses-0.2... Building ncurses-0.2... [1 of 4] Compiling UI.NCurses.Enums ( dist/build/UI/NCurses/Enums.hs, dist/build/UI/NCurses/Enums.o ) [2 of 4] Compiling UI.NCurses.Types ( UI/NCurses/Types.hs, dist/build/UI/NCurses/Types.o ) [3 of 4] Compiling UI.NCurses ( dist/build/UI/NCurses.hs, dist/build/UI/NCurses.o ) [4 of 4] Compiling UI.NCurses.Panel ( dist/build/UI/NCurses/Panel.hs, dist/build/UI/NCurses/Panel.o ) UI/NCurses/Panel.chs:49:1: Warning: The import of `UI.NCurses' is redundant except perhaps to import instances from `UI.NCurses' To import instances alone, use: import UI.NCurses() hsncurses-shim.c:5:29: error: ncursesw/curses.h: No such file or directory cabal: Error: some packages failed to install: ncurses-0.2 failed during the building phase. The exception was: ExitFailure 1 I'd settle for using hscurses, but it won't compile with GHC 7, and it has its own internal bugs besides. $ cabal install hscurses Resolving dependencies... Configuring hscurses-1.4.0.0... configure: WARNING: unrecognized options: --with-gcc checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for addnstr in -lcurses... yes checking for addnstr in -lncurses... yes checking for waddnwstr in -lncursesw... yes checking for iconv in -liconv... yes checking for libiconv in -liconv... yes checking for libiconv... yes checking how to run the C preprocessor... gcc -E checking for grep that handles long lines and -e... /usr/bin/grep checking for egrep... /usr/bin/grep -E checking for ANSI C header files... yes checking for sys/types.h... yes checking for sys/stat.h... yes checking for stdlib.h... yes checking for string.h... yes checking for memory.h... yes checking for strings.h... yes checking for inttypes.h... yes checking for stdint.h... yes checking for unistd.h... yes checking iconv.h usability... yes checking iconv.h presence... yes checking for iconv.h... yes checking curses.h usability... yes checking curses.h presence... yes checking for curses.h... yes checking ncurses.h usability... yes checking ncurses.h presence... yes checking for ncurses.h... yes checking ncursesw/ncurses.h usability... no checking ncursesw/ncurses.h presence... yes configure: WARNING: ncursesw/ncurses.h: present but cannot be compiled configure: WARNING: ncursesw/ncurses.h: check for missing prerequisite headers? configure: WARNING: ncursesw/ncurses.h: see the Autoconf documentation configure: WARNING: ncursesw/ncurses.h: section "Present But Cannot Be Compiled" configure: WARNING: ncursesw/ncurses.h: proceeding with the preprocessor's result configure: WARNING: ncursesw/ncurses.h: in the future, the compiler will take precedence checking for ncursesw/ncurses.h... yes checking locale.h usability... yes checking locale.h presence... yes checking for locale.h... yes checking langinfo.h usability... yes checking langinfo.h presence... yes checking for langinfo.h... yes checking wchar.h usability... yes checking wchar.h presence... yes checking for wchar.h... yes checking limits.h usability... yes checking limits.h presence... yes checking for limits.h... yes checking signal.h usability... yes checking signal.h presence... yes checking for signal.h... yes configure: creating ./config.status config.status: creating hscurses.buildinfo config.status: creating cbits/config.h configure: WARNING: unrecognized options: --with-gcc cabal: Missing dependency on a foreign library: * Missing (or bad) header file: HSCurses.h This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. If the header file does exist, it may contain errors that are caught by the C compiler at the preprocessing stage. In this case you can re-run configure with the verbosity flag -v3 to see the error messages. cabal: Error: some packages failed to install: hscurses-1.4.0.0 failed during the configure step. The exception was: ExitFailure 1 Cheers, Andrew Pennebaker www.yellosoft.us

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Using _ on the RHS of an equation?
by Jason Dagit 06 Apr '11

06 Apr '11

I think I got this idea from JHC, but I'm unable to find it documented in the JHC manual at the moment. The idea is that when _ appears on the RHS of an equation it is another name for undefined. Here is a hypothetical example, inspired by code I've seen in FFI definitions: \begin{code} {-# LANGUAGE EmptyDataDecls #-} data Bar instance Storable Bar where sizeOf _ = sizeOf #{size struct Bar} alignment _ = alignment (_ :: CDouble) -- here we could have just as succinctly said alignment (1 :: CDouble) data Foo -- the struct Foo on the C side contains a Bar instance Storable Foo where sizeOf _ = sizeOf #{size struct Foo} alignment _ = alignment (_ :: Bar) -- we can't instantiate a Bar \end{code} Is this something people would be interested in having as an extension in GHC? Or is it just too fluffy for anyone to really care? Thanks, Jason

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Re: [Haskell-cafe] ANN: stm-chans: Additional types of channels for STM.
by wren ng thornton 06 Apr '11

06 Apr '11

On 4/3/11 11:58 PM, Thomas DuBuisson wrote: > Wren, > Glad to see someone is doing a more complete packaging of STM helpers > and derivatives! > > I've done a little work on bounded TChans[1] (hackage "bounded-tchan" > package) and I think you should consider a few things: Ah, somehow I missed that in my searching. I'll give it a look and try to integrate things. > 1) Split the reader counter and writer counters as I've done in > bounded-tchan. This gives 2-5 times better performance (I benchmarked > using Criterion, a single reader, a single writer, and tested channels > with bounds of 10, 100, and 1000 elements). That's crazy. I wonder why... must be a contention issue... > 2) Implement a "tryWriteTBChan" for obvious reasons. Good idea. > 4) Help me figure out a good way to build dupTBChan [2]. It's not easy. I've yet to figure out a way of doing this that has the right semantics. Perhaps splitting the reader and writer counters will help, though it sounds like you haven't had much luck either. > [2] http://stackoverflow.com/questions/5446484/how-to-add-a-finalizer-on-a-tvar Have you taken a look at TwilightSTM? http://proglang.informatik.uni-freiburg.de/projects/syncstm/ http://hackage.haskell.org/package/twilight-stm Since I'm aiming to have stm-chans eventually incorporated into stm proper, that wouldn't work for me, though it sounds like it might solve the ideological question about adding finalizers to TVars -- Live well, ~wren

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Problems porting ghc6-6.10.1 to GNU/Hurd using 6.8.2
by Svante Signell 05 Apr '11

05 Apr '11

Hi, I'm currently trying to upgrade Debian GNU/Hurd ghc6.8.2 to 6.10.1 (and further from there, due to requirements of ghc and haddock versions), and get stuck on configuring the random-1.0.0.1 library in stage1. I have tried versions of ghc6 are 6.10.1+dfsg1-1 to 6.10.1+dfsg1-5 (the latest not requiring ghc6.10 due to haddock 2.4.2). The commands are shown below: (added the -verbose=3 parameter) cd ghc6-6.10.1+dfsg1/libraries (sorry for the missing \ due to cut-and-paste to the mailer) ( cd random && /home/srs/DEBs/ghc/ghc6-6.10.1+dfsg1/libraries/cabal-bin --verbose=3 /usr/bin/ghc6 /home/srs/DEBs/ghc/ghc6-6.10.1 +dfsg1/libraries/bootstrapping.conf configure --verbose=3 --prefix=/NONEXISTENT --bindir=/NONEXISTENT --libdir=/NONEXISTENT --libexecdir=/NONEXISTENT --datadir=/NONEXISTENT --docdir=/NONEXISTENT --haddockdir=/NONEXISTENT --htmldir=/NONEXISTENT --with-compiler=/home/srs/DEBs/ghc/ghc6-6.10.1 +dfsg1/ghc/stage1-inplace/ghc --with-hc-pkg=/home/srs/DEBs/ghc/ghc6-6.10.1 +dfsg1/utils/ghc-pkg/install-inplace/bin/ghc-pkg --libsubdir='$pkgid' --with-gcc=gcc --with-ld=/usr/bin/ld --with-hscolour=/usr/bin/HsColour --with-alex=/usr/bin/alex --with-happy=/usr/bin/happy --configure-option='--build' --configure-option='i486-gnu' --configure-option='--prefix=/usr' --configure-option='--with-ghc=/usr/bin/ghc6' --configure-option='build_alias=i486-gnu' --configure-option='CFLAGS=-g -O2' --configure-option='LDFLAGS=' --configure-option='CPPFLAGS=' --configure-option=--with-cc=gcc --with-hsc2hs=/home/srs/DEBs/ghc/ghc6-6.10.1 +dfsg1/utils/hsc2hs/install-inplace/bin/hsc2hs --haddock-options="--use-contents=../index.html --use-index=../doc-index.html" --enable-library-profiling --enable-split-objs ) && touch stamp/configure.library.build-profiling-splitting.random || touch random/unbuildable Output: ======= Creating dist (and its parents) ("/home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/ghc/stage1-inplace/ghc",["--numeric-version"]) /home/srs/DEBs/ghc/ghc6-6.10.1+dfsg1/ghc/stage1-inplace/ghc is version 6.10.1 ("/home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/utils/ghc-pkg/install-inplace/bin/ghc-pkg",["--version"]) /home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/utils/ghc-pkg/install-inplace/bin/ghc-pkg is version 6.10.1 ("/home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/ghc/stage1-inplace/ghc",["--supported-languages"]) Reading installed packages... ("/home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/utils/ghc-pkg/install-inplace/bin/ghc-pkg",["dump","--global"]) Hangs here! Running manually works OK!! /home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/utils/ghc-pkg/install-inplace/bin/ghc-pkg "dump" "--global" Gives a lot of output here... Below is the ghc-pkg script: cat /home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/utils/ghc-pkg/install-inplace/bin/ghc-pkg PKGCONF=/home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/inplace-datadir/./package.conf exec /home/srs/DEBs/ghc/ghc6-6.10.1\ +dfsg1/utils/ghc-pkg/install-inplace/libexec/ghc-pkg --global-conf\ $PKGCONF ${1+"$@"} I've tried changing the shell from dash to bash but no changes. I also tried to escape some of the variables in the gkc-pkg shell script but without any luck. Anybody can help me here??

2 2

Encoding of Haskell source files
by Roel van Dijk 05 Apr '11

05 Apr '11

Hello, The Haskell 2010 language specification states that "Haskell uses the Unicode character set" [1]. I interpret this as saying that, at the lowest level, a Haskell program is a sequence of Unicode code points. The standard doesn't say how such a sequence should be encoded. You can argue that the encoding of source files is not part of the language. But I think it would be highly practical to standardise on an encoding scheme. Strictly speaking it is not possible to reliably exchange Haskell source files on the byte level. If I download some package from hackage I can't tell how the source files are encoded from just looking at the files. I propose a few solutions: A - Choose a single encoding for all source files. This is wat GHC does: "GHC assumes that source files are ASCII or UTF-8 only, other encodings are not recognised" [2]. UTF-8 seems like a good candidate for such an encoding. B - Specify encoding in the source files. Start each source file with a special comment specifying the encoding used in that file. See Python for an example of this mechanism in practice [3]. It would be nice to use already existing facilities to specify the encoding, for example: {-# ENCODING <encoding name> #-} An interesting idea in the Python PEP is to also allow a form recognised by most text editors: # -*- coding: <encoding name> -*- C - Option B + Default encoding Like B, but also choose a default encoding in case no specific encoding is specified. I would further like to propose to specify the encoding of haskell source files in the language standard. Encoding of source files belongs somewhere between a language specification and specific implementations. But the language standard seems to be the most practical place. This is not an official proposal. I am just interested in what the Haskell community has to say about this. Regards, Roel [1] - http://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-150002.1 [2] - http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/separate-compil… [3] - http://www.python.org/dev/peps/pep-0263/

17 28

Re: [Haskell-cafe] Using _ on the RHS of an equation?
by Paul Keir 05 Apr '11

05 Apr '11

Hi Jason, I like the idea. I've seen some code from Oleg Kiselyov which uses __ (two underscores) in this way. The thing that stops me though, is when I get it wrong, and undefined fires somewhere, but I don't know where. Something like you propose, but with a line number, would be sweet. Paul The University of Glasgow, charity number SC004401

3 2

broken on build encoding
by Gang Yu 05 Apr '11

05 Apr '11

Hi, cafe, I always get the following build failure when I try to install encoding: Configuring encoding-0.6.4... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package haskell98-1.0.1.1 requires random-1.0.0.2 package HaXml-1.20.2 requires random-1.0.0.2 package random-1.0.0.2 requires time-1.1.4 package random-1.0.0.2 requires time-1.2.0.4 Preprocessing library encoding-0.6.4... Building encoding-0.6.4... Data/Encoding/ByteSource.hs:124:0: warning: extra tokens at end of #ifndef directive Data/Encoding/ByteSink.hs:104:0: warning: extra tokens at end of #ifndef directive [ 1 of 65] Compiling Data.Static ( Data/Static.hs, dist/build/Data/Static.o ) [ 2 of 65] Compiling Data.Array.Static ( Data/Array/Static.hs, dist/build/Data/Array/Static.o ) Data/Array/Static.hs:14:35: Warning: Bindings containing unlifted types must use an outermost bang pattern: { (I# ri) = index (s, e) i } *** This will be an error in GHC 6.14! Fix your code now! In the expression: let (I# ri) = index (s, e) i in extract addr ri In the definition of `!': ! (StaticArray s e addr) i = let (I# ri) = index ... i in extract addr ri [ 3 of 65] Compiling Data.Map.Static ( Data/Map/Static.hs, dist/build/Data/Map/Static.o ) [ 4 of 65] Compiling Control.Throws ( Control/Throws.hs, dist/build/Control/Throws.o ) [ 5 of 65] Compiling Data.Encoding.Exception ( Data/Encoding/Exception.hs, dist/build/Data/Encoding/Exception.o ) [ 6 of 65] Compiling Data.Encoding.ByteSource ( Data/Encoding/ByteSource.hs, dist/build/Data/Encoding/ByteSource.o ) Data/Encoding/ByteSource.hs:151:17: Not in scope: data constructor `State' cabal: Error: some packages failed to install: encoding-0.6.4 failed during the building phase. The exception was: ExitFailure 1https://mail.google.com/mail/?shva=1#compose I manually installed a ghc-6.12.3, a ubuntu ghc-6.12.1, all the same? anybody can provide helps? thanks Gang

3 7

ANN: Google Summer of Code student application period opens today
by Johan Tibell 05 Apr '11

05 Apr '11

Hi, The Google Summer of Code student application period starts today at 19:00 UTC. If you're a student and like to get paid to work on a Haskell project this summer I recommend you go find an interesting project [1] and start working on your application. You can find more information on the wiki [2]. Cheers, Johan 1. http://hackage.haskell.org/trac/summer-of-code/report/1 2. http://hackage.haskell.org/trac/summer-of-code/wiki/Soc2011

4 5

lhs2tex and comment
by Mitar 05 Apr '11

05 Apr '11

Hi! I have problems with lhs2tex and nested (long) comments. It formats it all in one long line and does not wrap it. Is there a way to define those comments so that it would maybe automatically wrap them at the end of the page? (And still take into the account possible indentation of comments.) Or at least to not ignore newlines in comments? I am using poly mode. Mitar

1 0

why is ghci trying to load hsc file ??
by briand＠aracnet.com 04 Apr '11

04 Apr '11

Howdy, I worked out a small hdf5 binding using cabal and bindings-DSL and sqlite3 as my example. Time to try it ! ghci -idist/build/ dist/build/Bindings/HDF5.o -lhdf5 -lhdf5_hl hdf5_pkg_test.hs GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading object (static) dist/build/Bindings/HDF5.o ... done Loading object (dynamic) hdf5 ... done Loading object (dynamic) hdf5_hl ... done final link ... done [1 of 2] Compiling Bindings.HDF5 ( dist/build/Bindings/HDF5.hs, interpreted ) src/Bindings/HDF5.hsc:49:8: parse error on input `import' Failed, modules loaded: none. Huh ? Why is it trying to read HDF5.hsc ?? What's even more interesting is that line 49 of that file doesn't have an import on it, so something is fubar. No idea how this could be happening. I've included a copy of my cabal file. BTW. I have to specify the hdf5 libraries, i.e. libhdf5 and libhdf5_hl on the command line. It seems like the build process should have taken care of that in some way, maybe... ? Certainly when I use something like sqlite3, I'm not specifying libsqlite3 on the command line. Thanks, Brian

7 11

Fast conversion between Vector Double and Vector CDouble
by Bas van Dijk 04 Apr '11

04 Apr '11

Hello, I just rewrote my levmar library[1] to use Vectors (from the vector package[2]) instead of lists. I was expecting to see a significant performance improvement. Unfortunately I only saw a 10% improvement. However, I noticed I had a lot of conversions from Vector Double to Vector CDouble and visa versa in my code: import Data.Vector.Storable ( Vector ) mapRealToFrac ∷ (Storable α, Storable β, Real α, Fractional β) ⇒ Vector α → Vector β mapRealToFrac = VS.map realToFrac When I replace this with: mapRealToFrac = unsafeCoerce My application computes the same result but does it 28 times faster! My question are: 1) Is this always safe? In other words: are the runtime representations of Double and CDouble always equivalent or do they vary between platforms? 2) Can the same improvement be accomplished using RULE pragma's? 3) Are there disadvantages of using CDouble instead of Double in the levmar API? For some reason it feels wrong to use CDouble in the API but I don't have a good argument against it yet. Thanks, Bas [1] http://hackage.haskell.org/package/levmar [2] http://hackage.haskell.org/package/vector

4 4

ANN: stm-chans: Additional types of channels for STM.
by wren ng thornton 04 Apr '11

04 Apr '11

-------------------------------------------- -- stm-chans 1.0.0 -------------------------------------------- The stm-chans package offers a collection of channel types, similar to Control.Concurrent.STM.TChan but with additional features. In particular it offers these types: * Control.Concurrent.STM.TBChan: Bounded FIFO channels. When the channel is full, writers will block/retry. This ensures that the writers do not get too far ahead of the readers, which helps to make sure that memory and cpu resources are used responsibly. * Control.Concurrent.STM.TMChan: Closeable FIFO channels. This is like TChan (Maybe a) but with a monotonicity guarantee that once Nothing is returned all future reads will be Nothing as well. * Control.Concurrent.STM.TBMChan: Bounded Closeable FIFO channels. This combines the capabilities of TBChan and TMChan. In addition, the stm-chans package offers a (partial) compatibility layer for some API improvements still making their way into the stm package[1]. These new functions include: * tryReadTChan :: TChan a -> STM (Maybe a) A version of readTChan which does not retry. Instead it returns Nothing if no value is available. * peekTChan :: TChan a -> STM a Get the next value from the TChan without removing it, retrying if the channel is empty. * tryPeekTChan :: TChan a -> STM (Maybe a) A version of peekTChan which does not retry. Instead it returns Nothing if no value is available. [1] http://article.gmane.org/gmane.comp.lang.haskell.libraries/15507 -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/stm-chans Darcs: http://community.haskell.org/~wren/stm-chans Haddock (Darcs version): http://community.haskell.org/~wren/stm-chans/dist/doc/html/stm-chans -- Live well, ~wren

3 3

Ann: contstuff, dnscache, ihttp, ismtp, netlines, yesod-tableview
by Ertugrul Soeylemez 04 Apr '11

04 Apr '11

Hello fellow Haskellers, (once again posted to the cafe, because the Haskell mailing list rejects my mails.) I have released quite a few libraries over time on Hackage. I feel they are stable enough now to announce them here. * contstuff: Fast and flexible monad transformers based on the CPS concept. Mainly it helps getting rid of large transformer stacks, because it unifies some monad transformers (like StateT and ContT), and it is also faster than the naive approach. This library is comparable to a combination of the transformers package with Edward Kmett's monad-ran package, but without hiding the CPS transformation. In early benchmarks ChoiceT turned out to be twice as fast as normal Haskell lists, and it is also a proper list monad transformer. Finally in the Control.ContStuff.Classes module you find lots of useful utility classes, which seem to be unique to contstuff. * dnscache: Handy DNS caching library. This library automatically manages a set of resolver threads for you, which share a reply cache. This allows very fast mass-resolution. The library also comes with a handy command line utility called 'massdns' for quick resolution of even very large lists of entities. Call massdns without arguments to get usage help. * ihttp: This is an HTTP library based on enumerators giving you maximum flexibility and control over the connection. Using proper iteratees you can simply implement e.g. proxy clients (my main use case), but also HTTP servers. Using proper enumeratees you can also encapsulate the connection in an SSL/TLS layer, but unfortunately there doesn't seem to be such an SSL/TLS implementation yet. * ismtp: This is an advanced ESMTP library, which allows you to structure your sessions as you like. It also features an automated resolver for MX records using the dnscache library. Right now there is support for most of the base functionalities of the protocol as specified in RFC 821 (SMTP) and RFC 1425 (service extensions). An update to RFC 2821 is planned, but is not necessary for practical purposes. The new RFC mainly removes some unneeded features and unifies the two mentioned RFCs. Right now there is no support for SMTL (SASL) authentication, but it is planned for a near future release. For the time being you can implement your own iteratees for this purpose, if you depend on authentication. * netlines: This is a library for writing implementations of text-based protocols. In particular it allows reading lines safely from untrusted sources with a maximum length in constant space. * yesod-tableview: For web applications using Michael Snoyman's Yesod web framework this library implements an easy to use table renderer mainly for database records. It is in an early stage of development right now, but as the need arises, I will extend it. To use the networking libraries, you should be familiar with John Millikin's 'enumerator' package. If you're not, I recommend studying it, because it is a useful library for all kinds of stream processing like network connections, files and concurrent channels. All mentioned libraries have been tested extensively for correctness and safety. Especially the networking libraries have undergone a lot of penetration testing. However, I'm only one developer, so I would be glad to hear about any vulnerabilities and other shortcomings you find. Usually I have a very short response time to bugs in these libraries, so please don't hesitate to contact me. Feature requests are also welcome, of course. =) Please note that major version numbers specify interface versions in my libraries. In other words, a new major version of a package usually means that the API has changed in a way, which is likely to break dependent packages. I would like to use this opportunity to thank a few people in particular (alphabetically): - Cale Gibbard, - Edward Kmett, - John Millikin, - Bryan O'Sullivan and - Michael Snoyman. I appreciate your efforts both in libraries and support. Many thanks, guys! Thanks also to the rest of the innovative and helpful Haskell community. Greets, Ertugrul -- Key-ID: E5DD8D11 "Ertugrul Soeylemez <es(a)ertes.de>" FPrint: 0F12 0912 DFC8 2FC5 E2B8 A23E 6BAC 998E CE40 2012 Keysrv: hkp://subkeys.pgp.net/ -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

2 1

ANNOUNCE: FreeType 2 Bindings
by Jason Dagit 04 Apr '11

04 Apr '11

I'm excited to announce the immediate availability of FreeType 2 bindings on hackage. These bindings are based off the 2.4.4 release of FreeType 2 and use the BSD licensing option for FreeType 2. This package should be of interest to OpenGL users as it provides a way to get nice looking fonts into your OpenGL programs. This binding is still in its infancy, hence version 0.0.1, but enough of the API is exposed that I was able to convert the example1.c program in the FreeType tutorial to Haskell. Here are a few relevant links: * What is FreeType: http://www.freetype.org/freetype2/index.html * Example program and tutorial: * http://www.freetype.org/freetype2/docs/tutorial/step1.html * http://www.freetype.org/freetype2/docs/tutorial/example1.c * Haskell version of tutorial: * https://github.com/dagit/freetype2/blob/cabalization/Main.hs * Example program output (yes, their tutorial example really does draw the bitmap as a grid of text, scroll around to find the word "blah"), more interesting examples to come later: * http://hpaste.org/45239/aoeu The current API level is "raw" and makes no attempts to present a good Haskell-like interface to the library. My plan is to build a higher level interface on top once a sufficient portion of the API is available to Haskell. You can find the source on github in the cabalization branch: * https://github.com/dagit/freetype2 Patches welcome! Thanks, Jason

2 2

llvm package on Windows
by Jacques Carette 04 Apr '11

04 Apr '11

I am trying to install llvm-0.9.0.1 on Windows (through cygwin), on top of a clean install of Haskell Platform 2010.2.0.0. I do cabal install llvm --configure-option --with-llvm-prefix="C:\\llvm" (which is where I unpacked llvm-gcc4.2-2.8-x86-mingw43.tar.bz2). This fails with .....mingw\bin\ld.exe: cannot find -lLLVMXCoreAsmPrinter (which is indeed the first -l option on the gcc line). [I also tried the llvm-ht package, same issue; I can't tell which of the 2 packages is 'best' from the online doc] Perhaps I need to 'install' llvm more thoroughly? [Most of the instructions online seem to assume you want to use llvm-gcc, which is not my case] Any clues? Jacques

2 1

Improvements to crypto-API: CTR CMAC and SIV modes
by klondike 04 Apr '11

04 Apr '11

Hi, The attached files provide some preliminary support for the CTR, CMAC and SIV modes of operation on the crypto-api for both strict and lazy bytestrings. My main worries here are that the implementations may be vulnerable to timing attacks, and that the code in CPolys may be subject to copyright restrictions since it may be a derivative work from "Table of Low-Weight Binary Irreducible Polynomials" by Gadiel Seroussi (since all I did was apply transformations to the original PDF to get the data). As a side note, if I have time I'll improve the dbl and shift operations so they can work directly over the bytestrings which may be faster than converting and back. I'll also try both parallelizing the implementation of the counter mode and make it work with transformations other than adding one.

1 0

Haskell programs as specifications
by Patrick Browne 03 Apr '11

03 Apr '11

Hi, Attached are two programs that represent one-way and two-way traffic on a road. I can get the programs to produce reasonable results that match our intuitive ideas of roads. However, I have 2 question about the programs: 1)I cannot get the result suggested by the author that t1 should give true in prog1 and false in prog2. 2)My second question is more theoretical. It is stated by the author that type checking and excitability provide verification. In this case verification probably means checking that an instance is consistent with its type class. Does verification using these techniques in Haskell have any firmer logical foundation than say doing the verification in Java? I am aware that Haskell uses inference for type checking, but is the net result superior to compilers that do not use inference? Also, is Haskell execution based purely on logic? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

3 4

fundeps => type family
by Tad Doxsee 03 Apr '11

03 Apr '11

Hi All, Last week I asked a question with the subject "object oriented technique". I got a lot very helpful answers and I thank all who contributed. At the end of my question, I alluded to some problems that I was having with what I wanted to do next, which was to add additional polymorphism. I figured out a solution, which is: ------------------------------------------------- {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-} data Rectangle a = MkRectangle { rx, ry, rw, rh :: a } deriving (Eq, Show) drawRect :: Show a => Rectangle a -> String drawRect r = "Rect (" ++ show (rx r) ++ ", " ++ show (ry r) ++ ") -- " ++ show (rw r) ++ " x " ++ show (rh r) data Circle a = MkCircle {cx, cy, cr :: a} deriving (Eq, Show) drawCirc :: Show a => Circle a -> String drawCirc c = "Circ (" ++ show (cx c) ++ ", " ++ show (cy c)++ ") -- " ++ show (cr c) r1 = MkRectangle 0 0 3 2 r2 = MkRectangle 1 1 4 5 c1 = MkCircle 0 0 5 c2 = MkCircle 2 0 7 class ShapeC a s | s -> a where draw :: s -> String copyTo :: s -> a -> a -> s {- -- GADT version data ShapeD a where MkShapeD :: ShapeC a s => s -> ShapeD a -} -- Existential Quantification version data ShapeD a = forall s . ShapeC a s => MkShapeD s instance ShapeC a (ShapeD a) where draw (MkShapeD s) = draw s copyTo (MkShapeD s) x y = MkShapeD (copyTo s x y) mkShape :: ShapeC a s => s -> ShapeD a mkShape s = MkShapeD s instance Show a => ShapeC a (Rectangle a) where draw = drawRect copyTo (MkRectangle _ _ rw rh) x y = MkRectangle x y rw rh instance Show a => ShapeC a (Circle a) where draw = drawCirc copyTo (MkCircle _ _ r) x y = MkCircle x y r r1s = MkShapeD r1 r2s = MkShapeD r2 c1s = MkShapeD c1 c2s = MkShapeD 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 ------------------------------------------------- The main difference with my previous version is that the above is polymorphic in the type for the origin and dimensions. (I used a Double previously.) Also, the above version uses existential quantification instead of GADTs, only because some said that that method is more "standard". I had to use functional dependencies in the defintion of class ShapeC to get it to compile and run. Ed Yang's excellent post (http://blog.ezyang.com/2011/03/type-tech-tree) says that type families are equivalent to multiparameter type classes + functional dependencies, so I tried to rewrite the above using type families, but I got stuck. Also the GHC documentaton (http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/type-families.h…) says that "Equality constraints ... enable a simple translation of programs using functional dependencies into programs using family synonyms instead. So I tried: class (T s ~ a) => ShapeC a s where type T s :: * draw :: s -> String copyTo :: s -> T s -> T s -> s but got a compile error: Alas, GHC 7.0 still cannot handle equality superclasses: T s ~ a So my question is, how does one convert the above code to use type families instead of functional dependencies? Is one technique preferable over another? Thanks, Tad

2 2

[ANNOUNCE] Data-flow based graph algorithms
by Mitar 03 Apr '11

03 Apr '11

Hi! Based on my Etage data-flow framework: http://hackage.haskell.org/package/Etage I have made a package to show how to implement graph algorithms on top of it: http://hackage.haskell.org/package/Etage-Graph I invite everybody to take a look and see how it is possible to implement known control-flow algorithms in a data-flow manner. For now, only shortest paths search is implemented (from all to all nodes). The nice feature is that such approach is easily parallelized. It quite heavily uses Haskell threads so it is also useful to benchmark and test them, especially for graphs with many nodes (> 300). There is documentation, but because of this bug it is not visible: http://hackage.haskell.org/trac/hackage/ticket/656 Mitar

1 0

Unboxing CFloat and GLfloat
by Jason Dagit 03 Apr '11

03 Apr '11

First a bit of background. Sometimes I would like to be able to create unboxed arrays of CFloats and GLfloats. Both of these are defined as newtypes around types that can be unboxed (e.g., Float). I'm assuming GHC here. As far as I can tell, the newtyping is there to help programmers pick a representation that matches the C compiler's representation on their platform without having to use CPP in their Haskell code. Would it be possible to add instances like these? instance MArray (STUArray s) GLfloat (ST s) instance MArray IOUArray GLfloat IO instance MArray (STUArray s) CFloat (ST s) instance MArray IOUArray CFloat IO I would also want instances for the other C types that map to Haskell types that are unboxed and the other GL types too. For example, in my program I imported all the right libraries and added this instance as an experiment: instance MArray (STUArray s) GLfloat (ST s) where {-# INLINE getBounds #-} getBounds (STUArray l u _ _) = return (l,u) {-# INLINE getNumElements #-} getNumElements (STUArray _ _ n _) = return n {-# INLINE unsafeNewArray_ #-} unsafeNewArray_ (l,u) = unsafeNewArraySTUArray_ (l,u) fLOAT_SCALE {-# INLINE newArray_ #-} newArray_ arrBounds = newArray arrBounds 0 {-# INLINE unsafeRead #-} unsafeRead (STUArray _ _ _ marr#) (I# i#) = ST $ \s1# -> case readFloatArray# marr# i# s1# of { (# s2#, e# #) -> (# s2#, unsafeCoerce (F# e#) #) } {-# INLINE unsafeWrite #-} unsafeWrite (STUArray _ _ _ marr#) (I# i#) e = ST $ \s1# -> let F# e# = unsafeCoerce e in case writeFloatArray# marr# i# e# s1# of { s2# -> (# s2#, () #) } I had to use an unsafeCoerce because the constructor for GLfloat is not exposed, and realToFrac has terrible performance because the following rules (suggested by Andy Gill) are missing from the OpenGL bindings: {-# RULES "realToFrac/a->GLfloat" realToFrac = \x -> GLfloat (realToFrac x) #-} {-# RULES "realToFrac/GLfloat->a" realToFrac = \(GLfloat x) -> realToFrac x #-} Even if I had those rules, I would still need to get access to the underlying representation (the e#) to have the right kind. GLfloat is defined in terms of CFloat which is defined in terms of other things. On my machine, CFloat happens to correspond to Float, although in general this need not be the case. So to do this instance correctly I would need to know, in general, what the underlying type is for CFloat. I think that means I would need to use the same macros as GHC, although I don't think that requires this instance to appear in base. I could just borrow the macros once I figure out the corresponding instance for CFloat. While the GLfloat instances don't need to be in base, the CFloat instance probably should be in base. Is this something I should make a library proposal for and submit patches? Is there an easier way to get these instances? Has someone already done this? Thanks, Jason

2 1

Call for contributions: Second edition of the Typeclassopedia
by Brent Yorgey 03 Apr '11

03 Apr '11

Hi all, It's time for a second edition of the Typeclassopedia [1], and I'd love to have your suggestions and contributions. You can send me suggestions and darcs patches [2] by email. For more detailed information, see my blog post [3]. -Brent [1] http://www.haskell.org/wikiupload/8/85/TMR-Issue13.pdf [2] http://code.haskell.org/~byorgey/typeclassopedia/ [3] http://byorgey.wordpress.com/2011/04/03/call-for-contributions-second-editi…

1 0

error to Fail Conversion
by Anwar Bari 03 Apr '11

03 Apr '11

HI Cafe I have problem with this function value :: Theorm-> [(String,Int)] -> Tem -> Int value _ env (Var s) = maybe (error "Unknown variable.") id (lookup s env) value mdl env (OP s _ _ l) = maybe (error "Function not defined.") id (lookup (map (value mdl env) l) (S.toList $ maybe (error "Uknown function.") id (fmap snd (multiLookup s (operations mdl))))) This function is working fine, but I want to change error function to fail function to be able use it in my GUI application. I can't change the output of value function to IO Int,any idea will help. Thanks Nour.

2 1

Can't connect to local yackage server
by Yitzchak Gale 03 Apr '11

03 Apr '11

On Mac OS X 10.6.7, after upgrading to yackage 0.1.0.1: After running yackage -l, I cannot connect to it even directly from localhost. I get the message: This Yackage server only talks to local clients On a Linux server it seems to work fine though. On the Mac, when I do "telnet localhost" I get: Trying ::1... Could it be that yackage 0.1.0.1 doesn't recognize the IPv6 notion of localhost? Thanks, Yitz

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7th Ghent Functional Programming Group meeting on Tuesday, April 26
by Jasper Van der Jeugt 03 Apr '11

03 Apr '11

Dear all, We are very glad to announce an exciting program for the 7th meeting of the Ghent Functional Programming Group, especially since we are celebrating our first year of existence today. Our program features no less than three interesting functional programming languages: Erlang, Haskell and Scheme. The meeting will take place on Tuesday, April 26 in the Technicum building of Ghent University (Sint-Pietersnieuwstraat 41, 9000 Gent) at 19h30. As before, to enter the building, you should go to the automatic sliding door on the far left of the building and dial the phone number that is provided on the note taped to the door. Someone will then open the door for you. Our program is as follows: 1. Tom Van Custem - Experiments with MapReduce in Erlang MapReduce is a programming model for large data processing popularized by, and in daily use at Google. The MapReduce model builds strongly on key tenets of functional programming such as higher-order functions and side-effect free execution. In this talk, we summarize this programming model and describe a didactic implementation in Erlang. Invented at Ericsson's research labs, Erlang is known for its massively concurrent programming model, and itself builds on a functional core language. The talk will not focus on Erlang as such, but we will describe its key features as needed to understand the MapReduce abstraction. 2. Tom Schrijvers - How you could have won the VPW 2011 contest with Haskell We all know that Functional Programming is great for writing concise solutions for programming problems. With some skill this can even be done quickly! Yet, there was little evidence of this at the 3rd edition of the Flemish Programming Contest (VPW 2011) that took place on March 23. Not so before the contest: The jury stress-tested all questions by writing various solutions in different languages. Haskell was used to solve most problems and invariably produced short solutions. In this talk I present my own Haskell solutions to several of this year's problems and discuss alternative solution strategies with the audience. After the talk you will be all set for winning next year's edition -- or at least enjoying it -- using Haskell. 3. Pieter Audenaert - Functional Geometry and a Graphical Language We will discuss a simple language for drawing images. During the presentation we will illustrate the power of data abstraction and algebraic closure, meanwhile using higher order procedures in an essential manner. The language has been designed to easy experimenting with patterns such as those appearing in typical M.C. Escher drawings where the artist repeats the pattern both moving it across the drawing and scaling it when applicable. In the language we use procedures to represent the data objects that will be combined in the final drawing and we make sure that all operations conducted on these procedures are algebraically closed. These features allow generating patterns of any complexity. For our implementation, we use the LISP functional programming language -- more accurately, the Scheme dialect. The presentation is based on "Structure and Interpretation of Computer Programs", Abelson & Sussman Hope to see you there! The GhentFPG organizing committee, Andy Georges Jeroen Janssen Jasper Van der Jeugt

2 2

using IO monad in Iteratee
by Dmitry Olshansky 03 Apr '11

03 Apr '11

Dear collegues, I use Iteratee with Monad Transformer stack in this way: type MyMonad a b = Iteratee a (StateT StateType IO) b I've wrote some Enumeratees using this type. Then I want to compose it with standard enumerator like Data.Enumerator.Binary.enumFile. But enumFile use IO monad instead of MonadIO class. I didn't see (maybe I'm blind...) a way to compose my Enumeratees with enumFile. How I can do that? Is it possible to change enumFile to using MonadIO class? Best regards, Dmitry

5 6

pool, persistent, persistent-sqlite: Space leak
by Ertugrul Soeylemez 03 Apr '11

03 Apr '11

Hello Michael, hello fellow Haskellers, there seems to be a space leak in either 'pool', 'persistent' or 'persistent-sqlite'. From the behaviour I suspect the bug to be in 'pool'. When I run a transaction in an infinite loop, my program keeps eating more and more memory, even if the transaction itself doesn't do anything: forever $ runSqlPool (return ()) pool This doesn't happen for non-pooled connections, i.e. the following code runs in constant space as expected: forever $ runSqlConn (return ()) conn Versions are pool-0.0.1, persistent-0.4.2, persistent-sqlite-0.4.0. It happens for both file and in-memory databases. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

2 2

darcs repo browser
by Radoslav Dorcik 02 Apr '11

02 Apr '11

Hi, there is one haskell web application based on happstack for viewing foreign darcs repos exposed only on pure HTTP servers. I need that for checking status of some repositories when I don't have access to darcs binary/shell... maybe it can be useful for somebody else too. Service is running on following URL: http://handra.rampa.sk/dawb/changes See more comments on http://rampa.sk/static/dawb.html Regarding RSS feeds - Please send me a note If you find the hourly cron job I run on some repos to produce feeds too expensive for your server resources. It does not obey robots.txt file yet. Regards, Rado

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Re: [Haskell-cafe] Asynchronous Arrows need Type Specialization - Help!
by Paul L 02 Apr '11

02 Apr '11

Sorry, forgot to CC the list. I wonder why Gmail doesn't default to reply-all. On Fri, Apr 1, 2011 at 9:48 PM, David Barbour <dmbarbour(a)gmail.com> wrote: > > If we ignore the 'delay' primitive (which lifts latency into program > logic), my model does meet all the arrow laws. Nonetheless, the issues > of physical synchronization still apply. It's important to recognize > that the Arrow Laws do not describe non-functional properties, such as > time-space performance. Well, when you throw the time stamp into the value domain, and according to your previous calculation of time stamps, (a1 *** a2) >>> first a3 will produce different result than (a1 >>> a3) *** a2. This is in direct conflict to arrow laws. Arrows by themselves do not impose physical synchronization. They are very often used to model computations about synchronous data streams, but that is a very different concept. >> In the actual implementation of such lifting (perhaps over multiple >> type classes), the calculation of the time stamp can be made precise. > > Indeed. And it is precisely the greater of the input timestamps. ;-) This is because you are using a single number as timestamp. Define your timestamp type differently, you'll get a different opinion. For example, you can use a pair of time stamps for pairs, and nested time stamps for nested values. -- Regards, Paul Liu

2 1

Setting up cabal on EC2
by Lyndon Maydwell 02 Apr '11

02 Apr '11

Hi all. I'm having some issues setting up cabal on EC2. I've installed ghc 7.0.2, however, the bootstrap.sh script for cabal-install is complaining about missing dependencies: > Linking Setup ... > Configuring Cabal-1.8.0.2... > Setup: At least the following dependencies are missing: > base >=4 && <3 && >=1 && <5, filepath >=1 && <1.2 > Setup: At least the following dependencies are missing: base >=4 && <3 && >=1 && <5, filepath >=1 && <1.2 ghc-pkg tells me that I have base and filepath installed: > /usr/local/lib/ghc-7.0.2/package.conf.d > base-4.3.1.0 > /usr/local/lib/ghc-7.0.2/package.conf.d > filepath-1.2.0.0 Could there be an environment variable I need to set somewhere?

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Re: [Haskell-cafe] [Haskell-beginners] how to modify show for a list of certain types
by Markus Läll 02 Apr '11

02 Apr '11

Check out "unlines"! On Sat, Apr 2, 2011 at 4:32 PM, Manfred Lotz <manfred.lotz(a)arcor.de> wrote: > Hi there, > I have a list of say type Record which I write to a file and > read back from the file if required. > > The list's content is a single line in the file it doesn't matter how > many records the list contains. > > I would like to modify show for type Record in a way that show does > work like the normal show but additionally adds a newline at the end of > the string. > > I'm a bit lost how to do that. Any idea? > > > -- > Manfred > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

1 0

Multi agent system
by Yves Parès 02 Apr '11

02 Apr '11

Hello, For the purpose of a simple game, I'm looking for a simple way to get agents communicating with each other, with agents possibly running on different threads: Basically, an agent is Agent :: * -> *, since it is parameterized with the type of messages it can receive, and can make two basic operations: fetch, which gives it the messages in its mailbox and send, which sends a message a to an Agent a Given that haskell threads are lightweight, is it reasonable to lauch a thread per agent and then implement fetch and send through Chans? It will be simple to implement and to use, but if I have, say, two hundreds of really dumb (sorry, *reactive *^^) agents, like ants, I will have to launch as many threads. I could use a neater abstraction than IO to model agents (FRP is perfect for that), but I would loose the benefits of concurrent programming (or I won't be able to keep things simple).

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Fwd: using IO monad in Iteratee
by Dmitry Olshansky 02 Apr '11

02 Apr '11

---------- Forwarded message ---------- From: Dmitry Olshansky <olshanskydr(a)gmail.com> Date: 2011/4/2 Subject: Re: [Haskell-cafe] using IO monad in Iteratee To: wren ng thornton <wren(a)freegeek.org> Well, in my case I can use probably withBinaryFile fn (\h-> run $ enumHandle h ...) 2011/4/2 wren ng thornton <wren(a)freegeek.org> > On 4/1/11 3:59 PM, Dmitry Olshansky wrote: > >> But enumFile use IO monad instead of MonadIO class. >> [...] >> >> Is it possible to change enumFile to using MonadIO class? >> > > Unless its changed significantly since I looked at it last (which it may > well have), it's not possible. The problem is that what we'd really need is > lowerIO :: m a -> IO a for the m in question; liftIO goes the wrong way. Of > course, getting a lowerIO with the right semantics will be tricky for most > monads. > > -- > Live well, > ~wren > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

Asynchronous Arrows need Type Specialization - Help!
by David Barbour 02 Apr '11

02 Apr '11

I was giving Control.Arrow a try for a reactive programming system. The arrows are agents that communicate by sending and returning time-varying state. Different agents may live in different 'vats' (event-driven threads) to roughly model distributed computing. For the most part, the state varies asynchronously - i.e. a file updates at a different rate than the mouse position. Anyhow, I ran into a problem: The (***) and (&&&) operations, as specified in Control.Arrow, are inherently synchronization points. Ideally one could do something like: (a1 *** a2) >>> first a3 and the output from a1 would be piped directly as input to a3, without touching a2. However, arrows also allow: (a1 *** a2) >>> arr f And, in this case, the current state of a1 and a2 must be combined into a stream of pairs so that f may be mapped over the stream. Obtaining both streams at a single place (a single vat) and time is a synchronizing operation. The synchronization operations are severely undermining the intended scalability and performance of this agent abstraction. Now, I believe I could make it work if I could somehow specialize the arrow type with aparametric structure... A (b,b') (c,c') = A b c = But I've not been able to make the Haskell type system happy with this. Type families allow some specialization, but they don't allow ambiguity or defaults. Given the 'arr' function, I must be able to instantiate an A involving any type at all. Today I explored an alternative version of Control.Arrow, using an opaque product type specialized to the arrow, but I still bumped into the issue of specializing the arrow type for this. I don't see how GADTs would help. I've been stuck on this for a couple weeks now. Ideas would be appreciated.

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Re: [Haskell-cafe] Asynchronous Arrows need Type Specialization - Help!
by Paul L 02 Apr '11

02 Apr '11

Thanks for the elaboration. I now have a much better understanding. FIrst of all, I agree that the system model as you laid out do not fit into the arrows abstraction with respect to the set of arrow laws. Then the choice is whether to shape the arrows to fit your model, or shape your model to fit arrows. You chose the former. But one thing I want to be clear is that the arrows abstraction didn't impose the kind of strictness to your model, your system design did. If you would like to consider the second approach, I have a few suggestions: 1. Enable a lazy evaluation strategy over a distributed system. This can be done by passing place holders between agents, and only fill them (either by sending over data or request-by-demand) when value is fully computed. There could be other alternatives, but I'm not familiar with the set of literatures in this particular area. Maybe somebody else could give pointers. 2. Statically schedule data availability and delivery from producer to consumer. I'm not sure what kind of application you try to model, but this would limit the scope to dataflow that can be statically analyzed before program runs. On the other hand, the limitation also bring benefits because we can be smart about whether to move data or to move computation. To do this properly requires a solution to the pure function problem. For the pure function example you gave: > let f = \ (fileState, queryResult) -> combineResponses in > (a1 *** a2) >>> arr f Didn't you just mention that you want to tag along the time stamp to the values? So fileState is really a lifted value over a plain file state, and so is queryResult. This means the combineResponse function is also a lifted function. In the actual implementation of such lifting (perhaps over multiple type classes), the calculation of the time stamp can be made precise. Only difficulty I see here is that we may need to lift time stamped values over tuples. A single Int value wouldn't suffice as the type for time stamp because it has to take the max of time stamps of the tuple elements (what is exactly what you are criticizing). It may require some type level magic, but I am quite positive that difficulties can be overcome. Regards, Paul Liu On Fri, Apr 1, 2011 at 6:00 PM, David Barbour <dmbarbour(a)gmail.com> wrote: > On Fri, Apr 1, 2011 at 2:57 PM, Paul L <ninegua(a)gmail.com> wrote: >> I now understand where you are coming from, but I don't quite get your >> motivation to develop new classes for arrows. Tupling is Haskell is of >> course very lazy, it does not evaluate any of its element. > > Assume three machines: Alice, Bob, and Charlie. Bob owns resource a1, > and Charlie owns resource a3. Bob and Charlie do not know about one > another. Alice runs 'a1 >>> a3' with some initial inputs. For an > 'efficient' implementation, I need the output from a1 to propagate > directly as input to a3, directly from Bob to Charlie - do not pass > Alice, do not waste precious bandwidth and latency. > > Now extend the example just a bit: > > (a1 *** a2) >>> first a3 > > Naively, Alice might take the outputs from a1 and a2 and build a tuple > (r1,r2), then promptly split the tuple back up and send r1 to Charlie. > But this wastes a lot of time and bandwidth and CPU bringing r1 and r2 > together onto Alice's machine. It doesn't matter that Alice never > looks at r1 herself... the fact that she even 'touches' the r1 > variable is killing performance and requires physical synchronization. > In short, laziness doesn't help here. What does help is optimizing > the above into ((a1 >>> a3) *** a2). > > I could go into even more reasons. (I.e. I'm actually using streams, > not simple values, so tupling and untupling is closer to zipping and > unzipping.) But I think the above argument is sufficient by itself. > > My model is designed for efficient, scalable, open, distributed > programming. I currently model distributed machines each as a thread > with an event-loop. But this is a proof-of-concept implementation: I'm > unwilling to 'cheat' by using any techniques that could not > efficiently and securely be implemented for a real open distributed > system. > >> >> As for the spatial and logical concepts, don't you think they are >> over-specifying what is really necessary for your system model? > > I do not believe I've overspecified. > > But I've also avoided explaining the full model. That sort of > explanation really requires a conference paper with a lot of > motivating examples, and it was not my intent to use Haskell-cafe as a > forum. > >> >> So I guess what I'm having trouble with is your arr f example. If f is >> sufficiently lazy, it does not evaluate the elements in its input >> tuple at all. Most pure functions we use to wire up arrows are like >> this: arr fst, arr snd, arr swap, arr (\x -> (x, x)), etc. > > For simple data plumbing that never needs to touch the input, I > currently suggest specialized arrows: > aconst :: c -> a b c > > afst :: a (b,c) b > asnd :: a (b,c) c > adup :: a b (b,b) > aswap :: a (b,c) (c,b) > > aleft :: a b (Either b c) > aright :: a c (Either b c) > amerge :: a (Either b b) b > amirror :: a (Either b c) (Either c b) > > Potentially, RULES pragmas could also convert from 'arr fst' to a > particular implementation of 'afst', though I'd not recommend RULES at > the typeclass level after having been hurt by those in Control.Arrow. > >> >> I also don't quite follow your example of passing a time stamp >> together with value. > > Logically, a 'future' is just a time-stamp paired with a value. > > let f = \ (fileState, queryResult) -> combineResponses in > (a1 *** a2) >>> arr f > > Here, 'arr f' is opaque to my implementation. I cannot look inside 'f' > and decide whether or not it needs both fileState and queryResult, or > just one of them, or none (const). So I must somehow wait for both > fileState and queryResult to be available before I can evaluate > combineResponses. How long do I wait? Well, until the last response is > available. > > In the reactive context, you don't really have futures, but you still > have latency before resource state becomes available, and must wait to > combine results. > >> >> If you are getting unwanted results, it is perhaps due to wrong >> computation, rather than the composition to a pure functions. >> Maybe you want to give a specific example of "f". > > Regards, > > David > -- Regards, Paul Liu

1 0

Missing MaybeT MonadFix instance
by Job Vranish 02 Apr '11

02 Apr '11

Is there a particular reason that the transformers<http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Co…>version of the MaybeT monad does not have a MonadFix instance? It seems like the following instance would be suitable: instance (MonadFix m) => MonadFix (MaybeT m) where mfix f = MaybeT $ mfix $ \a -> runMaybeT $ f $ case a of Just a -> a Nothing -> error "mfix MaybeT: Nothing" I don't think it's possible to hit the error case: In order to terminate f cannot be strict so it must return a value without evaluating its input. If f returns a Nothing, then you have your return value and you're done. If f returns a Just, then we don't hit the error case. - Job

1 0

is ppc64 arch registerised
by Jiri Skala 01 Apr '11

01 Apr '11

Hi all, I've read on this page http://hackage.haskell.org/trac/ghc/wiki/Platforms the ppc64 arch is registerised. I succeeded to build ghc-7.0.2 on ppc64 (Fedora) with GhcUnregisterised=YES. Switching off this option currently leads to failure. I'd like to know: - is the info about registerised ppc64 arch still valid? - could you provide me tips how to build it registerised? Thanks Skalnik

3 2

ArrowLoop and streamprocessors
by Mathijs Kwik 01 Apr '11

01 Apr '11

Hi all, I'm playing around a bit with arrows (more specifically, something like a CPS style streamprocessor as described in "Generalising Monads to Arrows" by John Hughes). A part of my program takes inputs/signals from 2 sources. The sources don't produce output at the same rate, and this part is able to handle inputs independently. So I figured I need Either A B ~> O for this part. handleA :: A ~> O handleB :: B ~> O someBox :: Either A B ~> O someBox = handleA ||| handleB So far so good. Now, further downstream (someBox >>> otherBox) there is. otherBox :: O ~> Either C B Let's say C are "normal" outputs, and B are control signals that need to get wired back to someBox. Control signals are rare, so maybe there's 1000 C outputs and only 1 B output in a certain timeframe. Also note that in this CPS style streamprocessing, there's no 1-on-1 relation between input and output, so on 1 input (O), otherBox might produce 2 outputs (2 times C), or 4 outputs (3 times C and 1 time B). To "wire back" B's to someBox, I'm pretty sure I need to use ArrowLoop. But (loop :: a (b, d) (c, d) -> a b c) uses tuples, which means the processing will only continue when both inputs are available. So if I turn someBox into (A, B) ~> O and otherBox into O ~> (C, B), the processing will instantly halt, waiting for a B to arrive after the A comes in. I know about the 'delay' trick that usually works for loops, where an output value is just inserted before the actual outputs, but that won't help in my case, because otherBox doesn't produce B's at the same rate that someBox receives A's, so by inserting a dummy B, someBox will only "run" once. Also, there's no relation between the number of A inputs someBox receives and the number of inputs for otherBox, so I also can't have otherBox just insert "noop" signals after every "run". 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? Thanks, Mathijs PS: I remember reading about (unfortunately didn't get the examples working) Reactive (FRP), which has the concept of Events and Behaviors I think Yampa has something like it too. A Behavior has a value at all times, which was updated by Events. That sounds a bit like it might solve my problem, since I need something that contains "Latest Control Signal was X" that updates when B's are produced, but that doesn't have to be put "on the wire" as a stream of events (because I can't determine how many events to put to match incoming A's. However, I hope there is another solution because I found Reactive and Yampa quite hard to grasp.

5 7

Encoding-aware System.Directory functions
by Michael Snoyman 01 Apr '11

01 Apr '11

Hi all, I think this is a well-known issue: it seems that there is no character decoding performed on the values returned from the functions in System.Directory (getDirectoryContents specifically). I could manually do something like (utf8Decode . S8.pack), but that presumes that the character encoding on the system in question is UTF8. So two questions: * Is there a package out there that handles all the gory details for me automatically, and simply returns a properly decoded String (or Text)? * If not, is there a standard way to determine the character encoding used by the filesystem, short of hard-coding in character encodings used by the major ones? For those curious: this is in regards to a bug in wai-app-static[1]. Thanks, Michael [1] http://hackage.haskell.org/package/wai-app-static

11 16

Computational Physics in Haskell
by Azeem -ul-Hasan 01 Apr '11

01 Apr '11

I started learning Haskell a little while ago. Although I am a novice I am still in love with it. I am physics major and primarily interested in Theoretical Physics and would like to use Haskell in this area. So, I just know to what has been done in this area, are there any libraries for simulating physical process in Haskell etc. Azeem Ul Hasan School of Science and Engineering, Lahore University of Management Sciences, Lahore, Pakistan

5 6

SoC project: advice requested
by Gábor Lehel 01 Apr '11

01 Apr '11

Hi all, I would very much like to apply for a Summer of Code project this year; however, I'm having trouble nailing down a project which is both (a) of appropriate scope and (b) which interests and motivates me. There's also obviously the question of needing a mentor for the project. Regarding (b), I'm finding it hard to get really excited about hacking on Cabal or other infrastructure: I'd much rather like to add some kind of new, interesting, and useful capability to GHC itself. Naturally, this comes into a kind of tension with (a). I'd love to work on something type system related, but have the feeling that going from 'amateur Haskeller' to 'type theory and GHC expert' within the course of a single summer is likely going to be too much. (Feel free to correct me on this point, though!) The two projects I've been able to think of which seem like they *might* be appropriate are: integrating the standard 'deriving' mechanism with Template Haskell to support deriving custom classes; and adding native FFI support for passing C structs / tuples (arbitrary products?) of FFI-able types by value, as discussed in [1]. Would either of these be not-too-big / would anyone be willing to mentor them? Alternately, I'd be very happy to receive suggestions about other GHC-related work which would be considered appropriate. (Or, heck, any other compiler.) A related problem is that, having done only minimal GHC hacking so far, drawing up a detailed plan / design in advance as part of the proposal would be difficult. If this is considered necessary and there is someone willing to mentor the project I'd be happy to research the problem in advance of the submission deadline so I can submit a more detailed proposal. Alternately, if it's deemed acceptable to learn the ropes / come up to speed as part of the SoC itself that's fine by me as well. (Wasn't this sort of the point originally?) Background info: I've taken part in the SoC once before, back in 2006 (when I applied to KDE to work on Krita). I don't yet have any Hackage packages to my name, however I'm working on a C++-to-Haskell bindings generator for my bachelor's thesis (the primary target being Qt*) which is likely to spawn quite a few. (I've avoided making any noise about this because I didn't want to put the cart before the horse: the plan was (and still is) to announce something once there is something worth announcing, and it's not at that point quite yet.) Apologies for not writing this email a couple weeks earlier; I have a day job and next to the thesis it didn't leave much time to brainstorm and research. (The job isn't Haskell-related and doesn't even pay as well, so if I get accepted I'll have no compunctions scaling it back (even completely). And the deadline for the thesis is May 15th, which is convenient enough.) Thanks, ~g [1] http://www.mail-archive.com/haskell-cafe@haskell.org/msg61034.html * Yes, I know qtHaskell exists. I have some, how to say, 'differences of opinion' with regards to its API, and it's not open source anyways. -- Work is punishment for failing to procrastinate effectively.

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Class constraints for associated type synonyms
by Jan-Willem Maessen 01 Apr '11

01 Apr '11

Hi all - I've been trying to construct a class declaration with an associated type synonym, but I'd like to constrain that type to belong to a particular class. Consider the following class: class Monoid m => Constructs c m | c -> m where construct :: m -> c This captures the idea that the collection c ought to be constructed using the monoid m (say if we're doing the construction using the Writer monad)--the functional dependency indicates the desire for the type c to injectively determine the choice of monoid m. For example: newtype ListBuilder a = Builder ([a] -> [a]) deriving (Monoid) instance Constructs [a] (ListBuilder a) where construct (Builder f) = f [] instance (Ord a) => Constructs (Set a) (Set a) where construct = id Now I'd like to be able to do the same thing using an associated type synonym, something like this: class Monoid (GeneratorOf a) => Generable a where type GeneratorOf a :: * -> * construct :: GeneratorOf a -> a Now, it seems I need FlexibleInstances to do this when I'm using an associated type synonym, but I don't need the flexibility when using a multiparameter type class. In both cases the instance constraint involves types that can be injectively inferred (if I have my terminology straight; work with me here) from a single type mentioned in the class head. In particular, I can imagine storing the dictionary for Monoid (GeneratorOf a) in the dictionary for Generable a, and thus allowing context reduction of (Monoid (GeneratorOf tyvar)) to (Generable tyvar). Meanwhile, I think there are things that are permitted by FlexibleInstances that I'd rather *not* have creeping into my programs. Is there a fundamental constraint I'm missing here that requires the full generality of FlexibleInstances? Do I need to use FlexibleInstances whenever I use associated types in my programs? -Jan-Willem Maessen

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is ppc64 arch registerised
by Jiri Skala 01 Apr '11

01 Apr '11

Hi all, I've read on this page http://hackage.haskell.org/trac/ghc/wiki/Platforms the ppc64 arch is registerised. I succeeded to build ghc-7.0.2 on ppc64 (Fedora) with GhcUnregisterised=YES. Switching off this option currently leads to failure. I'd like to know: - is the info about registerised ppc64 arch still valid? - could you provide me tips how to build it registerised? Thanks Skalnik

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Re: [Haskell-cafe] Strictness is observable
by oleg＠okmij.org 01 Apr '11

01 Apr '11

John Meacham wrote: > Error is not catchable in haskell 98. Only things thrown by raiseIO are. I see; so GHC, absent any LANGUAGE pragma, should have arranged for `error' to generate a non-catchable exception. Still, Haskell98 specifically allows for the behavior that lets us distinguish strict and non-strict functions (that is, the evaluation order in the pure code). Here is the relevant chapter and the verse from Haskell98 (PDF file found on Haskell.org) 21.2.3 File locking Implementations should enforce as far as possible, at least locally to the Haskell process, multiple- reader single-writer locking on files. That is, there may either be many handles on the same file which manage input, or just one handle on the file which manages output. If any open or semi- closed handle is managing a file for output, no new handle can be allocated for that file. If any open or semi-closed handle is managing a file for input, new handles can only be allocated if they do not manage output. Whether two files are the same is implementation-dependent, but they should normally be the same if they have the same absolute path name and neither has been renamed, for example. Warning: the readFile operation (Section 7.1) holds a semi-closed handle on the file until the entire contents of the file have been consumed. It follows that an attempt to write to a file (using writeFile, for example) that was earlier opened by readFile will usually result in failure with isAlreadyInUseError. The Report specifically warns of the possibility that an implementation that implements file locking, as encouraged by the Report, can distinguish if the entire contents has been consumed or not. The consumption happens in the pure code. Thus the pure Haskell evaluation order starts to matter for a Haskell98-compliant implementation. The following code demonstrates that fact. > -- Haskell98! -- at least as far as GHC implements the absence > -- of LANGUAGE pragmas > -- Strictness is observable > -- With no seq, no unsafe operations > > import System.IO > import Control.Exception > > -- fs and fns are both essentially (const True) functions, > -- but differ in strictness > fs,fns :: Bool -> Bool > > -- non-strict > fns x = True > > -- strict > fs True = True > fs x = True > > -- Any file name that one can create and not be sorry to override > file_name = "/tmp/foo" > > prepare :: IO () > prepare = writeFile file_name "xxx" > > > {- > test f = do > h <- openFile file_name ReadMode > lst <- hGetContents h >>= return . words > ns <- if f (head lst == "") then return "non-strict" else return "" > hClose h > if lst == [] then print ns else print "strict" > -} > > handler :: SomeException -> IO () > handler _ = print "non-strict" > > test f = handle handler $ do > h <- openFile file_name ReadMode > lst <- hGetContents h > ns <- if f (length lst == 0) then return "strict" else return "" > writeFile file_name "" > print ns > > main_s = prepare >> test fs > -- prints "strict" > > main_ns = prepare >> test fns > -- "non-strict" Yves Pare`s wrote: > Then if you turn : > fs True = True > fs x = True > to: > fs x = case x of > True -> True > x' -> True > Is it still strict, or does 'fs' wrap the case test and defer evaluation? Well, we have a test for that, don't we? > fs1 x = case x of > True -> True > x' -> True *Main> prepare >> test fs1 "strict"

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