November 2006 - Haskell

ANNOUNCE: PQC: QuickCheck in the Age of Concurrency
by dons＠cse.unsw.edu.au 24 Nov '06

24 Nov '06

------------------------------------------------------------------------ PQC: QuickCheck in the Age of Concurrency An SMP parallel QuickCheck driver ------------------------------------------------------------------------ Do you: * Have (or want) lots of QuickCheck properties? * Run them often (maybe on every darcs commit)? * Tired of waiting for the testsuite to finish? * Got a multi-core box with cpus sitting idle...? Yes? You need Parallel QuickCheck! PQC provides a single module: Test.QuickCheck.Parallel. This is a QuickCheck driver that runs property lists as jobs in parallel, and will utilise as many cores as you wish, with the SMP parallel GHC 6.6 runtime. It is simple, scalable replacement for Test.QuickCheck.Batch. An example, on a 4 cpu linux server, running 20 quickcheck properties. With 1 thread only: $ time ./a.out 1 1: sort1 : OK, 1000 tests. 1: sort2 : OK, 1000 tests. 1: sort3 : OK, 1000 tests. 1: sort4 : OK, 1000 tests. ... ./a.out 1 > x 18.94s user 0.01s system 99% cpu 18.963 total 18 seconds, 99% cpu. But I've got another 3 2.80GHz processors sitting idle! Let's use them, to run the testsuite faster. No recompilation required. 4 OS threads, 4 Haskell threads: $ time ./a.out 4 +RTS -N4 > /dev/null ./a.out 4 +RTS -N4 > /dev/null 20.65s user 0.22s system 283% cpu 7.349 total 283% cpu, not bad. We're getting close to being limited by the length of the longest running test. Or on a dual core macbook, thanks to Spencer Janssen for macbook data and testing: 1 thread: ./Example 1 17.256s 2 threads: ./Example 2 +RTS -N2 10.402s Get it! Homepage: http://www.cse.unsw.edu.au/~dons/pqc.html Haddocks: http://www.cse.unsw.edu.au/~dons/pqc/ Example : http://www.cse.unsw.edu.au/~dons/code/pqc/examples/Example.hs darcs get http://www.cse.unsw.edu.au/~dons/code/pqc Happy, quick checking, Don

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SYB Documentation is inconclusive.
by Arthur van Leeuwen 24 Nov '06

24 Nov '06

L.S., lately I've been playing with Data.Generics and records. I would like to write a generic method to output to e.g. CSV files. Given MissingH.CVS that problem reduces to a generic method to take lists of records to lists of lists with shown record fields, with as first element a list of the field names. That is, something quite like: > totable :: [RecordType] -> [[String]] > totable rs = fields (head rs) ++ map showrecord rs > where fields a = constrFields . head . dataTypeConstrs . dataTypeOf $ a > showrecord = gmapQ (show . toConstr) However, there are no guarantees whatsoever that the order of the fieldnames returned by constrFields corresponds to the order in which gmapQ returns the results for the children (that is the fields, in this case) of the records. Furthermore, I see no way of forcing such an order otherwise, other than writing boilerplate myself. So, is there something I just haven't seen in the lib yet, is something missing, or is just the documentation inconclusive and does the library behave as I want it to? Doei, Arthur. -- /\ / | arthurvl(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

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Re: [Haskell-cafe] SimonPJ and Tim Harris explain STM - video
by Cale Gibbard 23 Nov '06

23 Nov '06

On 23/11/06, Jason Dagit <dagit(a)eecs.oregonstate.edu> wrote: > A comment on that video said: > > ----- BEGIN QUOTE ---- > It seems to me that STM creates new problems with composability. > You create two classes of code: atomic methods and non atomic methods. > > Nonatomic methods can easily call atomic ones – the compiler could > even automatically inject the atomic block if the programmer forgot. > > Atomic methods and blocks cannot be allowed to call nonatomic code. > The nonatomic code could do I/O or other irrevocable things that would > be duplicated when the block had to retry. > ---- END QUOTE ---- > > I imagine an example like this (some pseudo code for a side effect > happy OO language): > > class Foo { > protected int counter; // assume this gets initialized to 0 > public doSomething() { > atomic{ > counter++; > Console.Write("called doSomething execution# " + counter); > // something which could cause the transaction to restart > } > } > public doOtherThing() { > atomic{ > doSomething(); > // something which could cause the transaction to restart > } > } > } > > Now imagine doSomething gets restarted, then we see the console output > once each time and counter gets incremented. So one solution would be > to move the side effects (counter++ and the console write) to happen > before the atomic block. This works for doSomething, but now what if > we called doOtherThing instead? We're back to having the extra > side-effects from the failed attempts at doSomething, right? We just > lost composability of doSomething? I'm assuming counter is only meant > to be incremented once per successful run of doSomething and we only > want to see the output to the log file once per successful run, but it > needs to come before the log output inside doSomething so that the log > makes sense. > > I realize STM is not a silver bullet, but it does seem like > side-effects do not play nicely with STM. What is the proposed > solution to this? Am I just missing something simple? Is the > solution to make it so that Console.Write can be rolled back too? > > Thanks, > Jason The solution is to simply not allow side effecting computations in transactions. They talk a little about it in the video, but perhaps that's not clear. The only side effects an atomic STM transaction may have are changes to shared memory. Another example in pseudocode: atomic x <- launchMissiles if (x < 5) then retry This is obviously catastrophic. If launchMissiles has the side effect of launching a salvo of missiles, and then the retry occurs, it's unlikely that rolling back the transaction is going to be able to put them back on the launchpad. Worse yet, if some variable read in launchMissiles changes, the transaction would retry, possibly causing a second salvo of missiles to be launched. So you simply disallow this. The content of a transaction may only include reads and writes to shared memory, along with pure computations. This is especially easy in Haskell, because one simply uses a new monad STM, with no way to lift IO actions into that monad, but atomically :: (STM a -> IO a) goes in the other direction, turning a transaction into IO. In other languages, you'd want to add some static typechecking to ensure that this constraint was enforced. - Cale

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help with happy/alex
by robert bauer 23 Nov '06

23 Nov '06

Hi, I have a .y and .x file. The alex -g _.x gives me a .hs file and happy -g -a -c _.y gives me a .hs file. I then use ghc -c alex.hs to get a a .o and a .hi file. This works great. Unfortunately ghc -c happy.y doesn't work -- it says that it the module name for the lexer doesn't match. I've attached the .y and .x files. Also, I cannot figure out what I need to do in the parser to call the lexer and I don't know how to kick start the parser -- I have no clue what to put in third, main.hs, module. Any help would be appreciated.

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Ranged Sets version 0.0.3
by Paul Johnson 23 Nov '06

23 Nov '06

The Ranged Sets library version 0.0.3 is now available, after a long hiatus. The CVS version on Sourceforge is now no longer used. Instead you can find a Darcs repository at http://ranged-sets.sourceforge.net/Ranged/ From the Changes.txt file: Version 0.0.3 ------------- Removed support for infinite sets. They sometimes still work, but generally are more trouble than they are worth. There is no simple set of rules for client applications to guarantee termination. Replaced the "deriving" clause for the Range type with instance declarations. Empty ranges created with different bounds will now test as equal. All empty ranges now compare as less than all non-empty ranges. "show" returns a string such as "3.5 < x <= 4.6", or "x < 23". Removed "maybeRange". Changed "rangeIntersection" to return a "Range" instead of a "Maybe Range". If the intersection is empty then it returns an empty range instead of Nothing. Renamed "rangeEmpty" to "rangeIsEmpty" for consistency with "rSetIsEmpty" Added "emptyRange" and "fullRange"

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Weird field name mangling.
by Arthur van Leeuwen 22 Nov '06

22 Nov '06

L.S., consider the following code: > module TestZ where > > import Data.Generics > > data TestZ = TestZ { testz :: Int } > deriving (Show, Read, Eq, Typeable, Data) > > main = print $ constrFields . toConstr $ TestZ { testz = 2 } I would expect this to output ["testz"]. Instead it outputs ["testzz"], using GHCi 6.4.1. Why is that z doubled? Doei, Arthur. -- /\ / | arthurvl(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

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Haskell Weekly News: November 22, 2006
by dons＠cse.unsw.edu.au 22 Nov '06

22 Nov '06

--------------------------------------------------------------------------- Haskell Weekly News http://haskell.org/haskellwiki/HWN Issue 50 - November 22, 2006 --------------------------------------------------------------------------- Welcome to issue 50 of HWN, a weekly newsletter covering developments in the Haskell community. This week a new release of the Haskell XML Toolbox, and details for the inaugural Haskell Hackathon are announced. And the HWN half century of issues is on the board! Announcements * Haskell XML Toolbox 7.0. Uwe Schmidt [1]released a new version of the [2]Haskell XML Toolbox. New in 7.0 is a module for XSLT transformation. The XSLT module implements most of the XSLT standard. The development of the XSLT module is done by Tim Walkenhost in his master thesis, describing the design of the transformer (and the limitations) is included in the distribution. HXT 7.0 works with ghc-6.4 and ghc-6.6. [3]A tutorial is available in the Haskell wiki. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14487 2. http://www.fh-wedel.de/~si/HXmlToolbox/index.html 3. http://www.haskell.org/haskellwiki/HXT * Hac: Haskell Hackathon 2007. The Hac organisers [4]announced that the inaugural Haskell Hackathon, [5]Hac 2007, will be held at Oxford University Computing Laboratory, January 10-12, 2007. The plan is to hack on Haskell infrastructure, tools, libraries and compilers. To attend please register, and get ready to hack those lambdas! 4. http://article.gmane.org/gmane.comp.lang.haskell.general/14489 5. http://haskell.org/haskellwiki/Hac_2007 * System.FilePath 0.11. Neil Mitchell [6]announced the release of [7]System.FilePath 0.11, a library for manipulating FilePath's correctly on both Posix and Windows. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14485 7. http://www-users.cs.york.ac.uk/~ndm/projects/libraries.php#filepath * Darcs release candidate. Tommy Pettersson [8]announced it's time for a new darcs release candidate, 1.0.9rc2. There will probably be yet another release candidate (rc3) before final 1.0.9. Get testing! 8. http://article.gmane.org/gmane.comp.version-control.darcs.devel/5010 * Safe library 0.1. Neil Mitchell [9]announced the release of [10]Safe library, 0.1. People often have a problem with pattern match errors, and the only helpful message they get is: 'pattern match error'. The Safe library hopes to eliminate some of the frustration this causes by providing versions of these partial functions with more helpful error messages. 9. http://article.gmane.org/gmane.comp.lang.haskell.general/14475 10. http://www-users.cs.york.ac.uk/~ndm/projects/libraries.php#safe * LocH, located errors 0.1. Don Stewart [11]announced the release of LocH, a small Haskell module providing source location-specific error messages and debugging strings for Haskell code. It uses the compiler-expanded 'assert' token, rather than cpp or m4, to provide a lightweight approach to generating source locations. No preprocessor is required. More information is available at [12]the LocH site, including [13]API documentation. 11. http://article.gmane.org/gmane.comp.lang.haskell.general/14475 12. http://www.cse.unsw.edu.au/~dons/loch.html 13. http://www.cse.unsw.edu.au/~dons/loch/Debug-Trace-Location.html * Starting your own Haskell project. Jason Dagit and Don Stewart [14]expanded on the document describing how best to [15]set up a new Haskell project, leading to the creation of [16]mkcabal,a new tool for setting up cabalised Haskell projects. 14. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16689/focus=16689 15. http://haskell.org/haskellwiki/How_to_write_a_Haskell_program 16. http://article.gmane.org/gmane.comp.lang.haskell.cabal.devel/269 Haskell' This section covers the [17]Haskell' standardisation process. * [18]Defaults * [19]Pattern guards and where clauses 17. http://hackage.haskell.org/trac/haskell-prime 18. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1907/focus=1907 19. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1904/focus=1904 Libraries This week's proposals and extensions to the [20]standard libraries. * [21]Add parsing (and some other changes) to the time package 20. http://haskell.org/haskellwiki/Library_submissions 21. http://article.gmane.org/gmane.comp.lang.haskell.libraries/5706 Discussion * New Arrows tutorial. Tim Newsham [22]wrote a small arrows tutorial, and is looking for feedback. 22. http://www.thenewsh.com/~newsham/x/arrow.txt * Yhc.Core backends for Haskell/Javascript/C. Neil Mitchell [23]asked about the direction the Yhc Core backends should take. 23. http://article.gmane.org/gmane.comp.lang.haskell.yhc/460 * The Burrows Wheeler Transformation. Jon Fairbairn [24]proposed a game to find the most elegant implementation of the Burrows Wheeler transformation and its inverse. 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16650/focus=16650 * More fusion. Claus Reinke [25]described a method for avoiding the O(n) overhead of cons in strict ByteStrings. Some alternative strategies were also suggested. 25. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/11267/focus=11… * Extending TypeEq: topEq for types. Nicolas Frisby [26]described a solution to an interesting type problem using TypeEq, Oleg followed up with an alternate solution 26. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16638/focus=16638 * Debugging by the rules. Don Stewart [27]described an idea to use rewrite rules to change mysterious failing functions to more useful variants. More details on using rewrite rules for various ticks have been [28]collected on the wiki. 27. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16577/focus=16577 28. http://www.haskell.org/haskellwiki/Playing_by_the_rules * Generalising zip. Jason Dagit [29]wondered about a generalisation of zip to encode functions like isPrefixOf 29. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16626/focus=16626 * Heterogeneous collections. Valentin Gjorgjioski [30]asked about techniques for heterogeneous data in Haskell. Several solutions were suggested, and a [31]new wiki page collects the details. 30. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16653/focus=16653 31. http://haskell.org/haskellwiki/Heterogenous_collections Blog noise [32]Haskell news from the blogosphere. * [33]The Maybe type in Java * [34]Erlang User Conference * [35]The Missing LINQ * [36]APLAS 06 * [37]The New Lambda Expressions Feature in C# 3.0 * [38]Edit Graphs * [39]A search engine (core) written in Haskell * [40]Vim indentation mode for Haskell * [41]Haskell Transactional Cache * [42]Haskell support added to Google Code Search * [43]The type system of ECMA Eiffel (modelled in Haskell) * [44]Diff in Haskell * [45]Computers were invented to keep track of boring things * [46]Lazy lists as iterators * [47]Library use in the Common Lisp community * [48]Local and global side effects with monad transformers * [49]Eratosthenes sieve * [50]Lazy streams for Lua * [51]Language design * [52]Overlap function in Haskell for rectangles 32. http://planet.haskell.org/ 33. http://blog.tmorris.net/maybe-in-java 34. http://www.process-one.net/index.php/en/comments/erlang_user_conference_200… 35. http://eamon.nerbonne.org/2006/11/gwc-missing-linq.html 36. http://blog.tmorris.net/aplas-06/ 37. http://www.developer.com/net/csharp/article.php/3598381 38. http://alangloria.blogspot.com/2006/11/edit-graphs-im-currently-hacking.html 39. http://haskell-web.blogspot.com/2006/11/search-engine-written-in-haskell.ht… 40. http://koweycode.blogspot.com/2006/11/haskell-vim-indentation.html 41. http://haskell-web.blogspot.com/2006/11/transactional-cache-for-haskell.html 42. http://groups.google.com/group/Google-Code-Search/browse_thread/thread/a1f8… 43. http://teameiffel.blogspot.com/2006/11/type-system-of-ecma-eiffel.html 44. http://alangloria.blogspot.com/2006/11/okay.html 45. http://dynamic.ropine.com/yesh/article/computers-were-invented-to-keep-trac… 46. http://community.livejournal.com/evan_tech/206250.html 47. http://sigkill.dk/blog/archives/240-Library-use-in-the-Common-Lisp-communit… 48. http://sigfpe.blogspot.com/2006/09/local-and-global-side-effects-with.html 49. http://kenta.blogspot.com/2006/11/eratosthenes-sieve.html 50. http://salimma.livejournal.com/72850.html 51. http://jlouisramblings.blogspot.com/2006/11/on-language-design.html 52. http://sancho-ex.livejournal.com/7056.html Quotes of the Week * Larry Wall: People who succeed in brushing up on Haskell do so by brushing up lazily. * xerox: [dons] then again, bots themselves are simple. [xerox] '...they are just a morphism in the category of IRC channels together with an endomapping ...' * Dave Love: Presumably people with the taste to program in Haskell will use Emacs anyway. * Baughn: I know I always have trouble explaining what the weather is like without involving foldr somehow. * Korollary: Darcs is free of chemical toxins of big pharma. It's made using all natural herbs and other woo. It's good for you. Cheers. * Pseudonym: A 'newbie', in Haskell, is someone who hasn't yet implemented a compiler. They've only written a monad tutorial. * astrolabe: I guess missile launching would have to be in IO * dons: Java's type system is unsuitable for young children, the elderly or infirm of constitution * earthy: The Haskell standard Prelude is ... a treasure trove. But it does require breaking your mind and reassembling the pieces to unlock the trove. * emu: Hijack the stack and take this compiler to Fun-land! * norpan: The comonad of the No monad is the Yes monad? * robreim: I'm in your base hacking all your lambdas * shapr: I was chatting with someone who writes Haskell and at some point they said they're not an academic, and I said that too. And then there was what I perceived to be a small disbelieving silence... * simonpj: Maybe if I had an Oleg implant I could express all this in the type system * skew: [On the subject of strictness] There is a sequet incantation. * ski: Please talk to your son or daughter about parametric polymorphism * wadler: The way we can tell it's C# instead of Haskell is because it's nine lines instead of two. Code Watch * Wed Nov 8 09:14:52 PST 2006. Alexey Rodriguez. [53]Addition of PAPI to the RTS. [54]PAPI is a library that supports use of the performance counters built into most CPUs for measuring things like cache misses and branch mispredictions. Alexey Rodriguez has added support for PAPI to the RTS, so we can take measurements of any of the CPU performance counters during the run of a Haskell program, and we even separate out results for the GC from results for the mutator. 53. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/17864 54. http://icl.cs.utk.edu/papi/ About the Haskell Weekly News Each week, new editions are posted to [55]the Haskell mailing list as well as to [56]the Haskell Sequence and [57]Planet Haskell. [58]RSS is also available, and headlines appear on [59]haskell.org. Headlines are available as PDF. The Haskell Weekly News is also [60]available in Spanish translation. To help create new editions of this newsletter, please see the [61]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get http://www.cse.unsw.edu.au/~dons/code/hwn 55. http://www.haskell.org/mailman/listinfo/haskell 56. http://sequence.complete.org/ 57. http://planet.haskell.org/ 58. http://sequence.complete.org/node/feed 59. http://haskell.org/ 60. http://haskell.org/haskellwiki/HWN/es 61. http://haskell.org/haskellwiki/HWN

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ANNOUNCE: Haskell Hackathon, Hac 07
by dons＠cse.unsw.edu.au 22 Nov '06

22 Nov '06

------------------------------------------------------------------------ Hac 2007 The 2007 Haskell Hackathon January 10-12, 2007 Oxford University Computing Laboratory http://haskell.org/haskellwiki/Hac_2007 ------------------------------------------------------------------------ Following the success of the GHC Hackathon at ICFP, we are pleased to announce the inaugural Haskell Hackathon, Hac 2007! The event will be held over 3 days, January 10-12 2007, at the Oxford University Computing Laboratory, in the UK. The plan is to hack on Haskell infrastructure, tools, libraries and compilers. To attend please register, and get ready to hack those lambdas! Code to hack on: * Hackage * Cabal * Porting foreign libraries * Porting compilers * Bug squashing * Performance improvements to compilers * You decide! Registration: We ask that you register you interest. Follow the instructions on the registration page: http://haskell.org/haskellwiki/Hac_2007/Registration Numbers for the hackathon are limited to 20, due to room size constraints. Should space be available after the registration period ends, additional attendees may register on a first-come first-served basis. Important dates: Registration deadline: December 6th, 2006 Confirmation: December 10th, 2006 Hackathon: January 10-12, 2007 Organisers: Duncan Coutts Ian Lynagh Don Stewart

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ANNOUNCE: Haskell XML Toolbox Version 7.0
by Uwe Schmidt 22 Nov '06

22 Nov '06

Haskell XML Toolbox 7.0 I would like to announce a new version of the Haskell XML Toolbox. New in 7.0 is a module for XSLT transformation. The XSLT module implements most of the XSLT standard. Only a few features are missing. The development of the XSLT module is done by Tim Walkenhost in his master thesis. The master thesis, describing the design of the transformer (and the limitations) is included in the distribution. HXT 7.0 works with ghc-6.4 and ghc-6.6 A tutorial is availiable in the Haskell wiki: http://www.haskell.org/haskellwiki/HXT More information and download: http://www.fh-wedel.de/~si/HXmlToolbox/index.html Please email comments, bugs, etc. to hxmltoolbox(a)fh-wedel.de Uwe -- University of Applied Sciences, Wedel, Germany http://www.fh-wedel.de/~si/index.html

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ANN: System.FilePath 0.11
by Neil Mitchell 20 Nov '06

20 Nov '06

Hi ANNOUNCEMENT: I am hereby announcing System.FilePath 0.11, which hopefully is pretty close to final now. This library manipulates FilePath's correctly on both Posix and Windows. http://www-users.cs.york.ac.uk/~ndm/projects/libraries.php#filepath (Includes a darcs repo, a .tar.gz, haddock documentation etc) CHANGES: I made some changes in response to comments by Simon Marlow: Removed addFileName, renamed the path splitting to searchPath, renamed shortPath to makeRelative. I did not follow two of his suggestions: removing isDirectory/asDirectory, removing splitPath. (I note these so it is easy to see which suggestions/point are outstanding easily) I also fixed some of the documentation (thanks to Evan) and added a few more properties. All in all, not much changed. COMPATABILITY: If you import System.FilePath.Version_0_10 then nothing will have changed. If you import System.FilePath.Version_0_11 then you'll get the new version. The original Version_0_09 will also continue to work unmodified. Hopefully no one will have any compatability issues. DISCUSSION: We seem to be getting towards the stage where most people who want a FilePath library in a similar spirit to this are reasonably happy with this one. Unless anyone comes up with anything too big, I'm going to propose this for the base library in a few days. Can everyone with an opinion on that issue please hold off for a few days. Thanks Neil

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