- Haskell-Cafe - Haskell.org

hmake and hat
by Joachim Breitner 26 Feb '10

26 Feb '10

Hi, I made hmake compile on Debian again, but it seems it’s not working yet properly. This also affects hat, as it uses hmake to compile. I guess we could just drop hmake and hat, but this is Debian, and we try to avoid dropping stuff that made it into a stable release. Is anyone interested in figuring out whats wrong with hmake and how to make hat compile again? Also, to haskell-cafe, are there any hat users out there? Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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Haskell and XML, need some tips from practioners
by Günther Schmidt 26 Feb '10

26 Feb '10

Hi everyone, I had to transform a 136 page word document into an XML-Document so that it can be imported into an application. The word document contained records in nested tables. Anyway through a very, very tedious process of xslt-transformations I finally have the XML document I need. But now I need to amend the attributes of some elements with looked up values from the outside, the lookup-key is a particular attribute value of the nodes. This I cannot do through xslt processing as the information needed is not within the xml document. I was thus going to use Haskell instead of an XSLT-processor for this final step. My question to those with experience of the Haskell-XML tools: which one should I use? Günther

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haddock installation weirdness?
by David Leimbach 26 Feb '10

26 Feb '10

I'm on Mac OS X Snow Leopard, and can't get haddock installed due to the following error: Resolving dependencies... Configuring haddock-2.6.0... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package haddock-2.6.0 requires Cabal-1.8.0.2 package ghc-6.12.1 requires Cabal-1.8.0.2 package bin-package-db-0.0.0.0 requires Cabal-1.8.0.2 Preprocessing library haddock-2.6.0... Preprocessing executables for haddock-2.6.0... unused terminals: 1 Building haddock-2.6.0... <command line>: cannot satisfy -package-id ghc-6.12.1-b691a185e99c62533666d9a28a9e1988: ghc-6.12.1-b691a185e99c62533666d9a28a9e1988 is unusable due to missing or recursive dependencies: Cabal-1.8.0.2-a08510b9460f1b65f9dee06ed53f0650 bin-package-db-0.0.0.0-0c559ebe951f9972c4e6dfe5ebd4ce6a (use -v for more information) cabal: Error: some packages failed to install: haddock-2.6.0 failed during the building phase. The exception was: ExitFailure 1 When I do a "ghc-pkg list | grep Cabal" I get the following: ghc-pkg list | grep Cabal Cabal-1.8.0.2 Cabal-1.8.0.2 I'm wondering if this means I have a copy in my .cabal, and another from GHC,and if that could be causing a problem? I'm trying to work on the Haddock docs for the NineP package I uploaded the other day, and would rather not have to finish uploading before previewing the result :-) Dave

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hdbc-mysql fails to compile
by Thomas Girod 26 Feb '10

26 Feb '10

Hi there. Looks like hdbc-mysql cannot compile against GHC-6.12 -- confirmed by the hackage build logs. [1] anyone know a way to dodge the problem ? cheers, Tom [1] http://hackage.haskell.org/packages/archive/HDBC-mysql/0.6/logs/failure/ghc…

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Re: [Haskell-cafe] Testing and module export lists
by Magnus Therning 25 Feb '10

25 Feb '10

On Wed, Feb 24, 2010 at 10:41, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Maybe you just want to test what's in your export list which > represents the public interface of your code. And if you cannot write > a test that exercise private implementation through the public > interface, then maybe there is a design problem... Possibly! The specific case I have is a module for parsing a small language. The module exports a single function, a function that takes a string and returns a tree. Of course the parser is built using parser combinators. I'd like to test each combinator I've built, but I don't want to export them all. The only way I see of achieving this is to split each module in two, one public and one internal, where the public one just re-exports the relevant pieces of the internal one. That's workable, but hardly aesthetically pleasing ;-) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Interested in the Dutch HUG Day (24 April 2010)?
by Sean Leather 25 Feb '10

25 Feb '10

In honor of the first anniversary of the Dutch Haskell Users Group (formed at Hac5 in Utrecht), we are planning a mini symposium to bring together Haskell and functional programming enthusiasts in the Netherlands and surrounding area. *The Dutch HUG Day will be held on Saturday, 24 April.* Please mark that date in your calendar. The remaining details are being worked out. In order to help us with this process, we have created a short survey. If you are even remotely considering attending, please take a moment to fill out a form with your name and the activities that interest you. * http://spreadsheets.google.com/viewform?formkey=dGVuVTdnZEl0ZDE3MEVweUtRdml… Note that this does not commit you to anything. We will only use this information to help plan the event (e.g. determine the number of seats at the location and the types of activities). For more information about the Dutch HUG, visit the wiki page and sign up for the mailing list. * http://www.haskell.org/haskellwiki/Dutch_HUG * http://groups.google.com/group/dutch-hug Thanks for your time! Sincerely, The Dutch HUG Day planning team

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Category Theory woes
by Mark Spezzano 25 Feb '10

25 Feb '10

Hi all, I'm trying to learn Haskell and have come across Monads. I kind of understand monads now, but I would really like to understand where they come from. So I got a copy of Barr and Well's Category Theory for Computing Science Third Edition, but the book has really left me dumbfounded. It's a good book. But I'm just having trouble with the proofs in Chapter 1--let alone reading the rest of the text. Are there any references to things like "Hom Sets" and "Hom Functions" in the literature somewhere and how to use them? The only book I know that uses them is this one. Has anyone else found it frustratingly difficult to find details on easy-to-diget material on Category theory. The Chapter that I'm stuck on is actually labelled Preliminaries and so I reason that if I can't do this, then there's not much hope for me understanding the rest of the book... Maybe there are books on Discrete maths or Algebra or Set Theory that deal more with Hom Sets and Hom Functions? Thanks, Mark Spezzano.

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Heterogeneous Data Structures - Nested Pairs and functional references
by Günther Schmidt 25 Feb '10

25 Feb '10

Hi, I've been thinking a lot lately about heterogeneous and extensible data structures for which HList certainly offers a solution. While HList is implemented through type-level programming I wonder if I can achieve similar results through value-level programming alone. This is where I was thinking of functional references. I wonder if or rather how one could do this: Let's say there was some clever monad ... someMonad = do h1 <- add "twenty" h2 <- add False h3 <- add 16 ..... modify h2 True and get a ("twenty",(True, 16)) back. And while *in* the monad some accessors available. Now come to think of it I think I actually read about this somewhere so I doubt this is truly my idea. Anybody got some thoughts on this? Günther

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Some great results on fused code with the LLVM backend
by Don Stewart 25 Feb '10

25 Feb '10

I tried out some of the vector and uvector fusion benchmarks with the new LLVM backend http://donsbot.wordpress.com/2010/02/21/smoking-fast-haskell-code-using-ghc… and got some great results for the tight loops generated through fusion. Up to 2x faster than gcc -O3 in some cases. The LLVM backend looks very promising -- considering we've not even begun to explore the optimization pipeline at the level. -- Don

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Re: Bug#570284: O: washngo
by Joachim Breitner 25 Feb '10

25 Feb '10

Hi, Am Mittwoch, den 17.02.2010, 15:35 -0600 schrieb John Goerzen: > I no longer use WASH, nor do I have time to maintain it anymore. > Orphaning it, and CCing debian-haskell in case someone there has > interest in it. we have two option: Taking it over by the Haskell Group, or dropping it from Debian. Therefore my question: Is anyone using Wash? Is it still useful, three years after the last change and with HAppS around? Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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RE: gtk2hs and ghc-6.10.4 on Windows (Vista)
by gladstein＠gladstein.com 25 Feb '10

25 Feb '10

In correspondance with Alistair Bayley I learned that there is in fact a gtk2hs installer for 6.10.4. One can find it by Googling "gtk2hs 6.10.4" (which I did not think to do). The URL is in http://www.mail-archive.com/gtk2hs-devel@lists.sourceforge.net/msg00340.html The installer doesn't work if the Haskell Platform is installed in the default location, which has spaces in the pathname. Reinstalling Haskell in, say, c:\HaskellPlatform makes it all work.

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Re: Testing and module export lists
by John Lato 25 Feb '10

25 Feb '10

> From: Magnus Therning <magnus(a)therning.org> > > How do people who like unit testing / property testing deal with export lists? > > I often find that I do want an export list to reduce clutter in the > finished code, but for testing I'd like to expose everything in a > module. Is there a nice way to deal with this (using the C > pre-processor would not qualify as "nice" ;-)? > Maybe there's a switch that causes GHC to simply ignore the export > list of a module and export everything? That would be a nice feature to deal with this problem. The best solution I have is to separate functions that don't belong in the public interface into separate modules that aren't exposed (they're listed in the other-modules section of the .cabal file). I then add the test executable to the .cabal file, so when it's built it can access the hidden modules (or I run the test from ghci). Of course this only works if it makes sense to structure the code this way. Cheers, John

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Lazy IO with recursive reads?
by Fabian Roth 24 Feb '10

24 Feb '10

Hi all, I am trying to read structured data from a socket and return a lazy list of records. However, the socket reading operation seems to be strict and never returns (until stack overflow). Here's some simplified code to reproduce the problem: -------------------- import Control.Monad main = do messages <- readLazy mapM_ (\x -> putStr $ show x ++ "\n") $ messages return () where readLazy :: IO [String] readLazy = do c <- fancyIORead liftM2 (++) (return c) readLazy fancyIORead :: IO [String] fancyIORead = return ["aa","bb"] -------------------- In my implementation fancyIORead reads blocks from the socket and returns a list of records. But it seems readLazy doesn't ever return. What could be the problem here? Also, if anyone has a better solution to write this thing, pls let me know. Thanks! Fabian

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ANN: concurrent-extra-0.2
by Roel van Dijk 24 Feb '10

24 Feb '10

Hello, We would like to announce an update of concurrent-extra [1]. Bug fixes: - A bug in RLock.acquire (thanks to Felipe Lessa). New features: - Broadcast: Wake multiple threads by broadcasting a value. This is a generalisation of Event. - Thread: Threads extended with the ability to wait for their termination. - delay: Arbitrarily long thread delays. - timeout: Wait arbitrarily long for an IO computation to finish. The lightweight thread wrapper was inspired by the threadmanager package [2]. The main advantage of our implementation is that we don't maintain an internal mapping from ThreadId to ThreadStatus. Instead we rely on the forked thread to broadcast its status to interested listeners. This should result in better performance (no lookup required). Every exported symbol is now documented. Regards, Roel & Bas van Dijk [1] http://hackage.haskell.org/package/concurrent-extra-0.2 [2] http://hackage.haskell.org/package/threadmanager-0.1.3

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ICFP 2010: Second call for papers
by Wouter Swierstra 24 Feb '10

24 Feb '10

===================================================================== Second Call for Papers ICFP 2010: International Conference on Functional Programming Baltimore, Maryland, 27 -- 29 September 2010 http://www.icfpconference.org/icfp2010 ===================================================================== Important Dates (at 14:00 UTC) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Submission: 2 April 2010 Author response: 24 -- 25 May 2010 Notification: 7 June 2010 Final papers due: 12 July 2010 Scope ~~~~~ ICFP 2010 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects or concurrency. Particular topics of interest include * Language Design: type systems; concurrency and distribution; modules; components and composition; metaprogramming; relations to object-oriented or logic programming; interoperability * Implementation: abstract machines; compilation; compile-time and run-time optimization; memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components or low-level machine resources * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling * Foundations: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects * Transformation and Analysis: abstract interpretation; partial evaluation; program transformation; program calculation; program proof * Applications and Domain-Specific Languages: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia programming; scripting; system administration; security; education * Functional Pearls: elegant, instructive, and fun essays on functional programming The conference also solicits Experience Reports, which are short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working in a particular application. Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ By 2 April 2010, 14:00 UTC, submit an abstract of at most 300 words and a full paper of at most 12 pages (6 pages for an Experience Report), including bibliography and figures. The deadline will be strictly enforced and papers exceeding the page limits will be summarily rejected. Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. A submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.acm.org/sigplan/republicationpolicy.htm. Proceedings will be published by ACM Press. Authors of accepted submissions are expected to transfer the copyright to the ACM. Presentations will be videotaped and released online if the presenter consents by signing an additional permission form at the time of the presentation. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. If this requirement is a hardship, make contact with the program chair at least one week before the deadline. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LATEX is available from SIGPLAN at http://www.acm.org/sigs/sigplan/authorInformation.htm. Submission: Submissions will be accepted electronically at a URL to be named later. Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 48-hour period, starting at 14:00 UTC on 24 May 2010, to read and respond to reviews. Special Journal Issue: There will be a special issue of the Journal of Functional Programming with papers from ICFP 2010. The program committee will invite the authors of select accepted papers to submit a journal version to this issue. Organization ~~~~~~~~~~~~ Conference Chair Paul Hudak, Yale University Program Chair Stephanie Weirich, University of Pennsylvania Program Committee: Umut Acar, Max Planck Institute for Software Systems Zena Ariola, University of Oregon James Cheney, University of Edinburgh Peter Dybjer, Chalmers University of Technology Robert Bruce Findler, Northwestern University Andy Gill, Kansas University Fritz Henglein, University of Copenhagen Michael Hicks, University of Maryland, College Park Patricia Johann, University of Strathclyde Andres Löh, Utrecht University Simon L. Peyton Jones, Microsoft Research Didier Rémy, INRIA Paris-Rocquencourt John Reppy, University of Chicago Manuel Serrano, INRIA Sophia-Antipolis Matthieu Sozeau, Harvard University

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gtk2hs and ghc-6.10.4 on Windows (Vista)
by Alistair Bayley 24 Feb '10

24 Feb '10

On 16 February 2010 17:57, Neil Mitchell <ndmitchell(a)gmail.com> wrote: > Hi Serguey, > > A GHC 6.10.4 version of Gtk2hs: > > http://www.mail-archive.com/gtk2hs-devel@lists.sourceforge.net/msg00340.html > > I used to recommend Gtk2hs over wxHaskell for GUI development as there > was always a version that worked on Windows with the latest GHC > release. I think I might have to switch back to recommending C# for > GUI development... Is there a trick to getting this installed? I have run the installer in a Vista machine, both as user and admin, and the installation is mostly successful, except that the packages are not registered with ghc-6.10.4. Also, the first step of the install complains that "GHC does not appear to be installed correctly, try reinstalling GHC version 6.10.4. Setup found what appears to be a non-working installation of GHC in ..." What test does it apply to determine if GHC is working or not? It can run the gtk-demo, so it's not all bad, but I can't build anything that uses it (like Yi). Alistair

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GHC RTS question
by Artyom Kazak 24 Feb '10

24 Feb '10

Hello everybody! I want to write a little program, that will receive a string as command-line argument and write it in the file. But if this string contains '+RTS', GHC runtime won't pass the rest of the string to my program. What can I do to avoid this?

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Problems linking hsql-mysql
by Maciej Podgurski 24 Feb '10

24 Feb '10

Hi, I have problems linking a simple test program that imports Database.HSQL.MySQL via ghc --make Test.hs. The error only occurs when importing this module of hsql-mysql-1.7.1. I pasted the building output here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=22911 (actually it was even longer but hpaste only takes 30k). When I do ghc -e main Test.hs or starting it from GHCi, everything works fine and no errors occur. So I guess all needed .lib files are there but GHC can't find them when linking (adding the flag -L"path to mysql/lib/opt" doesn't help). Anyone an idea? Best wishes, Maciej

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Building/installing OS X application bundles for GUIs - cabal-macosx
by Andy Gimblett 24 Feb '10

24 Feb '10

Hi all, Last week I quietly released v0.1.0 of cabal-macosx, providing support for building OSX application bundles (e.g. and in particular for GUIs created using wxHaskell and gtk2hs - hence all the cross-posting): http://hackage.haskell.org/package/cabal-macosx I'm now soliciting input on how to handle the install stage, leading (I hope) to a loudly-announcable release soon. :-) The current package provides a post-build hook which creates an application bundle at dist/build/foo.app (for example). It seems to work well. See the source repository for examples. However, running "runghc Setup install" copies the bare executable at dist/build/foo/foo (rather than the foo.app bundle) to the install location, which is no good. So I need to put in some install infrastructure to copy the .app, not the bare executable, to the right place. That raises the question: what is the right place? It seems to me that the defaults are not the right defaults for this case. --global should probably go to /Applications, and --user should perhaps go to ~/ Applications (though I'm less sure about that). Of course, end-users can explicitly set this on a per-install basis, but it seems that for bundles we'd want to override the default. Thus I'm currently asking: * What do people think about the default installation question? * Can anyone advise me on the right way to handle this (installing the .app, and overriding the default locations) with Cabal? I'm hoping I can just tweak part of the LocalBuildInfo and let the default machinery then handle the work, but at present it's not clear to me how to do that, and rather than spend ages trying to figure it out, perhaps someone who knows can help me out? I'm hoping I'm not going to have to completely override the install machinery - that would feel somewhat self-defeating. Many thanks, -Andy -- Andy Gimblett http://gimbo.org.uk/

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Linux ghci vs Windows ghci
by Donghee Nah 24 Feb '10

24 Feb '10

I'm using Windows 7 32bit Host OS(ghc 6.8.3) and Virtualbox Archlinux Guest OS(ghc 6.8.4) I feel that ghci code executing speed in guest os is 1.5~2x faster than host os The code: let t n = do {if n `mod` 100000 == 0 then print n else return ()} >> t (n+1) t 1 any clue?

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Profiling OpenGL applications
by Sönke Hahn 23 Feb '10

23 Feb '10

On Sunday, Andrew Coppin asked: > Is Thread Scope any use for profiling single-threaded programs? I used threadscope to look at eventlogs from a program that uses OpenGL to render multiple frames per second (compiled without "-threaded"). That means, there is CPU activity regularly (multiple times per second), and this can be observed via threadscope and i found it somewhat useful. There are some problems, though: 1. I can't definitely tell, which time span corresponds to CPU activity for the calculations for one frame. Is it possible to generate threadscope bookmarks from within the profiled program? (Or other events that can be shown in threadscope?) 2. The GPU activity cannot be observed. This is not a problem specific to Haskell, though. Is it even possible (with end user hardware) to get something similar to threadscope, not for CPU, but GPU activity? Any other ideas, that might help gathering information, about what's going on on the CPU(s) and the GPU? Many thanks, Sönke

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Parsing of bytestrings with non-String errors?
by Magnus Therning 23 Feb '10

23 Feb '10

I've looked at polyparse and attoparsec and they seem to have in common that the error always is a String. My current ideas for a project would be a lot easier if I could just return some other type, something that I can pattern match on. Is there a parser combinator library out there that works on bytestrings and allows using a custom error type? Or maybe there's some very basic reason why String is so commonly used? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Alex Lexer: Trying to get rid of "Alex"
by Amit Deshwar 23 Feb '10

23 Feb '10

Hi Haskell-cafe My problem: I'm trying to obtain the current position of the lexer once it reaches the end of the file (line and row number). I'm trying to do this in a function: getEndPosition = do (a,b,c) <- alexGetInput return a Unfortunately, the type of a is 'Alex AlexPosn' instead of just 'AlexPosn' How do I strip the Alex so I'm left with just a AlexPosn object? Thanks, Amit Deshwar University of Calgary

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grapefruit on windows or osX
by gladstein＠gladstein.com 23 Feb '10

23 Feb '10

I'm unable to get grapefruit going on osx or windows because (I think) I can't get the underlying GTK installed. Most recently I can't install gtk2hs over 6.10.4 because the installer tells me my haskell install isn't working and I should reinstall 6.10.1. It invites me to continue anyway and fix the DLL path myself. I continued and it doesn't work; cabal install grapefruit-ui-gtk still fails because it can't find a version of 'gtk'. The gtk demo program does work. Is the situation impossible, or do I just have to somehow tell something about some path? Hope that's all clear, and thanks in advance.

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Scarry parallel garbage collection behavior (6.12.1, Mac OSX)
by Pavel Perikov 22 Feb '10

22 Feb '10

Hi, list. This is a long post, sorry. The bottom line: specifying +RTS -Nn where n > number of cores slows the program compiled with 6.12.1 by orders of magnitude under Mac OSX. Turning off parallel garbage collections with -qg resolves the problem. Independent verification is appreciated. I have a very simple program: import Control.Parallel main=do let s = s1 `par` ( s2 `pseq` s1 + s2) s1 = sum [1..10000000] s2 = sum [1..10000005] print s compiled with ghc -O -threaded running it with without any options time ./clocktime produces the following times: real 0m3.779s user 0m3.644s sys 0m0.053s Giving -N option to RTS (or -N2, I have two core macbook) shows decent speedup: 100000060000015 real 0m2.678s user 0m4.762s sys 0m0.043s though increase in processor time by 30% seems suspicious. Things become weird if I increase the number of threads. I couldn't expect any further speedup for this task but what I get is absolutely unexpected: time ./clocktime +RTS -N3 100000060000015 real 0m57.071s user 1m40.910s sys 0m0.921s Almost a minute!! Adding more threads does not help either :) time ./clocktime +RTS -N4 100000060000015 real 1m48.281s user 3m10.832s sys 0m1.590s A bit of playing with Instruments/DTrace shows that runaway garbage collection puts the program on its knees. I the case of -N1 the top time consumers look like this: Self % Total % Self Run % Running % # Self # Samples ms Self Run ms Running Parent % Library Source Path Symbol Name 12.8 12.8 408 clocktime __gmpz_add 10.7 10.7 341 clocktime integer_cmm_plusIntegerzh 10 10 321 clocktime allocateLocal etc. This looks absolutely reasonable. Going to -N2 changes the picture to: Self % Total % Self Run % Running % # Self # Samples ms Self Run ms Running Parent % Library Source Path Symbol Name 15.6 15.6 989 clocktime gcWorkerThread 11.6 11.6 736 clocktime waitForGcThreads 9.5 9.5 603 clocktime scavenge_until_all_done 7.4 7.4 471 clocktime setContextSwitches 6.2 6.2 396 clocktime __gmpz_add -N3 Is awful: Self % Total % Self Run % Running % # Self # Samples ms Self Run ms Running Parent % Library Source Path Symbol Name 51.7 51.7 29514 clocktime gcWorkerThread 25.5 25.5 14591 clocktime scavenge_until_all_done 7.2 7.2 4114 clocktime waitForGcThreads 5.5 5.5 3180 clocktime GarbageCollect 5.2 5.2 2998 clocktime setContextSwitches 0.4 0.4 240 clocktime integer_cmm_plusIntegerzh Pavel

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A small oversight
by Andrew Coppin 22 Feb '10

22 Feb '10

I just discovered the highly useful function Data.Function.on. I vaguely recall a few people muttering a couple of years back that this would be a useful thing to have, but I had no idea it was in the standard libraries now. Anyway, while using it, I discovered a small omission from the Haskell libraries: We have min, max, minimum and maximum. We also have minimumBy and maximumBy. But there's no sign of minBy or maxBy. You can do, for example, (min `on` postcode) customer1 customer2 but that gives you the lowest postcode, not the customer to which this postcode belongs. By contrast, minimumBy (compare `on` postcode) [customer1, customer2] gives you the corresponding customer. But it seems silly to have to actually construct a list just for this. So... is there any danger of getting minBy and maxBy added? (I don't actually know off the top of my head where min and max are defined - the Prelude?) Also, constructions like sortBy (compare `on` foo) must surely be very common. Perhaps it would be an idea to define a family of functions like sortOn :: (Ord y) => (x -> y) -> [x] -> [x] sortOn foo = sortBy (compare `on` foo) Just an idea. Finally, take a look at this: newtype SwapOrd x = SwapOrd (unswap_ord :: x) deriving Eq instance Ord x => Ord (SwapOrd x) where compare x y = swap_ord $ compare x y swap_ord :: Ordering -> Ordering swap_ord o = case o of EQ -> EQ GT -> LT LT -> GT Just in case you wanted to sort things in reverse order. I think having swap_ord would be nice if nothing else...

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Pointfree composition for higher arity
by Sean Leather 22 Feb '10

22 Feb '10

I find myself often writing this pattern: someFun x y z = ... fun y z = runFun $ someFun someDefault y z > or, alternatively: fun y = runFun . someFun someDefault y > The second option approaches the ideal pointfreeness (or pointlessness if you prefer), but I'd like to go farther: (...) :: (c -> d) -> (a -> b -> c) -> a -> b -> d > (...) f g x y = f (g x y) > infixr 9 ... > > fun = runFun ... someFun someDefault > There, that's better. More points for fewer points (which means I should really change the name from fun to pun). Does anybody else care about this? What are some alternative solutions? I'd love to have something like this available in the Prelude or a library. (I have no strong feelings about the particular operator.) Regards, Sean

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Re: [Haskell] Functional Programming User Group Ghent
by minh thu 22 Feb '10

22 Feb '10

[moving to café from haskell] 2010/2/21 Jeroen Janssen <jejansse(a)gmail.com>: > Dear all, > We are in the progress of starting up a Functional Programming User Group in > Ghent. The basic idea of the group is to have occasional informal meetings > where people can give talks, where we have invited speakers, where we go to > a bar to chat about all things Functional Programming, to organize Haskell > Hacking events etc. > To see if enough people would be interested in such a group, and to make the > organizing process of the first meeting easier, we would like to ask anyone > interested to tick off the dates he/she is available on the Doodle-page > provided below. Note that the indicated time is only a rough guideline; if > you would prefer another time of day, you can indicate this in the Comments > section of the Doodle. > http://www.doodle.com/xq2i5hsueidwkh2f > If enough people are interested, and a date is decided upon, we will e-mail > the Haskell list once more to confirm the date and give anyone who was > interested, but could not fill in the doodle, a chance to be present. Also, > this ensures that you do not have to leave your e-mail address on Doodle. > If you have any questions, do not hecessitate to contact one of us at either > Bart.Coppens(a)UGent.be > jejansse(a)gmail.com > Kind Regards, > Bart Coppens > Jeroen Janssen Hi Jeroen, Great, a fp user group in Belgium! Alas my dutch is too rusty too even understand the Doodle poll description. Is the group planned only for dutch-speaking programmers ? Cheers, Thu

3 3

Haskell Weekly News: Issue 150 - February 22, 2010
by jfredett＠gmail.com 22 Feb '10

22 Feb '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100222 Issue 150 - February 22, 2010 --------------------------------------------------------------------------- Welcome to issue 150 of HWN, a newsletter covering developments in the [1]Haskell community. From the very pits of Harddrive failure I return... The Haskell Weekly News Reborn! Last week, on friday, my HWN machine suffered what can only be appropriately named a unbelievably poorly timed hard drive failure, hence the lack of an HWN. I trumphently return this week with a megaedition. Tons and tons of awesome stuff happened in the last two weeks. So I won't make you wait any longer, haskellers, your Haskell Weekly News! Announcements vty-ui 0.3. Jonathan Daugherty [2]announced a new version of vty-ui, featuring a richer rendering engine, generalized text transformations, and a more functional style. darcs 2.4 release candidate 2. Reinier Lamers [3]announced a new release candidate for darcs 2.4. iteratee-parsec 0.0.1. Maciej Piechotka [4]announced the release of iteratee-parsec, a library whih provides support for parsec parsers in an Iteratee monad. concurrent-extra-0.1. Roel van Dijk [5]announced the release of concurrent-extra, which provides a few new synchronization primitives. wyvern, a Dragon Go Server 'client'. wagnerdm [6]announced wyvern, a Dragon Go Server 'client', which plays moves for you on the [7]Dragon Go Server. atom-1.0.0. Tom Hawkins [8]announced a new major release of the atom library. Call for Copy: Monad.Reader Issue 16. Brent Yorgey [9]announced the call for copy for Monad.Reader issue 16. hmatrix 0.8.3. Alberto Ruiz [10]announced a new version of hmatrix. First Atlanta Function Programming Users Group Meeting. David Vollbracht [11]announced the first Atlanta FP Group meetup. TFP 2010 - Call for Papers and Participation. Simon Marlow [12]announced a call for papers and particpation in TFP 2010, the 11th Symposium on Trends in Functional Programming. CMCS 2010: Call for Short Submissions & Call for Participation. Alexandra Silva [13]announced a call for short submissions and participation in the 10th International Workshop on Coalgebraic Methods in Computer Science. WGP 2010 Call for Papers. Bruno Oliveira [14]announced a call for papers for Workshop on Generic Programming. ICE 2010: First call for papers. Alexandra Silva [15]announced the first call for papers for ICE2010, the 3rd Interation and Concurrency Experience workshop. Call for Papers: Haskell Symposium 2010. Jeremy.Gibbons [16]announced a call for papers for the 2010 Haskell Symposium. Dungeons of Wor - a largish FRP example and a fun game, all in one! Patai Gergely [17]announced Dungeons of Wor, a Game written in Haskell using the Elerea library and Functional Reactive Techniques. Discussion The Related monad and constant values in type classes. Jonas Almstroem Duregard [18]talked about a common pattern which occurs when associating data with a type (as opposed to a value). Heterogeneous Data Structures - Nested Pairs and functional references. Guenther Schmidt [19]asked about different ways to implement heterogeonous datastructures. Blog noise [20]Haskell news from the [21]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Michael Snoyman: [22]Four HTTP Request Body Interfaces (repost). * Brent Yorgey: [23]Math.Combinatorics.Multiset. * Darcs: [24]darcs weekly news #55. * Galois, Inc: [25]Tech Talk: Modern Benchmarking in Haskell. * Ketil Malde: [26]Tools for pyrosequencing analysis. * Arch Haskell News: [27]Arch Haskell News: February 2010. * Don Stewart (dons): [28]Migrating from uvector to vector. * GHC / OpenSPARC Project: [29]Memory Barriers and GHC 6.12.1. * Darcs: [30]darcs weekly news #54. * Gergely Patai: [31]Behind the dungeons. * David Amos: [32]How to find a strong generating set. Quotes of the Week * fasta: And by cool, I mean 'that is not supposed to happen'. * uncyclopedia: Calculations which are undefined are denoted by the _|_ symbol, pronounced Bottom, which the documentation explains as the compiler giving you the finger. * Draconx|Laptop: mathematics is the art of finding different ways to write "therefore". * medfly: Cale, #haskell wants to know all your personal preferences so we can all copy you * Cale: Gentoo is a massive waste of electricity. * hobophobe: So, I can only conclude that Haskell is a memetic virus, and monads are the eggs it lays out in innocent programming forums to entice others to become infected. * benmachine: unsafeCoerce is just a generalisation of id * jmcarthur: what good is a state monad if your state is basically 'i have no idea what the hell my state is'? About the Haskell Weekly News New editions are posted to [33]the Haskell mailing list as well as to [34]the Haskell Sequence and [35]Planet Haskell. [36]RSS is also available, and headlines appear on [37]haskell.org. To help create new editions of this newsletter, please see the information on [38]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [39]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70589 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70563 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70533 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70485 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70344 7. http://www.dragongoserver.net/ 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70328 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70314 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70290 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17802 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17799 13. http://article.gmane.org/gmane.comp.lang.haskell.general/17791 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17783 15. http://article.gmane.org/gmane.comp.lang.haskell.general/17779 16. http://article.gmane.org/gmane.comp.lang.haskell.general/17778 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70188 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/70492 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/70450 20. http://planet.haskell.org/ 21. http://haskell.org/haskellwiki/Blog_articles 22. http://snoyberg.wordpress.com/2010/02/21/four-http-request-body-interfaces-… 23. http://byorgey.wordpress.com/2010/02/20/math-combinatorics-multiset/ 24. http://blog.darcs.net/2010/02/darcs-weekly-news-55.html 25. http://www.galois.com/blog/2010/02/19/tech-talk-modern-benchmarking-in-hask… 26. http://blog.malde.org/index.php/2010/02/19/tools-for-pyrosequencing-analysi… 27. http://archhaskell.wordpress.com/2010/02/19/arch-haskell-news-february-2010/ 28. http://donsbot.wordpress.com/2010/02/15/migrating-from-uvector-to-vector/ 29. http://ghcsparc.blogspot.com/2010/02/memory-barriers-and-ghc-6121.html 30. http://blog.darcs.net/2010/02/darcs-weekly-news-54.html 31. http://just-bottom.blogspot.com/2010/02/behind-dungeons.html 32. http://haskellformaths.blogspot.com/2010/02/how-to-find-strong-generating-s… 33. http://www.haskell.org/mailman/listinfo/haskell 34. http://sequence.complete.org/ 35. http://planet.haskell.org/ 36. http://sequence.complete.org/node/feed 37. http://haskell.org/ 38. http://haskell.org/haskellwiki/HWN 39. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

1 0

Threading and FFI
by Yves Parès 22 Feb '10

22 Feb '10

Hello! I was wondering: is the Haskell default runtime (that which uses only one processor) scheduler able to interrupt a thread which is currently calling to a C function in order to enable another haskell thread to progress? I think it can't, because I have starvation problems when I have a thread which calls inside its loop to a C function which makes a little "sleep". BTW, what is the portable way to sleep a thread in Haskell ? Control.Concurrent.threadDelay is "GHC only". ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Threading-and-FFI-tp27611528p27611528.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

8 16

Installing wx via cabal
by Dušan Kolář 22 Feb '10

22 Feb '10

Dear all, Running cabal install --global wx stops very soon with the following error. Building wxcore-0.12.1.2... [ 1 of 22] Compiling Graphics.UI.WXCore.WxcObject ( src/haskell/Graphics/UI/WXCore/WxcObject.hs, dist/build/Graphics/UI/WXCore/WxcObject.o ) [ 2 of 22] Compiling Graphics.UI.WXCore.WxcDefs ( src/haskell/Graphics/UI/WXCore/WxcDefs.hs, dist/build/Graphics/UI/WXCore/WxcDefs.o ) [ 3 of 22] Compiling Graphics.UI.WXCore.WxcClassTypes ( src/haskell/Graphics/UI/WXCore/WxcClassTypes.hs, dist/build/Graphics/UI/WXCore/WxcClassTypes.o ) [ 4 of 22] Compiling Graphics.UI.WXCore.WxcTypes ( src/haskell/Graphics/UI/WXCore/WxcTypes.hs, dist/build/Graphics/UI/WXCore/WxcTypes.o ) [ 5 of 22] Compiling Graphics.UI.WXCore.WxcClassesAL ( src/haskell/Graphics/UI/WXCore/WxcClassesAL.hs, dist/build/Graphics/UI/WXCore/WxcClassesAL.o ) ghc: out of memory (requested 1048576 bytes) cabal: Error: some packages failed to install: It seems quite strange as the machine seems to have 16GB RAM and a free swap, can I somehow put options or so? I have ghc 10.4. installed. Thanks Dusan

3 2

games of chance
by Dupont Corentin 21 Feb '10

21 Feb '10

Hello, are you aware of a framework to find probabilities in a given game of chance, in Haskell? l would like to compute probabilities like: "roll 3 dices, sum the result, then roll 3 other dices, sum the result, what is the probability to obtain the same sum?" Cheers, Corentin

2 2

Re: Parsing of bytestrings with non-String errors?
by Permjacov Evgeniy 21 Feb '10

21 Feb '10

On 02/21/2010 11:57 PM, haskell-cafe-request(a)haskell.org wrote: > Message: 2 > Date: Sun, 21 Feb 2010 12:36:21 +0000 > From: Magnus Therning <magnus(a)therning.org> > Subject: [Haskell-cafe] Parsing of bytestrings with non-String errors? > To: haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: <4B8128C5.6030105(a)therning.org> > Content-Type: text/plain; charset="utf-8" > > I've looked at polyparse and attoparsec and they seem to have in common that > the error always is a String. My current ideas for a project would be a lot > easier if I could just return some other type, something that I can pattern > match on. > > Is there a parser combinator library out there that works on bytestrings and > allows using a custom error type? > > Or maybe there's some very basic reason why String is so commonly used? > > You can try to play with ParsecT / ErrorT

1 0

Re: [Haskell-cafe] Profiling
by Patai Gergely 21 Feb '10

21 Feb '10

> I tried to compile my program with -prof, but GHC just whines at me > that the packages I'm using haven't been compiled for profiling. Do I > really need to go recompile every single package I'm using with > profiling support before I can profile my program? How do I tell Cabal > to install the necessary code? (I really hope this doesn't involve > uninstalling and reinstalling everything...) Almost. You don't need to uninstall anything, just run cabal install <package> --reinstall -p for every relevant package. Gergely -- http://www.fastmail.fm - mmm... Fastmail...

4 7

STArray newListArray
by Vojtěch Knyttl 21 Feb '10

21 Feb '10

Hello, I am trying to create STArray with newListArray like this: la x y = newListArray ((1,1),(x,y)) [(x'+y') | x' <- [1..x], y' <- [1..y]] – but it does not work: No instance for (MArray a Field m) I tried to define the type like this, but it would not work either: la :: Int -> Int -> STArray (Int,Int) Field It is obvious that I don't get the syntax of using it, so I will appreciate any suggestions. V. K.

3 4

darcs 2.4 release candidate 2
by Reinier Lamers 21 Feb '10

21 Feb '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4 release candidate 2. darcs 2.4 will contain many improvements and bugfixes compared to darcs 2.3.1. Highlights are the faster operation of record, revert and related commands, and the experimental interactive hunk editing. This beta is your chance to test-drive these improvements and make darcs even better. Compared to darcs 2.4 beta 3, the performance of "darcs check" and "darcs repair" has been brought up to the level of darcs 2.3.1.` The easiest way to install darcs is using the Haskell Platform [1]. If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install --reinstall darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-beta-2.3.99.2.tar.gz , and build it by hand as explained in the README file. Interactive hunk editing ------------------------ To try out interactive hunk editing, press 'e' when you are prompted with a hunk patch by 'darcs record'. You will then be shown an editor screen in which you can edit the state you want to record between the last two ruler lines. You can find more information about the hunk editing feature on http://wiki.darcs.net/HunkEditor . Reporting bugs -------------- If you have an issue with darcs 2.4 release candidate 2, you can report it via the web on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net. What's New ---------- A list of important changes since 2.3.1 is as follows (please let me know if there's something you miss!): * Use fast index-based diffing everywhere (Petr) * Interactive patch splitting (Ganesh) * An 'optimize --upgrade' option to convert to hashed format in-place (Eric) * Hunk matching (Kamil Dworakowski, tat.wright) * Progress reporting is no longer deceptive (Roman) * A 'remove --recursive' option to remove a directory tree from revision control (Roman) * 'show files' accepts arguments to show a subset of tracked files (Luca) * A '--remote-darcs' flag for pushing to a host where darcs isn't called darcs * Many miscellaneous Windows improvements (Salvatore, Petr and others) * 'darcs send' now mentions the repository name in the email body (Joachim) * Handle files with boring names in the repository correctly (Petr) * Fix parsing of .authorspellings file (Tomáš) * Various sane new command-line option names (Florent) * Remove the '--checkpoint' option (Petr) * Use external libraries for all UTF-8 handling (Eric, Reinier) * Use the Haskell zlib package exclusively for compression (Petr) A list of issues resolved since 2.3.1: * 183: do not sort changes --summary output * 223: add --remote-darcs flag to specify name of remote darcs executable * 291: provide (basic) interactive patch splitting * 540: darcs remove --recursive * 835: 'show files' with arguments * 1122: get --complete should not offer to create a lazy repository * 1216: list Match section in ToC * 1224: refuse to convert a repo that's already in darcs-2 format * 1300: logfile deleted on unsucessful record * 1308: push should warn about unpulled patches before patch-selection * 1336: sane error message on --last "" (empty string to numbers parser) * 1362: mention repo name in mail send body * 1377: getProgname for local darcs instances * 1392: use parsec to parse .authorspelling * 1424: darcs get wrongly reports "using lazy repository" if you ctrl-c old-fashioned get * 1447: different online help for send/apply --cc * 1488: fix crash in whatsnew when invoked in non-tracked directory * 1548: show contents requires at least one argument * 1554: allow opt-out of -threaded (fix ARM builds) * 1563: official thank-you page * 1578: don't put newlines in the Haskeline prompts * 1583: on darcs get, suggest upgrading source repo to hashed * 1584: provide optimize --upgrade command * 1588: add --skip-conflicts option * 1594: define PREPROCHTML in makefile * 1620: make amend leave a log file when it should * 1636: hunk matching * 1643: optimize --upgrade should do optimize * 1652: suggest cabal update before cabal install * 1659: make restrictBoring take recorded state into account * 1677: create correct hashes for empty directories in index * 1681: preserve log on amend failure * 1709: fix short version of progress reporting * 1712: correctly report number of patches to pull * 1720: fix cabal haddock problem * 1731: fix performance regression in check and repair * 1741: fix --list-options when option has multiple names Kind Regards, the darcs release manager, Reinier Lamers [1]: You can download the Haskell platform from http://hackage.haskell.org/platform/

3 6

Restricted categories
by Sjoerd Visscher 21 Feb '10

21 Feb '10

Hi all, I want restricted categories, just like the rmonad package provides restricted monads. The ultimate goal is to have a product category: http://en.wikipedia.org/wiki/Product_category > {-# LANGUAGE TypeOperators, TypeFamilies, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, ScopedTypeVariables, UndecidableInstances #-} > import Prelude hiding ((.), id, fst, snd) > import qualified Prelude First lets create a clone of Data.Suitable from the rmonad package, but this time for types with two arguments: > class Suitable2 m a b where > data Constraints m a b > constraints :: m a b -> Constraints m a b > withResConstraints :: forall m a b. Suitable2 m a b => (Constraints m a b -> m a b) -> m a b > withResConstraints f = f (constraints undefined :: Constraints m a b) > withConstraintsOf :: Suitable2 m a b => m a b -> (Constraints m a b -> k) -> k > withConstraintsOf v f = f (constraints v) Now we can define a restricted category: > class RCategory (~>) where > id :: Suitable2 (~>) a a => a ~> a > (.) :: (Suitable2 (~>) b c, Suitable2 (~>) a b, Suitable2 (~>) a c) => b ~> c -> a ~> b -> a ~> c (->) (or "Hask") is an instance of this class: > instance Suitable2 (->) a b where > data Constraints (->) a b = HaskConstraints > constraints _ = HaskConstraints > instance RCategory (->) where > id = Prelude.id > (.) = (Prelude..) Now on to the product category. Objects of the product category are types that are an instance of IsProduct: > class IsProduct p where > type Fst p :: * > type Snd p :: * > fst :: p -> Fst p > snd :: p -> Snd p For example: > instance IsProduct (a, b) where > type Fst (a, b) = a > type Snd (a, b) = b > fst = Prelude.fst > snd = Prelude.snd Arrows from the product of category c1 and category c2 are a pair of arrows, one from c1 and one from c2: > data (c1 :***: c2) a b = c1 (Fst a) (Fst b) :***: c2 (Snd a) (Snd b) The instance for Suitable2 restricts objects to IsProduct, and requires the Fst and Snd parts of the objects to be suitable in c1 resp. c2: > instance (IsProduct a, IsProduct b, Suitable2 c1 (Fst a) (Fst b), Suitable2 c2 (Snd a) (Snd b)) => Suitable2 (c1 :***: c2) a b where > data Constraints (c1 :***: c2) a b = (IsProduct a, IsProduct b, Suitable2 c1 (Fst a) (Fst b), Suitable2 c2 (Snd a) (Snd b)) => ProdConstraints > constraints _ = ProdConstraints Finally the RCategory instance: > instance (RCategory c1, RCategory c2) => RCategory (c1 :***: c2) where > id = withResConstraints $ \ProdConstraints -> id :***: id > -- f@(f1 :***: f2) . g@(g1 :***: g2) = > -- withResConstraints $ \ProdConstraints -> > -- withConstraintsOf f $ \ProdConstraints -> > -- withConstraintsOf g $ \ProdConstraints -> > -- (f1 . g1) :***: (f2 . g2) Here I am running into problems. I get this error: Could not deduce (Suitable2 c2 (Snd a) (Snd b), Suitable2 c1 (Fst a) (Fst b)) from the context (IsProduct b, IsProduct c, Suitable2 c1 (Fst b) (Fst c), Suitable2 c2 (Snd b) (Snd c)) arising from a use of `withConstraintsOf' In the first argument of `($)', namely `withConstraintsOf g' I don't understand this, as I thought the constraints the error is complaining about is just what withConstraintsOf g should provide. I guess there's something about the Suitable trick that I don't understand, or possibly the type families Fst and Snd are biting me. Who can help me out? Thanks. greetings, Sjoerd

3 4

ANN: HaskellTorrent v0.0
by Jesper Louis Andersen 20 Feb '10

20 Feb '10

Hello, I am happy to announce HaskellTorrent v0.0, a concurrent bittorrent client. The current release is a rather early preview release: it can download torrents and seed them, but currently there are many optimizations and tweaks needed before it is ready for day-to-day use. In particular, we are not yet fully compliant with the bittorrent specification and need proper clients to connect to in order to operate. Basically, you install the cabal package and then run HaskellTorrent foo.torrent and it will then download the contents of the torrent to the current directory. We would like to hear about any kind of problems you might encounter. Send them to jesper.louis.andersen(a)gmail.com for now. We have only tested the software on GHC 6.10.4 and 6.12.1, mostly on Unix platforms. It is known to work on Debian/Unstable with GHC610 and GHC612. If you want to hack code, get in touch with me. There are something for any level of skill to do. *** HaskellTorrent is using the CML library to get concurrency. It easily runs some 200-300 processes when it operates and its process model is somewhat loosely inspired by the model of Erlang. Looking forward, the next goal is to optimize concurrency, memory usage and CPU usage in the client. It currently hogs resources due to naive data structure choices and some expensive correctness assertions. *** Links of interest: http://hackage.haskell.org/package/HaskellTorrent http://github.com/jlouis/haskell-torrent *** This release was brought to you by: Alex Mason: A lot of Wireprotocol and BCode serialization work through the use of the cereal package. Astro: GHC 6.12 support. Support for multi-file torrents. In addition, patches were contributed by: Jesper Louis Andersen John Gunderman Thomas Christensen -- J.

1 0

Computing sums
by Andrew Coppin 20 Feb '10

20 Feb '10

The other day, I found myself writing the following code: data Property x y = forall s. Property s (x -> s -> s) (s -> y) step :: x -> Property x y -> Property x y step x (Property s f g) = Property (f x s) f g read :: Property x y -> y read (Property s _ g) = g s pure :: (x -> y) -> Property x y pure f = Property undefined const f fold :: (x -> y -> y) -> y -> Property x y fold f y = Property y f id fold1 :: (x -> x -> x) -> Property x x fold1 f = Property Nothing (\x mx -> maybe (Just x) (Just . f) mx) fromJust (>==>) :: Property x y -> Property y z -> Property x z p0 >==> p1 = Property (p0,p1) (\x (q0,q1) -> let w0 = step x q0 in (w0, step (read w0) q1)) (\(_,q1) -> (read q1)) (>==<) :: Property x y -> Property x y' -> Property x (y,y') p0 >==< p1 = Property (p0,p1) (\x (q0,q1) -> (step x q0, step x q1)) (\(q0,q1) -> (read q0, read q1)) sum = fold (+) 0 count = pure (const 1) >==> sum minimum = fold1 min maximum = fold1 max Have I just invented arrows? (And if so, why is the above code nowhere to be seen in the libraries?)

4 6

What happened in Ohloh?
by Paul Johnson 20 Feb '10

20 Feb '10

If you go to http://www.ohloh.net/languages/compare?l0=haskell&measure=projects and look at the number (not percentage) of Haskell projects you see it rise exponentially until the start of 2008 and then suddenly drop away. Does anyone know what happened? Assuming this is just an artefact because they aren't scanning Haskell project hosts, can we get them to fix it? I'd like to use this kind of graph at work as evidence that Haskell is on a growth trajectory. Paul.

3 4

Haskell - Heterogeneous Data Structures - Nested Pairs and functional references
by Günther Schmidt 20 Feb '10

20 Feb '10

Hallo Heinrich, entschuldige bitte wenn ich Dich ausserhalb der Cafe-Liste kontaktiere, aber so langsam wird's mir peinlich.... Auf meinen Post "Heterogeneous Data Structures - Nested Pairs and functional references" vom 16.02.2010 in der Cafe-Liste habe ich eine interessante Antwort erhalten von Alexander Solla, die ich nicht verstanden haben. Als ich nachgehakt habe, hat er freundlicherweise sofort eine Antwort geliefert die ich noch weniger verstanden habe ... Jetzt trau' ich mich nicht mehr nochmals zu fragen, denn ich glaube ich würd's wieder nicht verstehen. Kannst Du mir vielleicht erklären was er mir da eigentlich geschickt hat? Grüße Günther

1 0

fast, strongly typed heterogeneous collections
by Alberto G. Corona 19 Feb '10

19 Feb '10

Hi What is the speed of hProjectByLabel/s in Data.HList.Record? . The documentation in hackage <http://hackage.haskell.org/packages/archive/HList/latest/doc/html/Data-HLis…>does not mention it. Perhaps this is because Hlist is designed for supposedly short records and this does not matter too much. looking at the code I guess that everything in HList is O(n) because it uses type-level list structure, but I think that nothing prevents from using a type level balanced tree, Data.Map style, instead. This would permit to implement large heterogeneous Maps with fast access and strongly typed. AFAIK, the alternative is to use a fast data container with data elements encapsulated within Data.Dynamic, that is weakly typed, produces type errors at runtime, which is unsatisfactory and dangerous. Any comments?

4 4

Cabal broken on Mac OS X?
by Oleg Lobachev 19 Feb '10

19 Feb '10

Hello all, I suspect that it's me and not the cabal, but everything looks fine and still fails. Would someone mind to tell me, what am I doing wrong? $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library $ ghc -V The Glorious Glasgow Haskell Compilation System, version 6.12.1 $ which ghc /usr/bin/ghc $ which cabal /usr/local/bin/cabal $ cabal install cabal-install cabal: failed to parse output of 'ghc-pkg dump' That's the problem! As it is Mac OS X 10.6.2, here is some information on the architecture and the executables used: $ uname -v Darwin Kernel Version 10.2.0: Tue Nov 3 10:37:10 PST 2009; root:xnu-1486.2.11~1/RELEASE_I386 $ file /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.1/ghc /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.1/ghc: Mach-O executable i386 $ file `which cabal` /usr/local/bin/cabal: Mach-O executable i386 $ file `which ls` /bin/ls: Mach-O universal binary with 2 architectures /bin/ls (for architecture x86_64): Mach-O 64-bit executable x86_64 /bin/ls (for architecture i386): Mach-O executable i386 $ ghc --make hello.hs [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... $ ./hello Hello world! $ file ./hello ./hello: Mach-O executable i386 The output of ghc-pkg dump is here: http://www.mathematik.uni-marburg.de/~lobachev/tmp/ghc-pkg.txt I have installed the Haskell Platform and then the new GHC via official pkg files. Greetings, Oleg

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Why is Haskell used so little in the industry?
by Gabi 19 Feb '10

19 Feb '10

I posted a question about it in StackOverflow (http://stackoverflow.com/questions/2284875/why-is-haskell-used-so-little-in…) Haskell gets some beating there. Please Hasellers, join the discussion and give your opinion there. -- Regards, Gabi http://bugspy.net

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ANNOUNCE: vty-ui 0.3
by Jonathan Daugherty 19 Feb '10

19 Feb '10

I'm happy to announce the release of vty-ui 0.3. Get it from Hackage: http://hackage.haskell.org/package/vty-ui Or get the source with darcs: http://repos.codevine.org/vty-ui Project homepage: http://codevine.org/vty-ui/ This version of vty-ui features a richer rendering engine, generalized text transformations, and a more functional style. If you've written any applications to use vty-ui, your type signatures and usage of a few of the built-in widget types will need to change but the API is largely the same. In particular, the largest change was a refactoring inspired by Luke Palmer's blog post, "Haskell Antipattern: Existential Typeclass," which targeted vty-ui (among other libraries) in its treatment of the topic: http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-… Here's a summary of changes: General * Replace Widget type class with a concrete Widget type * Move text widget into Graphics.Vty.Widgets.Text * Add support for generalized text "formatters" * Replace WrappedText widget type with a text-wrapping formatter * Provide a regex-based text-highlighting formatter (see the demo for an example) * Refactor the rendering engine to provide "address" information so you can get coordinates and size for rendered widgets * Add a few QuickCheck tests (and related fixes) Enjoy! -- Jonathan Daugherty

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Implementing "unionAll"
by Leon Smith 19 Feb '10

19 Feb '10

With the urging and assistance of Omar Antolín Camarena, I will be adding two functions to data-ordlist: mergeAll and unionAll, which merge (or union) a potentially infinite list of potentially infinite ordered lists, under the assumption that the heads of the non-empty lists appear in a non-decreasing sequence. Union takes two sorted lists and produces a new sorted list; an element occurs in the result as many times as the maximum number of occurrences in either list. The unionAll function generalizes this behavior to an infinite number of lists. A reasonable implementation of mergeAll is: > import Data.List.Ordered(merge, union) > mergeAll :: Ord a => [[a]] -> [a] > mergeAll = foldr (\(x:xs) ys -> x : merge xs ys) [] However, for many inputs, we can do better; the library implementation of mergeAll is based on H. Apfelmus's article "Implicit Heaps", which presents a simplification of Dave Bayer's "venturi" algorithm. The difference is that the foldr version uses a line of comparisons, whereas "venturi" uses a tree of comparisons. http://apfelmus.nfshost.com/articles/implicit-heaps.html http://www.mail-archive.com/haskell-cafe@haskell.org/msg27612.html However, as Omar pointed out to me, the following implementation of unionAll has a flaw: > unionAll :: Ord a => [[a]] -> [a] > unionAll = foldr (\(x:xs) ys -> x : union xs ys) [] Namely unionAll [[1,2],[1,2]] should return [1,2], whereas it actually returns [1,1,2]. After some work, I believe I have generalized H. Apfelmus's algorithm to handle this; however it seems a bit complicated. I would love feedback, especially with regard to simplifications, bugs, testing strategies, and optimizations: > unionAll' :: Ord a => [[a]] -> [a] > unionAll' = unionAllBy compare > data People a = VIP a (People a) | Crowd [a] > unionAllBy :: (a -> a -> Ordering) -> [[a]] -> [a] > unionAllBy cmp xss = loop [ (VIP x (Crowd xs)) | (x:xs) <- xss ] > where > loop [] = [] > loop ( VIP x xs : VIP y ys : xss ) > = case cmp x y of > LT -> x : loop ( xs : VIP y ys : xss ) > EQ -> loop ( VIP x (union' xs ys) : unionPairs xss ) > GT -> error "Data.List.Ordered.unionAll: assumption violated!" > loop ( VIP x xs : xss ) > = x : loop (xs:xss) > loop [Crowd xs] = xs > loop (xs:xss) = loop (unionPairs (xs:xss)) > > unionPairs [] = [] > unionPairs [x] = [x] > unionPairs (x:y:zs) = union' x y : unionPairs zs > > union' (VIP x xs) (VIP y ys) > = case cmp x y of > LT -> VIP x (union' xs (VIP y ys)) > EQ -> VIP x (union' xs ys) > GT -> error "Data.List.Ordered.unionAll: assumption violated!" > union' (VIP x xs) (Crowd ys) = VIP x (union' xs (Crowd ys)) > union' (Crowd []) ys = ys > union' (Crowd xs) (Crowd ys) = Crowd (unionBy cmp xs ys) > union' xs@(Crowd (x:xt)) ys@(VIP y yt) > = case cmp x y of > LT -> VIP x (union' (Crowd xt) ys) > EQ -> VIP x (union' (Crowd xt) yt) > GT -> VIP y (union' xs yt) -- Leon

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Re: [Haskell-cafe] Category Theory woes
by Michael Matsko 18 Feb '10

18 Feb '10

Nick, Actually, clopen is a set that is both closed and open. Not one that is neither. Except in the case of half-open intervals, I can't remember talking much in topology about sets with a partial boundary. Category theory-wise. No one seems to have mentioned MacLane's "Categories for the Working Mathematician." Although, I don't seem to recall instant enlightenment when I picked it up. Mike ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Michael Matsko" <msmatsko(a)comcast.net> Cc: haskell-cafe(a)haskell.org Sent: Thursday, February 18, 2010 4:54:03 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Hi Mike, of course... But in the same spirit, one could introduce a straightforward extension, «partially bordered», which would be as least as good as «clopen»... ;-) I must admit we've come a little off the topic -- how to introduce to category theory. The intent was to present some examples that mathematical terminology culture is not that exemplary as one should expect, but to motivate an open discussion about how one might «rename refactor» category theory (of 2:48 PM). I would be very interested in other people's proposals... :-) Michael Matsko wrote: Nick, That is correct. An open set contains no point on its boundary. A closed set contains its boundary, i.e. for a closed set c, Closure(c) = c. Note that for a general set, which is neither closed or open (say the half closed interval (0,1]), may contain points on its boundary. Every set contains its interior, which is the part of the set without its boundary and is contained in its closure - for a given set x, Interior(x) is a subset of x is a subset of Closure(x). Mike ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Michael Matsko" <msmatsko(a)comcast.net> Cc: haskell-cafe(a)haskell.org Sent: Thursday, February 18, 2010 3:15:49 PM GMT -05:00 US/Canada Eastern Subject: Re: Fwd: [Haskell-cafe] Category Theory woes Hi Mike, so an open set does not contain elements constituting a border/boundary of it, does it? But a closed set does, doesn't it? Cheers, Nick Michael Matsko wrote: ----- Forwarded Message ----- From: "Michael Matsko" <msmatsko(a)comcast.net> To: "Nick Rudnick" <joerg.rudnick(a)t-online.de> Sent: Thursday, February 18, 2010 2:16:18 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg, Topologically speaking, the border of an open set is called the boundary of the set. The boundary is defined as the closure of the set minus the set itself. As an example consider the open interval (0,1) on the real line. The closure of the set is [0,1], the closed interval on 0, 1. The boundary would be the points 0 and 1. Mike Matsko ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Gregg Reynolds" <dev(a)mobileink.com> Cc: "Haskell Café List" <haskell-cafe(a)haskell.org> Sent: Thursday, February 18, 2010 1:55:31 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg Reynolds wrote: On Thu, Feb 18, 2010 at 7:48 AM, Nick Rudnick < joerg.rudnick(a)t-online.de > wrote: IM(H??)O, a really introductive book on category theory still is to be written -- if category theory is really that fundamental (what I believe, due to its lifting of restrictions usually implicit at 'orthodox maths'), than it should find a reflection in our every day's common sense, shouldn't it? Goldblatt works for me. Accidentially, I have Goldblatt here, although I didn't read it before -- you agree with me it's far away from every day's common sense, even for a hobby coder?? I mean, this is not «Head first categories», is it? ;-)) With «every day's common sense» I did not mean «a mathematician's every day's common sense», but that of, e.g., a housewife or a child... But I have became curious now for Goldblatt... * the definition of open/closed sets in topology with the boundary elements of a closed set to considerable extent regardable as facing to an «outside» (so that reversing these terms could even appear more intuitive, or «bordered» instead of closed and «unbordered» instead of open), Both have a border, just in different places. Which elements form the border of an open set?? As an example, let's play a little: Arrows: Arrows are more fundamental than objects, in fact, categories may be defined with arrows only. Although I like the term arrow (more than 'morphism'), I intuitively would find the term «reference» less contradictive with the actual intention, as this term Arrows don't refer. A *referrer* (object) refers to a *referee* (object) by a *reference* (arrow). Categories: In every day's language, a category is a completely different thing, without the least Not necesssarily (for Kantians, Aristoteleans?) Are you sure...?? See http://en.wikipedia.org/wiki/Categories_(Aristotle) ... If memory serves, MacLane says somewhere that he and Eilenberg picked the term "category" as an explicit play on the same term in philosophy. In general I find mathematical terminology well-chosen and revealing, if one takes the trouble to do a little digging. If you want to know what terminological chaos really looks like try linguistics. ;-) For linguistics, granted... In regard of «a little digging», don't you think terminology work takes a great share, especially at interdisciplinary efforts? Wouldn't it be great to be able to drop, say 20% or even more, of such efforts and be able to progress more fluidly ? -g _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Fwd: [Haskell-cafe] Category Theory woes
by Michael Matsko 18 Feb '10

18 Feb '10

----- Forwarded Message ----- From: "Michael Matsko" <msmatsko(a)comcast.net> To: "Nick Rudnick" <joerg.rudnick(a)t-online.de> Sent: Thursday, February 18, 2010 2:16:18 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg, Topologically speaking, the border of an open set is called the boundary of the set. The boundary is defined as the closure of the set minus the set itself. As an example consider the open interval (0,1) on the real line. The closure of the set is [0,1], the closed interval on 0, 1. The boundary would be the points 0 and 1. Mike Matsko ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Gregg Reynolds" <dev(a)mobileink.com> Cc: "Haskell Café List" <haskell-cafe(a)haskell.org> Sent: Thursday, February 18, 2010 1:55:31 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg Reynolds wrote: On Thu, Feb 18, 2010 at 7:48 AM, Nick Rudnick < joerg.rudnick(a)t-online.de > wrote: IM(H??)O, a really introductive book on category theory still is to be written -- if category theory is really that fundamental (what I believe, due to its lifting of restrictions usually implicit at 'orthodox maths'), than it should find a reflection in our every day's common sense, shouldn't it? Goldblatt works for me. Accidentially, I have Goldblatt here, although I didn't read it before -- you agree with me it's far away from every day's common sense, even for a hobby coder?? I mean, this is not «Head first categories», is it? ;-)) With «every day's common sense» I did not mean «a mathematician's every day's common sense», but that of, e.g., a housewife or a child... But I have became curious now for Goldblatt... * the definition of open/closed sets in topology with the boundary elements of a closed set to considerable extent regardable as facing to an «outside» (so that reversing these terms could even appear more intuitive, or «bordered» instead of closed and «unbordered» instead of open), Both have a border, just in different places. Which elements form the border of an open set?? As an example, let's play a little: Arrows: Arrows are more fundamental than objects, in fact, categories may be defined with arrows only. Although I like the term arrow (more than 'morphism'), I intuitively would find the term «reference» less contradictive with the actual intention, as this term Arrows don't refer. A *referrer* (object) refers to a *referee* (object) by a *reference* (arrow). Categories: In every day's language, a category is a completely different thing, without the least Not necesssarily (for Kantians, Aristoteleans?) Are you sure...?? See http://en.wikipedia.org/wiki/Categories_(Aristotle) ... If memory serves, MacLane says somewhere that he and Eilenberg picked the term "category" as an explicit play on the same term in philosophy. In general I find mathematical terminology well-chosen and revealing, if one takes the trouble to do a little digging. If you want to know what terminological chaos really looks like try linguistics. ;-) For linguistics, granted... In regard of «a little digging», don't you think terminology work takes a great share, especially at interdisciplinary efforts? Wouldn't it be great to be able to drop, say 20% or even more, of such efforts and be able to progress more fluidly ? -g _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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The Related monad and constant values in type classes
by Jonas Almström Duregård 18 Feb '10

18 Feb '10

Hi, This literate haskell file was intended to be a quick question about a problem i have been pondering, but it developed into a short presentation instead. What i want to know is if there is already something like this (and suggestions for improvement of course). >{-#LANGUAGE GeneralizedNewtypeDeriving#-} Sometimes i find myself needing to associate a constant with a type or, more precisely, with a type class instance. Something like this would be nice: class Sized a where size :: Int instance Sized Int where size = 32 Of course this will not work since there is no way of knowing which instance i refer to when i use "size". A common work-around is to use a dummy parameter: >class SizedDummy a where > sizeDummy :: a -> Int > >instance SizedDummy Int where > sizeDummy = const 32 The size function is typically passed an undefined value. This is not very pretty, and somewhat unsafe. Another workaround is to define a newtype with a type parameter. >newtype SizeOf a = MkSize {toInt :: Int} >class SizedNewType a where > sizeNewType :: SizeOf a > >instance SizedNewType Int where > sizeNewType = MkSize 32 If we want the size of a pair to be the sum of it's components, something like this is needed: >instance (SizedNewType a, SizedNewType b) => SizedNewType (a,b) where > sizeNewType = sizeNewType' sizeNewType sizeNewType where > sizeNewType' :: SizeOf a -> SizeOf b -> SizeOf (a,b) > sizeNewType' a b = MkSize $ toInt a + toInt b This is way to much code say that "size = size a + size b". A more general solution can be achieved by making "Int" another type variable of "SizeOf". I call the resulting type "Related": >newtype Related a b = Related {unrelated :: b} deriving > (Eq,Ord,Show,Read,Bounded,Enum,Fractional,Num, > Real,Integral,RealFrac,Floating,RealFloat) This type is highly reusable and the GeneralizedNewtypeDeriving language extension is very practical (although the instances could be written manually). It can also be used as an Identity monad: >instance Functor (Related a) where > fmap f (Related a) = Related $ f a > >instance Monad (Related a) where > return = Related > (Related a) >>= f = f a This allows the Sized class and instances to be specified in a slim fashion using a familiar monadic interface: >class Sized a where > size :: Related a Int > >instance Sized Int where > size = return 32 > >instance (Sized a, Sized b) => Sized (a,b) where > size = do > a <- return size :: Sized a => Related (a,b) (Related a Int) > b <- return size :: Sized b => Related (a,b) (Related b Int) > return $ unrelated a + unrelated b This still requires a lot of type signatures, some additional magic is required. It is possible to write general versions of the type signatures above, which allows the following instance definition for (,,): >instance (Sized a, Sized b, Sized c) => Sized (a,b,c) where > size = do > a <- on3 size > b <- on2 size > c <- on1 size > return $ a + b + c With the derivation of Num, this can be done even more compact: >instance (Sized a, Sized b, Sized c, Sized d) => Sized (a,b,c,d) where > size = on1 size + on2 size + on3 size + on4 size The code for the onN functions: >rerelate :: Related a b -> Related c b >rerelate = return . unrelated >on1 :: Related a v -> Related (x a) v >on1 = rerelate >on2 :: Related a v -> Related (x a x0) v >on2 = rerelate >on3 :: Related a v -> Related (x a x0 x1) v >on3 = rerelate >on4 :: Related a v -> Related (x a x0 x1 x2) v >on4 = rerelate Regards, Jonas Almström Duregård

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How can i run darcs in ghci?
by Daryoush Mehrtash 18 Feb '10

18 Feb '10

I am trying to load darc.hs (from darcs 2.3.1, ghci 6.10.4 on Ubuntu) and I get the following :~/darcs-src/darcs-2.3.1/src$ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :load darcs Darcs/RepoPath.hs:59:0: error: impossible.h: No such file or directory phase `C pre-processor' failed (exitcode = 1) > What am I doing wrong? Thanks, Daryoush

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