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lambdabot install error
by epsilonhalbe 07 May '11

07 May '11

hey haskellers, i'm fresh into haskell and love it. so to do faster easier haskell i wanted to install lambdabot but had to face some errors - i don't understand them at all here is some error output from zsh: http://hpaste.org/46401/bot_install_error can anyone help me - i'm working on a debian (crunchbang) y.t epsilonhalbe (ε/2)

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Manatee-0.2.1 release!
by Lazycat Manatee 07 May '11

07 May '11

Hi all, Manatee-0.2.1 release! (Detail see: http://goo.gl/MkVw http://goo.gl/PnPbq http://goo.gl/OSpz8) New features in new version: * Show welcome page when you first time use Manatee, make manatee easier to use. * Keymap helper, you just need hit Ctrl+/ or Ctrl+? if you don't know keymap. * Window layout save/restore, you play Manatee and shutdown, all window layout will restore when you next startup. * Tab position save when you switch to other tab-group, and restore tab position when you switch back. * Use new input-framework instead manatee-anything package. * Many detail improve and bug fixed. Enjoy! -- Andy

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ANN: stm-chans 1.2.0
by wren ng thornton 07 May '11

07 May '11

-------------------------------------------- -- stm-chans 1.2.0 -------------------------------------------- The stm-chans package offers a collection of channel types, similar to Control.Concurrent.STM.TChan but with additional features. -------------------------------------------- -- Changes (since 1.0.0) -------------------------------------------- * (Version 1.1.0) Added tryWriteTBChan. * (Version 1.1.0) Added tryWriteTBMChan. * Some major performance improvements in TBChan and TBMChan, thanks to Thomas DuBuisson. In particular, these improvements reduce contention between readers and writers, improving throughput by up to 2--3 times when producers and consumers are running in separate OS threads. The cost for non-parallel programs is trivial (<1% worst-case running time, per Criterion). * Added estimateFreeSlotsTBChan, freeSlotsTBChan, estimateFreeSlotsTBMChan, and freeSlotsTBMChan. These functions return either the exact number or a lower-bound on the number of free slots available before the channels will block/retry. * Some other minor optimizations and code improvements. -------------------------------------------- -- Long description -------------------------------------------- In particular stm-chans offers these types: * Control.Concurrent.STM.TBChan: Bounded FIFO channels. When the channel is full, writers will block/retry. This ensures that the writers do not get too far ahead of the readers, which helps to make sure that memory and cpu resources are used responsibly. * Control.Concurrent.STM.TMChan: Closeable FIFO channels. This is like TChan (Maybe a) but with a monotonicity guarantee that once Nothing is returned all future reads will be Nothing as well. * Control.Concurrent.STM.TBMChan: Bounded Closeable FIFO channels. This combines the capabilities of TBChan and TMChan. In addition, the stm-chans package offers a (partial) compatibility layer for some recent API improvements to the stm package (see ticket #5104 [1]). These new functions include: * tryReadTChan :: TChan a -> STM (Maybe a) A version of readTChan which does not retry. Instead it returns Nothing if no value is available. * peekTChan :: TChan a -> STM a Get the next value from the TChan without removing it, retrying if the channel is empty. * tryPeekTChan :: TChan a -> STM (Maybe a) A version of peekTChan which does not retry. Instead it returns Nothing if no value is available. [1] http://hackage.haskell.org/trac/ghc/ticket/5104 -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/stm-chans Darcs: http://community.haskell.org/~wren/stm-chans Haddock (Darcs version): http://community.haskell.org/~wren/stm-chans/dist/doc/html/stm-chans -- Live well, ~wren

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Do exist globally available hosting project services?
by Facundo Domínguez 07 May '11

07 May '11

Dear haskellers, I was reading the terms of service of the Haskell Community Server and found this statement: "Users residing in countries on the United States Office of Foreign Assets Control sanction list, including Cuba, Iran, Libya, North Korea, Sudan and Syria, may not post or access CONTENT available through OUR services." I wonder whether there is a solution for hosting projects which does not discriminate people by their place of residence. http://code.haskell.org http://www.patch-tag.com https://github.com all seem to be ruled under USA laws. Would anybody recommend an alternative? Best, Facundo

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ANN: timeplot-0.3.0 - the analyst's swiss army knife for visualizing ad-hoc log files
by Eugene Kirpichov 06 May '11

06 May '11

Hello fellow haskellers, I announce the release of timeplot-0.3.0, the "analyst's swiss army knife for visualizing ad-hoc log files". Links: * http://jkff.info/presentation/two-visualization-tools - a presentation saying what the tools are all about and giving plenty of graphical examples on cluster computing use cases. At the end of the presentation there's also a couple of slides about installation. It is a little bit outdated, it corresponds to versions just before 0.3.0. * http://hackage.haskell.org/package/timeplot * http://github.com/jkff/timeplot * The sibling tool, splot - for visualizing the activity of many concurrent processes - http://hackage.haskell.org/package/splot and http://github.com/jkff/splot . It has also gotten a couple of new features since my last announcement. The major new feature of tplot is the introduction of subplots, the <<'within' plots>>. It allows one to plot data from several sub-tracks on one track of the graph: - several line- or dot-plots - several plots of sums or cumulative sums, perhaps stacked (to see how the sub-tracks contribute to the total sum - e.g. if your log speaks about different types of overhead and you wish to see how they contribute to the total) - stacked "activity count" plot - a generalization of the previous "activity count" plot, which allows you to, given a log saying like "Machine started servicing job JOB1 ... Machine finished servicing job JOB1" etc, plot how many machines are servicing each job at any moment, in a stacked fashion - so, how loads by different jobs contribute to the whole cluster's load. The "activity frequency" plot plots the same on a relative scale. The syntax is, for example: "within[.] dots" or "within[.] acount" or even "within[.] duration cumsum stacked" etc. Note that these are of course just example use cases and the tool is universal, it is not in any sense specialized to clusters, jobs, overheads or actually even to logs. I'd like to encourage you to give it a try and look around for a use case :) If you do give the tool a try, please tell me if something goes wrong, be it an installation problem or a bug (the version is fresh released, so this is quite possible). -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Can't build Gtk2hs on Windows
by Andrew Coppin 06 May '11

06 May '11

Today I tried to install Gtk2hs. Big mistake! Last time I tried it, it was quite easy. Now that it uses Cabal, even on Windows you can compile this stuff from source fairly easily. It's just that you have to fiddle with environment variables to make it find stuff. However... 1. Since the move of haskell.org, the gtk2hs homepage has vanished off the face of the Earth. Is there any danger we might get this back some day? 2. http://hackage.haskell.org/trac/gtk2hs/ticket/1203 (In other words, every time you try to cabal install a GTK-related package, it fails during the register step, and you need to hand-edit Gtk2HsSetup.hs to fix the issue.) 3. http://hackage.haskell.org/trac/gtk2hs/ticket/1209 (In other words, certain GTK-related packages just plain fail to build due to undefined names or missing header files or...) In summary, it's just utterly broken. Which is very frustrating, given that not so long ago it was working really well. (I never did get Glade to work though... although it looks like that might be fixed now, if I could just get past all the other issues.)

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ANN: syntactic-0.1
by Emil Axelsson 06 May '11

06 May '11

I'm happy to announce the first release of syntactic: http://hackage.haskell.org/package/syntactic providing generic abstract syntax and utilities for embedded languages. To get an idea of what it's about, check out the tiny(!) implementation of (simplified) Feldspar in the Examples directory: http://projects.haskell.org/syntactic/Examples/MuFeldspar The library is intended to provide a growing set of utilities for embedded languages. In particular, I hope to be able to provide safe interfaces to various unsafe techniques for speeding things up. Comments, patches and clients are welcome! / Emil

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CFP: APLAS 2011, Kenting, Taiwan
by Shin-Cheng Mu 06 May '11

06 May '11

========================================================== APLAS 2011 Call For Papers Ninth Asian Symposium on Programming Languages and Systems Kenting, Taiwan, December 5--7, 2011 (co-located with CPP 2011) http://flolac.iis.sinica.edu.tw/aplas11/ ========================================================== BACKGROUND: APLAS aims at stimulating programming language research by providing a forum for the presentation of latest results and the exchange of ideas in topics concerned with programming languages and systems. APLAS is based in Asia, but is an international forum that serves the worldwide programming language community. APLAS is sponsored by the Asian Association for Foundation of Software (AAFS) founded by Asian researchers in cooperation with many researchers from Europe and the USA. The past APLAS symposiums were successfully held in Shanghai ('10), Seoul ('09), Bangalore ('08), Singapore ('07), Sydney ('06), Tsukuba ('05), Taipei ('04) and Beijing ('03) after three informal workshops. Proceedings of the past symposiums were published in Springer-Verlag's LNCS 6461, 5904, 5356, 4807, 4279, 3780, 3302, and 2895. The 2011 edition will be held at Kenting, a seaside resort in Southern Taiwan. TOPICS: The symposium is devoted to both foundational and practical issues in programming languages and systems. Papers are solicited on, but not limited to, the following topics: + semantics, logics, foundational theory; + design of languages and foundational calculi; + type systems; + compilers, interpreters, abstract machines; + program derivation and transformation; + program analysis, verification, model-checking, software security; + concurrency, constraints, domain-specific languages; + tools for programming, verification, implementation. APLAS 2011 is not limited to topics discussed in previous symposiums. Papers identifying future directions of programming and those addressing the rapid changes of the underlying computing platforms are especially welcome. Demonstration of systems and tools in the scope of APLAS are welcome to the System and Tool presentations category. Authors concerned about the appropriateness of a topic are welcome to consult with Program Chair prior to submission. IMPORTANT DATES: Abstract Deadline: June 13, 2011 Submission Deadline: June 17, 2011 (GMT) Notification: August 29, 2011 Camera-Ready: September 19, 2011 INVITED SPEAKERS: Nikolaj Bjorner, Microsoft Research Redmond Ranjit Jhala, UC San Diego Peter O'Hearn, Queen Mary U. London Sriram Rajamani, Microsoft Research India SUBMISSION INFORMATION: We solicit submissions in two categories: Regular research papers, describing original research results, including tool development and case studies, from a perspective of scientific research. Regular research papers should not exceed 16 pages in the Springer LNCS format, including bibliography and figures. They should clearly identify what has been accomplished and why it is significant. Submissions will be judged on the basis of significance, relevance, correctness, originality, and clarity. In case of lack of space, proofs, experimental results, or any information supporting the technical results of the paper could be provided as Appendix or a link to a web page. System and Tool presentations, describing systems or tools that support theory, program construction, reasoning, and/or program execution in the scope of APLAS. Unlike presentations of regular research papers, presentation of accepted papers in this category is expected to be centered around a demonstration. The paper and the demonstration should identify the novelties of the tools and use motivating examples. System and Tool papers should not exceed 8 pages in the Springer LNCS format, including bibliography and figures. Submissions will be judged based on both the papers and the systems or tools as described in the papers. It is highly desirable that the tools are available on the web. Papers should be submitted electronically via the submission web page at http://www.easychair.org/conferences/?conf=aplas2011. Acceptable formats are PostScript or PDF, viewable by Ghostview or Acrobat Reader. Submitted papers must be unpublished and not submitted for publication elsewhere. Papers must be written in English. The proceedings are planned to be published as a volume in Springer-Verlag's Lecture Notes in Computer Science series. Accepted papers must be presented at the conference. GENERAL CHAIR: Tyng-Ruey Chuang, Academia Sinica, Taiwan PROGRAM CHAIR: Hongseok Yang, Queen Mary U. London aplas2011(a)easychair.org PROGRAM COMMITTEE: Lars Birkedal (ITU, Denmark) James Brotherston (Imperial College, UK) Kung Chen (National Chengchi U., Taiwan) Wenguang Chen (Tsinghua U., China) Wei-Ngan Chin (NUS, Singapore) Javier Esparza (TUM, Germany) Xinyu Feng (USTC, China) Jerome Feret (INRIA, France) Matthew Fluet (RIT, USA) Rajiv Gupta (UC Riverside, USA) Masahito Hasegawa (Kyoto U., Japan) Radha Jagadeesan (Depaul U., USA) Naoki Kobayashi (Tohoku U., Japan) Daniel Kroning (U. Oxford, UK) Rupak Majumdar (MPI-SWS, Germany) Andrzej Murawski (U. Leicester, UK) Paulo Oliva (Queen Mary U. London, UK) Doron Peled (Bar Ilan U., Israel) Sukyoung Ryu (KAIST, Korea) Sriram Sankaranarayanan (U. Colorado, USA) Armando Solar-Lezama (MIT, USA) Sam Staton (U. Cambridge, UK) Viktor Vafeiadis (MPI-SWS, Germany) Kapil Vaswani (MSR, India) Martin Vechev (IBM, USA) Peng Wu (IBM, USA) Hongseok Yang (Queen Mary U. London, UK) Pen-Chung Yew (Academia Sinica, Taiwan) ORGANIZERS: Mike Dodds, U. Cambridge (Poster chair) Shin-Cheng Mu, Academia Sinica, Taiwan (Local arrangement chair) Noam Rinetzky, Queen Mary U. London (Publicity chair) Yih-Kuen Tsay, National Taiwan University (Finance chair)

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Usage of rewrite rule specialization in Hackage
by Scott Kilpatrick 06 May '11

06 May '11

I'm looking for "real" code that uses the kind of GHC rewrite rule that specializes a polymorphic function with another one, as described here <http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite- rules.html#rule-spec>. Can anyone point me to any popular packages in Hackage that make use of this? Thanks, Scott

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Haskell statistics for use of TypeFamilies and FunctionalDependencies
by Gwern Branwen 05 May '11

05 May '11

Continuing my investigation of my local copies of patch-tag+github+c.h.o+misc repositories (, I rewrote my haskell-src-exts script to examine LANGUAGE extensions in use (ignoring any .cabal flags): import Language.Haskell.Exts import qualified Data.Foldable as F (concat) import Control.Monad (when) main :: IO () main = do args <- fmap lines $ getContents mapM_ (\a -> checkAndPrint a `catch` (\_ -> return ())) args checkAndPrint :: FilePath -> IO () checkAndPrint fs = do x <- readFile fs let exts = F.concat $ readExtensions x let exts' = filter (\y -> y == FunctionalDependencies || y == TypeFamilies) exts when (not $ null exts') $ putStr (fs++":") >> (putStrLn $ unwords $ map show exts) return () The attachment is the gzipped* result of `find . -type f -name "*?hs" | sort | runhaskell ~/extensions.hs >> ~/exts.txt`. Notes: - 3597 files matched either LANUAGE - there were: 2685 users of TypeFamilies 1030 users of FunctionalDependencies 118 users of TypeFamilies+FunctionalDependencies 2.7k TypeFamilies may be surprisingly large; if so, it may be due to GHC and its duplicates; 947 lines match the string 'ghc' and 721 lines match both 'ghc' and 'TypeFamilies'. * 500k is a bit much for an attachment -- gwern http://www.gwern.net

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How often is the planet updated?
by Magnus Therning 05 May '11

05 May '11

I see that Planet Haskell hasn't been updated since April 26. Is something wrong with it, or does it really not update more often than that? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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2nd CFP: ACM SIGPLAN 7th Workshop on Generic Programming (WGP 2011)
by Shin-Cheng Mu 05 May '11

05 May '11

====================================================================== CALL FOR PAPERS WGP 2011 7th ACM SIGPLAN Workshop on Generic Programming Tokyo, Japan Sunday, September 18th, 2011 http://flolac.iis.sinica.edu.tw/wgp11/ Collocated with the International Conference on Functional Programming (ICFP 2011) ====================================================================== Goals of the workshop --------------------- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, and, for at least 20 years, generic programming techniques have been a specific focus of research in the functional and object-oriented programming communities. Generic programming has gradually spread to more and more mainstream languages, and today is widely used in industry. This workshop brings together leading researchers and practitioners in generic programming from around the world, and features papers capturing the state of the art in this important area. We welcome contributions on all aspects, theoretical as well as practical, of * generic programming, * programming with (C++) concepts, * meta-programming, * programming with type classes, * programming with modules, * programming with dependent types, * polytypic programming, * adaptive object-oriented programming, * component-based programming, * strategic programming, * aspect-oriented programming, * family polymorphism, * object-oriented generic programming, * and so on. Organizers ---------- Co-Chair Jaakko Järvi, Texas A&M University, USA Co-Chair Shin-Cheng Mu, Academia Sinica, Taiwan Programme Committee ------------------- Dave Abrahams, BoostPro Computing, USA Magne Haveraaen, Universitetet i Bergen, Norway Akimasa Morihata, Tohoku University, Japan Pablo Nogueira, Universidad Politécnica de Madrid, Spain Ulf Norell, Chalmers University of Technology and University of Gothenberg, Sweden Ross Paterson, City University London, UK Rinus Plasmeijer, Radboud University Nijmegen, The Netherlands Sibylle Schupp, Technische Universität Hamburg-Harburg, Germany Andrew Sutton, Kent State University, USA Tarmo Uustalu, Institute of Cybernetics, Estonia Important Information --------------------- We plan to have formal proceedings, published by the ACM. Submission details Deadline for submission: Monday 2011-06-06 Notification of acceptance: Tuesday 2011-07-01 Final submission due: Monday 2011-07-25 Workshop: Sunday 2011-09-18 Authors should submit papers, in postscript or PDF format, formatted for A4 paper, to the WGP11 EasyChair instance by the above deadline. The length should be restricted to 12 pages in standard (two-column, 9pt) ACM format. Accepted papers are published by the ACM and will additionally appear in the ACM digital library. History of the Workshop on Generic Programming ---------------------------------------------- This year: * Tokyo, Japan 2011 (affiliated with ICFP11) Earlier Workshops on Generic Programming have been held in * Baltimore, Maryland, US 2010 (affiliated with ICFP10) * Edinburgh, UK 2009 (affiliated with ICFP09) * Victoria, BC, Canada 2008 (affiliated with ICFP), * Portland 2006 (affiliated with ICFP), * Ponte de Lima 2000 (affiliated with MPC), * Marstrand 1998 (affiliated with MPC). Furthermore, there were a few informal workshops * Utrecht 2005 (informal workshop), * Dagstuhl 2002 (IFIP WG2.1 Working Conference), * Nottingham 2001 (informal workshop), There were also (closely related) DGP workshops in Oxford (June 3-4 2004), and a Spring School on DGP in Nottingham (April 24-27 2006, which had a half-day workshop attached). Additional information: The WGP steering committee consists of J. Gibbons, R. Hinze, P. Jansson, J. Järvi, J. Jeuring, B. Oliveira, S. Schupp, and M. Zalewski

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Haskell Weekly News - Issue 180
by Daniel Santa Cruz 05 May '11

05 May '11

Welcome to issue 180 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of April 24 to 30, 2011. Announcements Eric Kow suggested (http://goo.gl/bc4JM) the creation of a Haskell User Group starter kit. If you have ideas in this department, please see what Eric has started! Christopher Done wrote (http://goo.gl/2m7AJ) a response to a blog from Cedric entitled "Why Scala's Option and Haskell's Maybe types won't save you from Null." A lively conversation followed. Henning Thielemann started an very interesting thread (http://goo.gl/FWyDE) with the title "Python is lazier than Haskell." (See the quotes of the week for the full quote) Quotes of the Week These quotes are taken from the #haskell channel, and from the haskell/haskell-cafe mailing lists. * Henning Thielemann: If Haskell is great because of its laziness, then Python must be even greater, since it is lazy at the type level. * Ben Lippmeier: Laziness at the value level causes space leaks, and laziness at the type level causes mind leaks. Neither are much fun. When people start wanting laziness at the kind level we'll have to quarantine them before the virus spreads... * Gregg Reynolds: If the designers could find a way to support laziness at the programmer level, I for one would be very grateful. * Felipe Almeida Lessa: But I am sure that john-millikin-is-great.txt must be increasing like a memory leak ;) * James Cook: I think Haskell questions on SO tend to the opposite extreme; no matter how poorly thought-out the question, the Haskell community will descend on it like a swarm of helpful piranhas. * Philippa: C++ seeing a renaissance as a functional language, is this a bit like rusty nails seeing a renaissance as sex toys for masochists? * #haskell: <aristid> preflex: seen FunctorSalad * preflex_ is now known as preflex. * copumpkin is now known as postflex. * xplat is now known as circumflex. <circumflex> FunctorSalad was never seen on #haskell-world-conquest -4 years, 3 days, 7i minutes and 0 seconds ago, saying: RELEASE THE KRAKEN! * tolkad: Without a standard you are lost, adrift in a sea of unspecified semantics. Hold fast to the rules of the specification lest you be consumed by ambiguity. * Saizan: it's much easier via type theory, 1) prove bottom 2) ???? 3) quodlibet * dons: Think of a monad as a spacesuit full of nuclear waste in the ocean next to a container of apples. Now, you can't put oranges in the space suite or the nuclear waste falls in the ocean, *but* the apples are carried around anyway, and you just take what you need. * tolkad: Perhaps by using it we could modernise haskell by applying the principles of the new kind of science Top Reddit Stories * The Irish Times: "Functional programming is more than just esoteric; it's becoming somewhat cool." Domain: irishtimes.com, Score: 44, Comments: 6 On Reddit: http://goo.gl/cfUbb Original: http://goo.gl/1nHEY * Why Silkapp uses Haskell Domain: blog.silkapp.com, Score: 43, Comments: 0 On Reddit: http://goo.gl/IFREi Original: http://goo.gl/1SwNa * Harper again: The real point of laziness Domain: existentialtype.wordpress.com, Score: 38, Comments: 89 On Reddit: http://goo.gl/vA8oD Original: http://goo.gl/hqcpy * Patent 5,893,120 reduced to mathematical formulae (via Haskell) Domain: paulspontifications.blogspot.com, Score: 34, Comments: 13 On Reddit: http://goo.gl/PmB4K Original: http://goo.gl/QknmV * IO evaluates the Haskell heap Domain: blog.ezyang.com, Score: 33, Comments: 3 On Reddit: http://goo.gl/FzaGV Original: http://goo.gl/KNHku * Why Haskell? Domain: blog.mired.org, Score: 33, Comments: 19 On Reddit: http://goo.gl/t1r8I Original: http://goo.gl/cV6KL * Filter and Visualize Data in Seconds with Silk (Haskell-based startup) Domain: thenextweb.com, Score: 33, Comments: 18 On Reddit: http://goo.gl/hA9HF Original: http://goo.gl/NIR3U * Haskell libraries you should use Domain: blog.johantibell.com, Score: 29, Comments: 3 On Reddit: http://goo.gl/PPTWG Original: http://goo.gl/LmbT6 * Tying the Knot - a really mind bending Haskell technique Domain: haskell.org, Score: 27, Comments: 8 On Reddit: http://goo.gl/iEdSM Original: http://goo.gl/5Zg6f * haskell.org GSoC accepted projects are up. Domain: self.haskell, Score: 26, Comments: 32 On Reddit: http://goo.gl/mnrl9 Original: /r/haskell/comments/gxj1h/haskellorg_gsoc_accepted_projects_are_up/ Top StackOverflow Questions * What are the best Haskell libraries to operationalize a program? votes: 57, answers: 3 Read on SO: http://goo.gl/rYQiL * How do you design programs in Haskell or other functional programming languages? [migrated] votes: 24, answers: 0 Read on SO: http://goo.gl/IW7Z5 * Haskell: why the convention to name a helper function "go"? votes: 22, answers: 3 Read on SO: http://goo.gl/CE1O7 * What haskell topics need to be addressed in a Real-World-Haskell style? votes: 21, answers: 5 Read on SO: http://goo.gl/3tgE4 * What are some interesting uses of high order functions? votes: 14, answers: 14 Read on SO: http://goo.gl/WdGeF * How are mutable arrays implemented in Haskell? votes: 10, answers: 3 Read on SO: http://goo.gl/LMK8u * Performance problem with Euler problem and recursion on Int64 types votes: 10, answers: 6 Read on SO: http://goo.gl/CLXD2 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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Can't install haskellnet with ghc7
by Charles-Pierre Astolfi 05 May '11

05 May '11

Hi -cafe, Did anybody managed to install haskellnet from hackage with ghc7? I tried on windows and mac and I get the following type error: [ 4 of 11] Compiling Network.HaskellNet.SMTP ( Network/HaskellNet/SMTP.hs, dist/build/Network/HaskellNet/SMTP.o ) Network/HaskellNet/SMTP.hs:269:25: Couldn't match expected type `Data.Text.Internal.Text' with actual type `[Char]' Expected type: Data.Text.Internal.Text Actual type: String In the first argument of `simpleMail', namely `to' In a stmt of a 'do' expression: myMail <- simpleMail to from subject plainBody htmlBody attachments Am I missing something? -- Cp

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warning - Euler problem spoiler enclosed
by Barbara Shirtcliff 05 May '11

05 May '11

Hi, In the following solution to problem 24, why is nub ignored? I.e. if you do lexOrder of "0012," you get twice as many permutations as with "012," even though I have used nub. puzzled, Bar -- file Euler.hs module Euler where import Data.List {- problem 24 A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are: 012 021 102 120 201 210 What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9? -} lexI :: Char -> String -> Int lexI c s = maybe 1 (id) $ elemIndex c s lexOrder :: [Char] -> [[Char]] lexOrder s | length s == 1 = [s] | length s == 2 = z : [reverse z] | otherwise = concat $ map (\n -> h n) [0..((length s) - 1)] where z = sort $ nub s -- why is the nub ignored here? h :: Int -> [String] h n = map (z!!n :) $ lexOrder $ filter (\c -> lexI c z /= n) z p24 = (lexOrder "1234567890")!!999999 main :: IO() main = do putStrLn $ show $ p24

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Advertisement: the Haskell Stack Overflow Q & A site
by Don Stewart 04 May '11

04 May '11

Hey all, I thought I'd just make a quick advertisement for the Haskell Stack Overflow community: http://stackoverflow.com/questions/tagged/haskell as a forum for questions and answers on beginner to advanced Haskell problems. The site is very active, with roughly as many questions being asked on SO as on haskell-cafe these days. One of the benefits of a site like SO as a forum is the ability to record and link to prior work, edit for technical errors, and easily search and categorize past answers. It is also less prone to noise, for those suffering from cafe overload. Cheers, Don

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Another good video tutorial on monads
by C K Kashyap 04 May '11

04 May '11

http://vimeo.com/20717301 I really liked how he starts off with the let statement. Regards, Kashyap

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Re: [Haskell-cafe] Please add a method for optimized concat to the Semigroup class
by John Lato 04 May '11

04 May '11

> > From: Edward Kmett <ekmett(a)gmail.com> > > On Tue, May 3, 2011 at 3:43 PM, Yitzchak Gale <gale(a)sefer.org> wrote: > > > I'm sure there are countless other natural examples of semigroups > > in the wild, and that the typical non-trivial ones will benefit > > from an optimized sconcat. > > > > Sold! (modulo the semantic considerations above) > Somewhat academic, but would there be a case for implementing sconcat in terms of Foldable (or similar)? John L

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Fwd: Re: Binary and Serialize classes
by Alberto G. Corona 04 May '11

04 May '11

---------- Forwarded message ---------- De: "Alberto G. Corona" <agocorona(a)gmail.com> Fecha: 03/05/2011 11:24 Asunto: Re: [Haskell-cafe] Binary and Serialize classes Para: "Antoine Latter" <aslatter(a)gmail.com> E1610With the exception of heavy serialization usage, for example, in very optimized RPC applications (and even there SOAP shows that this is not ever the case), text serialization is better. The unwritten rules of good design says that data representation and compression must be orthogonal. The binary formats were designed for performing both functionalities in the times when memory were measured in Kbytes. 2011/4/28 Antoine Latter <aslatter(a)gmail.com> > > On Thu, Apr 28, 2011 at 10:00 AM, Evan Laforge <...

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runST readSTRef type error
by Ken Takusagawa II 04 May '11

04 May '11

I run into the following type error: foo :: ST s (STRef s Int) -> Int foo p = (runST (p >>= readSTRef)) with ghc 6.12.1 st.hs:8:16: Couldn't match expected type `s1' against inferred type `s' `s1' is a rigid type variable bound by the polymorphic type `forall s1. ST s1 a' at st.hs:8:9 `s' is a rigid type variable bound by the type signature for `foo' at st.hs:7:10 Expected type: ST s1 (STRef s Int) Inferred type: ST s (STRef s Int) In the first argument of `(>>=)', namely `p' In the first argument of `runST', namely `(p >>= readSTRef)' However, if I add {-# LANGUAGE RankNTypes #-} and change the type signature to foo :: (forall s.ST s (STRef s Int)) -> Int it works. I don't fully understand what's going on here. Is this the "right" way to fix the problem? Are there other options? My gut feeling is, for such a simple use case of the ST monad, I shouldn't need such a big hammer as RankNTypes. --ken

3 2

ANN: Leksah 0.10.0
by Hamish Mackenzie 04 May '11

04 May '11

Yesterday we uploaded our official 0.10.0 release (0.10.0.4) to Hackage and we have Windows and OS X installers at http://leksah.org/download.html Anyone using an earlier version should upgrade. This release is mainly small improvements and bug fixes. Some of the improvements include: * Better messages for some common problems. * Improved grep (now searches all source directories in your workspace) * Updated build system for more reliable rebuilding of dependent packages in your workspace We have automated the building of the Leksah installers and this should allow us to do more frequent releases in the future. There is a lot more to do and we would love to hear from anyone who has time to help out. Worth mentioning because they can be hard to find: * <Ctrl> R adds missing imports * <Ctrl> <Enter> evaluates current line or selection in GHCi * Right click menu in the editor has other debug functions * Comment and uncomment are now <Ctrl> D and <Ctrl> <Shift> D To build Leksah yourself: * Install Gtk and GtkSourceView2 (C libraries and headers) * Install The Haskell Platform (or GHC >= 6.12.3) * cabal update * cabal install gtk2hs-buildtools * cabal install leksah * leksah Making a Gtk application look nice on Windows and OS X is not easy so we recommend using one of these prebuilt binaries: Windows users using GHC 7.0.3 http://leksah.org/packages/leksah-0.10.0.4-ghc-7.0.3.exe Windows users using GHC 6.12.3 http://leksah.org/packages/leksah-0.10.0.4-ghc-6.12.3.exe OS X 10.6 users using GHC 7.0.3 http://leksah.org/packages/leksah-0.8.0.6.dmg (These installers do not install Gtk2Hs packages on your system) Please consult http://leksah.org/download.html for more details about installation! Background information can be found in the Leksah manual http://leksah.org/leksah_manual.pdf. Known bugs and problems: * Leksah does not run with GHC 6.12.2 (http://http://hackage.haskell.org/trac/ghc/ticket/4038) Have fun Jürgen & Hamish See you on #leksah IRC

11 19

Filtering / branching enumeratee
by Skirmantas Kligys 04 May '11

04 May '11

Hello, I am using enumerator-0.4.10, and I need to distribute processing of different parts of the incoming stream to different iteratees (I am parsing a huge XML file, and different sub-trees have different processing logic). Only a single iteratee will be active at a time since the sub-trees don't intersect. I wrote a simple example that filters the stream and passes the result to one iteratee; please see below. However, with multiple nested iteratees it seems to me that I can no longer use an enumeratee. Do I need to write my own multi-enumeratee that holds multiple inner iteratees? Any better ideas? Here is my (beginner's) code for a single nested iteratee: module Main ( main ) where import qualified Data.Enumerator as E ( Enumeratee, Step(..), Stream(..), checkDone, checkDoneEx, continue, enumList, joinI, run_, yield ) import Data.Enumerator ( ($$), (>>==) ) import qualified Data.Enumerator.List as EL ( consume ) -- cribbed from EL.concatMap concatMapAccum :: Monad m => (s -> ao -> (s, [ai])) -> s -> E.Enumeratee ao ai m b concatMapAccum f s0 = E.checkDone (E.continue . step s0) where step _ k E.EOF = E.yield (E.Continue k) E.EOF step s k (E.Chunks xs) = loop s k xs loop s k [] = E.continue (step s k) loop s k (x:xs) = case f s x of (s', ais) -> k (E.Chunks $ ais) >>== E.checkDoneEx (E.Chunks xs) (\k' -> loop s' k' xs) passFromTo :: Monad m => ((a -> Bool), (a -> Bool)) -> Bool -> E.Enumeratee a a m b passFromTo (from, to) pass0 = concatMapAccum updatePass pass0 where updatePass pass el = case (pass, from el, to el) of (True, _, to_el) -> (not to_el, [el]) (False, True, _) -> (True, [el]) (False, False, _) -> (False, []) main :: IO() main = do E.run_ (E.enumList 3 [1..20] $$ E.joinI $ passFromTo ((\e -> e == 3 || e == 13), (\e -> e == 7 || e == 17)) False $$ EL.consume) >>= print $ ./dist/build/StatefulEnumeratee/StatefulEnumeratee [3,4,5,6,7,13,14,15,16,17]

1 0

Robert Harper on monads and laziness
by Ketil Malde 04 May '11

04 May '11

I'm following Harper's blog, Existential Type¹, which I find to be an enjoyable and entertainingly written tirade about the advantages of teaching functional programming - specifically ML - to students. Of course, he tends to be critical of Haskell, but it's nice to get some thought provoking opinion from somebody who knows a bit about the business. Recently, he had a piece on monads, and how to do them in ML, and one statement puzzled me: "There is a particular reason why monads had to arise in Haskell, though, which is to defeat the scourge of laziness." My own view is/was that monads were so successful in Haskell since it allowed writing flexible programs with imperative features, without sacrificing referential transparency. Although people are quick (and rightly so) to point out that this flexibility goes way beyond IO, I think IO was in many ways the killer application for monads. Before IO, we had very limited functionality (like 'interact' taking a 'String -> String' function and converting it into an exectuable program) to build real programs from. Laziness does require referential transparency (or at least, it is easier to get away with the lack of RT in a strict language), so I can see that he is indirectly correct, but RT is a goal in itself. Thus, I wonder if there are any other rationale for a statement like that? -k ¹ http://existentialtype.wordpress.com/ -- If I haven't seen further, it is by standing in the footprints of giants

9 11

Please add instance Semigroup Text
by Yitzchak Gale 04 May '11

04 May '11

Hi Edward, Could you please add a Semigroup instance for Text? Once you're doing that, I suppose you'd also want to add it for lazy Text and both kinds of ByteStrings. But what I currently need is strict Text. The reason, of course, is that in complex calculations <> is *so* much more readable than `mappend`. Obviously WrappedMonoid is useless there. Hmm, and now for Semigroup and Monoid we start down the same path as for Functor and Monad... Or should we give <> back to Monoid and use something different for Semigroup? That doesn't seem very nice either... Well, in the meantime, you really will make my life easier if you could add some more Semigroup instances for popular Monoids. Thanks, Yitz

5 6

Re: [Haskell-cafe] Re: Functional dependencies and Peano numbers (and hoogle-bug?)
by Oscar Finnsson 03 May '11

03 May '11

Thanks for the great feedback. The bijective example was especially interesting. While reading "Fun with Type Functions" I notices GNum as an interesting alternative to the Num type class but I couldn't find any such package on hackagedb. Do anyone know if there is anything like GNum on hackagedb? On an unrelated note: I hoogled "to" (i.e. http://haskell.org/hoogle/?hoogle=to) and just got a blank page. Nothing. Nil (not even <html>...</html>). Is this a bug or a "feature"? :) The reason I hoogled it was because I'm searching for something like explicit casting found in many other languages, something similar to > class To f t where > to :: f -> t > > instance To a a where > to x = x > > instance (Real a, Fractional b) => To a b where > to = realToFrac > > instance (Read a) => To String (Maybe a) where > to = maybeRead -- not from cgi :) so I can write something like > ("23.2" `to`) :: Maybe Double or > ((42 :: Integer) `to`) :: Float Anyone made a module/package that solves this problem already? I cannot be the first that needs generic type safe conversion... . -- Oscar

3 2

Is Harper right that Haskell cannot model the natural numbers?
by Richard O'Keefe 03 May '11

03 May '11

In one of his blog posts, Robert Harper claims that the natural numbers are not definable in Haskell. SML datatype nat = ZERO | SUCC of nat Haskell data Nat = Zero | Succ Nat differ in that the SML version has strict constructors, and so only finite instances of nat can be constructed, whereas Haskell has lazy constructors, so inf = Succ inf is constructible, but that's not a natural number, and it isn't bottom either, so this is not a model of the natural numbers. Fair enough, but what about data Nat = Zero | Succ !Nat where the constructors *are* strict? It's perfectly good Haskell 98 as far as I can see. Now "Nat" itself isn't _quite_ a model of the naturals because it includes bottom, but then an SML function returning nat can also fail, so arguably SML's nat could or should be thought of as including bottom too. What am I missing?

5 4

"Computational Semantics w/ Functional Programming", Jan van Eijck and Christina Unger, 2010. It's in #Haskell. :)
by caseyh＠istar.ca 03 May '11

03 May '11

This may have already been mentioned but it is worth mentioning n times: "Computational Semantics w/ Functional Programming", Jan van Eijck and Christina Unger, 2010. It's in #Haskell. :)

2 1

Idea for Haddock: Specifying a syntax field when using template splicing
by Mathew de Detrich 03 May '11

03 May '11

I just had an idea while I was working on a webserver in Haskell, where I not only enter Haskell code (obviously) but I also enter Javascript and CSS code in functions that use QuasiQuoting (i.e. TemplateHaskell). The idea is basically that with standard Haddock comments you can specify a 'tag' for function argument that has a QuasiQuoter as a type, the 'tag' specifying the syntax of the code inside of what is being spliced (for that specific function). The generated haddock documentation would then display the syntax tag for that specific function. The purpose for this would be that IDE's and Editors would pick up that the code going through the QuasiQuoter has a certain syntax, and the IDE/Editors can then apply syntax coloring (and indenting rules and whatnot) to that piece of code I guess this tag concept can be applied generally to any type of function (and any type of tag, not just tags for syntax) but there should be some standardization, (tag types especially should be static, tag values don't have to) Thoughts?

1 0

Binary and Serialize classes
by Evan Laforge 03 May '11

03 May '11

It's been remarked to me that relying on the Binary and Serialize classes is dangerous, because there's no guarantee they won't maintain a consistent format. So if my app uses the Serialize instances that come with cereal, it could suddenly fail to read old saves after an upgrade to cereal. However, neither binary nor cereal expose the underlying serialization algorithms for various types except through the instances, so I would have to copy and paste the code over if I want control over it. If I don't trust 'instance Serialize String' to not change behind my back, maybe I could at least trust 'Data.Serialize.stringToUtf8' to not change since if it did the name would now be wrong. Are these fears justified? I imagine if the Int instance for Serialize changed there would be an uproar and it would probably have to be changed back. I sent a bug to the maintainers of data-binary a long time ago about the Double instance not serializing -0, and they ignored it, probably because it would be bad to change the instance. So can I use the included instances without fear of them changing between releases? Of course I still run the risk of an instance from some other package changing, but I'm less likely to be using those. Speaking of that Double instance... both data-binary and cereal use decodeFloat and encodeFloat which mean they suffer from the same problems as realToFrac, namely that -0 becomes 0 and NaN becomes -Infinity (I'm not sure why the latter happens since the decoded values differ... must be a problem with encodeFloat). It's tempting to just get the ieee754 bitmap out and serialize that. I know I've seen this question around before, but is it possible to somehow cast a D# directly to bytes? I know I can write a C function and FFI that in, but it would be tidier to do it all in haskell. I guess I can probably use castPtr and memCpy, but I don't see the required addressOf. I.e. how would I write 'memcpy(buf, &d, sizeof(double));'?

6 7

Re: [Haskell-cafe] Interfacing C++ iterators does not work as expected
by Huynh Huu Long 03 May '11

03 May '11

Big thanks, that works for me as well. Doesn't GHC check whether a type is allowed (like CInt,...) or not (like Bool,...)? 2011/5/2 Gábor Lehel <illissius(a)gmail.com>: > It works if I replace the 'bool' return type of 'finished' with int > and CInt on the C++ and Haskell side, respectively, and test for (/= > 0).

2 1

Reformatter for Show
by Alexey Khudyakov 03 May '11

03 May '11

Hello everyone! Haskell have nice automatically derivable Show type class. It's quite useful for debugging with one exception. String representation of even moderately large value is completely unreadable (example below). My question is there any tool for reformatting result of show so it could be read by human beings? Real life example of world salad produced by show > Module (SrcLoc {srcFilename = "rec.hs", srcLine = 1, srcColumn = 1}) > (ModuleName "Main") [OptionsPragma (SrcLoc {srcFilename = "rec.hs", > srcLine = 1, srcColumn = 1}) (Just GHC) "-F -pgmF ./preprocessor "] > Nothing (Just [EVar (UnQual (Ident "main"))]) [] [DataDecl (SrcLoc > {srcFilename = "rec.hs", srcLine = 3, srcColumn = 1}) DataType [] > (Ident "Foo") [] [QualConDecl (SrcLoc {srcFilename = "rec.hs", > srcLine = 3, srcColumn = 12}) [] [] (RecDecl (Ident "Foo") [([Ident > "asd"],UnBangedTy (TyCon (UnQual (Ident "Int")))),([Ident > "fgh"],UnBangedTy (TyCon (UnQual (Ident "String"))))])] [(UnQual > ...

5 5

versioning of Haskell packages
by Daniil Elovkov 02 May '11

02 May '11

Hello list I have an idea on versioning of Haskell packages and a small question about release model of Haskell Platform. Since the latter is shorter let's start with it. So, what is the release model of Haskell Platform? Is it released every N months and frozen at that point? Or some intermediate package/compiler updates can make the new version to appear out of schedule? Now, about package versioning. Haskell packages are flourishing and evolving at a high speed. Packages depend on other package and they evolve asynchronously. In result it is easy to end up in need of multiple versions of the same package in one program or maybe stumble upon other conflicts/problems. I'm expressing myself a little vaguely because I don't have a good example in my head right now. However, the idea is to maintain not just multi-digit version names that don't carry any semantics except that 1.1.3 is later than 1.1.2, but also somehow describe compatibility between versions. This way, even if the package A has jumped from 1.0 to 1.5 and the dependent package B has been stagnant all this time (and B depends on A 1.0), if we have information that 1.5 is compatible with 1.0, then we can safely use A 1.5 while building B. Or we could use whatever version of A is found in the system as long as its compatible with A 1.0 that B depends on. The compatibility relation can be made more fine grained and be applied not at the level of packages but that of modules, for example. This way, if we only use modules that are compatible with the version we depend on, it's fine to go with the newer version. Even if the package as a whole is not compatible. Also, when doing this at the level of modules, package authors could deliberately maintain compatibility by leaving the API of old modules intact and making them sort of wrappers to the new API which itself is not compatible with the older version. Of course, if the case when a newer version is backwards compatible is rare for most Haskell packages, then the idea doesn't make a lot of sense. But if it's common, then this could simplify building packages/programs and maintaining installed packages. What do you think? -- Daniil Elovkov

4 4

Interfacing C++ iterators does not work as expected
by Huynh Huu Long 02 May '11

02 May '11

Hi, I tried to access C++ iterators from Haskell via the FFI. There are two example programs attached, in Haskell as well as in C++. Both utilise the same iterator wrapper example.cpp, but when running the Haskell program it prints an empty list, where it should print the contents of some vector. The C++ program meanwhile runs fine though. What does not seem to work is this part: bool finished (std::vector<int>::iterator *begin, std::vector<int>::iterator *end) { return *begin == *end; } In Haskellland, "finished :: Ptr Iterator -> Ptr Iterator -> IO Bool" always returns IO True. Is this a bug lurking in my program or is it a GHC issue? Long

2 1

GLUT error while using Gloss
by Artyom Kazak 02 May '11

02 May '11

Hi Cafe! When I'm running examles from Gloss graphics library, all examples except gloss-hello are crashing with the following error: "GLUT: Warning in gloss-easy: GL error: gloss-easy: <stderr>: hPutChar: invalid argument (character is not in the code page)". A window with produced picture is showed for one second and then it crashes. gloss-hello, strangely, is working perfectly. I'm using Windows 7 (but on XP there is same behaviour), Haskell Platform 2011.2, OpenGL 2.4.0.1, GLUT 2.2.2.0, Gloss 1.3.1.2. At the first time, exampes didn't want to run at all, but I placed glut32.dll from http://www.xmission.com/~nate/glut.html in my .dlls folder. I tried another glut32.dll (from the Raincat game), but it didn't change anything. How can it be fixed?

2 2

hello:
by Eran L. 02 May '11

02 May '11

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1 0

Odd "Ambiguous type variable `a0' in the constraint:" error (with Yesod)
by Mathew de Detrich 02 May '11

02 May '11

Ok so I have a problem that was described here ( http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431) in regards to returning a "Ambiguous type variable `a0' in the constraint error" when compiling. Originally I thought it was due to the way I was coding that part of the code (or to be more accurate the code specifically for creating the so called widgets with addHTML/addWidget,addHamlet). So I copied the example code given here exactly (http://www.yesodweb.com/book/example-widgets) Compiling this worked fine, so at the next point I changed the definition of getRootR to getRootR = defaultLayout $ wrapper $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|I was added with addHtml|] addHamlet [$hamlet|I was added with addHamlet|] addWidget [$hamlet|I was added with addWidget|] and then to getRootR = defaultLayout $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|I was added with addHtml|] addHamlet [$hamlet|I was added with addHamlet|] addWidget [$hamlet|I was added with addWidget|] Both times compiled fine, so the issue wasn't what I originally thought that it was (as described in http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431) The problem is, that when I use the above example code in my WebServer, I get this Ambigious type error when compiling (even though I have set up the route the exact same way). Unfortunatley I can't provide the code for my webserver, since its commercially owned (and it would be pointless because its just renamed variables, but otherwise its the same), so does anyone have any ideas what could possibly cause such an error (such as a missing/extra import or some package or something), or possibly some missing instances? Also sorry for creating another mailing list, but its a different issue then what I thought it was originally (and I also wanted to put it on haskell-cafe since its a more general issue)

2 8

Links into list
by John Sneer 02 May '11

02 May '11

Hello all, I have rather strange question. I wonder whether there is an obvious solution or not in Haskell. I tried but without defining quite ugly tree-like structures and incorporating IORefs it seems to me like impossible. Note: I use ASCII art to explain. let l be a Haskell list +-+-+-------------------------------------+ l = | | | .... | +-+-+-------------------------------------+ let li be another Haskell list +-+-+-+ li = |a|b|c| +-+-+-+ let hi be a hook/pointer list such, that it links some/all members of the list li +--+--+--+ hi = |pa|pb|pc| +--+--+--+ | | | V V V +--+--+--+ li = |a |b |c | +--+--+--+ When I insert li into l, I would like to get something like: +--+--+--+ hi = |pa|pb|pc| +--+--+--+ | | | V V V +-+-+----------+--+--+--+---------------------------+ l' = | | | .... |a |b |c | | +-+-+----------+--+--+--+---------------------------+ where l' is still a Haskell list. Now, I would like to modify, directly via pb the list l' to have: +--+--+--+ hi = |pa|pb|pc| +--+--+--+ | | +-------+ V V V +-+-+----------+--+--+-+-+-+-+--+---------------------------+ le = | | | .... |a |b |q|w|e|r|c | | +-+-+----------+--+--+-+-+-+-+--+---------------------------+ where le is a Haskell list, but, in fact accessible through the same ID/reference all the time. Simply: I would like to have direct access into several places in a very long list. Moreover, the efficiency and features of Haskell list and all the functions around it are to be used as well (not all, but many, nevertheless, I could make them new, if I can achieve good performance, not a big trouble). Is it possible, or not? Could you possibly point me to some resource I can get more information about such "tricks". Thanks John -- John Sneer johnsneer(a)operamail.com -- http://www.fastmail.fm - A no graphics, no pop-ups email service

4 4

Re: [Haskell-cafe] Links into list
by Ivan Lazar Miljenovic 02 May '11

02 May '11

Re-CCing the list. On 2 May 2011 11:33, John Sneer <johnsneer(a)operamail.com> wrote: > I took a look at > http://cvs.haskell.org/Hugs/pages/libraries/base/Data-Sequence.html > but it doesn't seem to be helpful - cannot insert sequence into the > sequence, just > modification of a single element is supported. Moreover, indexing via > Ints would > require extra recalculation when something is somewhere inserted. > > So, either it is not the solution or I have not understood the Sequence > well. :-( Well, you can split the list (probably using splitAt) and then insert another one in between (using ><). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

How to make ghc 7 with llvm?
by Magicloud Magiclouds 02 May '11

02 May '11

Hi, As I recalled, ghc started to support llvm from version 7. But there is a problem: there is no option to make ghc with llvm. So Library within ghc source will be in gcc's binary format. Then when I install other packages, they may complain that the binary format is not llvm, so they install some libraries again. Any way I could make ghc 7 with llvm? -- 竹密岂妨流水过山高哪阻野云飞

6 7

Posix IO & Unicode lookup exception
by Lee Pike 01 May '11

01 May '11

Hi, I wanted to see if anyone had an idea about why the following program results in an exception. The issue seems to be related to stdin to a POSIX file descriptors for a subprocess, and GHC's Unicode encoding lookup. I've tried to make the program as simple as possible to make the issue obvious. Consider: > module Pipe where > > import System.Process > import System.IO > import System.Posix.IO > > main = sequence_ $ replicate 100000 f > > f = do > (Just hin, _, _, prc) <- > createProcess (shell "./pipe_c.out") { std_in = CreatePipe > , std_out = UseHandle stdout > , std_err = UseHandle stderr} > > pin <- handleToFd hin > _ <- fdWrite pin ("h\n") > closeFd pin > waitForProcess prc and a trivial C program to read in the data (but doing nothing with it): --------------------------------------- #include <stdio.h> #define SIZE 10 int main(void) { char in[SIZE]; if(fgets(in, SIZE , stdin) != NULL); return 0; } ---------------------------------------- and executing > gcc -Wall -o pipe_c.out pipe_c.c > ghci Pipe.hs > main ... *** Exception: malloc: resource exhausted (out of memory) iconvOpen ("UTF-8","UTF-8"): resource exhausted (Cannot allocate memory) My best guess is that the GHC is calling iconv_open() <http://www.kernel.org/doc/man-pages/online/pages/man3/iconv_open.3.html>, but not making a subsequent call to iconv_close() <http://www.kernel.org/doc/man-pages/online/pages/man3/iconv_close.3.html>. (Actually, my best guess is I'm making a silly mistake, but I'm having trouble seeing it. :) ) I am running 2.6.35-28-generic (Ubuntu) and GHC 7.0.2, but I've observed the same issues with GHC 6.12.3. Lee

2 1

Wai and http-enumerator not as lazy as I'd like
by Erik de Castro Lopo 01 May '11

01 May '11

Hi all, I'm using Wai and http-enumerator to build a http proxy. The core of the code looks like this: import qualified Network.HTTP.Enumerator as HE import qualified Network.Wai as Wai serveRequest :: forall (m :: * -> *). (MonadControlIO m, Failure HE.HttpException m) => HE.Request m -> m Wai.Response serveRequest request = do HE.Response sc rh bs <- HE.withManager $ HE.httpLbsRedirect request return $ Wai.responseLBS (mkStatus sc) rh bs This works but does not run in constant space as I would have hoped. The thing is, HE.httpLbsRedirect returns a lazy ByteString and Wai.responseLBS writes a lazy ByteString, so why isn't the whole thing lazy? I'd appreciate any clues. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

3 4

want to start a page of Haskell examples to go with Skiena's book "The Algorithm Design Manual" and also tie in Bird's equational reasoning for functional programming; ...
by caseyh＠istar.ca 01 May '11

01 May '11

I want to start a page of Haskell examples to go with Skiena's book "The Algorithm Design Manual" and also tie in Bird's equational reasoning for functional programming; where is a good place for such examples.

3 2

GHC optimizer consuming memory
by mike frai 30 Apr '11

30 Apr '11

Hi, While using Michael Snoyman's persistent package, I discovered that a certain bit of code generated from it made the GHC optimizer consume a ridiculous amount of memory (ie. only when using the "-O" flag). I know very little about the GHC compiler and Michael Snoyman recommended I post my findings here to see what others thought. I've done what I can to extract the code generated from the persistent package and simplify/strip it down to the bare minimum of what's needed to cause this issue. You can find it in the attachment FromTest.hs. From my testing, removing five fields from Rec and the equivalent from the fromValue function - the memory consumption drops dramatically and I am able to compile. For your convenience, I've also attached FromTest.out, which is the output I get when compiling FromTest.hs using "ghc -O -v3". This is just the output up to the point it starts consuming a lot of memory. I've never seen anything after this point because my system slows down to a crawl (due to swapping) and I end up killing the process. Please let me know what you guys think or if you need any more information about this. Thanks, - Mike

2 1

How to keep cabal and ghci package versions in sync?
by Gracjan Polak 30 Apr '11

30 Apr '11

Hi all, I have a project with a .cabal file listing package dependencies using the usual version constraints ==X.Y.* <Z.W or >=K.J syntax. Standard route cabal configure; cabal build works correctly as it is able to select working set of package versions. I have also a .ghci file. When I run GHCi it uses all latest installed packages in the system. This prevents the project from loading. I tried to use 'cabal-dev ghci', but this still selects latest global packages. So, how to I load up ghci with the same package versions as selected by cabal? Thanks! -- Gracjan

5 7

Template Haskell support for GADTs
by Boris Lykah 30 Apr '11

30 Apr '11

Hi all! I am writing a library which allows to refer to the separate fields of a datatype. The fields are described as GADT with one constructor for each field. The constructors return GADT with the field type. The auxiliary data structures for this should be generated automatically via Template Haskell. I found that the GADTs produced by TH are not equivalent to the usual ones. They require additional extension -XTypeFamilies(for the equality constraints) along with -XGADTs, and, which is more important, are less type-safe. To describe the data structure I used ForallC as it was suggested in the closed ticket http://hackage.haskell.org/trac/ghc/ticket/3497 (Template Haskell support for GADTs) This is simplified code for data Sample = Sample {foo::String, bar::Int} written manually: data SampleField a where FooField :: SampleField String BarField :: SampleField Int and a GADT with similar structure generated via TH. $(do let gadt = mkName "THSampleField" let tv = mkName "a" let con1 = forallC [plainTV tv] (cxt $ [equalP (varT tv) (conT ''String)]) $ normalC (mkName "THFooField") [] let con2 = forallC [plainTV tv] (cxt $ [equalP (varT tv) (conT ''Int)]) $ normalC (mkName "THBarField") [] result <- dataD (cxt []) gadt [plainTV tv] [con1, con2] [] return [result] ) which produces data THSampleField a where THFooField :: a ~ String => THSampleField a THBarField :: a ~ Int => THSampleField a The expression asTypeOf FooField BarField fails to compile as expected because String cannot match Int, but asTypeOf THFooField THBarField is a valid expression of type (THSampleField a), which is very confusing and breaks the existing code. Am I missing something? If this is the only way to create the GADTs then I think we should consider reopening the ticket. -- Regards, Boris

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More ideas for controlled mutation
by Edward Z. Yang 30 Apr '11

30 Apr '11

Laziness can be viewed as a form of controlled mutation, where we overwrite a thunk with its actual value, thus only running the code once and reaping great time benefits. I've been toying around with some ideas where we do alternative forms of controlled mutation. One such idea has to do with memoization. One way we memoize functions is by building up a data-structure that covers the entirety of input domain of the function, with the values in each slot being thunks for the corresponding function call. Now, this is all very nice and well, but for some types of inputs (like machine integers) we end up making a very big structure for a function for which, in practice, we'll only store and retrieve a few values of the domain. Hash tables take advantage of this fact by simply chaining together values in a linked list if they land in the same bucket. Could we have similarly bucketized memoization? What we want here is for a *thunk to possibly evaluate to different values, but calls to the API be observationally equivalent.* That is, if the only way I can inspect a dictionary list is do a lookup, I don't care if my representation is [(1,4),(2,2)] or [(2,2),(1,4)]. An obvious way to do this is to use unsafePerformIO to read out an IORef stating the value currently being looked up, and have the thunk evaluate to the pair of that key and the result. There are some synchronization concerns, of course: ideally we would only take out a lock on the thunk once we realize that the value doesn't already exist in the memotable, but I don't think there's a way in GHC Haskell to observe if a value is a thunk or not (maybe such a mechanism would be useful?) This seems related to lazy IO, where thunks are co-opted into performing input-output effects. After all, mutation is just a controlled form of IO. Is lazy IO evil or not? One consensus is that for operations that involve limited resources (file descriptors, etc.) lazy IO is too opaque for its own good. It seems we also haven't cracked it's performance problems either (for the relatively un-objectionable generation of an infinite stream of random numbers, Don Stewart notes: "There are real overheads here. Consider eagerly filling chunks and extracting elements piecewise."). But code that looks pure and can be reasoned about like pure code, but has the efficiency benefits of mutation is a very attractive thing indeed. I think it's worth some thought, though the programming world at large has a hard enough time reasoning about laziness even when everything is completely pure! Cheers, Edward P.S. An obvious question is whether or not we could use this technique to implement splay heaps or hash tables with pure interfaces. My opinion is no, because these structures demand you be able to *observe* the mutation.

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Parsing binary 'hierachical' objects for lazy developers
by John Obbele 30 Apr '11

30 Apr '11

Hi Haskellers, I'm currently serializing / unserializing a bunch of bytestrings which are somehow related to each others and I'm wondering if there was a way in Haskell to ease my pain. The first thing I'm looking for, is to be able to automatically derive "Serializable" objects, for example: --------------------------------------------------------------- import Data.Serialize -- using cereal as an example data MyFlag = One | Two | Three instance Serialize [MyFlag] where put = putWord16le . marshalFlags get = unmarshal `fmap` getWord16le data ObjectA = ObjectA { attribute0 :: Word8 , attribute1 :: Word16le , attribute2 :: [MyFlag] } deriving (Serialize) -- magic goes here! --------------------------------------------------------------- Unfortunately ghci complains that 'Serialize' is not a derivable class. Yet, deriving the Serialize instance for ObjectA should be simple, since all the three attributes are already serializable themselves... Second issue, I would like to find a way to dispatch parsers. I'm not very good at expressing my problem in english, so I will use another code example: --------------------------------------------------------------- -- let's say we have two objects with almost the same structure: data ObjectA = ObjectA { objLength :: Int , objType :: TypeId , attribute2a :: [MyFlag] } data ObjectB = ObjectB { objLength :: Int , objType :: TypeId , attribute2b :: Word32le } --------------------------------------------------------------- When we begin to deserialize theses objects, we don't know their final type, we just know how to read their length and their typeId. Only then can we determine if what we are parsing is an ObjectA or an ObjectB. Once we now the object type, we can resume the parsing and return either an ObjectA or ObjectB. Oki, so I may have read too much of Peter Seibel's chapter on binary-data parsing in Common Lisp or spent too much time working on object-oriented code, but currently, I have no idea on how to write this 'simply' in Haskell :( any help would be welcome /john

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Re: [Haskell-cafe] Iteratee: manyToOne
by John Lato 29 Apr '11

29 Apr '11

> > From: Felipe Almeida Lessa <felipe.lessa(a)gmail.com> > > On Thu, Apr 28, 2011 at 1:10 PM, Felipe Almeida Lessa > <felipe.lessa(a)gmail.com> wrote: > > On Thu, Apr 28, 2011 at 12:09 PM, Felipe Almeida Lessa > > <felipe.lessa(a)gmail.com> wrote: > >> I foresee one problem: what is the leftover of 'manyToOne xs' if each > >> x in xs needs different lengths of input? > >> > >> One possible untested-but-compiling solution: > > [snip] > > > > Like I said, that manyToOne implementation isn't very predictable > > about leftovers. ?But I guess that if all your iteratees consume the > > same input OR if you don't care about leftovers, then it should be > > okay. > > Sorry for replying to myself again. =) > > I think you can actually give predictable semantics to manyToOne: > namely, the leftovers from the last iteratee are returned. This new > implementation should be better: > If you do this, the user needs to take care to order the iteratees so that the last iteratee has small leftovers. Consider: manyToOne [consumeALot, return ()] In this case, the entire stream consumed by the first iteratee will need to be retained and passed on by manyToOne. In many cases, the user may not know how much each iteratee will consume, which can make these semantics problematic. Iteratee has 'enumPair', (renamed 'zip' in HEAD) which returns the leftovers from whichever iteratee consumes more. This avoids the problem of retaining extra data, and seems simpler to reason about. Although if you really need to consume a predictable amount of data, the safest is probably to run the whole thing in a 'take'. John Lato

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Why not Darcs?
by Andrew Coppin 29 Apr '11

29 Apr '11

I'm sure this must be a VFAQ, but... There seems to be universal agreement that Darcs is a nice idea, but is unsuitable for "real" projects. Even GHC keeps talking about getting rid of Darcs. Can anybody tell me what the "problems" with Darcs actually are?

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Inputs to classic FRP: unsafeInterleaveIO/unsafePerformIO
by Edward Amsden 29 Apr '11

29 Apr '11

As far as I can tell, with classic FRP implementations (those which use behaviors as a first-class abstraction), the only way to create a behavior or event based on some external input (for instance keypresses or microphone input) is to do something with unsafePerformIO or unsafeInterleaveIO. A behavior is a value, which when evaluated at a specific time would have to either block its evaluation until input could be read, or check the input at that particular time. Is there any other way of implementing external behaviors besides that? -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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