October 2006 - Haskell

Fun in the Afternoon: Thurs 16th Nov in Oxford
by Jeremy.Gibbons＠comlab.ox.ac.uk 11 Oct '06

11 Oct '06

Dear colleagues, Graham Hutton and Conor McBride at Nottingham and I are organizing a termly seminar "Fun in the Afternoon" on functional programming and related topics. The idea is to have a small number of talks as an antidote to mid-term blues, three afternoons a year. The hope is that talks will be informal and fun, and that there will be plenty of scope for discussion and chat as well. Fun in the Afternoon will be peripatetic. The first meeting will be in Oxford University Computing Laboratory on Thursday 16th November, and Phil Wadler of the University of Edinburgh will be opening proceedings. All are welcome, but if you'd like to come, could you please drop me (Jeremy Gibbons) a line so that I have an idea of numbers? If you'd like to give a talk, please also propose a title and a duration. We haven't yet fixed the timetable, because it depends in part on what offers of talks we get. But our current plan is to have talks 14:00-14:45 and 15:30-17:00. Brings sandwiches to eat together beforehand, and join us for drinks in a pub afterwards. There's a webpage with more information at http://sneezy.cs.nott.ac.uk/fun/ This page also explains how to subscribe to the mailing list, to which all further announcements will be sent. Directions to OUCL are at http://web.comlab.ox.ac.uk/oucl/about/directions.html Jeremy, Graham and Conor

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ANNOUNCE: Cabal version 1.1.6
by Duncan Coutts 11 Oct '06

11 Oct '06

The Haskell Cabal The Common Architecture for Building Applications and Libraries. http://haskell.org/cabal/ Cabal version 1.1.6 is now available. It is included in GHC version 6.6. For other Haskell implementations or older versions of GHC you can install it separately: DOWNLOAD: http://www.haskell.org/cabal/download.html http://www.haskell.org/cabal/release/latest/cabal-1.1.6.tar.gz DOCUMENTATION: http://www.haskell.org/cabal/release/latest/doc/users-guide/ http://www.haskell.org/cabal/release/latest/doc/API/ See both the README file and the changelog for interface changes: http://www.haskell.org/cabal/release/latest/changelog BUGS: Report bugs using our bug tracker: http://hackage.haskell.org/trac/hackage/newticket or to the libraries(a)haskell.org mailing list.

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ANNOUNCE: GHC version 6.6
by Ian Lynagh 11 Oct '06

11 Oct '06

=========================================================== The (Interactive) Glasgow Haskell Compiler -- version 6.6 =========================================================== The GHC Team is pleased to announce a new release of GHC. There have been many changes since the 6.4.2 release. For details, see the release notes here: http://haskell.org/ghc/docs/6.6/html/users_guide/release-6-6.html How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language; the current language version is Haskell 98, agreed in December 1998 and revised December 2002. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ GHC developers' home page http://hackage.haskell.org/trac/ghc/ Haskell home page http://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: http://hackage.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://www.haskell.org/ghc/docs/latest/html/building/building-guide.html Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: http://hackage.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at http://www.haskell.org/mailman/listinfo/glasgow-haskell-users http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see http://www.haskell.org/mailman/listinfo/ Some GHC developers hang out on #haskell on IRC, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

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Classes with no data type
by Thomas Davie 10 Oct '06

10 Oct '06

Hi, I've met an interesting problem in terms of how to type a data structure and the functions that operate upon it. The problem centres around a single data type. This data type can be constructed in multiple ways using different functions, depending on the options the user specifies. That's all simple enough. The problem really comes later on. Depending on the function used generate the data structure I want to use different functions later on for example, to display the data. Thus I have a typical classes problem, in that I have several implementations of essentially the same function for different circumstances. The problem is, they must all operate on the same data type, so I cannot define them as seperate instances. Anyone got any ideas how to type this? Bob

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Re: [Haskell] Classes with no data type
by Chad Scherrer 10 Oct '06

10 Oct '06

Bob, If you want to display them differently depending on how they were generated, I would wrap each one in a newtype. Then you can make the newtypes instances of different typeclasses. The wrapper remembers how the structure was generated, and using typeclasses, you should be able to treat each in the appropriate way pretty easily. -Chad > Hi, > I've met an interesting problem in terms of how to type a data > structure and the functions that operate upon it. > > The problem centres around a single data type. This data type can be > constructed in multiple ways using different functions, depending on > the options the user specifies. That's all simple enough. The > problem really comes later on. Depending on the function used > generate the data structure I want to use different functions later > on for example, to display the data. > > Thus I have a typical classes problem, in that I have several > implementations of essentially the same function for different > circumstances. The problem is, they must all operate on the same > data type, so I cannot define them as seperate instances. > > Anyone got any ideas how to type this? > > Bob > > > ------------------------------ > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > > > End of Haskell Digest, Vol 38, Issue 6 > ************************************** > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Expecting more inlining for bit shifting
by roconnor＠theorem.ca 10 Oct '06

10 Oct '06

Consider the following GHC code: module Main where import GHC.Word import GHC.Base import GHC.Prim import Random a `shiftRLT` b | b >=# 32# = int2Word# 0# | otherwise = a `uncheckedShiftRL#` b (W32# x#) `shift` (I# i#) | i# >=# 0# = W32# (narrow32Word# (x# `shiftL#` i#)) | otherwise = W32# (x# `shiftRLT` negateInt# i#) x `shiftR` i = x `shift` (-i) shift7 x = x `shiftR` 7 shift6 (W32# x) = (W32# (x `uncheckedShiftRL#` 6#)) main = do xs <- sequence (replicate 1000000 (fmap (shift7 . fromIntegral) (randomIO::IO Int))) print (sum xs) I have copied the definition of `shiftR` for Word32 into this file. Suppose we want to shift a series of numbers by 7 bits. One would expect GHC's inliner to notice that (-7) is indeed not greater than 0, and eliminate the branch in the definition of `shift`. Further one would expect GHC to notice that 7 is indeed not gtreater than 32, and eliminate the branch in shiftRLT. Thus one would expect the code generated by using shift7 to be identical to that being generated by shfit6 (with 7 replaced by 6). But this appears not to be the case. The code generated for shift7 (if I can read the C code correctly) is: Sp[-1] = (-0x7U); Sp[-2] = R1.p[1]; *Sp = (W_)&s2za_info; Sp=Sp-2; JMP_((W_)&Main_zdwshift_info); while the code generated for shift6 is the lovely: Hp=Hp+2; if ((W_)Hp > (W_)HpLim) goto _c2Aa; _s2xq = (R1.p[1]) >> 0x6U; Hp[-1] = (W_)&GHCziWord_W32zh_con_info; *Hp = _s2xq; R1.p=Hp-1; Sp=Sp+1; JMP_(*Sp); _c2Aa: HpAlloc = 0x8U; JMP_(stg_gc_enter_1); My question is, why the discrepency? -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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ANNOUNCE: Haddock version 0.8
by Simon Marlow 10 Oct '06

10 Oct '06

Announcing a new release of the Haddock documentation tool, version 0.8. Changes in this release: * Haddock has a Cabal build system, and will build on Windows without Cygwin or MSYS. GHC 6.4 or later is required. * New options: --comments-base, --comments-module, --comments-entity for generating links to pages (eg. wiki) for collecting user comments. * New options: --source-base, --source-module, --source-entity for generating links to source code. Haddock now understands {-# LINE #-} pragmas, which means it can figure out the correct name for the source file even if the file is preprocessed. * Haddock generates output for populating the Hoogle search engine. * New markup <<url>> for including images. Documentation, source code and binaries are available from http://www.haskell.org/haddock Cheers, Simon

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ANNOUNCE: STM invariants and exceptions
by Tim Harris (RESEARCH) 10 Oct '06

10 Oct '06

Hi, I've checked a number of changes to the transactional memory support into GHC head: 1. The main change is to add support for dynamically checked data invariants of the kind described in http://research.microsoft.com/~tharris/papers/2006-transact.pdf. There are two operations: "always X :: STM Bool -> STM ()" and "alwaysSucceeds X :: STM a -> STM ()". Invariants are expressed as STM actions which must hold (i) when passed to always/alwaysSucceeds, and (ii) at the point that every subsequent transaction commits. An invariant passed to "always" signals failure by returning False or by raising an exception. An invariant passed to "alwaysSucceeds" signals failure just by raising an exception. Tests conc062 and conc063 try out some corner cases, and the paper discusses the details (what if an invariant calls "retry", what if it updates TVars, what if it loops, and so on etc...) 2. If an atomic block contains "catch X Y" and X raises an exception then the exception is passed to Y after rolling back the effects of X. Previously we would retain the effects of X, requiring the programmer to manually compensate for the portion of X that had run before raising the exception. Rolling back X seems more consistent with the treatment of exceptions that propagate out of atomic blocks, and the argument that transactional memory can bring ease-of-programming benefits by automating the construction of compensating code for memory operations. The post-publication version of the "Composable memory transactions" paper has more details http://research.microsoft.com/~tharris/papers/2005-ppopp-composable.pdf. Test conc061 tests some of the corner cases. 3. There are some bug-fixes to the implementation of "X `orElse` Y" in which, under high contention, X and Y may not appear to execute against the same view of the heap. I'm not sure if this problem has ever come up in practice! We'd love to hear about it if you're using these features, Cheers, Tim

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Haskell Weekly News: October 10, 2006
by dons＠cse.unsw.edu.au 10 Oct '06

10 Oct '06

--------------------------------------------------------------------------- Haskell Weekly News http://haskell.org/haskellwiki/HWN Issue 44 - October 10, 2006 --------------------------------------------------------------------------- Welcome to issue 44 of HWN, a weekly newsletter covering developments in the Haskell community. Developments this week include Lennart Kolmodin's new inotify bindings for Haskell, work begins on Spanish translations of Haskell literature, and new versions of Darcs and Cabal are tagged Announcements * hinotify 0.1. Lennart Kolmodin [1]announced hinotify 0.1, a library to [2]inotify which has been part of the Linux kernel since 2.6.13. inotify provides file system event notification, simply add a watcher to a file or directory and get an event when it is accessed or modified. [3]API and [4]source. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14345/ 2. http://www.kernel.org/pub/linux/kernel/people/rml/inotify/ 3. http://haskell.org/~kolmodin/code/hinotify/docs/api/ 4. http://haskell.org/~kolmodin/code/hinotify/ * Monad Transformer Tutorial. Martin Grabmueller [5]published a small tutorial on using monad transformers. In contrast to others approaches, it concentrates on using them, not on their implementation. [6]PDF and Literate Haskell source available. 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/15714 6. http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html * Speaking Haskell in Spanish. Luis Araujo [7]announced [8]a project to make Haskell documentation more available to Spanish speakers. The idea is to [9]collect information in Spanish about Haskell, including news and tutorials, and to translate [10]Haskell wiki pages. 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/15713/ 8. http://haskell.org/haskellwiki/Haskell.es 9. http://www.haskell.org/haskellwiki/Haskell.es 10. http://haskell.org/haskellwiki/Special:Popularpages * Haskell Packages 6.6. Isaac Jones [11]announced that the Cabal package tools for Haskell are in a good state, with almost 30 packages already in [12]the database. Time to start testing packages, starting with the cabal release candidate that'll go into GHC 6.6, to make sure they work nicely together! 11. http://article.gmane.org/gmane.comp.lang.haskell.cabal.devel/175 12. http://hackage.haskell.org/packages/ * Cabal-1.1.6 release candidate. Duncan Coutts [13]released a tarball for the next 1.16 Cabal release candidate. Let's get this tested before GHC 6.6 arrives! 13. http://article.gmane.org/gmane.comp.lang.haskell.libraries/5213/ * Darcs 1.0.9 release candidate. Tommy Pettersson [14]announced the first release candidate for next stable [15]darcs, 1.0.9rc1. This will mainly be a bug fix version to get things right that got wrong or didn't get right in 1.0.7 and 1.0.8, but there are some new features and optimizations too. 14. http://article.gmane.org/gmane.comp.version-control.darcs.user/10332 15. http://darcs.net/ * Haskell and Vim. Marc Weber [16]wrote some Vim scripts to ease various Haskell coding tasks in Vim. 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/15707 Haskell' This section covers the [17]Haskell' standardisation process. * [18]Status report * [19]Stand-alone deriving declarations (part 1) * [20]Stand-alone deriving declarations (part 2) 17. http://hackage.haskell.org/trac/haskell-prime 18. http://hackage.haskell.org/trac/haskell-prime/wiki/Status' 19. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1717/focus=1717 20. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1726/focus=1726 Discussion * Allowing both prefix unary minus and right section subtraction. Michael Shulman [21]described a technique for writing operators that can be used both as infix or postfix operators, using the new postfix support in GHC 6.6. 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/15710/ * Google Summer of Code Summit and Haskell. Don Stewart [22]sought feedback on this year's Google Summer of Code Haskell projects, in preparation for Haskell.org's attendance at the Google SoC Summit. 22. http://article.gmane.org/gmane.comp.lang.haskell.general/14339/ * GHC under Wine. Robert Marlow [23]described his experience setting up GHC under Wine to produce Windows binaries from Linux. 23. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/10857 * Function lists and arguments. Joel Koerwer [24]described a puzzle to try to apply a function of type a function of type (a -> a -> ... -> a -> a), to a list of arguments of the same length. [25]Some solutions were suggested. 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/15681/focus=15681 25. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/15683/focus=15683 Conference roundup * APLAS'06. Manuel Chakravarty [26]announced the call for participation [27]4th Asian Symposium on Programming Languages and Systems, to be held in Sydney. 26. http://article.gmane.org/gmane.comp.lang.haskell.general/14346 27. http://www.cse.unsw.edu.au/~aplas06/ Jobs * Open assistant professorship at Utrecht. Doaitse Swierstra [28]announced a 5 year position for an assistant professor at Utrecht. Current areas of interest include: domain specific embedded languages, programming language design and implementation, generic programming, program verification, advanced type systems. [29]More info. 28. http://article.gmane.org/gmane.comp.lang.haskell.general/14343/ 29. http://www.cs.uu.nl/vacatures/en/62612.html * PhD studentship. Shengchao Qin [30]announced that applications are invited for a PhD student to work on resource analysis and verification for dependable embedded software, at the University of Durham. The successful applicant should have a good background in program analysis, type systems and type theory, software verification and embedded systems. 30. http://article.gmane.org/gmane.comp.lang.haskell.general/14347/ * Post-doctoral position. Stephan Merz [31]announced that applications are invited for a post-doctoral fellowship on the modeling and verification of domain-specific languages in the [32]MOSEL team at LORIA. The project aims to model domain-specific languages in the specification language [33]TLA+ and to develop verification techniques for programs written in DSLs against high-level correctness properties. Candidates must hold a PhD and should have demonstrated research interest in formal methods and formal reasoning. They should preferably have experience with interactive proof assistants. 31. http://lists.seas.upenn.edu/pipermail/types-announce/2006/000298.html 32. http://www.loria.fr/equipes/mosel/ 33. http://research.microsoft.com/users/lamport/tla/tla.html Blog noise Haskell news from the blogosphere. * [34]Roll your own Haskell IRC bot * [35]Understanding Monads Via Python List Comprehensions * [36]Mercury and imperative programming * [37]Languages as first-class design decisions * [38]Becoming a Haskell developer via YHC * [39]JAOO Conference: Functional Programming And Monads * [40]CompSci.ca does Functional Programming 34. http://haskell.org/haskellwiki/Roll_your_own_IRC_bot 35. http://lukeplant.me.uk/blog.php?id=1107301643 36. http://www.mercury.cs.mu.oz.au/mailing-lists/mercury-users/mercury-users.99… 37. http://www.recursivethought.com/blog/view/languages-as-design 38. http://jaortega.wordpress.com/2006/10/08/becoming-a-haskell-developer/ 39. http://best-practice-software-engineering.blogspot.com/2006/10/event-jaoo-c… 40. http://compsci.ca/blog/compsci-does-functional-programming/ Quotes of the Week * Larry Wall: Perl is turning into Haskell. Someone shoot me. * Tim Berners-Lee [and the w3c]: Functional languages such as Haskell ... facilitate the creation of programs that may be easier to analyze than their imperative equivalents. * lispy: > init . map snd . filter fst . zip (fix ([True, False] ++)) . ap (zipWith (+)) tail . map length . group . fix $ show * lambdabot: [2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536, 131072,... * Verity Stob: Lisp is still #1 for key algorithmic techniques such as recursion and condescension. * dcoutts: (:[]) looks like a monster * largos: [on programming languages] ...and I'd rather not own as much rope as c/c++ gives you. * ozone: When will bytestring be O(1) for all operations? * skew: [Monads are] just like the sequences of statements in other languages. Except very rarely have they thought so deeply about what that sequencing means Code Watch * Stand-alone deriving declarations added. bringert. Add [41]support for stand-alone 'deriving' declarations. The main motivation for this is to allow you to use the instance deriving mechanism for data types and newtypes declared in other modules, for example if you want to use generics with existing code that does not derive Typeable and Data, or want to use Show for debugging. The syntax is: deriving Class for Type, and for multi-parameter type classes:: deriving (Class t1 ... tn) for Type 41. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/17038/ About the Haskell Weekly News Each week, new editions are posted to [42]the Haskell mailing list as well as to [43]the Haskell Sequence and [44]Planet Haskell. [45]RSS is also available, and headlines appear on [46]haskell.org. To help create new editions of this newsletter, please see the [47]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 42. http://www.haskell.org/mailman/listinfo/haskell 43. http://sequence.complete.org/ 44. http://planet.haskell.org/ 45. http://sequence.complete.org/node/feed 46. http://haskell.org/ 47. http://haskell.org/haskellwiki/HWN

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BCS FACS Christmas Meeting, Teaching Formal Methods, 15 December 2006; papers deadline 20 October 2006
by Clare Martin 09 Oct '06

09 Oct '06

REMINDER: deadline for submissions 20 October 2006 Proceedings will be published in BCS Electronic Workshops in Computing (eWiC) series ---------------------------------------------------------- Call for Contributions and Participation ---------------------------------------------------------- BCS-FACS Christmas Meeting 2006 TEACHING FORMAL METHODS: PRACTICE AND EXPERIENCE A One Day Workshop at BCS London Offices, First Floor, The Davidson Building, 5 Southampton Street, London WC2E 7HA FRIDAY 15 DECEMBER 2006 Organised by Oxford Brookes University (OBU) and BCS-FACS Invited speaker sponsored by Formal Methods Europe (FME) (http://www.fmeurope.org) http://www.bcs-facs.org/events/xmas2006.html The workshop will give teachers of formal methods an opportunity to discuss their experiences in this area, to share successes and failures, to identify issues in teaching formal methods and discuss how they might be addressed. Topics to be covered include: + how to motivate the study of formal methods; + techniques for teaching formal methods; + handling students with limited mathematics backgrounds; + linking formal methods and software development; + tools for teaching formal methods; + how to assess formal methods. Papers (4-6 sides A4) are invited outlining experience from which others could benefit; a selection will be chosen by the Programme Committee for presentation (ca. 20 mins) and inclusion in the proceedings to be published in the BCS Electronic Workshops in Computing (eWiC) series after the event. Paper copies of the proceedings will be made available to delegates. Papers should be prepared according to the eWiC guidelines. Style files are available for Latex, Word and HTML. Authors of accepted papers will be required to sign a licence to publish. All these documents are available at http://ewic.bcs.org/organisers/index.htm. Papers should be submitted as PDF files and sent by email to daduce(a)brookes.ac.uk. Organisations and vendors wishing to display literature or demonstrate tools and products are asked to contact the organisers (email: daduce(a)brookes.ac.uk) DEADLINES 20 October: Submission of papers; send by email to daduce(a)brookes.ac.uk, in PDF format 17 November: Notification of selection for presentation/ inclusion in proceedings 24 November: Final versions of papers for proceedings 6 December: Registration closes 15 December: Workshop [registration to 10.30 am, close approx. 5.00 pm] CO-CHAIRS: David Duce (Oxford Brookes University), Paul Boca (BCS-FACS and Programming Research Ltd) PROGRAMME COMMITTEE: Ian Bayley (OBU), Raymond Boute (Ghent University, Belgium), Jonathan Bowen (London South Bank University), Sharon Curtis (OBU), David Duce (OBU), Ian East (OBU), Mark Green (OBU), Henri Habrias (University of Nantes, France), David Lightfoot (OBU), Clare Martin (OBU), Jose Oliveira (University of Minho, Portugal), Steve Schneider (University of Surrey), Andrew Simpson (Oxford University), Dyke Stiles (Utah State University, USA), Muhammad Younas (OBU), Hong Zhu (OBU) COST The cost of the workshop will be Â£60 for paid up BCS-FACS members and students, and Â£85 for non-FACS members. The cost includes lunch and refreshments, and a printed copy of the proceedings. All delegates will receive free membership to the FACS Specialist Group for 2007. FURTHER DETAILS Please see the web site http://www.bcs-facs.org/events/xmas2006.html 2003 WORKSHOP Proceedings of the Teaching Formal Methods 2003 workshop, held at Oxford Brookes University in December 2003 are available at http://cms.brookes.ac.uk/tfm2003/

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