- Haskell - Haskell.org

(last) call for participation: HAL next Tuesday
by Johannes Waldmann 30 Nov '06

30 Nov '06

Dear all, this is a reminder for the Haskell meeting to be held in Leipzig, Germany, Tuesday next week (December 5th) http://www.iba-cg.de/haskell.html If you want to come, register now. As we have a number of non-local attendees already, we might append a "functional breakfast" the next morning. -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- ---- http://www.imn.htwk-leipzig.de/~waldmann/ -------

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LDTA 07 CfP - deadline extended
by Eric Van Wyk 29 Nov '06

29 Nov '06

[DEADLINE EXTENDED: Title and Abstract due Dec 4, Papers due Dec 11] Call for Papers for Seventh Workshop on Language Descriptions, Tools and Applications LDTA 2007 A satellite event of ETAPS 2007 March 25, 2007 in Braga, Portugal (Cooperation with ACM SIGPLAN to be confirmed) http://www.di.uminho.pt/ldta07 Scope: The aim of this one-day workshop is to bring together researchers from academia and industry interested in the field of formal language definitions and language technologies, with a special emphasis on tools developed for or with these language definitions. This active area of research involves the following basic technologies: - Program analysis, transformation, and generation - Formal analysis of language properties - Automatic generation of language processing tools For example, language definitions can be augmented in a manner so that not only compilers or interpreters can be automatically generated but also other tools such as syntax-directed editors, debuggers, partial evaluators, test generators, and documentation generators. Although various specification formalisms like attribute grammars, action semantics, operational semantics, and algebraic approaches have been developed, they are not widely exploited in current practice. It is the aim of the LDTA workshops to bridge this gap between theory and practice. Among others, the following application domains can benefit from advanced language technologies: - Software component models and modeling languages - Re-engineering and re-factoring - Aspect-oriented programming - Domain-specific languages - XML processing - Visualization and graph transformation - Programming environments such as Eclipse, NetBeans and Visual Studio - Modern runtime platforms including .Net, Rotor, Java Virtual Machine The workshop welcomes contributions on all aspects of formal language definitions, with special emphasis on applications and tools developed for or with these language definitions. Experience papers describing novel or compelling uses of language definition-based methods in real world projects are particularly sought. Invited Speaker: The invited speaker for LDTA 2007 is Uwe Assmann from TU Dresden. Important Dates: - Submission deadline: December 4, 2006 (title and abstract) December 11, 2006 (paper) - Notification: January 16, 2007 - Final version due: February 16, 2007 - Workshop: March 25, 2007 Submission Procedure and Publication: Submission will be open from autumn 2006. Three classes of papers are solicited: research papers, experience reports and short tool-demo papers. Experience reports must describe the use of a language-based tool to solve a non-trivial applied problem with an emphasis on the advantages and disadvantages of the tool. Tool-demo papers should contain a brief description of the tool and include a section that clearly explains what will be demonstrated. Research papers and experience reports should be at most 15 pages in length and tool-demo papers should be at most 4 pages in length. All classes of paper should be submitted electronically as PostScript or PDF files to both of the program committee chairs, Tony Sloane at asloane(a)ics.mq.edu.au and Adrian Johnstone at adrian(a)cs.rhul.ac.uk. The message should also contain a text-only abstract and contact author information. Additional submission details, along with LaTeX style files, are available on the LDTA 2007 web page: http://www.di.uminho.pt/ldta07. The final versions of accepted papers will be published in Electronic Notes in Theoretical Computer Science (ENTCS), Elsevier Science, and will be made available during the workshop. The authors of the best full-length papers will be invited to write a journal version of their paper which will be separately reviewed and, assuming acceptance, be published in journal form. As in past years, this will be done in a a special issue devoted to LDTA 2007 of the journal Science of Computer Programming (Elsevier Science). Program Committee: - Judith Bishop, University of Pretoria, South Africa - Claus Brabrand, BRICS, University of Aarhus, Denmark - Nigel Horspool, University of Victoria, Canada - Johan Jeuring, Utrecht University, The Netherlands - Adrian Johnstone, Royal Holloway, University of London, UK (co-chair), adrian(a)cs.rhul.ac.uk - Steven Klusener, Vrije Universiteit, The Netherlands - Kent Lee, Luther College, USA - Brian Malloy, Clemson University, USA - Terence Parr, University of San Francisco, USA - Michael Schwartzbach, BRICS, University of Aarhus, Denmark - Tony Sloane, Macquarie University, Australia (co-chair), asloane(a)ics.mq.edu.au - Jurgen Vinju, CWI, The Netherlands Organizing Committee: - Thomas Noll, RWTH Aachen University, Germany, noll(a)cs.rwth-aachen.de - Alcino Cunha, University of Minho, Braga, Portugal, alcino(a)di.uminho.pt

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PLAN-X 2007: Call for Participation
by Torsten Grust 29 Nov '06

29 Nov '06

Call for Participation P L A N - X 2 0 0 7 Programming Language Technologies for XML An ACM SIGPLAN Workshop collocated with POPL 2007 Nice, France -- January 20, 2007 www.plan-x-2007.org Please join us for PLAN-X 2007, the fifth workshop in the PLAN-X series, dedicated to the interaction and integration of programming language technology and the world of XML. The XML data model and its associated languages add interesting twists to programming language practice as well as theory. Just like its four predecessors, the PLAN-X 2007 workshop turns the spotlight on how programming language technology can embrace and explain streaming XML transformations, types for XPath and XML updates, web service contracts, tree patterns in XQuery, LINQ and XML Schema, and more. PLAN-X 2007 will feature eight talks, three system demonstrations, extensive opportunity for discussion, and a keynote address (speaker to be announced). PLAN-X 2007 will be held in the Plaza Hotel (Nice, France) all-day on Saturday, January 20, 2007, just after and collocated with POPL 2007, the ACM SIGPLAN - SIGACT Symposium on Principles of Programming Languages (January 17-19, 2007). -- ACCEPTED PAPERS Kazuhiro Inaba, Haruo Hosoya XML Transformation Language Based on Monadic Second Order Logic Nate Foster, Benjamin C. Pierce, Alan Schmitt A Logic Your Typechecker Can Count On: Unordered Tree Types in Practice Alain Frisch, Keisuke Nakano Streaming XML Transformations Using Term Rewriting Giuseppe Castagna, Nils Gesbert, Luca Padovani A Theory of Contracts for Web Services Sebastian Maneth, Helmut Seidl Deciding Equivalences of Top-Down XML Transformations in Polynomial Time Jan Hidders, Philippe Michiels, Jerome Simeon, Roel Vercammen How to Recognise Different Kinds of Tree Patterns From Quite a Long Way Away Pierre Geneves, Nabil Layaida, Alan Schmitt XPath Typing Using a Modal Logic with Converse for Finite Trees James Cheney Lux: A Lightweight, Statically Typed XML Update Language -- ACCEPTED SYSTEM DEMONSTRATIONS Jorge Coelho, Mario Florido XCentric: A Logic-Programming Language for XML Processing Kazuhiro Inaba, Haruo Hosoya MTran: An XML Transformation Language Based on Monadic Second Order Logic Ralf Lämmel LINQ to XSD -- REGISTRATION PLAN-X 2007 is held in cooperation with POPL 2007. You can register for the workshop via the POPL 2007 registration process (online or offline). Please visit http://www.regmaster.com/conf/popl2007.html Registration rates are shown below. The rates are unaffected by the POPL 2007 early bird registration deadline. Note that you can upgrade an existing POPL 2007 registration to include PLAN-X 2007. Workshop-only registration is possible as well. ACM or SIGPLAN Member $89 Non-Member $99 Student $89 Your registration includes a copy of the PLAN-X 2007 informal proceedings, coffee breaks, and lunch. -- PLAN-X 2007 Workshop Chairs - General Chair - Program Chair Torsten Grust Giorgio Ghelli TU München U Pisa Munich, Germany Pisa, Italy grust(a)in.tum.de ghelli(a)di.unipi.it -- PLAN-X 2007 Program Committee - Michael Benedikt (Lucent, USA) - Daniela Florescu (Oracle, USA) - Alain Frisch (INRIA Roquencourt, France) - Giorgio Ghelli, Chair (U Pisa, Italy) - Haruo Hosoya (U Tokyo, Japan) - Anders Møller (U Aarhus, Denmark) - Mukund Raghavachari (IBM T.J. Watson Research Center, USA) - Alan Schmitt (INRIA Rhône-Alpes, France) - Sophie Tison (U Lille, France) - Philip Wadler (U Edinburgh, UK)

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ANN: hpodder 0.99.0
by John Goerzen 28 Nov '06

28 Nov '06

Hi, I'm happy to announce hpodder 0.99.0, the first beta candidate for making an eventual 1.0.0 release of hpodder. hpodder is a podcast downloader that happens to be written in Haskell. This version introduces two major new features: nicer apt-like output and multithreaded downloading. You can download it from http://quux.org/devel/hpodder Thanks, -- John Goerzen Author, Foundations of Python Network Programming http://www.amazon.com/exec/obidos/tg/detail/-/1590593715

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ANN: MissingH 0.16.3
by John Goerzen 28 Nov '06

28 Nov '06

Hello, I am happy to annouce the release of MissingH 0.16.3. The changes in this version include: * New module MissingH.ProgressTracker which tracks the progress of long-running tasks. It can also give estimated time to completion and overall speed statistics, as well as support dividing a task into many subtasks. * New module MissingH.ProgressMeter which displays one or more ProgressTrackers as a status bar on a console. * New module MissingH.Quantity which renders numbers according to a quantification system such as SI or binary * New functions forkRawSystem and pOpen3Raw in MissingH.Cmd. Instead of taking functions to run as actions, instead returns the PID of the child process and leaves it up to the programmer to properly handle the result. As usual, you may download it from http://quux.org/devel/missingh This will be the last release of MissingH as we know it. See the discussion in -cafe for details. The next release will feature major modules split off, and the start of the effort to integrate smaller features into the base Haskell libraries. It will likely also have a new name. -- John

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SimonPJ and Tim Harris explain STM - video
by Bayley, Alistair 28 Nov '06

28 Nov '06

http://channel9.msdn.com/Showpost.aspx?postid=231495 The links to the video are a couple of yellow buttons at the bottom of the article: "Watch" or "Download". I haven't watched this yet (it's nearly an hour long, I think). Found via reddit (http://reddit.com) Haskeller's on TV (sort of...) woot woot! Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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ANNOUNCE: How to make a type-safe alloca: a new construction of a restricted IO monad
by Stefan O'Rear 27 Nov '06

27 Nov '06

Introducing RIO, an experimental library for extensible restricted IO in Haskell. Selling features: * Automatic permissions inference. If you define a function: fun fd r = do { str <- hGetContents fd ; modifyRIORef r (+1) ; return str } RIO can infer the requirements as part of the type signature: fun :: (Elem (ReadHandle a) o, Elem (ReadRef b) o, Elem (WriteRef b) o, Num c) => RHandle a -> RIORef b c -> RIO o String * Recursive context aquisition. If we have: foo ref1 = withContextRef $ \p -> do ref2 <- newRIORef p (undefined :: Int) -- do some stuff with ref1 and ref2 it works. * Potential for region allocation. When you use withContextRef, any variables allocated cannot be type- correctly accessed after withContextRef returns. Hence, they can be deallocated at that time. It is possible to write a function withNewRef that allocates a reference and does not grant the capability to allocate more in the same context; this could be implemented using *stack* allocation. * It is extremely extensible. It is possible to implement a new set of restricted actions in a separate module with very little code. See the file README.LONG for a detailed demonstration. http://members.cox.net/stefanor/RIO-0.1.tar.gz

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Haskell Weekly News: November 28, 2006
by dons＠cse.unsw.edu.au 27 Nov '06

27 Nov '06

--------------------------------------------------------------------------- Haskell Weekly News http://haskell.org/haskellwiki/HWN Issue 51 - November 28, 2006 --------------------------------------------------------------------------- Welcome to issue 51 of HWN, a weekly newsletter covering developments in the Haskell community. Automated testing fever strikes the camp, with three new QuickCheck-related libraries and tools released. Announcements * QuickCheck 2 development version. Bjorn Bringert [1]announced that the development version of QuickCheck 2 is now available in a public darcs repository. Highlights of the new QuickCheck version include: shrinking of failing test cases; supports testing monadic code; handles exceptions gracefully; coarbitrary has moved to a separate class; type-level modifiers for changing test data generation (e.g. NonNegative); function table printing; and user-defined actions when properties fail. The source is [2]available via darcs. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14511 2. http://www.cs.chalmers.se/~bringert/darcs/QuickCheck/ * PQC: QuickCheck in the Age of Concurrency. Don Stewart [3]announced PQC: Parallel QuickCheck. [4]PQC provides a single module: [5]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. 3. http://article.gmane.org/gmane.comp.lang.haskell.general/14503 4. http://www.cse.unsw.edu.au/~dons/pqc.html 5. http://www.cse.unsw.edu.au/~dons/pqc/Test-QuickCheck-Parallel.html * cabal-test: automatic testing for Cabal projects. David Himmelstrup [6]announced cabal-test, the automatic tester for Cabal projects. The cabal-test tool is capable of testing embedded QuickCheck properties in any and all cabalized projects. The tests are currently executed in parallel with PQC. QuickCheck properties can reside anywhere in the code and don't have to be exported. The [7]darcs repo is available. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14519 7. http://darcs.haskell.org/~lemmih/cabal-test * Streams 0.1.7. Bulat Ziganshin [8]announced Streams version 0.1.7, a fast extensible [9]I/O and serialization library. Changes include: GHC 6.6 support, support for files larger than 4G on Windows, haddock documentation. 8. http://article.gmane.org/gmane.comp.lang.haskell.general/14504 9. http://haskell.org/haskellwiki/Library/Streams * Ranged Sets 0.0.3. Paul Johnson [10]announced the 0.0.3 release of [11]Ranged Sets. Ranged sets allow programming with sets of values described by a list of ranges. A value is a member of the set if it lies within one of the ranges. The ranges in a set are ordered and non-overlapping, so the standard set operations can be implemented by merge algorithms in O(n) time. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14492/ 11. http://ranged-sets.sourceforge.net/Ranged/ * Type-class overloaded functions. Oleg Kiselyov [14]presented functions polymorphic over classes of types. Each instance of such (2-polymorphic) function uses ordinary 1-polymorphic methods, to generically process values of many types, members of that 2-instance type class. The typeclass constraints are thus manipulated as first-class entities. We also show how to write typeclass instances with back-tracking: if one instance does not apply, the typechecker will chose the `next' instance -- in the precise meaning of `next'. 14. http://thread.gmane.org/gmane.comp.lang.haskell.general/14482/focus=14483 * Cabal mode for emacs. Matthew Danish [15]released a small (and developing) major mode for editing Cabal files in emacs. 15. http://mapcar.org/haskell/cabal-mode/ * YCR2JS Programmers Guide Draft. Dimitry Golubovsky [16]announced the draft of low-level [17]programming guide for Yhc Core to Javascript converter. Everyone interested in future use of this tool is encouraged to read and review the Guide. Its purpose is to give some ideas about interaction of Haskell programs converted into Javascript with a web browser on the lowest possible level, without application frameworks and support libraries (just because these haven't been developed). 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/16764 17. http://haskell.org/haskellwiki/Yhc/Javascript/Programmers_guide * NeHe Tutorials in Haskell. Jason Dagit [18]announced the availability of the (somewhat) famous NeHe tutorials for OpenGL have been ported to HOpenGL. A [19]darcs repository is provided. 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16870/focus=16870 19. http://codersbase.com/index.php/Nehe-tuts Haskell' This section covers the [20]Haskell' standardisation process. * [21]Standard (core) libraries initiative: rationale 20. http://hackage.haskell.org/trac/haskell-prime 21. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1919/focus=1919 Discussion * The Future of MissingH. John Goerzen [22]opened a discussion on the future development of MissingH, in order to improve usability and adoption by the community. It was felt that the current monolithic structure, and nondescript naming raises the barrier to use. 22. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16819/focus=16819 * Controlling backtracking with the list monad. Isto [23]posed an interesting question about whether the backtracking in a list monad could be controlled, producing a nice way to build branch and bounding algorithms 23. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16776/focus=16776 * Lifting conditionals. Dougal Stanton [24]asked about lifting if/then/else into the IO monad, revealing some interesting subtleties in the implementation 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16779/focus=16779 * Cheap commits for improving the library documentation. Don Stewart [25]created a new page, for non-core contributors to submit suggestions on improving library documentation. Rather than require the user to produce a darcs patch, than can instead add their change to the wiki page. Ideally, a wiki-editable version of the entire haddock docs would be desired 25. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16848/focus=16848 * Optimising a hash function. Ivan Tomac [26]asked about optimising low-level bit shifting code to run close to C speed. 26. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16849/focus=16849 * Common subexpression elimination. Christian Maeder [27]asked about common subexpression elimination in GHC. 27. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/11318/focus=11… * Passing arrays between Haskell and C without overhead. Brian Hulley. [28]asked for advice on how best to bind to a C library, from Haskell, such that data can be passed with zero copying. 28. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/11313/focus=11… * Creating Loadable Dynamic libraries Libraries in OSX. Alfonso Acosta. [29]described how to create dynamic loadable libraries (.so) with GHC 6.6 under Linux X86 using some C glue code and linking with -optl -shared. Advice was sought on porting this to OSX. 29. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/11297/focus=11… Conference roundup [30]Fun in the Afternoon 30. http://sneezy.cs.nott.ac.uk/fun/nov-06/ * Links: web programming without tiers. Phil Wadler. * Mechanized Metatheory Model-Checking. James Cheney. Syntactic techniques based on operational semantics and type systems are extremely useful for studying the metatheory of logics and programming languages. However, proving properties of such systems is often an onerous task, because there are often a large number of 'standard' or straightforward cases. It is tempting to cut corners and do careful proofs only for cases that seem interesting, but this can easily lead to a counterexample being missed. It therefore seems attractive to try to formalize such proofs in order to ensure their validity. However, this cure is often worse than the disease, since theorem proving tools tend to have a high learning curve. Moreover, theorem proving generally addresses the uncommon case of formalizing an informal proof of a well-understood system which has stood the test of time, rather than the common case of finding a bug in a poorly-understood system that may still be under development. * A Principled Approach to Version Control. Wouter Swierstra. Version control systems are essential for managing the distributed development of large software pro jects. We present a formal model for reasoning about version control. In particular, we give a general definition of patch. Patches abstract over the data on which they operate, making our framework equally suited for version control of everything from highly-structured XML files to blobs of bits. We model repositories as a multiset of patches. The mathematical definitions of patches and repositories enable us to reason about complicated issues such as conflicts and conflict resolution. * C# is a functional programming language. Andrew Kennedy. Polymorphic functions, parameterized types, lambda expressions, type inference, streams, GADTs, and combinator libraries: these are features usually associated with functional languages such as Haskell and ML. In this talk I will show how the design of C# 3.0, launching next year, was strongly influenced by ideas from functional programming, and supports all these features. I will use classic examples from the functional programming literature to illustrate the talk. Blog noise [31]Haskell news from the blogosphere. * [32]Scoping, and my Scheme Interpreter * [33]XP Day Benelux: the report * [34]From L�b's Theorem to Spreadsheet Evaluation * [35]A search engine written in Haskell: part two * [36]OCaml - First Impressions * [37]House: an Operating System Written in Haskell * [38]Reverse Engineering Machines with the Yoneda Lemma * [39]Haskell and functional programming * [40]Query for readers: Interested in Haskell? * [41]Why Haskell? * [42]What rocks * [43]Why pointers and references are a bad thing * [44]Reasons to learn C * [45]Some Thoughts on Haskell * [46]Simon Peyton Jones and Tim Harris explain STM [video] 31. http://planet.haskell.org/ 32. http://computativestylings.blogspot.com/2006/11/scoping-and-my-scheme-inter… 33. http://matteo.vaccari.name/blog/?p=59 34. http://sigfpe.blogspot.com/2006/11/from-l-theorem-to-spreadsheet.html 35. http://haskell-web.blogspot.com/2006/11/search-engine-written-in-haskell-pa… 36. http://triple.aeoth.net/2006/11/24/ocaml-first-impressions/ 37. http://www.osnews.com/story.php?news_id=16554 38. http://sigfpe.blogspot.com/2006/11/yoneda-lemma.html 39. http://chanson.livejournal.com/156835.html 40. http://scienceblogs.com/goodmath/2006/11/query_for_readers_interested_i.php 41. http://scienceblogs.com/goodmath/2006/11/why_haskell.php 42. http://n3dst4.livejournal.com/50393.html 43. http://kawagner.blogspot.com/2006/11/why-pointers-and-references-are-bad.ht… 44. http://www.nekomancer.net/blog/archives/reasons-to-learn-c 45. http://geeklikemetoo.blogspot.com/2006/11/some-haskell-basics.html 46. http://channel9.msdn.com/Showpost.aspx?postid=231495 Quotes of the Week * Cale: GADT pattern match in non-rigid context. There are prescriptions available for that sort of thing. * Bulat: Base lib includes everything, from 'const' to starship control * Cale: I should actually think before coding, but the type system is so good. * Syzygy-: Why do I get the feeling that the arrow code was written during 'Speak like a pirate day'? * SamB: [on the subject of fromJust] thinks that unJust would be a more fun name for that * apfelmus: Ah, yet another UndeadArray necromancer exhausting his stack of bones. May the forces of light suggest to structure the incantation of darkness? * dons: Ah, it *is* like Haskell. I see: Jiffy Pop is the family fun treat. No other brand of popcorn offers a self-contained popping pan. It can even be used outdoors. * thetallguy: Using and advocating Haskell is like being Calvin (and Hobbes). To you, it's alive, real, a true delight. To those who know better, it's a stuffed tiger. Code Watch * Mon Nov 27 05:06:02 PST 2006. Wolfgang Thaller. [47]Initial support for x86_64-darwin. Basic -fvia-C code generation is there, not much testing. 47. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18000 * Fri Nov 24 15:05:48 PST 2006. simonpj. [48]Fix constraint handling for lazy patterns. Lazy patterns are quite tricky! Consider f ~(C x) = 3. Can the Num constraint from the 3 be discharged by a Num dictionary bound by the pattern? Definitely not! See Note 'Hopping the LIE in lazy patterns' in TcPat. The type checker wasn't ensuring this, and that was causing all manner of strange things to happen. It actually manifested as a strictness bug reported by Sven Panne. 48. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/17969 About the Haskell Weekly News Each week, new editions are posted to [49]the Haskell mailing list as well as to [50]the Haskell Sequence and [51]Planet Haskell. [52]RSS is also available, and headlines appear on [53]haskell.org. Headlines are available as PDF. The Haskell Weekly News is also [54]available in Spanish translation. To help create new editions of this newsletter, please see the [55]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [56]http://www.cse.unsw.edu.au/~dons/code/hwn 49. http://www.haskell.org/mailman/listinfo/haskell 50. http://sequence.complete.org/ 51. http://planet.haskell.org/ 52. http://sequence.complete.org/node/feed 53. http://haskell.org/ 54. http://haskell.org/haskellwiki/HWN/es 55. http://haskell.org/haskellwiki/HWN 56. http://www.cse.unsw.edu.au/~dons/code/hwn

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Re: base libraries
by Paul Johnson 27 Nov '06

27 Nov '06

Duncan Coutts wrote: > Cabal can be upgraded using Cabal. This is important. I think there are two things here. First, its important that it can be upgraded. Most distributions already have a package management system that can handle this. I just got GHC, Haddock and a bunch of other stuff onto my new FC6 installation via Pirut, the FC6 graphical installer (which is coded in Python and would probably be much faster and better if it was in Haskell, but thats another story). If Cabal and its supporting libraries are split out from the main distribution into separate packages then I can get an upgraded Cabal the same way without having to get those libraries as well. > Unlike well specified libraries like ByteString, Cabal tackles an > ill-specified problem and we're still learning how best to solve it, so > the api and features change much more frequently. So if I become a Cabal developer rather than just a user, or I want the source for some other reason, then of course I can do a cabal-get cabal and all the relevant libraries get installed for me. In short, I think that Cabal should work like any other application: you can just install the binary and go, or you can install the source (via Cabal) and hack it if you want to. Binary installs should be handled by the distribution (Windows users can have a .msi, Mac users can have whatever the Mac equivalent is). Compiler packages should generally include the Cabal binaries along with the compiler binary, Haddock binary and so on. Paul.

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polymorphism and existential types
by Louis-Julien Guillemette 27 Nov '06

27 Nov '06

Supposing a polymorphic value (of type, say, forall a . ExpT a t) is stored inside an existential package (of type, say, forall a . Exp a), I wonder how to recover a polymorphic value when eliminating the existential. The ``natural way'' to write this doesn't work: {-# OPTIONS -fglasgow-exts #-} data ExpT a t data Exp a = forall t . Exp (ExpT a t) f :: (forall a . ExpT a t) -> () f e = () g :: (forall a . ExpT a t) -> () g e = let e1 :: forall a . Exp a e1 = Exp e in case e1 of Exp e' -> f e' {- Test.hs:18:17: Inferred type is less polymorphic than expected Quantified type variable `a' is mentioned in the environment: e' :: ExpT a t (bound at Test.hs:18:11) In the first argument of `f', namely `e'' In the expression: f e' In a case alternative: Exp e' -> f e' Failed, modules loaded: none. -} In particular, the solution of pushing the forall inside doesn't work for me (it breaks other parts of my code...) : data Exp' = forall t . Exp' (forall a . ExpT a t) g' :: (forall a . ExpT a t) -> () g' e = let e1 :: Exp' e1 = Exp' e in case e1 of Exp' e' -> f e' Any help welcome! Louis-Julien

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