July 2011 - Haskell-Cafe

GHC API output files options problem
by Marianna Rapoport 15 Jul '11

15 Jul '11

Hi all, we are working on a Haskell support plugin for IntelliJ IDEA, and need to provide correct compilation of Haskell programs. We compile the latter using GHC API, with options specifying .hi and .o paths. For some reason, unlike a standalone GHC compilation, GHC API ignores the values of the outputFile and outputHi flags. Please find attached a minimal example where "err_test.hs" uses GHC API to compile "src/A.hs". Thus, "test_ghc.sh" and "test_my.sh" should do the same, but the first works correctly while the latter puts "use.o" and "use.hi" to the "src" folder. To launch "test_my.sh", please change the value of lib in "err_test.hs" according to your GHC installation path. Thank you very much in advance! Marianna Rapoport, amaurremi(a)gmail.com Oleg Sobolev, sobolev_ov(a)mail.ru

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HaskellDB DB Layout Description
by Tom Murphy 15 Jul '11

15 Jul '11

Hi, I've found good explanations of the HaskellDB combinators, but I can't find good information about how to correctly define the database layout. Can anyone point me to a resource, or give a quick example? Thanks! Tom

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Request for comments: dbus-core 0.9
by John Millikin 15 Jul '11

15 Jul '11

I think I'm pretty close to releasing the next version of my D-Bus library, dbus-core. This is a big release that changes a lot of the API, so I'd like to see if anybody 1) has any problems with the new APIs or 2) has any suggested improvements. haddock: https://john-millikin.com/temp/dbus-core_0.9/ tarball: https://john-millikin.com/temp/dbus-core_0.9.tar.gz pdf: https://john-millikin.com/temp/dbus-core_0.9.pdf examples: https://john-millikin.com/temp/dbus-core_0.9/examples/ Notable improvements in this release are: • Merges dbus-core and dbus-client. The modules "DBus.Client" and "DBus.Client.Simple" provide a very nice, high-level API to common D-Bus operations. Note: I've reverted to the dbus-client_0.3 API for this, since not many users liked the monad-based API introduced in 0.4. • The class Variable was split into IsAtom, IsValue, and IsVariant. Together these let the rest of the type conversion API go away -- you no longer have to worry about Array, Structure, or Dictionary (except in a few, rare cases). • Support for custom transports (eg, a pipe or in-memory buffer). • Improved support for custom authentication mechanisms. Also, while the docs are obviously not finished, I'm interested to know if anyone finds any parts of the library confusing -- those'll get extra doc love.

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Call for GUI examples - Functional Reactive Programming
by Heinrich Apfelmus 14 Jul '11

14 Jul '11

Dear Haskellers, Can GUI programming be liberated from the IO monad? Functional Reactive Programming (FRP) promises as much, and I'm trying to make this dream a reality with my [reactive-banana][] library. Having released version 0.4.0.0, I am now looking for example programs to direct the future evolution of the library. Do you know any *small GUI programs* that you would *like* to see *implemented with Functional Reactive Programming?* I'm sure that some of you * are interested in FRP and would like to learn from a few simple but beautiful examples * or have written small GUI programs that they are unhappy with because they were not purely functional * or have thought about FRP before and concocted a few examples that are very tricky to implement with FRP * or have written a nice little GUI application that simply makes a great example. I would love to hear your examples, so that I can try to convert them to FRP style and test my library against them! Strictly speaking, it doesn't have to be an example with a graphical interface, I'm also interested in audio and animation examples, though I'm currently focusing on GUIs. Here a few examples of what I am looking for: * I think that Tim Docker's minimal step sequencer [hbeat][] simply makes a great example. I hope that wxHaskell offers a platform-independent way to play sound. * While editing the [Haskell Weekly News][hwn], Brent Yorgey wrote a little command-line [program to gather newsworthy items][zipedit]. But wouldn't a GUI be great? Since I'm only interested in the GUI, someone would have to supply the feed parser for this example to be viable. Maybe the current editor, Daniel Santa Cruz, might be interested? * Notes of a musical performance can be modeled as event streams (MIDI), as Henning Thielemann has [done with great effect][midi streams]. Surely, reactive-banana should be up to the task, but writing an arpeggiator seems impossible at the moment. [reactive-banana]: http://hackage.haskell.org/package/reactive-banana [zipedit]: http://byorgey.wordpress.com/2008/06/21/zipedit/ [hbeat]: http://dockerz.net/twd/hBeat [midi streams]: http://hackage.haskell.org/package/streamed [hwn]: http://www.haskell.org/haskellwiki/Haskell_Weekly_News Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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Re: [Haskell-cafe] FFI for a beginner
by Donn Cave 14 Jul '11

14 Jul '11

>> The docs <http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…> >> say that -#include pragmas no longer work, but fail to explain how to >> load code without them. Suffice to say I have no recourse but trial >> and error." > > Ah, now that is a GHC documentation question. Can you tell us what > should the docs should say instead? Then we can fix the docs. - I believe I would say (agreeing with M Parès) that hsc2hs serves this purpose - put "#include <termios.h>" in the .hsc file and use hsc2hs features like #const and (#peek x, y). The termios example could cover a lot more ground by operating on the termios struct and calling tcsetattr directly from Haskell, using hsc2hs # macros (I append an example implementation), but you'd also want to explain that hsc2hs is only needed for these # macros, not for FFI in general. But hsc2hs as of GHC 7.0.3 generates a "{-# INCLUDE" pragma, which of course is cause for complaint from ghc. So there might be more to be fixed, beyond just the documentation. That line probably has no actual purpose other than to annoy ghc, and as a workaround it can simply be removed from the .hs file. - My impression is that Greencard isn't the best way to get started with FFI and hasn't been for a decade or so, so it should probably be purged from docs. - The "root" page needs some attention from someone who understands what "safe" and "unsafe" mean. Currently (under "Improving efficiency") it cites the conventional interpretation that a function must be "safe" if it calls back into the runtime, but neglects to mention that it must also be "safe" if it is not to block execution of other threads, including bound OS threads - so related to efficiency issues you might want to make any slow I/O operation "safe" to avoid this. There may be other points about "unsafe" that need to be documented. Donn -------- {-# LANGUAGE ForeignFunctionInterface #-} module TTY (clearICANON) where import Data.Word (Word32) import Foreign import Foreign.C #include <termios.h> type TCFlag = (#type tcflag_t) type Speed = (#type speed_t) data Termios = Termios { termios_c_iflag :: TCFlag , termios_c_oflag :: TCFlag , termios_c_cflag :: TCFlag , termios_c_lflag :: TCFlag , termios_c_cc :: [CChar] , termios_c_ispeed :: Speed , termios_c_ospeed :: Speed } deriving Show instance Storable Termios where sizeOf _ = #size struct termios alignment _ = alignment (undefined::CDouble) peek a = do iflag <- (#peek struct termios, c_iflag) a oflag <- (#peek struct termios, c_oflag) a cflag <- (#peek struct termios, c_cflag) a lflag <- (#peek struct termios, c_lflag) a cc <- peekArray 20 ((#ptr struct termios, c_cc) a) ispeed <- (#peek struct termios, c_ispeed) a ospeed <- (#peek struct termios, c_ospeed) a return (Termios iflag oflag cflag lflag cc ispeed ospeed) poke a (Termios iflag oflag cflag lflag cc ispeed ospeed) = do (#poke struct termios, c_iflag) a iflag (#poke struct termios, c_oflag) a oflag (#poke struct termios, c_cflag) a cflag (#poke struct termios, c_lflag) a lflag pokeArray ((#ptr struct termios, c_cc) a) (take 20 (cc ++ repeat 0)) (#poke struct termios, c_ispeed) a ispeed (#poke struct termios, c_ospeed) a ospeed foreign import ccall "tcgetattr" tcgetattr :: CInt -> Ptr Termios -> IO CInt foreign import ccall "tcsetattr" tcsetattr :: CInt -> CInt -> Ptr Termios -> IO CInt setLFlag :: TCFlag -> Termios -> Termios setLFlag c a = a { termios_c_lflag = ((termios_c_lflag a) .|. c) } clearLFlag :: TCFlag -> Termios -> Termios clearLFlag c a = a { termios_c_lflag = ((termios_c_lflag a) .&. (complement c)) } setTTYAttr :: CInt -> (Termios -> Termios) -> IO () setTTYAttr fd fn = alloca $ \ termiosp -> do status <- tcgetattr fd termiosp if status == 0 then do termios <- peek termiosp poke termiosp (fn termios) status <- tcsetattr fd (#const TCSAFLUSH) termiosp if status == 0 then return () else throwErrno "tcgetattr" else throwErrno "tcgetattr" clearICANON = setTTYAttr 0 (clearLFlag (#const ICANON))

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[iteratee] empty chunk as special case of input
by Sergey Mironov 14 Jul '11

14 Jul '11

Hi community, hi John. I find myself reading bindIteratee[1] function for a several days.. there is something that keeps me away from completely understanding of the concept. The most noticeble thing is \nullC\ guard in the definition. To demonstate the consequences of this solution, let me define an iterator like myI = Iteratee $ \onDone _ -> onDone 'a' (Chunk "xyz") It is a bit unusial, since myI substitutes real stream with a fake one (xyz). Now lets define two actions producing different results in unusual manner: printI i = enumPure1Chunk ['a'..'g'] i >>= run >>= print i1 = (return 'b' >> myI >> I.head) -- myI substitutes the stream, last /I.head/ produces 'x', OK i2 = (I.head >> myI >> I.head) -- produces 'b'! I expected another 'x' here but myI's stream was ignored by >>= Well, I understand that this is probably an expected behaviour, but what is it for? Why we can't handle null input like non-null? Iterator may just stay in it's current state in that case. Thanks in advance Sergey -- [1] - bindIteratee (basically, >>=) code from Data.Iteratee.Base.hs bindIteratee :: (Monad m, Nullable s) => Iteratee s m a -> (a -> Iteratee s m b) -> Iteratee s m b bindIteratee = self where self m f = Iteratee $ \onDone onCont -> let m_done a (Chunk s) | nullC s = runIter (f a) onDone onCont m_done a stream = runIter (f a) (const . flip onDone stream) f_cont where f_cont k Nothing = runIter (k stream) onDone onCont f_cont k e = onCont k e in runIter m m_done (onCont . (flip self f .))

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Compiler support for orphan instances
by Henning Thielemann 14 Jul '11

14 Jul '11

Yesterday I tried to install fix-imports and ran into another example of problems with changing transitive closures of orphan instance propagation: $ cabal install fix-imports Resolving dependencies... Configuring fix-imports-0.1.2... Preprocessing executables for fix-imports-0.1.2... Building fix-imports-0.1.2... [1 of 6] Compiling Types ( src/Types.hs, dist/build/FixImports/FixImports-tmp/Types.o ) [2 of 6] Compiling Util ( src/Util.hs, dist/build/FixImports/FixImports-tmp/Util.o ) [3 of 6] Compiling Index ( src/Index.hs, dist/build/FixImports/FixImports-tmp/Index.o ) src/Index.hs:48:15: No instance for (Monad (Either [Char])) arising from a use of `sequence' at src/Index.hs:48:15-40 Possible fix: add an instance declaration for (Monad (Either [Char])) ... It is a repeating problem that you use an instance without noting it (here Monad (Either [Char])) and import it without noting it (you import a module, that possibly by accident imported and thus re-exported that instance). If the modules you import, decide to no longer use the Monad Either instance, then your imports will ignore the instance. I thought that some compiler support could reduce this problem. Proposal 1 GHC might warn, if you use an orphan instance but import it from a module, that does not define it. Example: module A where instance Msg s => Monad (Either s) ... module B where import A module C where {- B does not define instance Monad Either, but propagates that instance from A. -} import B test :: Either String Int test = do fail "bla" return 5 {- Here GHC should emit a warning, since the Monad Either instance is imported from B but not directly from A. If B drops the 'import A' this will no longer work. -} Proposal 2 Sometimes a module is actually intended for re-exporting an instance. Sometimes an instance is moved from module A to module Z, but A still re-exports the instance in order to maintain compatibility. In these cases proposal 1 would yield false alarms. So maybe there should be a pragma for tagging instances that are intentionally re-exported. Problem: How to identify instances? This is certainly related to earlier ideas of explicit named instance imports. module A where instance Msg s => Monad (Either s) ... module B where import A {- Explicitly propagate the instance from A, which is as good as defining it here. If the instance is not in scope, then compilation should fail at this pragma. This way it is asserted, that the instance is really exported. -} {-# INSTANCE-EXPORT Monad (Either s) #-} module C where import B test :: Either String Int test = do fail "bla" return 5 {- No warning here, because B intentionally propagated the Monad Either instance. -} Proposal 3 Forget about all those proposals and implement named instances. However the proposals 1 and 2 let you write code that works on all Haskell 98 compilers. Compilers may just not warn or ignore the INSTANCE-EXPORT pragma. Proposal 3 requires a language extension and using it will work only on the compilers that support that extension. Proposal 4 Forget about flexible, undecidable, overlapping, orphan and other instances and create a system, where classes are replaced by explicit method dictionaries and some rules for inserting these dictionaries, where the rules can be declared by the package author in the modules.

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Re: [Haskell-cafe] FFI for a beginner
by Jason Dagit 14 Jul '11

14 Jul '11

I don't know how to make ghc load them without using either hsc2hs or c2hs. I've had better experiences with hsc, but your mileage may vary. On Apr 8, 2011 3:34 AM, "Andrew Pennebaker" <andrew.pennebaker(a)gmail.com> wrote: ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples>are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…>say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error. GitHub: charm <https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm> <https://github.com/mcandre/charm>$ make cp /usr/include/charm.c . ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib [1 of 2] Compiling Charm ( charm.hs, charm.o ) [2 of 2] Compiling Main ( hellocharm.hs, hellocharm.o ) Linking hellocharm ... ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o collect2: ld returned 1 exit status make: *** [hellocharm] Error 1 Cheers, Andrew Pennebaker www.yellosoft.us _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Haskell Weekly News: Issue 190
by Daniel Santa Cruz 13 Jul '11

13 Jul '11

Welcome to issue 190 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 3 to 9, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/07/haskell-weekly-news-issue-190.h… Announcements Karel Gardas announced that he, Stephen Blackheath, and David Terei are "working together on ARM registerised port of GHC. The port is using LLVM as a code generator and in kind of working already." Testing is much appreciated! [1] http://goo.gl/TVvGD Matthias Fischmann inveted people to the HacPhi satellite in Berlin on July 29-31. See the post for details. Admission is free, but there is limited space, so don't wait too long! [2] http://goo.gl/XqdvI Quotes of the Week * chrisdone: [on hpaste as motivational tool] When I added hlint I was like 'hey Neil, check this', and he was all 'Oh noes! I must fix all the bugs!' * *amacleod started excising GOTOs from his BASIC code when he was 10. *roconnor started excising GOTOs from his Pascal code when he was 11. *luite used goto's in his C code last year. * AlanPerlis: One can't proceed from the informal to the formal by formal means. * Axman6: well, if C++ is anything to go by, the next Haskell will have lambdas and easy concurrency! * Cale: Constructing a definition from a type is formally the same thing as constructing a proof from a theorem. (Just with different logical systems in place :) * xplat: im in ur Sierpinski space, classifyin' ur subobjects * cmccann: in ContT, flow controls you * [Kron] What is the meaning of life? D: [joe6] learning agda? * camccann: Applying readFile to a file path doesn't return a String value any more than putting a steak next to a meat grinder magically turns them into a hamburger. * blackdog: the type system makes you strong, makes you hard. Top Reddit Stories * GHC on ARM Domain: ghcarm.wordpress.com, Score: 35, Comments: 4 On Reddit: [3] http://goo.gl/idJ7s Original: [4] http://goo.gl/D5zjE * The Monad.Reader Issue 18 Domain: themonadreader.wordpress.com, Score: 32, Comments: 3 On Reddit: [5] http://goo.gl/XA67w Original: [6] http://goo.gl/tzVX2 * Haskell vs. ML in the context of "Worse is Better" Domain: news.ycombinator.com, Score: 31, Comments: 25 On Reddit: [7] http://goo.gl/LpTbP Original: [8] http://goo.gl/bNU4w * Do (side) effects really need a total order? Domain: pchiusano.blogspot.com, Score: 31, Comments: 8 On Reddit: [9] http://goo.gl/jfxWl Original: [10] http://goo.gl/5tmUZ * Call for GUI examples - Any small GUI applications that you would like to see implemented with Functional Reactive Programming? Domain: apfelmus.nfshost.com, Score: 30, Comments: 13 On Reddit: [11] http://goo.gl/MMZJb Original: [12] http://goo.gl/X7IXP * Impredicative Polymorphism - A Use Case Domain: augustss.blogspot.com, Score: 23, Comments: 6 On Reddit: [13] http://goo.gl/QCwpt Original: [14] http://goo.gl/owsJT * Yesod :: Testimonials from a developer and designer Domain: yesodweb.com, Score: 19, Comments: 9 On Reddit: [15] http://goo.gl/5HOKo Original: [16] http://goo.gl/Zad2d * Parallelizing a nonogram solver Domain: jaspervdj.be, Score: 14, Comments: 2 On Reddit: [17] http://goo.gl/5LFlF Original: [18] http://goo.gl/BUbUA * A Haskell newbie's guide to Snap, Part 2 Domain: learnmeahaskell.blogspot.com, Score: 13, Comments: 0 On Reddit: [19] http://goo.gl/KTW50 Original: [20] http://goo.gl/rF1B1 * Losing my mind: Is Rhask (R language+Haskell) a real entity? Domain: self.haskell, Score: 11, Comments: 8 On Reddit: [21] http://goo.gl/O95KN Original: [22] http://goo.gl/O95KN Top StackOverflow Questions * How to gain control of a 5GB heap in Haskell? votes: 27, answers: 1 Read on SO: [23] http://goo.gl/OdZIG * Using higher-order Haskell types in C# votes: 16, answers: 2 Read on SO: [24] http://goo.gl/qzvok * Why applicative functors in Haskell and functional programming votes: 16, answers: 6 Read on SO: [25] http://goo.gl/72kE7 * Is there OS written in Haskell? votes: 16, answers: 3 Read on SO: [26] http://goo.gl/PgOeO * A good way to avoid “sharing”? votes: 15, answers: 1 Read on SO: [27] http://goo.gl/9GLzu * Hard Time Learning Haskell [migrated] votes: 13, answers: 0 Read on SO: [28] http://goo.gl/hhonY * Creating a Haskell REPL within a Haskell application votes: 11, answers: 1 Read on SO: [29] http://goo.gl/jZU7I About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90890 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90922 3. http://www.reddit.com/r/haskell/comments/ija21/ghc_on_arm/ 4. http://ghcarm.wordpress.com/2011/07/06/armv7-thumb-vfpv3-support-and-github… 5. http://www.reddit.com/r/haskell/comments/ihz6h/the_monadreader_issue_18/ 6. http://themonadreader.wordpress.com/2011/07/05/issue-18/ 7. http://www.reddit.com/r/haskell/comments/ig4yk/haskell_vs_ml_in_the_context… 8. http://news.ycombinator.com/item?id=2725386 9. http://www.reddit.com/r/haskell/comments/iihrl/do_side_effects_really_need_… 10. http://pchiusano.blogspot.com/2011/07/do-side-effects-really-need-total-ord… 11. http://www.reddit.com/r/haskell/comments/ijsnv/call_for_gui_examples_any_sm… 12. http://apfelmus.nfshost.com/blog/2011/07/08-call-for-gui-examples.html 13. http://www.reddit.com/r/haskell/comments/il5g5/impredicative_polymorphism_a… 14. http://augustss.blogspot.com/2011/07/impredicative-polymorphism-use-case-in… 15. http://www.reddit.com/r/haskell/comments/iitr1/yesod_testimonials_from_a_de… 16. http://www.yesodweb.com/page/testimonials 17. http://www.reddit.com/r/haskell/comments/ihdbh/parallelizing_a_nonogram_sol… 18. http://jaspervdj.be/posts/2011-07-05-parallelizing-a-nonogram-solver.html 19. http://www.reddit.com/r/haskell/comments/iiksr/a_haskell_newbies_guide_to_s… 20. http://learnmeahaskell.blogspot.com/2011/07/haskell-newbies-guide-to-snap-p… 21. http://www.reddit.com/r/haskell/comments/ig5mk/losing_my_mind_is_rhask_r_la… 22. http://www.reddit.com/r/haskell/comments/ig5mk/losing_my_mind_is_rhask_r_la… 23. http://stackoverflow.com/questions/6623391/how-to-gain-control-of-a-5gb-hea… 24. http://stackoverflow.com/questions/6562378/using-higher-order-haskell-types… 25. http://stackoverflow.com/questions/6570779/why-applicative-functors-in-hask… 26. http://stackoverflow.com/questions/6638080/is-there-os-written-in-haskell 27. http://stackoverflow.com/questions/6614023/a-good-way-to-avoid-sharing 28. http://stackoverflow.com/questions/6618081/hard-time-learning-haskell-migra… 29. http://stackoverflow.com/questions/6588999/creating-a-haskell-repl-within-a…

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ANN: fix-imports-0.1.2
by Evan Laforge 13 Jul '11

13 Jul '11

Not sure if anyone else is using this, but I've fixed some bugs and bumped fix-imports by a few more versions. Here's copy paste from the package description: A small standalone program to manage the import block of a haskell program. It will try to add import lines for qualified names with no corresponding import, remove unused import lines, and sort the import block according to some heuristic you can define. This only works for qualified imports! Unqualified imports are left untouched. It's most convenient if bound to an editor key. Changes since last announcement: Mon Jul 11 23:00:19 PDT 2011 qdunkan(a)gmail.com * version 0.1.2 Mon Jul 11 22:58:56 PDT 2011 qdunkan(a)gmail.com * fix but where multiple comments above an import are lost Fri Jul 1 15:16:40 PDT 2011 qdunkan(a)gmail.com * don't reload file if no changes were made Fri Jul 1 14:43:02 PDT 2011 qdunkan(a)gmail.com * sort and format the import block even if imports weren't added or removed Wed Jun 15 12:39:12 PDT 2011 qdunkan(a)gmail.com * version 0.1.1 Wed Jun 15 12:14:31 PDT 2011 qdunkan(a)gmail.com * upgrade to haskell-src-exts 1.11, which removes the mandatory fixities Wed Jun 15 12:11:27 PDT 2011 qdunkan(a)gmail.com * fix bug causing qualified vs. non-qualified imports to be sorted inconsistently Thu Jun 2 12:10:08 PDT 2011 qdunkan(a)gmail.com * more specific versions for base

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