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Budapest Haskell meetup Feb 14
by Zsolt Dollenstein 09 Feb '13

09 Feb '13

[sorry for the possible double-post, apparently posting through the google groups interface is futile] Hi, In case someone is around Budapest on the 14th of Feb (Thursday), there is going to be a Haskell meetup in the heart of the city. Hamish Mackenzie will talk about yet-to-be-released Leksah features and David Terei will explain how his memcache client works. Details and sign up at http://www.meetup.com/fp-bud/events/102245252/ Hope to see you there :) Zsolt

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1st CFP SBLP 2013 (17th Brazilian Symposium on Programming Languages)
by Andre Rauber Du Bois 09 Feb '13

09 Feb '13

[Apologies if you receive multiple copies of this CFP] ======================================================= CALL FOR PAPERS 17th BRAZILIAN SYMPOSIUM ON PROGRAMMING LANGUAGES Brasília, Distrito Federal, Brazil September 29th to October 4th, 2013 http://cbsoft2013.cic.unb.br/sblp ======================================================== IMPORTANT DATES Paper abstract submission (15 lines): April 19th Full paper submission: April 26th, 2013 Notification of acceptance: May 31st, 2013 Final papers due: June 28th, 2013 INTRODUCTION The 17th Brazilian Symposium on Programming Languages, SBLP 2013, will be held in Brasília, Brazil, on September 29th to October 4th, 2013. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. The symposium will be part of the 4th Brazilian Conference on Software: Theory and Practice, CBSoft 2013, http://cbsoft2013.cic.unb.br/, which will host four traditional, well-established symposia: * XXVII Brazilian Symposium on Software Engineering (SBES) * XVII Brazilian Symposium on Programming Languages (SBLP) * XVI Brazilian Symposium on Formal Methods (SBMF) * VII Brazilian Symposium on Components, Software Architecture and Software Reuse (SBCARS) SBLP 2013 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. We solicit papers that should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. Full papers submitted in English will be published in a volume of Lecture Notes in Computer Science (LNCS), by Springer (pending approval). For this reason, all papers must be prepared using the LNCS template, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. We encourage the submission of short papers reporting on partial results of on-going master dissertations or doctoral theses. All accepted papers will be published in the conference proceedings distributed in a digital media by the CBSOFT organizers. Submissions should be done using SBLP 2013 installation of the EasyChair conference mangement system at http://www.easychair.org/conferences/?conf=sblp2013. As in previous editions, a journal special issue, with selected papers from accepted contributions, is anticipated. Selected papers from 2003 to 2008 editions of SBLP were published in special issues of the Journal of Universal Computer Science, by Springer. The post-proceedings of SBLP from 2009 to 2012, also with selected papers from the conference proceedings, are being edited as special issues of Science of Computer Programming, published by Elsevier. CBSOFT CHAIRS Genaina Nunes Rodrigues, UnB Rodrigo Bonifácio, UnB Diego Aranha, UnB PROGRAMME CHAIRS André Rauber Du Bois, UFPel Phil Trinder, Glasgow University PROGRAMME COMMITTEE Alberto Pardo, Univ. de La Republica Alex Garcia, IME Alvaro Freitas Moreira, UFRGS André Rauber Du Bois, UFPel (co-chair) Andre Santos, UFPE Carlos Camarao, UFMG Christiano Braga, UFF Edwin Brady, University of St. Andrews Fernando Castor Filho, UFPE Fernando Quintão Pereira, UFMG Francisco Heron de Carvalho Junior, UFC Hans-Wofgang Loidl, Heriot-Watt University Jeremy Singer, Glasgow University Joao Saraiva, Universidade do Minho João F. Ferreira, Teesside University Lucilia Figueiredo, UFOP Luis Soares Barbosa, Univ. do Minho Manuel António Martins, Univ. de Aveiro Marcelo A. Maia, UFU Marcello Bonsangue, Leiden Univ/CWI Marcelo d'Amorim, UFPE Marco Tulio Valente, UFMG Mariza A. S. Bigonha, UFMG Martin A. Musicante, UFRN Noemi Rodriguez, PUC-Rio Peter Mosses, Swansea University Phil Trinder, Glasgow University (co-chair) Qiu Zongyang, Beijing University Rafael Dueire Lins, UFPE Renato Cerqueira, PUC-Rio Ricardo Massa, UFPE Roberto S. Bigonha, UFMG Roberto Ierusalimschy, PUC-Rio Sandro Rigo, UNICAMP Sergio Soares, UFPE Simon Thompson, Univ. of Kent Varmo Vene, Univ. de Tartu

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cabal-dev add-source
by JP Moresmau 08 Feb '13

08 Feb '13

Hello, I'm trying to understand cabal-dev, and I seem to be missing some basic point, because I can't get dependencies between projects working properly. I have two projects, let's call them P1 and P2. P2 depends on P1, as indicated by its cabal file build-depends field. I run cabal-dev add-source ..\P1 inside P2 then cabal-dev install Everything works fine, and my project compiles, with modules in P2 calling functions from P1. Then I add a new function if an exposed module of P1. I rerun the cabal-dev add-source and cabal-dev install commands to tell P2 of the change, as per the cabal-dev documentation. I change a file in P2 to use the new function, the compilation fails. If I check into the cabal-dev folder, I see the .hi file for my changed module has not been updated (old date). What am I missing? Thanks -- JP Moresmau http://jpmoresmau.blogspot.com/

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ICFP 2013: Call for papers
by David Van Horn 08 Feb '13

08 Feb '13

===================================================================== 18th ACM SIGPLAN International Conference on Functional Programming ICFP 2013 Boston, MA, USA, 25-27 September 2013 http://www.icfpconference.org/icfp2013 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Submissions due: Thursday, 28 March 2013 23:59 UTC-11 (Pago Pago, American Samoa, time) Author response: Wednesday, 22 May 0:00 UTC-11 Friday, 24 May 2013 23:59 UTC-11 Notification: Friday, 7 June 2013 Final copy due: Friday, 5 July 2013 Scope ~~~~~ ICFP 2013 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, and 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, concurrency, or parallelism. Topics of interest include (but are not limited to): * Language Design: concurrency and distribution; modules; components and composition; metaprogramming; interoperability; type systems; relations to imperative, object-oriented, or logic programming * Implementation: abstract machines; virtual machines; interpretation; 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; program verification; dependent types * Analysis and Transformation: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation * 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: teaching introductory programming; parallel programming; mathematical proof; algebra * Functional Pearls: elegant, instructive, and fun essays on functional programming * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working If you are concerned about the appropriateness of some topic, do not hesitate to contact the program chair. Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By Thursday, 28 March 2013, 23:59 UTC-11 (American Samoa time), submit a full paper of at most 12 pages (6 pages for an Experience Report), including bibliography and figures. The deadlines 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. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.acm.org/sigplan/republicationpolicy.htm * Authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. Overall, 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. 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. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. 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:http://www.acm.org/sigs/sigplan/authorInformation.htm Submission: Submissions will be accepted on the web athttps://www.easychair.org/conferences/?conf=icfp2013 . 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 72-hour period, starting at 0:00 UTC-11 on Wednesday, 22 May 2013, to read reviews and respond to them. Special Journal Issue: There will be a special issue of the Journal of Functional Programming with papers from ICFP 2013. The program committee will invite the authors of select accepted papers to submit a journal version to this issue. General Chair: Greg Morrisett, Harvard University Program Chair: Tarmo Uustalu, Institute of Cybernetics, Tallinn Program Committee: Thorsten Altenkirch, University of Nottingham Olaf Chitil, University of Kent Silvia Ghilezan, University of Novi Sad Michael Hanus, Christian-Albrechts-Universität zu Kiel Fritz Henglein, University of Copenhagen Mauro Jaskelioff, Universidad Nacional de Rosario Alan Jeffrey, Alcatel-Lucent Bell Labs Shin-ya Katsumata, Kyoto University Shriram Krishnamurthi, Brown University John Launchbury, Galois Ryan Newton, Indiana University Sungwoo Park, Pohang University of Science and Technology Sam Staton, University of Cambridge Nikhil Swamy, Microsoft Research, Redmond, WA Dimitrios Vytiniotis, Microsoft Research, Cambridge

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Extensible Type Unification
by Leon Smith 08 Feb '13

08 Feb '13

I've been toying with some type-level programming ideas I just can't quite make work, and it seems what I really want is a certain kind of type unification. Basically, I'd like to introduce two new kind operators: kind Set as -- a finite set of ground type terms of kind as kind Range as = Range (Set as) (Set as) -- a greatest lower bound and a least upper bound A type expression of kind (Set as) would either be a type variable of kind (Set as), or set of *ground* type terms of kind (as). A type expression of kind (Range as) would be a type variable of kind (Range as), or two type expressions of kind (Set as). To unify ground terms of these types, one would compute as follows: unifySets xs ys | xs == ys = Just xs | otherwise = Nothing unifyRanges (Range glb0 lub0) (Range glb1 lub1) | glb' `subset` lub' = Just (Range glb' lub') | otherwise = Nothing where glb' = glb0 `union` glb1 lub' = lub0 `isect` lub1 I say sets of ground types, because I have no idea what unification between sets of non-ground types would mean, and I really don't need it. Among applications that came to mind, one could use this to create a restricted IO abstraction that could tell you which kinds of actions might be taken (read from mouse, talk to network, etc), and be able to run only those scripts that are restricted to certain resources. Or, one could define a singular GADT that represents messages/events decorated by various categories, and then define functions that only operate on selected categories of messages. E.G. for something vaguely IRC-ish, one could write something like: data EventCategory = Channel | Presence | Information | Conversation data Event :: Range EventCategory -> * where ChanCreate :: ... -> Event (Range (Set '[Channel]) a) ChanJoin :: ... -> Event (Range (Set '[Channel]) a) ChanLeave :: ... -> Event (Range (Set '[Channel]) a) PresAvailable :: ... -> Event (Range (Set '[Presence]) a) PresAway :: ... -> Event (Range (Set '[Presence]) a) WhoisQuery :: ... -> Event (Range (Set '[Information]) a) WhoisResponse :: ... -> Event (Range (Set '[Information]) a) Message :: ... -> Event (Range (Set '[Conversation]) a) And then be able to write functions such as dispatch :: Event (Range a (Set '[Channel, Conversation])) -> IO () dispatch e = case e of ChanCreate{..} -> ... ChanJoin{..} -> ... ChanLeave{..} -> ... Message{..} -> ... In this case, the case analysis tool would be able to warn me if I'm missing any possible events in this dispatcher, or if I have extraneous events that I can't be passed (according to my type.) Anyway, I've been trying to see if I can't come up with something similar using existing type-level functionality in 7.6, with little success. (Though I'm not very experienced with type-level programming.) If not, might it be possible to add some kind of extensible unification mechanism, in furtherance of type-level programming? Best, Leon

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Announce: LA Haskell User Group February Meeting
by Phil Freeman 07 Feb '13

07 Feb '13

Dear Haskellers, There will be a meeting of the LA Haskell User Group on Wednesday February 13th at 7pm. The details, including a list of discussion topics, as they become available, can be found here: http://www.meetup.com/Los-Angeles-Haskell-User-Group/events/102199892/ Thanks, Phil Freeman.

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Either example
by Jacob Thomas 07 Feb '13

07 Feb '13

Hello I'm new to Haskell, and need help with figuring out the Either type... Any example to show how this works? Jacob

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Sol vingt simple problem in various languages and systems
by Doaitse Swierstra 07 Feb '13

07 Feb '13

At https://gist.github.com/ckirkendall/2934374 you find solutions in many languages to a simple programming problem: the evaluation of an expression in an environment. I want to point your attention to the currently last three entries: - one using the UUAGC compiler to solve the problem in an aspect oriented way (entered by Jeroen Bransen (jbransen)) - one using some Haskell features, libraries and extensions (entered by me (doaitse)) - one using our attribute grammar combinators from the AspectAG package, which expresses the various aspects in an attribute grammar style (entered by Marcos Viera (marcosomarviera)) In case you did not manage to avoid getting wound up in a discussion about programming languages you can find ammunition here. Doaitse

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GHCi (7.4.2) is working on ARM
by kenny lu 07 Feb '13

07 Feb '13

I've got a Cubieboard a few weeks back. I started it off with Linaro (Ubuntu) 12.06. Today I started to upgrade the OS to 12.11, which surprisingly came with ghc 7.4.2. And ghci magically works too. Here are the links http://www.cubieboard.org sudo apt-get install update-manager-core sudo apt-get update sudo apt-get upgrade sudo do-release-upgrade Regards, Kenny

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Haskell Weekly News: Issue 257
by Daniel Santa Cruz 07 Feb '13

07 Feb '13

Welcome to issue 257 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of January 27 to February 02, 2013. Quotes of the Week * shachaf: Everyone forgets about Agda Lovelace, the first constructivist. * applicative: I have my new ghc and am reinstalling my feeble world * wuttf: My world is starting to feel like isToyLanguage x = x /= Haskell * PenguinOfDoom: Being enlightened gentlemen, we split all programming languages into two groups, sucks and doesn't-suck and put all of them into the first group. * Taneb: lens has got to be the only library with more contributors than people who know how it works * shachaf: I like to put all my application's modules under GHC.* That's because I use the GHC API, and I want them not to conflict. Top Reddit Stories * The FP Complete School of Haskell goes beta Domain: fpcomplete.com, Score: 84, Comments: 45 On Reddit: [1] http://goo.gl/dEEyU Original: [2] http://goo.gl/0eKLt * My reaction to: How To Exclude Women From Your Technical Community: A Tutorial Domain: self.haskell, Score: 75, Comments: 145 On Reddit: [3] http://goo.gl/dVnVv Original: [4] http://goo.gl/dVnVv * Edward Yang: The GHC scheduler Domain: blog.ezyang.com, Score: 72, Comments: 1 On Reddit: [5] http://goo.gl/pkCmD Original: [6] http://goo.gl/SP1s3 * GHC HEAD now has support for named holes; an extension of -XTypeHoles. Feedback welcome! Domain: github.com, Score: 44, Comments: 32 On Reddit: [7] http://goo.gl/87Pbw Original: [8] http://goo.gl/cqX9N * LiquidHaskell - Safely Catching A List By Its Tail Domain: goto.ucsd.edu, Score: 44, Comments: 2 On Reddit: [9] http://goo.gl/uxU64 Original: [10] http://goo.gl/9aS35 * ProseDoc: A literate programming tool for Haskell inspired by docco Domain: shangaslammi.github.com, Score: 43, Comments: 12 On Reddit: [11] http://goo.gl/x4YGE Original: [12] http://goo.gl/sM4bN * ANNOUNCE: GHC version 7.6.2 Domain: haskell.org, Score: 39, Comments: 16 On Reddit: [13] http://goo.gl/BS6Sm Original: [14] http://goo.gl/RPF00 * What you consider "advanced" Haskell topics? Domain: self.haskell, Score: 34, Comments: 45 On Reddit: [15] http://goo.gl/JRgzG Original: [16] http://goo.gl/JRgzG * How to write unmaintainable Haskell code Domain: github.com, Score: 34, Comments: 49 On Reddit: [17] http://goo.gl/V0nxC Original: [18] http://goo.gl/G8wNv * Introduction to Haskell, Lecture 3 is Live (Types and Typeclasses) Domain: shuklan.com, Score: 32, Comments: 5 On Reddit: [19] http://goo.gl/JyH81 Original: [20] http://goo.gl/bve7N * Proposal: A trivial Cabal package signing utility for use with Hackage Domain: github.com, Score: 32, Comments: 61 On Reddit: [21] http://goo.gl/R2l9g Original: [22] http://goo.gl/IpYSo Top StackOverflow Questions * Haskell/GHC UndecidableInstances - example for non-terminating type check? votes: 19, answers: 2 Read on SO: [23] http://goo.gl/KX8oj * Efficient parallel strategies votes: 15, answers: 2 Read on SO: [24] http://goo.gl/aDnW2 * Is it possible to implement MonadFix for `Free`? votes: 15, answers: 2 Read on SO: [25] http://goo.gl/uKOrO * Do Haskell imports have side effects? votes: 15, answers: 1 Read on SO: [26] http://goo.gl/sjltX * What monads can be expressed as Free over some functor? votes: 10, answers: 1 Read on SO: [27] http://goo.gl/Q8MF9 * What is the point of having a lazy/strict version of Writer? votes: 10, answers: 1 Read on SO: [28] http://goo.gl/InUsm * Why can applicative functors have side effects, but functors can't? votes: 9, answers: 4 Read on SO: [29] http://goo.gl/LxUi5 * How to write fixed point build rules in Shake, e.g. Latex votes: 9, answers: 1 Read on SO: [30] http://goo.gl/Ef82G * Event handling in Netwire compared to conventional FRP frameworks votes: 9, answers: 2 Read on SO: [31] http://goo.gl/mA4Kc Until next time, Daniel Santa Cruz References 1. http://fpcomplete.com/school-of-haskell-goes-beta/ 2. http://www.reddit.com/r/haskell/comments/17ngtq/the_fp_complete_school_of_h… 3. http://www.reddit.com/r/haskell/comments/17jy0e/my_reaction_to_how_to_exclu… 4. http://www.reddit.com/r/haskell/comments/17jy0e/my_reaction_to_how_to_exclu… 5. http://blog.ezyang.com/2013/01/the-ghc-scheduler/ 6. http://www.reddit.com/r/haskell/comments/17fd8c/edward_yang_the_ghc_schedul… 7. https://github.com/ghc/ghc/commit/677144b858f4a425e77399bdfbfcd43dbabd1488 8. http://www.reddit.com/r/haskell/comments/17l9xm/ghc_head_now_has_support_fo… 9. http://goto.ucsd.edu/~rjhala/liquid/haskell/blog/blog/2013/01/31/safely-cat… 10. http://www.reddit.com/r/haskell/comments/17ohbm/liquidhaskell_safely_catchi… 11. http://shangaslammi.github.com/prose-doc/ 12. http://www.reddit.com/r/haskell/comments/17keu0/prosedoc_a_literate_program… 13. http://www.haskell.org/pipermail/glasgow-haskell-users/2013-January/023661.… 14. http://www.reddit.com/r/haskell/comments/17jyw9/announce_ghc_version_762/ 15. http://www.reddit.com/r/haskell/comments/17gavl/what_you_consider_advanced_… 16. http://www.reddit.com/r/haskell/comments/17gavl/what_you_consider_advanced_… 17. https://github.com/quchen/unmaintainable_haskell/blob/master/README.md 18. http://www.reddit.com/r/haskell/comments/17idma/how_to_write_unmaintainable… 19. http://shuklan.com/haskell/lec03.html 20. http://www.reddit.com/r/haskell/comments/17ic23/introduction_to_haskell_lec… 21. https://github.com/chrisdone/cabal-sign 22. http://www.reddit.com/r/haskell/comments/17njda/proposal_a_trivial_cabal_pa… 23. http://stackoverflow.com/questions/14661073/haskell-ghc-undecidableinstance… 24. http://stackoverflow.com/questions/14624376/efficient-parallel-strategies 25. http://stackoverflow.com/questions/14636048/is-it-possible-to-implement-mon… 26. http://stackoverflow.com/questions/14666666/do-haskell-imports-have-side-ef… 27. http://stackoverflow.com/questions/14641864/what-monads-can-be-expressed-as… 28. http://stackoverflow.com/questions/14644830/what-is-the-point-of-having-a-l… 29. http://stackoverflow.com/questions/14576543/why-can-applicative-functors-ha… 30. http://stackoverflow.com/questions/14622169/how-to-write-fixed-point-build-… 31. http://stackoverflow.com/questions/14654385/event-handling-in-netwire-compa… 32. https://plus.google.com/105107667630152149014/about

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A list of Haskell-related quotes?
by Petr Pudlák 06 Feb '13

06 Feb '13

Dear haskellers, over the time I've read many funny or inspiring quotes related to Haskell, but I forgot them later. For example I vaguely remember: - "What I really like about Haskell: It's completely unlike PHP." - "To learn Haskell your brain will have to get seriously rewired." Does anybody collect them or know about such a collection? Thanks, Petr Pudlak

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Re: [Haskell-cafe] How far compilers are allowed to go with optimizations?
by Artyom Kazak 06 Feb '13

06 Feb '13

Ouch, forgot the Cafe. Would you object to this particular optimisation (replacing an algorithm with an entirely different one) if you were guaranteed that the space behaviour would not change?

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Next Meetup
by Christopher Olah 05 Feb '13

05 Feb '13

Hey All, I think we were planning on meeting on the second Wednesday of this month (Feb 13) but Hacklab has a members meeting that day. How do people feel about the 14th or 15th? Chris

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CfP: OdHac (Haskell Hackathon)
by Roman Cheplyaka 05 Feb '13

05 Feb '13

------------------------------------------------------------------------ OdHac International Haskell Hackathon May 3-5, 2013 Odessa, Ukraine http://haskell.org/haskellwiki/OdHac Co-organized with Provectus IT ------------------------------------------------------------------------ We are pleased to announce the upcoming Haskell Hackathon! The event will be held over 3 days, May 3-5, in Odessa, Ukraine. This is a great opportunity to meet your fellow haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! To attend, please register at http://bit.ly/OdHacReg. Please propose projects at http://www.haskell.org/haskellwiki/OdHac/Projects Consider participating in our accommodation program: http://www.haskell.org/haskellwiki/OdHac/Accommodation

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Video! Duncan Coutts on Cloud Haskell
by Theo 05 Feb '13

05 Feb '13

Back in October 2012 Well-Typed and Haskell expert Duncan Coutts delivered a talk on 'Cloud Haskell' at the 2K12 edition of the Haskell eXchange. Cloud Haskell takes Erlang's successful approach to distributed concurrency and implements it in Haskell as a library. We will look at what this means and what possibilities it opens up. You should also get a flavour of what distributed programming in Haskell is like. The initial Cloud Haskell idea and a prototype implementation were introduced last year. Since then we have been working on a new from-scratch implementation with a focus on robustness and flexibility Watch the video on the Skills Matter website - http://bit.ly/VB9tcy

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Re: [Haskell-cafe] FFI - Approaches to C/C++
by Casey Basichis 05 Feb '13

05 Feb '13

Hi Ertugrul, I'm not entirely sure what you mean. I'm intending on using Ogre for GUI - for which there is the Hogre bindings, but after emailing the DEV about it, I didn't get the impression from his advice that I should be using it for production code. Here is what he suggested: "It depends, really. Hogre is good for running Ogre from within Haskell, but it has its limitations. The number one thing people have been struggling with is handling input with hogre - there's Hois (Haskell wrapper for OIS) but it's not perfect (it misses input events), and the other option is having to write some C++ glue. Hogre is a solid proof of concept and you can do some demos with it, but if you're e.g. writing a game it might be a bit of a struggle. In the end it's about how much you value being able to write code in Haskell (or how allergic to C++ you are)." I'm on iOS so I imagine those difficulties are compounded. I am using several other C++ libraries for which there are no existing bindings and no Haskell alternative packages that are even remotely close. Are you suggesting it would be better to write all my own FFI bindings for all the needed libraries? Everything I read suggests that Haskells strengths are in transformation and that interaction is not its strong suit. I am interested in your thoughts and I am open to whatever, but you are the first to suggest that the mix is a bad idea. Thanks, Casey >>* I'm working on a project in Haskell and C++ where the former is the*>>* brains and the latter is for UI, interaction etc.* >That's a rather odd choice. Not exactly answering your question, but >questioning your project decisions, why would you do UI and interaction >in C++? You have the necessary Haskell bindings and libraries to write >everything cleanly in Haskell. > > >Greets, >Ertugrul

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haskell build phase is very slow
by Junior White 04 Feb '13

04 Feb '13

Hi Cafe, I came from the C/C++ world, recently I play with a haskell editor named yi-editor, I find haskell build phase is very slow: 1.Compile one .hs file is slow 2. When change a little, it will recompile many many files Compare to c, compile a c source file seems much quick then compile a haskell source file. And if i change a .c file, it only recompile the file i changed, and relink, it's awesome. How can i make my build phase more quick? Thanks!

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<package>-inplace is shadowed by package <package>-<hexstring>
by Alexander Bernauer 04 Feb '13

04 Feb '13

Hi I am trying to write criterion benchmarks for the pretty library. Unfortunately, building the benchmark executable fails at some point during the development with the following error message: ---8<--- <command line>: cannot satisfy -package-id pretty-1.1.1.1-inplace: pretty-1.1.1.1-inplace is shadowed by package pretty-1.1.1.1-7bd0114e9691fca3d3447fc0133701cf --->8--- I don't understand the cause of the error, let alone what I am supposed to do next. Any help is much appreciated. Here are the steps that lead to the above quoted error. Sorry, if this is not the smallest counter example possible. As I said, I hardly understand what is going on. (the two patch files mentioned are attached to this email, virthualenv is version 0.2.1) ---8<--- git clone https://github.com/haskell/pretty.git cd pretty git apply /tmp/0001-hello-world-setup.patch export PATH=/opt/ghc-7.4.2/install/bin:/opt/haskell-platform-2012.4.0.0/install/bin:$PATH ~/.cabal/bin/virthualenv source .virthualenv/bin/activate cabal configure --user cabal build cabal install cabal install criterion cabal configure --user --enable-benchmarks cabal build git apply /tmp/0002-replaced-example-with-real-benchmark.patch cabal install language-c cabal configure --user --enable-benchmarks cabal build # error --->8--- So the first build of the benchmark executable works, but the second one "suddenly" fails. Executing "cabal build -v" next yields ---8<--- $ cabal build -v creating dist/build creating dist/build/autogen Building pretty-1.1.1.1... Preprocessing library pretty-1.1.1.1... Building library... creating dist/build /opt/ghc-7.4.2/install/bin/ghc --make -package-name pretty-1.1.1.1 -hide-all-packages -fbuilding-cabal-package -package-conf /home/alex/tmp/pretty/.virthualenv/ghc_pkg_db -i -idist/build -isrc -idist/build/autogen -Idist/build/autogen -Idist/build -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package-id base-4.5.1.0-66f22db3dfcd87541c9c7e50e7095d26 -O -Wall -fwarn-tabs -XHaskell98 -XCPP -XBangPatterns Text.PrettyPrint Text.PrettyPrint.HughesPJ Linking... /usr/bin/ar -r dist/build/libHSpretty-1.1.1.1.a dist/build/Text/PrettyPrint.o dist/build/Text/PrettyPrint/HughesPJ.o /usr/bin/ar: creating dist/build/libHSpretty-1.1.1.1.a /usr/bin/ld -x --hash-size=31 --reduce-memory-overheads -r -o dist/build/HSpretty-1.1.1.1.o dist/build/Text/PrettyPrint.o dist/build/Text/PrettyPrint/HughesPJ.o Registering pretty-1.1.1.1... /opt/ghc-7.4.2/install/bin/ghc-pkg update - --global --user --package-conf=/home/alex/tmp/pretty/.virthualenv/ghc_pkg_db --package-conf=dist/package.conf.inplace Preprocessing benchmark 'bench-pretty' for pretty-1.1.1.1... Building benchmark bench-pretty... creating dist/build/bench-pretty creating dist/build/bench-pretty/bench-pretty-tmp /opt/ghc-7.4.2/install/bin/ghc --make -o dist/build/bench-pretty/bench-pretty -hide-all-packages -fbuilding-cabal-package -package-conf /home/alex/tmp/pretty/.virthualenv/ghc_pkg_db -package-conf dist/package.conf.inplace -i -idist/build/bench-pretty/bench-pretty-tmp -ibench -idist/build/autogen -Idist/build/autogen -Idist/build/bench-pretty/bench-pretty-tmp -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build/bench-pretty/bench-pretty-tmp -hidir dist/build/bench-pretty/bench-pretty-tmp -stubdir dist/build/bench-pretty/bench-pretty-tmp -package-id base-4.5.1.0-66f22db3dfcd87541c9c7e50e7095d26 -package-id criterion-0.6.2.1-5ae981935ea0c9773e0df464b5922b27 -package-id language-c-0.4.2-fe58279e43af097994ce7b7010785c69 -package-id pretty-1.1.1.1-inplace -O -O -XHaskell98 bench/Bench.hs <command line>: cannot satisfy -package-id pretty-1.1.1.1-inplace: pretty-1.1.1.1-inplace is shadowed by package pretty-1.1.1.1-7bd0114e9691fca3d3447fc0133701cf (use -v for more information) --->8--- What does this mean? What am I doing wrong? How can I fix this? Greetings, Alex

1 0

Text.PrettyPrint.HughesPJ is sloooow (and wl-pprint-text is fast)
by Johannes Waldmann 04 Feb '13

04 Feb '13

I was running into serious performance problems when printing moderately sized Doc and Xml data (HaXml goes via Doc). So I replace Text.PrettyPrint.HughesPJ with Text.PrettyPrint.Leijen.Text, and patched the HaXml printer in the same way. Now it is running much faster. This confirms some of the comments here: http://stackoverflow.com/questions/9761507/which-pretty-print-library If you want to see for yourself, here is a test case. It constructs and prints a document tree with about 10^4 nodes, rendered to an Xml document with 10^6 (nonblank) chars: https://github.com/jwaldmann/haskell-tpdb/blob/master/test/speed.hs Of the 4 output methods (fin the main program), only one takes < 1 second. Rendering via HughesPJ needs extra stack (!) and takes about 20 seconds. Rendering via Text.XML.HaXml.ByteStringPP seems much slower still (!) (ghc-7.6.2, pretty-1.1.1.0, HaXml-1.23.3, wl-pprint-text-1.1.0.0, amd64, fedora 18, kernel 3.7.4)

2 1

Problems with code blocks in the description field in a .cabal file
by Josef Svenningsson 04 Feb '13

04 Feb '13

Hi, I'm putting together a cabal package and I'd like to have some code examples in my description file. In particular I would like to have a code block containing markdown containing a code block of Haskell, like this: > ~~~{ .haskell } > module Main where > > main = putStrLn "Hello World!" > ~~~ When I put the above code in my .cabal file and do `cabal haddock --executables` I get the following error: haddock: failed to parse haddock prologue from file: dist/doc/html/codeExtract/codeExtract/haddock-prolog31969.txt In general I can provoke the parse error from haddock whenever I have something inside curly braces. So the error seems to stem from haddock. I've tried to track down what happens inside haddock but I've run out steam. I'd like to know if there is anything I can do to be able to write something like the above as part of the description in my .cabal file. Thanks, Josef

3 6

Yet another Conduit question
by Simon Marechal 04 Feb '13

04 Feb '13

I am working with bulk sources and sinks, that is with a type like: Source m [a] Sink [a] m () The problem is that I would like to work on individual values in my conduit. I can have this: concat :: (Monad m) => Conduit [a] m a concat = awaitForever (mapM_ yield) But how can I do it the other way around ? I suppose it involves pattern matching on the different states my conduit might me in. But is that even possible to do it in a "non blocking" way, that is catenate data while there is something to read (up to a certain threshold), and send it as soon as there is nothing left to read ? Or doesn't that make any sense in the context of Conduits (in the sense that this conduit will be recheck for input before the upstream conduits will have a chance to operate) ? Another approach would be to have a map equivalent: conduitMap :: Conduit i m o -> Conduit [i] m [o] But I am not sure how to do this either ...

4 10

Re: [Haskell-cafe] Monadic parser vs. combinator parser
by Jan Stolarek 04 Feb '13

04 Feb '13

> Of course that is fine. But keep in mind that all parser combinator based > libraries are based on a top-down parsing strategy which in most places, > BUT NOT ALL, fits Haskell's syntax. Try to use the chaining combinators > where-ever possible. In case of doubt you can always spy on how things are > done in the UHC. Luckily I'm not building a parser for Haskell, but for a very small set of instructions given to a program from the command line. In case of emergency I will be looking at the source code of uulib and uu-parsinglib (which, by the way, could use some line wrapping to fit in 80 columns). Janek

1 0

Call for Nominations: Haskell Prime language committee
by Malcolm Wallace 04 Feb '13

04 Feb '13

Dear Haskell lovers, The Haskell Prime process for standardisation of new versions of the Haskell language is at something of an impasse. Since the Haskell 2010 Report was issued (at the end of 2009), there has been very little momentum to formalise existing extensions and generalisations, nor appetite to decide on whether any such extensions should be adopted as part of the core language standard. We therefore seek nominations for new members of the Haskell Prime language committee; people who have the enthusiasm to take this forward, as well as some relevant experience. If you think you would like to contribute, please nominate yourself by sending a email to haskell-2011-committee(a)haskell.org describing who you are, and what you think the committee might hope to achieve. The address above is populated by the existing committee members, but is now (temporarily) open to all during the nomination period. Nominations close in 3 weeks time, i.e. to be received by end of Sunday 24th February 2013. The process for deciding a new language committee is described at http://hackage.haskell.org/trac/haskell-prime/wiki/Committee Regards, Malcolm

1 0

Parsec Vs. src exts
by Sean Cormican 03 Feb '13

03 Feb '13

Hi, I've been spending a bit of time looking into using Parsec as a parser for a subset of the syntax of the Haskell language, a high level description of what I'm looking to create is a parser which confirms whether a given Haskell code file is valid or in the case that it isn't returns some helpful error messages to the user. (Please bear with me, I know this is a strange implementation) My question is that while I have a modest understanding of Parsec from tutorials, the haskell-src package has also been recommended of which I can find very limited documentation in comparison. However, the issue arises from both my limited understanding of haskell and lack of information on the usage of haskell-src. Can anybody provide me with some guidance on whether to try and create the parser using Parsec or if haskell-src is a better option. In the case that haskell-src is a better option, are there any tutorials or documents that I would benefit from reading? Thanks in advance, Seán

4 4

Most used functions in hackage
by Casey Basichis 03 Feb '13

03 Feb '13

Hi, I'm guessing this is a long shot, but Is there any link that counts the use of all functions in all packages in Hackage and lists them by frequency or by other stats? I'm still new to haskell but I've been working my way through tons and tons of tutorials and books. It would be very helpful to target in on the current reality of the most critical functions. Anything like this out there? Thanks, Casey

6 13

linking errors while compile hugs98 in macos
by Junior White 03 Feb '13

03 Feb '13

Hi Cafe, I downloaded the latest hugs98 source package, unzip and build, I get the following link errors. It seems many symbols are not defined, am I missing same depending libraries? This is my machine info: ➜ hugs98-plus-Sep2006 git:(master) ✗ uname -a Darwin lan-seimatoMacBook-Air.local 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64 ================================ Preprocessing Network/Hackage/Version Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccuTP291.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/C/Error.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/C/Error.so" "libraries/bootlib/Foreign/C/Error.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c' Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccYZo1zT.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/Marshal/Alloc.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/Marshal/Alloc.so" "libraries/bootlib/Foreign/Marshal/Alloc.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c packages/base/cbits/dirUtils.c packages/base/cbits/consUtils.c'

4 7

why GHC cannot infer type in this case?
by oleg＠okmij.org 03 Feb '13

03 Feb '13

Dmitry Kulagin wrote: > I try to implement little typed DSL with functions, but there is a problem: > compiler is unable to infer type for my "functions". One way to avoid the problem is to start with the tagless final representation. It imposes fewer requirements on the type system, and is a quite mechanical way of embedding DSL. The enclosed code re-implements your example in the tagless final style. If later you must have a GADT representation, one can easily write an interpreter that interprets your terms as GADTs (this is mechanical). For more examples, see the (relatively recent) lecture notes http://okmij.org/ftp/tagless-final/course/lecture.pdf {-# LANGUAGE TypeOperators, KindSignatures, DataKinds #-} {-# LANGUAGE NoMonomorphismRestriction, TypeFamilies #-} module TestExp where -- types of DSL terms data Ty = Int16 | TFun [Ty] Ty | TSeq [Ty] -- DSL terms class Exp (repr :: Ty -> *) where int16:: Int -> repr Int16 add :: repr Int16 -> repr Int16 -> repr Int16 decl :: (repr (TSeq ts) -> repr t) -> repr (TFun ts t) call :: repr (TFun ts t) -> repr (TSeq ts) -> repr t -- building and deconstructing sequences unit :: repr (TSeq '[]) cons :: repr t -> repr (TSeq ts) -> repr (TSeq (t ': ts)) deco :: (repr t -> repr (TSeq ts) -> repr w) -> repr (TSeq (t ': ts)) -> repr w -- A few convenience functions deun :: repr (TSeq '[]) -> repr w -> repr w deun _ x = x singleton :: Exp repr => (repr t -> repr w) -> repr (TSeq '[t]) -> repr w singleton body = deco (\x _ -> body x) -- sample terms fun = decl $ singleton (\x -> add (int16 2) x) -- Inferred type -- fun :: Exp repr => repr (TFun ((:) Ty 'Int16 ([] Ty)) 'Int16) test = call fun (cons (int16 3) unit) -- Inferred type -- test :: Exp repr => repr 'Int16 fun2 = decl $ deco (\x -> singleton (\y -> add (int16 2) (add x y))) {- inferred fun2 :: Exp repr => repr (TFun ((:) Ty 'Int16 ((:) Ty 'Int16 ([] Ty))) 'Int16) -} test2 = call fun2 (int16 3 `cons` (int16 4 `cons` unit)) -- Simple evaluator -- Interpret the object type Ty as Haskell type * type family TInterp (t :: Ty) :: * type instance TInterp Int16 = Int type instance TInterp (TFun ts t) = TISeq ts -> TInterp t type instance TInterp (TSeq ts) = TISeq ts type family TISeq (t :: [Ty]) :: * type instance TISeq '[] = () type instance TISeq (t1 ': ts) = (TInterp t1, TISeq ts) newtype R t = R{unR:: TInterp t} instance Exp R where int16 = R add (R x) (R y) = R $ x + y decl f = R $ \args -> unR . f . R $ args call (R f) (R args) = R $ f args unit = R () cons (R x) (R y) = R (x,y) deco f (R (x,y)) = f (R x) (R y) testv = unR test -- 5 tes2tv = unR test2 -- 9

2 1

Monadic parser vs. combinator parser
by Jan Stolarek 03 Feb '13

03 Feb '13

I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this: https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser Monadic parser seems extremely verbose and not very straightforward compared to this one. I started to wonder whether I should use monadic parser for the sake of it being monadic or should I just go with the combinator approach? Any thoughts will be appreciated before I shoot myself in the foot :) Janek

6 9

A question about "An introduction to Category theory in four easy movements"
by Jan Stolarek 03 Feb '13

03 Feb '13

Hi all, I was thinking about reading "An introduction to Category theory in four easy movements" available online at http://www.cs.man.ac.uk/~hsimmons/BOOKS/books.html I noticed however that these notes were published as a book. Looking at the table of contents at Amazon, it seems that organization of material is different. About 40% of online version are solutions to the exercises and this is seems not to be the case for the book. Were these solutions removed from the printed version or are they just contained within chapters? Are there any significant differences in scopes of these two versions? I would be thankful if anyone owning the book could give a short comparison with online notes. Janek

1 0

Substantial (1:10??) system dependencies of runtime performance??
by Nick Rudnick 03 Feb '13

03 Feb '13

Dear all, for quite a while now, I have experienced this issue with some curiosity; yesterday I had it again, when a program that took well over one hour before only needed about ten minutes, after a system reboot (recent Ubuntu) and with no browser started -- finally deciding to post this. I still can't reproduce these effects, but there is indication it is connected with browser use (mostly Google Chrome, with usually 10's of windows and ~100 folders open) and especially use of video players; closing or killing doesn't seem to set free resources, a reboot or at least suspend to disk seems to be necessary (suspend to RAM doesn't seem enough). Roughly, I would say the differences in runtime can reach a factor as much as 1:10 at many times -- and so I am curious whether this subject has already been observed or even better discussed elsewhere. I have spoken to somebody, and our only plausible conclusion was that software like web browsers is able to somewhat aggressively claim system resources higher in the privilege hierarchy (cache?? register??), so that they are not available to other programs any more. I hope this is interesting to others, too, I guess it is an issue for anybody programming computation intensive code to be run on standard systems with other applications running there, too, and having to predict the estimated runtime to the client. Maybe I have overseen some libs which are already able to scan the system state in this regard, or even tell Haskell to behave 'less nice' when other applications are known to be of lower priority?? Thanks a lot in advance, Nick

5 6

Announce: Leksah 0.13.2.0 (still a bit experimental)
by Hamish Mackenzie 02 Feb '13

02 Feb '13

Version Control --------------- Harald and Stephan's Version Control integration has been merged in! If you want to help out on this there are still some cool things that could be added. I for one would love something to show my changes vs. the repo as I edit. Harald and Stephan also added a number of other nice UI changes. Unfortunately I do not have a list (but my favourite is that selecting some text highlights other occurrences). Better Haddock and GHCi support ------------------------------- While writing documentation for jsc, I made a few changes to help make writing Haddock docs easier.... * Documentation pane - New pane that uses WebKit to display your packages Haddock docs (Package -> Open Documentation). * Auto build Haddock docs - If you click the tick button on the toolbar it will automatically build your docs and reload the Documentation pane while you edit your code. If you have background build switched off then Ctrl+B will run tests and build docs if the tick is on. * Auto run - While you are in GHCi mode background build will also reevaluate the last expression sent to GHCi with Ctrl+Enter (also works with Ctrl+B if you have background build off). * Haddock & Ctrl+Enter - Haddock code block comments ( -- > and -- >>> ) are removed before the current line or selected text is sent to GHCi. * Support for choosing which executable or test suite GHCi should run (expand the package in the workspace pane to see the list and activate the one you want to load in GHCi). Hop on #leksah if you have any questions. OS X ---- Sorry, I am still having trouble building WebKitGTK+ on OSX. If you want a challenge... https://github.com/jralls/gtk-osx-build https://github.com/leksah/leksah/blob/master/osx/gtk.sh Alternatively we might have to build Leksah with -f-webkit to leave out the webkit stuff. Windows ------- Choose the version that matches your installed GHC http://leksah.org/packages/leksah-0.13.2.0-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.13.2.0-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.13.2.0-ghc-7.4.1.exe http://leksah.org/packages/leksah-0.13.2.0-ghc-7.4.2.exe http://leksah.org/packages/leksah-0.13.2.0-ghc-7.6.1.exe Linux ----- Follow the steps in the .travis.yml file... https://github.com/leksah/leksah/blob/master/.travis.yml Should go something like this... https://travis-ci.org/leksah/leksah

1 0

Why does not zipWith' exist
by Kazu Yamamoto 02 Feb '13

02 Feb '13

Hello, Many texts explain the following Fibonacci code: fibs :: [Integer] fibs = 0 : 1 : zipWith (+) fibs (tail fibs) But this code is very slow because evaluation of (+) is done lazily. If we have the following strict zipWith', the code above becomes much faster. zipWith' f (a:as) (b:bs) = x `seq` x : zipWith' f as bs where x = f a b zipWith' _ _ _ = [] Data.List defines foldl' against foldl. But it does not define zipWith'. I'm curious why zipWith' does not exist in the standard libraries. --Kazu

4 8

Optimizing performance problems with Aeson rendering large Text arrays
by Oliver Charles 01 Feb '13

01 Feb '13

Hello, In summary, i'm working on an application that responds to a users query, a sequence index, with the union of a list of UUIDs that have "changed" since that same sequence index, split into 6 sections. I wish to respond to these queries via JSON to provide an easy to use web service, and for the most part, what I have works. The problem I am having is that profiling seems to show that the majority of the time spent in my application is encoding this to JSON, and also that the application is only 60% productive with 40% allocations happening in Data.Aeson.encode (and friends). Here's an overview of what I'm doing, the full code can be found at the end of this email. I am storing my data in memory as an IntMap, from sequence index to a changeset: IntMap ChangeSet Where a ChangeSet is essentially a tuple of HashSet's of UUIDs: data ChangeSet = ChangeSet { artistChanges :: !(HashSet MBID) , labelChanges :: !(HashSet MBID) , recordingChanges :: !(HashSet MBID) , releaseChanges :: !(HashSet MBID) , releaseGroupChanges :: !(HashSet MBID) , workChanges :: !(HashSet MBID) } deriving (Generic) The MBID newtype is just a newtype around Text, but you can only create MBIDs by parsing a UUID fromString - just to enforce a bit more correctness, but without the cost of having to serialize the UUID to JSON. When I query, I splitLookup on the IntMap to get the requested change set by sequence index, and all future changesets. I union all of these, and then render the response back to the client: let (_, !cs, !futureCs) = IntMap.splitLookup csId changeSets writeLBS $ encode $ mconcat $ catMaybes $ map Just (IntMap.elems futureCs) ++ [ cs ] None of this shows up in profiling however, and here's what I see: Thu Jan 31 17:03 2013 Time and Allocation Profiling Report (Final) Main +RTS -p -RTS total time = 4.75 secs (4748 ticks @ 1000 us, 1 processor) total alloc = 4,329,582,160 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc encode Data.Aeson.Encode 23.5 17.4 string Data.Aeson.Encode 18.5 35.1 break Data.Aeson.Encode 17.5 2.3 mconcat Main 15.1 9.7 fromValue/Array Data.Aeson.Encode 9.2 14.8 toJSON Main 5.7 9.0 send.loop Snap.Internal.Http.Server.HttpPort 3.0 0.0 mapIter Snap.Iteratee 2.1 2.3 parseJSON Main 1.7 3.0 writeLBS Snap.Internal.Types 1.1 4.9 COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 216 0 0.0 0.0 100.0 100.0 main Main 433 0 0.0 0.0 100.0 100.0 main.since Main 1063 1 0.0 0.0 75.3 78.7 encode Data.Aeson.Encode 1391 0 23.5 17.4 75.3 78.7 fromValue/Object Data.Aeson.Encode 1395 254 0.0 0.0 46.0 52.2 fromValue/Array Data.Aeson.Encode 1420 757 7.9 12.0 36.4 25.7 fromValue/String Data.Aeson.Encode 1422 3089095 0.7 0.0 28.5 13.7 string Data.Aeson.Encode 1423 3089095 10.2 11.4 27.8 13.7 break Data.Aeson.Encode 1425 3089095 17.5 2.3 17.5 2.3 string Data.Aeson.Encode 1396 884 8.3 23.7 9.6 26.6 fromValue/Array Data.Aeson.Encode 1421 0 1.3 2.9 1.3 2.9 fromValue/String Data.Aeson.Encode 1424 0 0.0 0.0 0.0 0.0 break Data.Aeson.Encode 1397 884 0.0 0.0 0.0 0.0 toJSON Main 1393 127 5.7 9.0 5.7 9.0 Unless I'm reading this incorrectly, this shows that 75% of the time is spent in encode, along with almost 80% of my allocations. While the performance of my application is actually satisfactory (I respond in around 0.04s), I'd still like to do better - if only for the practical experience of learning how to optimize. Any ideas what I can do about this? I feel like I might get better performance if fromValue/Array new that I had a vector of Text values, and they could just be intercalated with ", " - but I have no idea how the internals of Text works so this might really perform the same as the fold that is currently used. I am compiling for benchmarking purposes with: ghc -Wall -fno-warn-orphans -Werror -O2 -rtsopts \ -hide-package hashable-1.2.0.5 Main.hs And I run with: ./Main Though I have tried with +RTS -A2M, -A4M and -A8M - none of which seem to make a huge difference. -N2 seems to performance worse with a single connection, though that might perform better for many concurrent connections - I haven't yet looked. I'd love to hear your thoughts on what I can do to get this even faster!If you need any more information, don't hesitate to ask. Thanks, - ocharles -------------------- Here is the code I am using. If you want to run this, you will need a directory 'change-sets', which contain JSON files named according to the pattern (\d+).json. I've tared up my test directory and uploaded it to http://ocharles.org.uk/change-sets.tar.gz, if you want to try running this. You will need to install: cabal install aeson unordered-containers containers text uuid snap-core snap-server I run a benchmark with this Perl script, which just does 30 requests and then prints out the average request time. #!/usr/bin/perl use 5.10.0; use List::Util qw( sum ); use LWP::UserAgent; use Time::HiRes qw( gettimeofday tv_interval ); my $lwp = LWP::UserAgent->new; my @intervals; for my $i (0..30) { my $t0 = [ gettimeofday ]; $lwp->get('http://0.0.0.0:8000/since/65527'); push @intervals, tv_interval($t0); } say sum(@intervals) / $#intervals; I think that's it! {-# LANGUAGE BangPatterns, FlexibleContexts, GeneralizedNewtypeDeriving, OverloadedStrings, TypeOperators #-} import Prelude hiding (readFile) import Control.Applicative import Control.Monad (forM, mzero) import Data.Aeson import Data.ByteString.Lazy (readFile) import Data.IntMap.Strict (IntMap) import Data.Hashable import Data.List import Data.Maybe (catMaybes, fromMaybe) import Data.Monoid import Data.HashSet (HashSet) import Data.Text (Text) import Data.UUID import Snap.Core import Snap.Http.Server import System.Directory import System.FilePath import qualified Data.IntMap as IntMap import qualified Data.HashSet as HashSet import qualified Data.Text as Text import qualified Data.Text.Encoding as Encoding data ChangeSet = ChangeSet { artistChanges :: !(HashSet MBID) , labelChanges :: !(HashSet MBID) , recordingChanges :: !(HashSet MBID) , releaseChanges :: !(HashSet MBID) , releaseGroupChanges :: !(HashSet MBID) , workChanges :: !(HashSet MBID) } newtype MBID = MBID Text deriving (Eq, Show, Ord, Hashable, ToJSON) instance Monoid ChangeSet where mempty = ChangeSet mempty mempty mempty mempty mempty mempty a `mappend` b = ChangeSet { artistChanges = artistChanges a `mappend` artistChanges b , labelChanges = labelChanges a `mappend` labelChanges b , recordingChanges = recordingChanges a `mappend` recordingChanges b , releaseGroupChanges = releaseGroupChanges a `mappend` releaseGroupChanges b , releaseChanges = releaseChanges a `mappend` releaseChanges b , workChanges = workChanges a `mappend` workChanges b } mconcat as = ChangeSet { artistChanges = HashSet.unions (map artistChanges as) , labelChanges = HashSet.unions (map labelChanges as) , recordingChanges = HashSet.unions (map recordingChanges as) , releaseChanges = HashSet.unions (map releaseChanges as) , releaseGroupChanges = HashSet.unions (map releaseGroupChanges as) , workChanges = HashSet.unions (map workChanges as) } instance FromJSON ChangeSet where parseJSON (Object j) = j .: "data" >>= go where go o = ChangeSet <$> (HashSet.fromList <$> o .: "artist") <*> (HashSet.fromList <$> o .: "label") <*> (HashSet.fromList <$> o .: "recording") <*> (HashSet.fromList <$> o .: "release") <*> (HashSet.fromList <$> o .: "release_group") <*> (HashSet.fromList <$> o .: "work") parseJSON _ = mzero instance ToJSON ChangeSet where toJSON c = object [ "data" .= object [ "artist" .= artistChanges c , "label" .= labelChanges c , "recording" .= recordingChanges c , "release" .= releaseChanges c , "release_group" .= releaseGroupChanges c , "work" .= workChanges c ] ] instance FromJSON MBID where parseJSON (String s) = maybe mzero (const $ return (MBID s)) $ fromString (Text.unpack s) parseJSON _ = mzero loadChangeSets :: FilePath -> IO (IntMap ChangeSet) loadChangeSets d = do changeSetFiles <- filter (isSuffixOf ".json") <$> getDirectoryContents d fmap IntMap.fromList $ forM changeSetFiles $ \f -> do let csId = read $ reverse $ drop 5 $ reverse f changeSet <- fromMaybe (error "Failed to decode") . decode' <$> readFile (d </> f) return (csId, changeSet) main :: IO () main = do changeSets <- loadChangeSets "change-sets" quickHttpServe $ route [("/since/:x", since changeSets)] where since changeSets = do Just csId <- fmap (read . Text.unpack . Encoding.decodeUtf8) <$> getParam "x" let (_, !cs, !futureCs) = IntMap.splitLookup csId changeSets writeLBS $ encode $ mconcat $ catMaybes $ map Just (IntMap.elems futureCs) ++ [ cs ]

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Re: [Haskell-cafe] Ticking time bomb
by Alexander Kjeldaas 01 Feb '13

01 Feb '13

Forgot the list. On Fri, Feb 1, 2013 at 10:21 AM, Alexander Kjeldaas < alexander.kjeldaas(a)gmail.com> wrote: > > Trying to avoid the wrath of Ketil I'll refrain from suggesting to do > anything, I'll just explain why git is good at this, and not arbitrary. :-) > > Most systems that I know of to verify *anything*, use merkle trees, or > something very similar. > http://en.wikipedia.org/wiki/Hash_tree > > For example the TPM chip on your motherboard, used for example to ensure > the integrity of the Google Chromebook and Windows BitLocker > http://en.wikipedia.org/wiki/Trusted_Platform_Module > (simplified example: in secure memory it stores H1=hash(microcode), then > H2=hash(H1 || BIOS), then H3=hash(H2 || MBR), then H4=hash(H3 || kernel), > ...). > > Or the integrity of the bitcoin currency. > https://en.bitcoin.it/wiki/Protocol_specification#Merkle_Trees > > So these are pretty different systems, but it all boils down to doing > cryptographic secure hashes over a previous hash + new data to ensure > integrity of the new combined data. Given only one verified hash in such a > system, no part of the data, nor its history of mutation can be forged. > "History" can mean which software runs on your computer (TPM), which > transactions are valid (Bitcoin), or which commits have been done in a SCM > (git, mercurial). > > So git is not magical, it is just a practical implementation of something > that works. Any other *general* solution will be based on similar basic > principles. Mercurial does this and there is a GPG extension for it. > > Bazaar does not use a SHA1-based content addressable storage, so while a > signed commit signs the tree, it does not represent the history (no "hash > of hash", only "hash" if you look at it as a merkle tree), but it does > chain commits. To verify a tree + history, *all* commits must be signed, > which is fragile IMO. > > Regarding Darcs, my understanding is that it deliberately rejects hashing > the tree, so it is not clear to me how to verify tree+history. Patches can > be signed, but as long as patches are independent, there is no "hash of > hash" component which makes it difficult to see how one can verify the > tree. My understanding of darcs is very limited though. > > But to be *practical* the rest of the workflow should be secure as well, > so you need: > > 1. A way to distribute the merkle tree (git pull/clone/push). > Distribution is of the data that is to be signed is required for > security, because otherwise the representation of the data itself (web view > or 'git diff') can be compromised. Signatures have no meaning if you > cannot trust that you know what you sign. > 2. A way to sign a change to the merkle tre (git commit -S, git tag -s etc) > 3. A way to have multiple signatures on a given hash (i.e. commit, or tag, > or whatever it is called in a particular merkle tree implementation). > This is required to avoid catastrophic "owning" of core developers. > If required, I do think that multiple signatures can be emulated by a > structured set of commits that have single signatures though. > 3. A way to reliably do code reviews on the changes to the data (git diff) > This is really the same as 1). We cannot reliably do 'git diff' > unless the developers do it on their own equipment, thus the system must be > distributed. > 4. Given the requirement for a distributed merkle tree, some merge > strategy is needed. It is thus practical, though not required, to have > good support for this. > (Btw, even the bitcoin hash chain has a merge strategy - the tree with > the most compute power will win, and others are forced to "rebase" their > transactions on that tree) > > > So my choice of git is not arbitrary. The way git works is pretty > fundamental to verifying the integrity of stuff. > > Though when I have looked through the other options, mercurial might be a > better fit since it is supported on Windows. > > Trying to solve this problem from scratch might not be such a good idea, > because it might be very close to a reimplementation of git or mercurial. > Or maybe it is a good idea for someone who has some time on their hands. > Just be aware that the requirements for verifying anything is very close > to what existing distributed SCM systems do. > > Alexander > > > On Fri, Feb 1, 2013 at 3:32 AM, Kevin Quick <quick(a)sparq.org> wrote: > >> Git has the ability to solve all of this. >>> >> ... >> >> 2. Uploads to hackage either happen through commits to the git >>> repository, >>> or an old-style upload to hackage automatically creates a new anonymous >>> branch in the git repository. >>> 3. The git repository is authorative. Signing releases, code reviews >>> etc. >>> all happens through the git repositories. This gives us all the >>> flexibility of a git-style trust model. >>> >> ... >> >> 5. Who owns which package names can be held in a separate meta-tree git >>> repository, and can have consensus requirements on commits. >>> 6. This special meta-tree can also contain suggested verification keys >>> for >>> commits to the other hackage git trees. It can even contain keys that >>> protect Haskell namespaces in general, so that no hackage package can >>> overwrite a protected Haskell namespace. >>> 7. As backward compatibility, the meta-tree can sign simple hashes of >>> already existing packages on hackage. >>> >> ... >> >> 1. There could be some git magic script that downloads the signed git tag >>> objects only (small data set). Then another script would generate a >>> git-compatible SHA1 of the extracted tarball, given that the tarball was >>> fetched from hackage. >>> 2. Or cabal-install could fetch directly from git repositories and use >>> standard git verification. >>> 3. Or a trusted machine creates tarballs from the git repositories, signs >>> them and uploads them to hackage. >>> >> >> Without details of git's trust/verification model, it's difficult to see >> how this particular SCM tool provides the trust capabilities being >> discussed any better than a more focused solution. Additionally, the use >> of git is also difficult for many Windows users (80MB installed footprint, >> last I tried). git has a much broader solution space than simply ensuring >> the integrity of package downloads, especially when "there could be some >> git magic script" that is still not identified and appears to have the same >> insecurities as the package download/upload itself. >> >> Instead of using the "git" solution and looking for problems to solve >> with it, IMHO we should work from clearly defined problem to solution in >> general terms as our class, and then determine what specific tools >> represent an instance of that solution class. >> >> -- >> -KQ >> >> >> ______________________________**_________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >> > >

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Teaching haskell and fuzzy logic to high school students
by claudio 31 Jan '13

31 Jan '13

I'm an Italian IT teacher and I would like to introduce functional programming using haskell. As classroom activity, I would like to build-up a simple fuzzy inference engine. Does anyone know if someone has experience in teaching haskell to high school students or something has already been done with fuzzy logic and haskell? I found only this page: http://www.haskell.org/haskellwiki/AI/Logic/Fuzzy with some information and a link to a pubblication (with fee, I didn't download yet). Thanks Claudio

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Type classes, collections, sum types, closures, and a massive headache
by Bob Hutchison 31 Jan '13

31 Jan '13

Hi, I'm relatively new to Haskell, and consider myself to be towards the beginner side of the scale. Nevertheless, I've got this Haskell program I've been working on that's sitting around 11k lines right now. The pattern has been to let it grow to then knock it back by 'refactoring' or whatever you want to call it… doing it right the second time maybe… or the third time. All I want to get across is that though I consider myself a Haskell beginner I've still managed to produce something that is actually quite complex and of reasonable size in about three months. I'm still getting caught by stuff that I should not be caught by. So. Today I thought it was about time to simplify how new 'things' of a certain kind are added to the system. These things are some a cross between an event and an assertion of a fact in a rule based system. There are many different kinds of these things. I already have more than a dozen commonplace ones, and I expect there's a much larger number of more specialized ones that a user will want to add on their own. While they start out quite differently, they end up satisfying a common interface and follow the identical three or four state lifecycle. This sounded like a type class to me, and in fact, easily implemented as such. Now, this is how I got caught: it seems to be impossible to have collections of things with a common type class if they have different types. How is it that I've written that many lines of code in Haskell and I'm just noticing this now? (If I wasn't so annoyed, I'd look for something clever to reflect how loc count obviously doesn't mean much… but clever seems to be beyond me today). Is this true? Are there any GHC extensions that will let me around this? The immediate problem is mapping an input to the system, some json message containing a reference to the 'thing' (like a key of some kind). I have to take that reference and find the thing and operate on it. All operations are easily accommodated by a type class. However, since I can't have a collection with mixed types even if the types satisfy a type class, I can't quite see how to actually store the things so I can find them. So there are a couple of obvious ways to handle this. I could use an ADT and a sum type of all the known kinds of thing, but I already know that this has to be extended and that's going to be problematic with users doing this on their own. And the type signatures look ugly. So I think that's not the best. I could use an ADT that contains functions that correspond to the functions of the type class, and that close over the 'thing' in question. I think this could be made to work, but I'm concerned with walking into more nasty surprises… If anyone is able to make sense of what I wrote and has any suggestions I'd really appreciate hearing them. Thanks, Bob

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Call for Participation - ACCAT 2013
by Ulrike Golas 31 Jan '13

31 Jan '13

[We apologize if you receive multiple copies of this message.] CALL FOR PARTICIPATION ========== 8th International Workshop on Applied and Computational Category Theory ACCAT 2013 http://accat2013.zib.de/ Satellite Event of ETAPS 2013, Rome, March 17 2013 ========== Scope ===== Since the 1960s, the use of category theory in computer science has been a fruitful one, including applications to different areas such as automata theory, algebraic specification, and programming languages, among others. In recent years, techniques and methods from CT have been adopted as a standard research tool, and considered as such in different venues around the world. The ACCAT workshop on "Applied and Computational Category Theory" has been one of these venues. Since its inception in 2006, ACCAT provided a forum where invited contributors presented their own research on different facets of category theory applied to computer science. Following the tradition, the program of this year edition will include eight invited talks by top researchers in the area: Samson Abramsky Robin B. Cockett Barbara Koenig Ugo Montanari Till Mossakowski Dusko Pavlovic Andrzej Tarlecki Glynn Winskel Despite ACCAT's success, we believe that the current formula of the workshop should be deeply revised. Indeed, we believe that a fully fledged conference is missing where all kinds of applications of category theory to computer science can be presented (like the former CTCS conference, which somehow ended in 2006). Therefore, after the presentations, the workshop will end up with a general discussion among the invited speakers and the attendees. We hope that the outcome of the discussion could be a decision whether to push for such high-level workshop/conference on the application of category theory to computer science, or at least to verify the viability of a further meeting focusing on this issue. For more information, please contact Fabio Gadducci (gadducci(a)di.unipi.it) or Ulrike Golas (golas(a)zib.de) -- Dr. Ulrike Golas Konrad-Zuse-Zentrum für Informationstechnik Berlin Takustr. 7, 14195 Berlin, Germany Tel. +49 30 84185 - 318 golas(a)zib.de -- www.zib.de/golas

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why GHC cannot infer type in this case?
by Dmitry Kulagin 31 Jan '13

31 Jan '13

Hi Cafe, I try to implement little typed DSL with functions, but there is a problem: compiler is unable to infer type for my "functions". It seems that context is clear, but still GHC complains "Could not deduce...". It is sad because without type inference the DSL will be very difficult to use. Could someone explain me why this happens and how it can be avoided? I use GHC 7.4.2 I tried to distill the code to show the problem (and full error message below it). Thank you! Dmitry {-# LANGUAGE GADTs, KindSignatures, DataKinds #-} module TestExp where -- Sequence of DSL's types to represent type of tuple data TSeq where TSeqEmpty :: TSeq TSeqCons :: TypeExp -> TSeq -> TSeq -- All types allowed in DSL data TypeExp where -- Primitive type TInt16 :: TypeExp -- Function type is built from types of arguments (TSeq) and type of result TFun :: TSeq -> TypeExp -> TypeExp -- Sequence of expressions to represent tuple data Seq (t :: TSeq) where SeqEmpty :: Seq TSeqEmpty SeqCons :: Exp w -> Seq v -> Seq (TSeqCons w v) data Exp (r :: TypeExp) where Decl :: (Seq args -> Exp r) -> Exp (TFun args r) Call :: Exp (TFun args r) -> Seq args -> Exp r Int16 :: Int -> Exp TInt16 Add :: Exp TInt16 -> Exp TInt16 -> Exp TInt16 -- Assumed semantics: fun x = x + 2 -- The following line must be uncommented to compile without errors -- fun :: Exp (TFun (TSeqCons TInt16 TSeqEmpty) TInt16) fun = Decl $ \(SeqCons x SeqEmpty) -> Add (Int16 2) x -- eval (Int16 x) = x -- eval (Call (Decl f) seq) = eval (f seq) -- eval (Add x y) = eval x + eval y -- test = eval $ Call fun (SeqCons (Int16 3) SeqEmpty) ----------------- There is full error message: ---------------------------- TestExp.hs:30:53: Could not deduce (w ~ 'TInt16) from the context (args ~ TSeqCons w v) bound by a pattern with constructor SeqCons :: forall (w :: TypeExp) (v :: TSeq). Exp w -> Seq v -> Seq (TSeqCons w v), in a lambda abstraction at TestExp.hs:30:16-33 or from (v ~ 'TSeqEmpty) bound by a pattern with constructor SeqEmpty :: Seq 'TSeqEmpty, in a lambda abstraction at TestExp.hs:30:26-33 `w' is a rigid type variable bound by a pattern with constructor SeqCons :: forall (w :: TypeExp) (v :: TSeq). Exp w -> Seq v -> Seq (TSeqCons w v), in a lambda abstraction at TestExp.hs:30:16 Expected type: Exp 'TInt16 Actual type: Exp w In the second argument of `Add', namely `x' In the expression: Add (Int16 2) x

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"branching" conduits
by Simon Marechal 31 Jan '13

31 Jan '13

Hello, I have found the Conduit abstraction to be very well suited to a set of problems I am facing. I am however wondering how to implement "branching" conduits, and even conduit pools. I am currently in the process of rewriting parts (the simple parts) of the Logstash tool. There is a sample program that I use here: https://github.com/bartavelle/hslogstash/blob/deprecateUtils/examples/Redis… As it can be seen, it uses a "Redis" source, a conduit that decodes the JSON ByteString into a LogstashMessage, a conduit that stores it into Elasticsearch and outputs the result of that action as an Either, and finally a sink that prints the errors. My problem is that I would like more complex behaviour. For example, I would like to route messages to another server instead of putting them into Elasticsearch when the LogstashMessage has some tag set. But this is just an example, and it is probable I will want much more complex behavior soon. I am not sure how to proceed from here, but have the following ideas: * investigate how the Conduits are made internally to see if I can create a operator similar to $$, but that would have a signature like: Source m (Either a b) -> Sink a m r -> Sink b m r and would do the branching in a binary fashion. I am not sure this is even possible. * create a "mvars" connectors constructor, which might have a signature like this: Int -- ^ branch count (LogstashMessage -> Int) -- ^ branching function (Sink LogstashMessage m (), [Source m LogstashMessage]) -- ^ a suitable sink, several sources for the other conduits it would internally create a MVar (Maybe LogstashMessage) for each branch, and put putMVar accordingly to the branching function. When the Conduit is destroyed, it will putMVar Nothing in all MVars. the sources would takeMVar, check if it is Nothing, or just proceed as expected. The MVar should guarantee the constant space property, but there is the risk of inter branch blocking when one of the branches is significantly slower than the others. It doesn't really matter to me anyway. And all the branch Sinks would have to have some synchronization mechanism so that the main thread waits for them (as they are going to be launched by a forkIO). This is the simplest scheme I have thought of, and it is probably not a very good one. I am very interested in suggestions here.

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Announcement - Haskell User Group Frankfurt
by Peter Althainz 31 Jan '13

31 Jan '13

Dear Haskellers, for all who are located near Frankfurt, there is a new Meetup kicked off: http://www.meetup.com/Frankfurt-Haskell-User-Group regards Peter Althainz

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FFI - Approaches to C/C++
by Casey Basichis 31 Jan '13

31 Jan '13

Hi, I'm working on a project in Haskell and C++ where the former is the brains and the latter is for UI, interaction etc. I've read this http://www.altdevblogaday.com/2012/04/26/functional-programming-in-c/ and a number of other haskell posts suggesting the OOP is not the way to go. Without trying to emulate functional programming through templates or boost::phoenix, what approaches do you all favor when designing parts of an application in C++? Patterns to embrace or avoid? Should I just use functions and handle things with name spaces? I was thinking about handling the callbacks with boosts signals and slots 2 I know this is not entirely haskell centric, but it is a question for haskell users. Thanks, Casey -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis(a)gmail.com 310.387.7540

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Re: [Haskell-cafe] FFI - Approaches to C/C++
by Casey Basichis 31 Jan '13

31 Jan '13

Hi Ertugrul, Thank you for the detailed reply. From what you wrote, partial FFI still seems like the way to go. Unfortunately Ogre isn't the only large library I'm using, so "difficult" several times over sounds like a good way to handicap the project early on. I'm perfectly happy to use Haskell for the strengths that will most benefit my project. I can always go back and try to bring the C++ specific parts into the fold once a prototype is up and running. As it seems there is a great deal of c/c++ to do either way, I would really appreciate so thoughts towards my original question. What practices in C++ are preferred by Haskell users, in the know, for the parts of the app that will not be pure Haskell? Should I be looking to avoid OOP? Dependency Injection? I wont reiterate all the facets of the first post, but it would help me immensely to zero in on a few patterns and strategies that can minimized the damage I inflict in c++ land. Thanks, Casey p.s. With "That used to be true, but the reason has nothing to do with the language. The problem was that the libraries weren't there." What do you mean? Which packages should I be looking at? I am on iOS like I said, its a stage 1 GHC compiler so I don't have access to GHCI or template haskell. >Casey Basichis <caseybasichis at gmail.com> wrote: > >> I'm not entirely sure what you mean. >> >> I'm intending on using Ogre for GUI - for which there is the Hogre >> bindings, but after emailing the DEV about it, I didn't get the >> impression from his advice that I should be using it for production >> code. Here is what he suggested: >> >> "It depends, really. Hogre is good for running Ogre from within >> Haskell, but it has its limitations. The number one thing people have >> been struggling with is handling input with hogre - there's Hois >> (Haskell wrapper for OIS) but it's not perfect (it misses input >> events), and the other option is having to write some C++ glue. Hogre >> is a solid proof of concept and you can do some demos with it, but if >> you're e.g. writing a game it might be a bit of a struggle. In the end >> it's about how much you value being able to write code in Haskell (or >> how allergic to C++ you are)." >> >> I'm on iOS so I imagine those difficulties are compounded. >> >> I am using several other C++ libraries for which there are no existing >> bindings and no Haskell alternative packages that are even remotely >> close. >> >> Are you suggesting it would be better to write all my own FFI bindings >> for all the needed libraries? > >That's not what I'm suggesting. It was just too little information to >properly judge the difficulty of doing everything in Haskell. > >Binding to Ogre (or C++ in general) is indeed difficult. If Hogre >doesn't work or is too limited, your best option might be to write a C >wrapper around the Hogre functionality you need. Another option is to >use SDL/OpenGL directly, which may be easier or harder depending on your >application. > >However, if you can build the bridge between your rendering library and >Haskell, then Haskell is certainly the better choice. > > >> Everything I read suggests that Haskells strengths are in >> transformation and that interaction is not its strong suit. >> >> I am interested in your thoughts and I am open to whatever, but you >> are the first to suggest that the mix is a bad idea. > >That used to be true, but the reason has nothing to do with the >language. The problem was that the libraries weren't there. Nowadays >you can write all sorts of interactive applications in Haskell, >including GUIs, TUIs, games, simulations and web applications. However, >I've long been waiting for useful bindings to Ogre or Irrlicht, but I'm >afraid that it's not going to happen any time soon. > >Ultimately it's your choice. Let me summarize the possiblities: > > * C wrapper around Ogre. Easy integration, but need to write the > rendering code in C/C++. > > * Full FFI bindings to Ogre. Difficult integration, but you can write > your rendering code in Haskell. > > * Partial FFI bindings to Ogre. Integration may be somewhat easy, if > you do the initialization in Haskell and the actual rendering in > C/C++. However, this again requires to write the rendering in > C/C++. > > * Using SDL/OpenGL directly: Everything available for Haskell. May > be difficult, because you need to write OpenGL code. > >I hope, this helps. > > >Greets, >Ertugrul -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis(a)gmail.com 310.387.7540

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(no subject)
by Casey Basichis 31 Jan '13

31 Jan '13

Hi Ertugrul, Thank you for the detailed reply. From what you wrote, partial FFI still seems like the way to go. Unfortunately Ogre isn't the only large library I'm using, so "difficult" several times over sounds like a good way to handicap the project early on. I'm perfectly happy to use Haskell for the strengths that will most benefit my project. I can always go back and try to bring the C++ specific parts into the fold once a prototype is up and running. As it seems there is a great deal of c/c++ to do either way, I would really appreciate so thoughts towards my original question. What practices in C++ are preferred by Haskell users, in the know, for the parts of the app that will not be pure Haskell? Should I be looking to avoid OOP? Dependency Injection? I wont reiterate all the facets of the first post, but it would help me immensely to zero in on a few patterns and strategies that can minimized the damage I inflict in c++ land. Thanks, Casey p.s. With "That used to be true, but the reason has nothing to do with the language. The problem was that the libraries weren't there." What do you mean? Which packages should I be looking at? I am on iOS like I said, its a stage 1 GHC compiler so I don't have access to GHCI or template haskell. >Casey Basichis <caseybasichis at gmail.com> wrote: > >> I'm not entirely sure what you mean. >> >> I'm intending on using Ogre for GUI - for which there is the Hogre >> bindings, but after emailing the DEV about it, I didn't get the >> impression from his advice that I should be using it for production >> code. Here is what he suggested: >> >> "It depends, really. Hogre is good for running Ogre from within >> Haskell, but it has its limitations. The number one thing people have >> been struggling with is handling input with hogre - there's Hois >> (Haskell wrapper for OIS) but it's not perfect (it misses input >> events), and the other option is having to write some C++ glue. Hogre >> is a solid proof of concept and you can do some demos with it, but if >> you're e.g. writing a game it might be a bit of a struggle. In the end >> it's about how much you value being able to write code in Haskell (or >> how allergic to C++ you are)." >> >> I'm on iOS so I imagine those difficulties are compounded. >> >> I am using several other C++ libraries for which there are no existing >> bindings and no Haskell alternative packages that are even remotely >> close. >> >> Are you suggesting it would be better to write all my own FFI bindings >> for all the needed libraries? > >That's not what I'm suggesting. It was just too little information to >properly judge the difficulty of doing everything in Haskell. > >Binding to Ogre (or C++ in general) is indeed difficult. If Hogre >doesn't work or is too limited, your best option might be to write a C >wrapper around the Hogre functionality you need. Another option is to >use SDL/OpenGL directly, which may be easier or harder depending on your >application. > >However, if you can build the bridge between your rendering library and >Haskell, then Haskell is certainly the better choice. > > >> Everything I read suggests that Haskells strengths are in >> transformation and that interaction is not its strong suit. >> >> I am interested in your thoughts and I am open to whatever, but you >> are the first to suggest that the mix is a bad idea. > >That used to be true, but the reason has nothing to do with the >language. The problem was that the libraries weren't there. Nowadays >you can write all sorts of interactive applications in Haskell, >including GUIs, TUIs, games, simulations and web applications. However, >I've long been waiting for useful bindings to Ogre or Irrlicht, but I'm >afraid that it's not going to happen any time soon. > >Ultimately it's your choice. Let me summarize the possiblities: > > * C wrapper around Ogre. Easy integration, but need to write the > rendering code in C/C++. > > * Full FFI bindings to Ogre. Difficult integration, but you can write > your rendering code in Haskell. > > * Partial FFI bindings to Ogre. Integration may be somewhat easy, if > you do the initialization in Haskell and the actual rendering in > C/C++. However, this again requires to write the rendering in > C/C++. > > * Using SDL/OpenGL directly: Everything available for Haskell. May > be difficult, because you need to write OpenGL code. > >I hope, this helps. > > >Greets, >Ertugrul

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list comprehansion performance has hug different
by Junior White 31 Jan '13

31 Jan '13

Hi Cafe, I have two programs for the same problem "Eight queens problem", the link is http://www.haskell.org/haskellwiki/99_questions/90_to_94. My two grograms only has little difference, but the performance, this is my solution: -- solution 1------------------------------------------------------------ queens1 :: Int -> [[Int]] queens1 n = map reverse $ queens' n where queens' 0 = [[]] queens' k = [q:qs | q <- [1..n], qs <- queens' (k-1), isSafe q qs] isSafe try qs = not (try `elem` qs || sameDiag try qs) sameDiag try qs = any (λ(colDist, q) -> abs (try - q) == colDist) $ zip [1..] qs -- solution 2-------------------------------------------------------------- queens2 :: Int -> [[Int]] queens2 n = map reverse $ queens' n where queens' 0 = [[]] queens' k = [q:qs | qs <- queens' (k-1), q <- [1..n], isSafe q qs] isSafe try qs = not (try `elem` qs || sameDiag try qs) sameDiag try qs = any (λ(colDist,q) -> abs (try - q) == colDist) $ zip [1..] qs the performance difference is: (set :set +s in ghci) *Main> length (queens1 8) 92 (287.85 secs, 66177031160 bytes) *Main> length (queens2 8) 92 (0.07 secs, 17047968 bytes) *Main> The only different in the two program is in the first is "q <- [1..n], qs <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]". Does sequence in list comprehansion matter? And why?

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Haskell Weekly News: Issue 256
by Daniel Santa Cruz 31 Jan '13

31 Jan '13

Welcome to issue 256 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the weeks of January 20 to 26, 2013. Quotes of the Week * elliott: cmccann: unfortunately it is too perfect an abstraction to be useful. * SamanthaD: shachaf: you're one of those dirty imperative communists who want the state to dictate everything! * monochrom: I refuse camel case and mark zuckerberg. same level. not negotiable. * mauke: a newtype is like an existing type but wearing glasses and a fake mustache and a sign saying "you've never seen me before" Top Reddit Stories * Taking magic out of GHC or: Tracing compilation by transformation (intro to Core transformations, inlining,.. Domain: ics.p.lodz.pl, Score: 59, Comments: 2 On Reddit: [1] http://goo.gl/lJmsb Original: [2] http://goo.gl/IbJ5O * Introduction to Haskell IO Domain: haskellforall.com, Score: 57, Comments: 26 On Reddit: [3] http://goo.gl/fPky4 Original: [4] http://goo.gl/Xymbr * The longest type signature I have ever seen (package Nussinov78) Domain: i.imgur.com, Score: 48, Comments: 26 On Reddit: [5] http://goo.gl/ePwNs Original: [6] http://goo.gl/B74yW * There is now a patch for the GHC runtime system which implements a scheduler which can handle threads with priorities. Domain: hackage.haskell.org, Score: 47, Comments: 12 On Reddit: [7] http://goo.gl/22XgH Original: [8] http://goo.gl/8iC8w * Success story: novice haskeller moves console cursor with “netwire” FRP library Domain: danbst.wordpress.com, Score: 43, Comments: 8 On Reddit: [9] http://goo.gl/dm8ga Original: [10] http://goo.gl/Ue7pK * Haskell as pseudo code Domain: arxiv.org, Score: 33, Comments: 2 On Reddit: [11] http://goo.gl/JL8Mf Original: [12] http://goo.gl/5D4zV * Generalizing do notation Domain: self.haskell, Score: 28, Comments: 19 On Reddit: [13] http://goo.gl/teYAD Original: [14] http://goo.gl/teYAD * Tutorial: Up-front Unit Testing in Haskell Domain: github.com, Score: 26, Comments: 21 On Reddit: [15] http://goo.gl/kjzYJ Original: [16] http://goo.gl/oB2jg * The Design of a Practical Proof Checker for a Lazy Functional Language [PDF] Domain: people.cs.missouri.edu, Score: 26, Comments: On Reddit: [17] http://goo.gl/sj5Aj Original: [18] http://goo.gl/eEUOj * Introduction to Haskell, Lecture 2 is Live Domain: shuklan.com, Score: 24, Comments: 4 On Reddit: [19] http://goo.gl/1nUVv Original: [20] http://goo.gl/rCbfx * Hugs in your browser Domain: tryhaskell.org, Score: 24, Comments: 21 On Reddit: [21] http://goo.gl/zNtex Original: [22] http://goo.gl/wMPPW * Free functors - the reason Free and operational are not equivalent Domain: self.haskell, Score: 24, Comments: 12 On Reddit: [23] http://goo.gl/E7qe7 Original: [24] http://goo.gl/E7qe7 * Scribbling, smudging and smearing with Haskell Domain: self.haskell, Score: 22, Comments: 108 On Reddit: [25] http://goo.gl/KBw0V Original: [26] http://goo.gl/KBw0V * Touch and Keyboard API (announcing Elm 0.7.1) Domain: elm-lang.org, Score: 20, Comments: 3 On Reddit: [27] http://goo.gl/OhFaH Original: [28] http://goo.gl/rV9bU * (re)implementing the generalised Functor strength function using lenses. Domain: statusfailed.com, Score: 20, Comments: 3 On Reddit: [29] http://goo.gl/gEYHr Original: [30] http://goo.gl/TDX2A Top StackOverflow Questions * Can using UndecidableInstances pragma locally have global consequences on compilation termination? votes: 20, answers: 1 Read on SO: [31] http://goo.gl/PcQF1 * Why doesn't GHC Haskell support overloaded record parameter names? votes: 14, answers: 1 Read on SO: [32] http://goo.gl/BJ71T * How much does it cost for Haskell FFI to go into C and back? votes: 14, answers: 2 Read on SO: [33] http://goo.gl/kUCKn * Compile unsafe Haskell votes: 13, answers: 1 Read on SO: [34] http://goo.gl/nGaCX * Why GADT/existential data constructors cannot be used in lazy patterns? votes: 12, answers: 2 Read on SO: [35] http://goo.gl/zhpCh * Correct design for Haskell exception handling votes: 11, answers: 2 Read on SO: [36] http://goo.gl/zeWoH * How to use phase control of inlining in haskell? votes: 10, answers: 0 Read on SO: [37] http://goo.gl/dyuGI * Looking for resources that help testing a Haskell implementation for standard conformance votes: 9, answers: 1 Read on SO: [38] http://goo.gl/DL7vY * Generate a C struct based on a complex Haskell type votes: 8, answers: 1 Read on SO: [39] http://goo.gl/7E8OT * How to structure a Haskell project? votes: 7, answers: 1 Read on SO: [40] http://goo.gl/J4GuA * Difference between two functions creating a singleton list votes: 7, answers: 3 Read on SO: [41] http://goo.gl/ZXzw0 Until next time, Daniel Santa Cruz References 1. http://ics.p.lodz.pl/~stolarek/blog/2013/01/taking-magic-out-of-ghc-or-trac… 2. http://www.reddit.com/r/haskell/comments/17bu54/taking_magic_out_of_ghc_or_… 3. http://www.haskellforall.com/2013/01/introduction-to-haskell-io.html 4. http://www.reddit.com/r/haskell/comments/17712e/introduction_to_haskell_io/ 5. http://i.imgur.com/Zv9JH9A.png 6. http://www.reddit.com/r/haskell/comments/178jwh/the_longest_type_signature_… 7. http://hackage.haskell.org/trac/ghc/ticket/7606 8. http://www.reddit.com/r/haskell/comments/1725g0/there_is_now_a_patch_for_th… 9. http://danbst.wordpress.com/2013/01/23/novice-netwire-user/ 10. http://www.reddit.com/r/haskell/comments/173cxi/success_story_novice_haskel… 11. http://arxiv.org/abs/1301.5076 12. http://www.reddit.com/r/haskell/comments/176xk0/haskell_as_pseudo_code/ 13. http://www.reddit.com/r/haskell/comments/16zrsp/generalizing_do_notation/ 14. http://www.reddit.com/r/haskell/comments/16zrsp/generalizing_do_notation/ 15. https://github.com/kazu-yamamoto/unit-test-example/blob/master/markdown/en/… 16. http://www.reddit.com/r/haskell/comments/1750i9/tutorial_upfront_unit_testi… 17. http://people.cs.missouri.edu/~harrisonwl/papers/tfp2012.pdf 18. http://www.reddit.com/r/haskell/comments/179367/the_design_of_a_practical_p… 19. http://shuklan.com/haskell 20. http://www.reddit.com/r/haskell/comments/172h5q/introduction_to_haskell_lec… 21. http://tryhaskell.org/hugs/ 22. http://www.reddit.com/r/haskell/comments/177a56/hugs_in_your_browser/ 23. http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_fr… 24. http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_fr… 25. http://www.reddit.com/r/haskell/comments/175mxw/scribbling_smudging_and_sme… 26. http://www.reddit.com/r/haskell/comments/175mxw/scribbling_smudging_and_sme… 27. http://elm-lang.org/blog/announce/version-0.7.1.elm 28. http://www.reddit.com/r/haskell/comments/179nwl/touch_and_keyboard_api_anno… 29. http://statusfailed.com/blog/2013/01/26/haskells-strength-generalising-with… 30. http://www.reddit.com/r/haskell/comments/17b5ak/reimplementing_the_generali… 31. http://stackoverflow.com/questions/14476230/can-using-undecidableinstances-… 32. http://stackoverflow.com/questions/14422169/why-doesnt-ghc-haskell-support-… 33. http://stackoverflow.com/questions/14519905/how-much-does-it-cost-for-haske… 34. http://stackoverflow.com/questions/14429966/compile-unsafe-haskell 35. http://stackoverflow.com/questions/14538255/why-gadt-existential-data-const… 36. http://stackoverflow.com/questions/14538294/correct-design-for-haskell-exce… 37. http://stackoverflow.com/questions/14446368/how-to-use-phase-control-of-inl… 38. http://stackoverflow.com/questions/14484569/looking-for-resources-that-help… 39. http://stackoverflow.com/questions/14500582/generate-a-c-struct-based-on-a-… 40. http://stackoverflow.com/questions/14421178/how-to-structure-a-haskell-proj… 41. http://stackoverflow.com/questions/14530555/difference-between-two-function… 42. https://plus.google.com/105107667630152149014/about

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TIPS: To Insure Package Sanity
by Albert Y. C. Lai 31 Jan '13

31 Jan '13

If you possess multiple instances (may be different versions, may be same version different builds) of a package, life can be hard and confusing. The problems are explained in my http://www.vex.net/~trebla/haskell/sicp.xhtml and faced by many people regularly. (Just read this mailing list.) cabal-install has a mechanism for not adding more instances. It is available since version 0.14.0, or earlier. It is just little known. It is also a bit manual. You have to give 1 instruction for each package. If you want to say it for n packages, you have to give n instructions. Also, if you give such an instruction for a package you do not already have, there is a problem: now you can't install that package. To insure package sanity, add these lines to your $HOME/.cabal/config, one line per package you want to protect. (Cannot merge into one line.) constraint: array installed constraint: bytestring installed constraint: Cabal installed constraint: containers installed ... Generally, do it for every package that comes with GHC, and every package that comes with the Haskell Platform if you have it, plus every package that you want stable. (You may omit packages that cannot possibly be on hackage, e.g., integer-gmp.) Remember to revise those lines whenever you switch GHC versions or Haskell Platform versions, since their package lists change. Alternatively, if you want even more manual work (in exchange for more fine-tuning perhaps?): every time you use "cabal install": cabal install --constraint="array installed" --constraint="bytestring installed" --constraint="Cabal installed" --constraint="containers installed" ...

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Package conflicts using cabal-dev
by Arnaud Bailly 30 Jan '13

30 Jan '13

Hello, I am running into a strange issue that reminds me of Java's classloader black magic havoc :-) My code compiles fine using cabal-dev install, but when I try to compile an individual file with ghc, I got the following error: $ ghc -package-conf cabal-dev/packages-7.4.1.conf YakGraph.hs [1 of 1] Compiling YakGraph ( YakGraph.hs, YakGraph.o ) YakGraph.hs:13:30: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' In the first argument of `unpack', namely `s' In the expression: unpack s In an equation for `fromGraphId': fromGraphId (Str s) = unpack s YakGraph.hs:24:57: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' In the first argument of `unpack', namely `name' In the expression: unpack name In a case alternative: (Just (Str name)) : _ -> unpack name The code leading to this error is here: https://gist.github.com/4677191 Any insights on this would be appreciated. Arnaud

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Heads up: planned removal of String instances from HTTP package
by Ganesh Sittampalam 30 Jan '13

30 Jan '13

Hi, tl;dr: I'm planning on removing the String instances from the HTTP package. This is likely to break code. Obviously it will involve a major version bump. The basic reason is that this instance is rather broken in itself. A String ought to represent Unicode data, but the HTTP wire format is bytes, and HTTP makes no attempt to handle encoding. This was discussed on the libraries@ list a while back, but I'm happy to discuss further if there's a general feeling that this is a bad thing to do: http://www.haskell.org/pipermail/libraries/2012-September/018426.html I will probably upload the new version in a week or two. Cheers, Ganesh

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Darcs Hacking Sprint #8 (February 15-17, Paris)
by Guillaume Hoffmann 30 Jan '13

30 Jan '13

Dear hackers, the Darcs project is glad to announce that the next Hacking Sprint will take place in Paris on February 15-17, at the IRILL (near Place d'Italie). Please check out the details at: http://darcs.net/Sprints/2013-02 Here are three things to know 1. Everybody is welcome to join us. We'd love to have you, whatever your Haskell or Darcs hacking experience. Also, if you've got a wacky idea for the future of version control, or a cool use for the Darcs library, you should join us too :-) 2. Please let us know if you're attending. Just add your name to http://wiki.darcs.net/Sprints/2013-02 and you're good to go. You can also send us an email. 3. We can reimburse travel costs (within reason!). Let us know if you'd like a reimbursement, and save your receipts. Many thanks to everybody who participated in our fundraising drives or who gave money on the side. Thanks also to the Software Freedom Conservancy for making fundraising and reimbursements so painless! If you can't join us in person, but you'd like to cheer us on, say hello at http://darcs.net/Donations ! Looking forward to seeing you! Guillaume

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Handling exceptions or gracefully releasing resources
by Thiago Negri 30 Jan '13

30 Jan '13

`Control.Exception.bracket` is a nice function to acquire and release a resource in a small context. But, how should I handle resources that are hold for a long time? Should I put `Control.Exception.finally` on every single line of my finalizers? What exceptions may occur on an IO operation? Every IO function has the risk of throwing an exception? Thanks, Thiago.

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