October 2007 - Haskell

Re: Trying to install binary-0.4
by Udo Stenzel 25 Oct '07

25 Oct '07

Don Stewart wrote: > Since you're not using ghc 6.8, you should use binary 0.3 :) That was PC for "sorry, GHC 6.6 is no longer supported and don't even ask about 6.4" The other day I tried to install the tar library on a GHC 6.4. It's nearly impossible. The old base library gets in the way of installing bytestring, binary 0.4 absolutely needs bytestring 0.9 and tar absolutely needs binary 0.4, but still won't compile because System.Posix is now called System.PosixCompat which actually makes it incompatible. Oh, and everything needs a new cabal, but the setup scripts still picks up the wrong one (apparently, since Distribution.Simple is supposed to be there but GHC doesn't find it). Wanna know the solution? - install exactly one version of cabal, 1.1.6.2, and *remove* all others, - ask ghc-pkg for the description of base, then edit Data.ByteString out of that and re-register it, - install bytestring 0.9, - install binary 0.4, - edit the Setup.lhs of tar (it still won't run, even though it should), - edit the source of tar, changing System.PosixCompat to System.Posix, - install it. Should I mention that this is made even more difficult by not being root on the machine in question? All this happened with libraries that look as if they are supposed to be stable, but absolutely nothing works right out of the box. Oh, and Cabal Configurations will make it worse, not better. Here's what should be done, imho: - Rename 'base' ASAP and especially before GHC 6.8 comes out, call it 'foundation' or something else. If you want to keep the name 'base', make sure Cabal considers 'base-2.x' a different library than 'base-3.x'. - Provide a replacement configuration for GHC 6.6 and 6.4 (yes, that one is still alive!) that removes the conflict between 'base' and 'bytestring' and pretends to provide bytestring, containers, array, etc. - Provide a known good cabal. Make sure it installs on GHC 6.6 and 6.4. - Start fixing dependencies. - Refrain from renaming stuff. System.Posix is a fine name. - Refrain from always using the latest interface of everything. While we're at it, the ability to have multiple versions of a library installed under the same name is a recipe for desaster, too. It should be dropped, instead implementing Eternal Compatibility in Theory by encoding version numbers in module names. Sorry for the rant, but the situation was actually better before Cabal tried to fix everything and in the process broke both versions of GHC that are in widespread use right now. -Udo

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ANNOUNCE: hslogger4j 0.1.1
by Bjorn Buckwalter 19 Oct '07

19 Oct '07

All, I'm pleased to share my unpolished hslogger4j library, available now from the project homepage[1] or HackageDB. "Hslogger4j provides handlers for hslogger (John Goerzen's Haskell logging framework) that are compatible with log4j's XMLLayout. In particular log messages created by the handlers can be published directly to the GUI-based log viewer Chainsaw v2." The motivation behind the library is to leverage tools developed for the widely used Apache logging framework log4j. In my case I've been monitoring my hslogger[2] logs in real time using the GUI-based Log viewer Chainsaw v2[3]. The library also includes some optional supporting Java files (most importantly a jar) necessary to use Chainsaw with the richer hslogger levels. The README should clarify usage. Thank you, Bjorn Buckwalter [1] http://hslogger4j.googlecode.com [2] http://software.complete.org/hslogger [3] http://logging.apache.org/chainsaw

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ICFP 2008 Call for Workshop Proposals
by Simon Marlow 19 Oct '07

19 Oct '07

[ posting on behalf of Michael Sperber <sperber(a)deinprogramm.de> ] CALL FOR WORKSHOP PROPOSALS ICFP 2008 13th ACM SIGPLAN International Conference on Functional Programming September 22-24, 2008 Victoria, British Columbia, Canada http://www.icfpconference.org/icfp2008/index.html The 13th ACM SIGPLAN International Conference on Functional Programming (ICFP 2008) will be held in Victoria, Canada from 22nd to 24th September 2008. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops to be affiliated with ICFP 2008 and sponsored by SIGPLAN. These workshops should be more informal and focused than ICFP itself, include sessions that enable interaction among the workshop attendees, and be fairly low cost. The preference is for one-day workshops, but other schedules can also be considered. The workshops themselves will be held between September 18th and September 27th, as hotel capacity allows. ---------------------------------------------------------------------- Submission details Deadline for submission: 21th November 2007 Notification of acceptance: 19th December 2007 Prospective workshop organisers are invited to submit a completed workshop proposal form in plain text format to the ICFP 2008 workshop co-chairs (Mike Sperber and Graham Hutton), via email to icfp08-workshops at deinprogramm.de by 21th November 2007. Please note that this is a firm deadline. Organisers will be notified if their proposal is accepted by 19th December 2007, and if successful are required to produce a final report after the workshop has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available here: http://www.icfpconference.org/icfp2008/workshops/icfp08-workshops-form.txt Further information about SIGPLAN sponsorship is available at here: http://acm.org/sigplan/sigplan_workshop_proposal.htm ---------------------------------------------------------------------- Selection committee The workshop proposals will be evaluated by a committee comprising the following members of the ICFP 2008 organising committee, together with the members of the SIGPLAN executive committee. Mike Sperber DeinProgramm Workshops co-chair Graham Hutton University of Nottingham Workshops co-chair Jim Hook Portland State University General chair Peter Thiemann University of Freiburg Program chair ---------------------------------------------------------------------- Further information Any queries regarding ICFP 2008 workshop proposals should be addressed to the workshops co-chairs (Mike Sperber and Graham Hutton), via email to icfp08-workshops at deinprogramm.de. ----------------------------------------------------------------------

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ANNOUNCE: Lazy SmallCheck 0.1
by Matthew Naylor 18 Oct '07

18 Oct '07

Announcing Lazy SmallCheck 0.1, a library for exhaustive, demand-driven testing of Haskell programs. Lazy SmallCheck is based on the idea that if a property holds for a partially-defined input then it must also hold for all fully-defined instantiations of that input. Compared to `eager' input generation in SmallCheck, Lazy SmallCheck may require significantly fewer test-cases to verify a property for all inputs up to a given depth. There is a webpage for Lazy SmallCheck: http://www-users.cs.york.ac.uk/~mfn/lazysmallcheck/ There you'll find a more detailed description, a worked example, a comparison with SmallCheck on a number of benchmarks, and link to download the library. The library was developed together with Fredrik Lindblad during his recent visits to York. Suggestions, experiences and bug reports are welcome! Matthew.

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[[spam]]
by guzinu＠omu.edu.tr 18 Oct '07

18 Oct '07

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[Fwd: undecidable & overlapping instances: a bug?]
by Jorge Marques Pelizzoni 18 Oct '07

18 Oct '07

Hi, all! It's been a while since I sent the message below to the GHC list, which I believe is its rightful forum, but I've had no reply so far. So I am giving the haskell list a try. Thanks in advance. Cheers, Jorge. --------------------------- Mensagem Original ---------------------------- Assunto: undecidable & overlapping instances: a bug? De: "Jorge Marques Pelizzoni" <jpeliz(a)icmc.usp.br> Data: Sab, Outubro 13, 2007 5:59 am Para: "GHC users" <glasgow-haskell-users(a)haskell.org> -------------------------------------------------------------------------- Hi, all! I am quite intrigued at the behaviour examplified in the attached module. It's true I am a newbie and probably don't quite get the whole consequence spectrum of -fallow-undecidable-instances, but why providing that dummy instance (commented out) get the thing to compile? By the way, I'm using GHC 6.6 on WinXP (actually, latest Visual Haskell with MS Visual Studio 2005) and the error message I get is: FooModule.hs:13:9: Could not deduce (Show a) from the context (Concrete a b) arising from use of `bar' at FooModule.hs:13:9-13 Possible fix: add (Show a) to the class or instance method `foo' In the expression: bar x In the definition of `foo': foo x = bar x In the definition for method `foo' A second question: which kinds of overlapping are covered by -fallow-overlapping-instances? It seems that the following (also in the attached module) is not allowed: instance (Show a, Abstract a b) => Concrete a b where foo x = show x instance (Abstract a b) => Concrete a b which gives me the message: FooModule.hs:17:0: Duplicate instance declarations: instance [overlap ok] (Show a, Abstract a b) => Concrete a b -- Defined at FooModule.hs:17:0 instance [overlap ok] (Abstract a b) => Concrete a b -- Defined at FooModule.hs:30:0 Thanks in advance for any pointers. Cheers, Jorge.

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ANNOUNCE: xmonad 0.4
by Don Stewart 18 Oct '07

18 Oct '07

The xmonad dev team is pleased to announce the 0.4 release of xmonad! http://xmonad.org xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard: a mouse is optional. xmonad is written, configured and extensible in Haskell. Custom layout algorithms, key bindings and other extensions may be written by the user in Haskell, in config files. Window layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled across several physical screens. Features: * Simple, intuitive user interface * Stable, fast and small memory footprint * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating windows * XRandR support to rotate, add or remove monitors dynamically * Per-workspace layout algorithms * Per-screens custom status bars * Easy, powerful customisation and reconfiguration * Large extension library * Extensive documentation and support for hacking This is a big release, with every feature we originally proposed for xmonad implemented. Since xmonad 0.3, the following notable improvments have appeared: * Powerful "rules" system: config files may specify rules about how particular applications are handled. For example, gimp might always be placed in the floating layer, or firefox always placed on workspace 2. Gnome dock apps (such as kicker or gnome-panel) might always be placed in a status bar gap and unmanaged. * User-specified workspace tag types (not restricted to numeric keys). E.g. "web" or "www" are valid workspace tags now. * Layouts algorithms serialisable. Current workspace state preserved across restarts/reconfiguration, including user-written modules' state * User written code sandboxed from the core applicuation further, making it safer to experiment with new extensions * 100% code coverage of logic core of xmonad by its testsuite, thanks to QuickCheck and HPC. * More hooks to make writing expressive extensions easier * mod-m to shift focus to the `master' window * many new binary distributions, for various flavours of Linux and BSD, OSX is also well supported. * extensions: xmonad comes with a huge library of extensions (3 times the size of xmonad itself), contributed by viewers like you. Extensions enable pretty much arbitrary window manager behaviour to be implemented by users, in Haskell, in the config files. For more information on using and writing extensions, see the webpage. Get it! More information, screenshots, documentation, tutorials and community resources are available from the xmonad home page: http://xmonad.org The 0.4 release, and its dependencies, are available from hackage.haskell.org, here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad Brought to you by the xmonad team: Spencer Janssen Don Stewart Jason Creighton Andrea Rossato David Roundy with code contributions from: Brandon Allbery Chris Mears Shachaf Ben-Kiki Eric Mertens Alec Berryman Neil Mitchell Gwern Branwen Devin Mullins Joachim Breitner Daniel Neri Alexandre Buisse Stefan O'Rear Nick Burlett Simon Peyton Jones Peter De Wachter Hans Philipp Annen Aaron Denney Karsten Schoelzel Nelson Elhage Michael Sloan Shae Erisson Ivan Tarasov Joachim Fasting Alex Tarkovsky Michael Fellinger Christian Thiemann David Glasser Joe Thornber Kai Grossjohann Matsuyama Tomohiro Dave Harrison Daniel Wagner Juraj Hercek Ferenc Wagner Sam Hughes Jamie Webb Miikka Koskinen Brent Yorgey David Lazar nornagon Lucas Mai timthelion Robert Marlow Klaus Weidner As well as the support of a cast of hundreds on the #xmonad and #haskell IRC channels, and the wider Haskell, FP and window manager communities. Thanks to everyone for their support!

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Announce: generating free theorems, online and offline
by Janis Voigtlaender 17 Oct '07

17 Oct '07

A while back I announced a library and tools for generating free theorems from Haskell types: http://www.haskell.org/pipermail/haskell/2006-August/018426.html There is now an improved version with new features. To try it out online, visit: http://linux.tcs.inf.tu-dresden.de/~voigt/ft Highlights are: - support for type classes (e.g., enter "elem" and note the generated respects-restrictions) - three different language subsets to choose from, including one that takes selective strictness into account and thus gives theorems that are valid even in the presence of seq and friends (e.g., enter "filter" and compare the outputs for the three language subsets) - equational as well as inequational free theorems, the latter often coming with less restrictive preconditions, and thus being applicable where equational free theorems are not (e.g., enter "head" and compare the outputs in equational vs. inequational style for the second or third language subset) The source code of the library is available from: http://wwwtcs.inf.tu-dresden.de/~voigt/dist.tar.gz This package additionally contains a shell-like interface to the library, giving more control than the web interface. In particular, using it one can declare one's own algebraic data types, type synonyms, type renamings and type classes, and then generate free theorems for types involving those. Credits for the implementation are due to Sascha Boehme! Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Module system question
by Iavor Diatchki 17 Oct '07

17 Oct '07

Hello, I have a question concerning Haskell's module system. Consider the following example which defines three modules A,B, and C: > module A where { data X = X } > module B where { class X a } > module C where { import A; import B; data Y = Y X } The question is: "Is there an ambiguity error in module C?". It seems that the answer depends on how we interpret "ambiguous". In the context of module C, the name X may refer to either the class defined in module B, or the datatype defined in module A. Therefore, we could consider X to be ambiguous, and indeed, this is what happens in GHC 6.6. On the other hand, the name X is used in a context where we are expecting a type constructor and not a class name, and therefore the name X could be unambiguously taken to refer to the datatype X, which is what seems to happen in Hugs. I like the Hugs behavior because it accepts more programs. OTOH, GHC's behavior may be a bit simpler to explain and implement(?). Any thoughts? -Iavor

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IN Doubt...
by Manuel Hernandez 16 Oct '07

16 Oct '07

Dear Haskellers, why this program is not accepted by Hugs? equal a a = True equal a b | not(a==b) = False Best, Manher

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