- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] attoparsec and backtracking
by wren ng thornton 16 Mar '13

16 Mar '13

On 3/15/13 3:29 PM, Evan Laforge wrote: > However, which error msg shows up depends on the order of the (<|>) > alternatives, and in general the global structure of the entire > parser, because I think it just backtracks and then picks the last > failing backtrack. Even after carefully rearranging all the parsers > it seems impossible to get this particular error to bubble up to the > top. The thing is, as soon as I see an unexpected suffix I know I can > fail entirely right there, with exactly that error msg, but since > there's no way to turn off backtracking I think there's no way to do > that. I had some similar issues recently. The trick is figuring out how to convince attoparsec to commit to a particular alternative. For example, consider the grammar: A (B A)* C; where if the B succeeds then we want to commit to parsing an A (and if it fails then return A's error, not C's). To simplify things, let's drop the leading A since it's not part of the problem. And let's try to parse an invalid string like "BX" (or "BABX"). The key point is that, bad = (pB *> pure (:) <*> pA <*> bad) <|> (pC *> pure []) is different than, good = do e <- eitherP pB pC -- (Left <$> pB) <|> (Right <$> pC) case e of Left _ -> (:) <$> pA <*> good Right _ -> pure [] In particular, the first one is bad (for our purposes) because due to hoisting the choice up high, after parsing the B we fail to commit, so when parsing A fails we'll backtrack over the B and try C instead. Assuming C doesn't overlap with B, we'll then report C's error. Whereas the latter is good because due to pushing the choice down, once we've parsed B (or C) we're committed to that choice; so when A fails, we'll report A's error (or backtrack to the lowest choice that dominates the call to good). Attoparsec does indeed just report the failure generated by the final parse, so you'll have to refactor things to recognize which sort of token you're looking for (e.g., p_num vs p_identifier or whatever), and then commit to that choice before actually parsing the token. It's not very modular that way, but I think that's the only option right now. It shouldn't be too hard to design a combinator for doing a hard commit (by discarding the backtrack continuation); but that has modularity issues of its own... Another option, of course, is to drop down to performing lexing on the ByteString itself (e.g., [1]) and then wrap those individual lexers to work as attoparsec Parsers. Even if using attoparsec for the heavy lifting, this is a good approach for maximizing performance of the lexing step. [1] <http://hackage.haskell.org/package/bytestring-lexing> -- Live well, ~wren

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Open-source projects for beginning Haskell students?
by Brent Yorgey 15 Mar '13

15 Mar '13

Hi everyone, I am currently teaching a half-credit introductory Haskell class for undergraduates. This is the third time I've taught it. Both of the previous times, for their final project I gave them the option of contributing to an open-source project; a couple groups/individuals took me up on it and I think it ended up being a modest success. So I'd like to do it again this time around, and am looking for particular projects I can suggest to them. Do you have an open-source project with a few well-specified tasks that a relative beginner (see below) could reasonably make a contribution towards in the space of about four weeks? I'm aware that most tasks don't fit that profile, but even complex projects usually have a few "simple-ish" tasks that haven't yet been done just because "no one has gotten around to it yet". If you have any such projects, I'd love to hear about it! Just send me a paragraph or so describing your project and explaining what task(s) you could use help with --- something that I could put on the course website for students to look at. Here are a few more details: * The students will be working on the projects from approximately the end of this month through the end of April. During the next two weeks they would be contacting you to discuss the possibility of working on your project. * By "relative beginner" I mean someone familiar with the material listed here: http://www.cis.upenn.edu/~cis194/lectures.html and just trying to come to terms with Applicative and Monad. They definitely do not know much if anything about optimization/profiling, GADTs, the mtl, or Haskell-programming-in-the-large. (Although part of the point of the project is to teach them a bit about programming-in-the-(medium/large)). * What I would hope from you is a willingness to exchange email and/or chat with the student(s) over the course of the project, to give them a bit of guidance/mentoring. I am certainly willing to help on that front, but of course I probably don't know much about your particular project. Thanks! -Brent

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PPDP 2013: Call for Papers
by Tom Schrijvers 15 Mar '13

15 Mar '13

===================================================================== Call for papers 15th International Symposium on Principles and Practice of Declarative Programming PPDP 2013 Special Issue of Science of Computer Programming (SCP) Madrid, Spain, September 16-18, 2013 (co-located with LOPSTR 2013) http://users.ugent.be/~tschrijv/PPDP2013/ ====================================================================== PPDP 2013 is a forum that brings together researchers from the declarative programming communities, including those working in the logic, constraint and functional programming paradigms, but also embracing a variety of other paradigms such as visual programming, executable specification languages, database languages, and knowledge representation languages. The goal is to stimulate research in the use of logical formalisms and methods for specifying, performing, and analysing computations, including mechanisms for mobility, modularity, concurrency, object-orientation, security, verification and static analysis. Papers related to the use of declarative paradigms and tools in industry and education are especially solicited. Topics of interest include, but are not limited to: * Functional programming * Logic programming * Answer-set programming * Functional-logic programming * Declarative visual languages * Constraint Handling Rules * Parallel implementation and concurrency * Monads, type classes and dependent type systems * Declarative domain-specific languages * Termination, resource analysis and the verification of declarative programs * Transformation and partial evaluation of declarative languages * Language extensions for security and tabulation * Probabilistic modelling in a declarative language and modelling reactivity * Memory management and the implementation of declarative systems * Practical experiences and industrial application This year the conference will be co-located with the 23nd International Symposium on Logic-Based Program Synthesis and Transformation (LOPSTR 2013) and held in cooperation with ACM SIGPLAN. The conference will be held in Madrid, Spain. Previous symposia were held at Leuven (Belgium), Odense (Denmark), Hagenberg (Austria), Coimbra (Portugal), Valencia (Spain), Wroclaw (Poland), Venice (Italy), Lisboa (Portugal), Verona (Italy), Uppsala (Sweden), Pittsburgh (USA), Florence (Italy), Montreal (Canada), and Paris (France). Papers must describe original work, be written and presented in English, and must not substantially overlap with papers that have been published or that are simultaneously submitted to a journal, conference, or workshop with refereed proceedings. Work that already appeared in unpublished or informally published workshop proceedings may be submitted (please contact the PC chair in case of questions). Proceedings will be published by ACM Press* After the symposium, a selection of the best papers will be invited to extend their submissions in the light of the feedback solicited at the symposium. The papers are expected to include at least 30% extra material over and above the PPDP version. Then, after another round of reviewing, these revised papers will be published in a special issue of SCP with a target publication date by Elsevier of 2014. Important Dates Abstract Submission: May 27, 2013 Paper submission: May 30, 2013 Notification: July 4, 2013 Camera-ready: July 21, 2013 Symposium: September 16-18, 2013 Invites for SCP: October 2, 2013 Submission of SCP: December 11, 2013 Notification from SCP: February 22, 2014 Camera-ready for SCP: March 14, 2014 Authors should submit an electronic copy of the paper (written in English) in PDF. Each submission must include on its first page the paper title; authors and their affiliations; abstract; and three to four keywords. The keywords will be used to assist us in selecting appropriate reviewers for the paper. Papers should consist of no more than 12 pages, formatted following the ACM SIG proceedings template (option 1). The 12 page limit must include references but excludes well-marked appendices not intended for publication. Referees are not required to read the appendices, and thus papers should be intelligible without them. Program Committee Sergio Antoy Portland State University, USA Manuel Carro IMDEA Software Institute, Spain Iliano Cervesato Carnegie Mellon University, Qatar Agostino Dovier Universita degli Studi di Udine, Italy Maria Garcia de la Banda Monash University, Australia Ralf Hinze University of Oxford, UK Yukiyoshi Kameyama University of Tsukuba, Japan Oleg Kiselyov USA Yanhong Annie Liu State University of New York at Stony Brook, USA Stefan Monnier Universite de Montreal, Canada Alan Mycroft University of Cambrige, UK Bruno C. d. S. Oliveira National University of Singapore, Singapore Alberto Pettorossi Universita di Roma Tor Vergata, Italy Enrico Pontelli New Mexico State University, USA Kristoffer Rose IBM Research, USA Sukyoung Ryu KAIST, South Korea Vitor Santos Costa University of Porto, Portugal Torsten Schaub University Potsdam, Germany Tom Schrijvers Ghent University, Belgium Martin Sulzmann Hochschule Karlsruhe, Germany Wouter Swierstra Universiteit Utrecht, The Netherlands Tarmo Uustalu Institute of Cybernetics, Estonia Janis Voigtlaender University of Bonn, Germany Meng Wang Chalmers University of Technology, Sweden Jan Wielemaker Universiteit van Amsterdam, The Netherlands Program Chair Tom Schrijvers Department of Applied Mathematics and Computer Science Ghent University 9000 Gent, Belgium General Chair Ricardo Pena Facultad de Informatica Universidad Complutense de Madrid 28040 Madrid, Spain * Confirmation pending

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Lazy object deserialization
by Scott Lawrence 15 Mar '13

15 Mar '13

Hey all, All the object serialization/deserialization libraries I could find (pretty much just binary and cereal) seem to be strict with respect to the actual data being serialized. In particular, if I've serialized a large [Int] to a file, and I want to get the first element, it seems I have no choice but to deserialize the entire data structure. This is obviously an issue for large data sets. There are obvious workarounds (explicitly fetch elements from the "database" instead of relying on unsafeInterleaveIO to deal with it all magically), but it seems like it should be possible to build a cereal-like library that allows proper lazy deserialization. Does it exist, and I've just missed it? Thanks, -- Scott Lawrence Linux baidar 3.7.10-1-ARCH #1 SMP PREEMPT Thu Feb 28 09:50:17 CET 2013 x86_64 GNU/Linux

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Where's the case? or The difference between simpl and prep
by Tom Ellis 15 Mar '13

15 Mar '13

The -ddump-simpl output below doesn't contain a case corresponding to the seq in sum', but the -ddump-prep does. Isn't the output from simpl the input to prep? If so, where does the case reappear from? If not, how are simpl and prep related? It seems to have something to do with "Str=DmdType SS" but I don't understand. This seems to come from the IdInfo on the Id which is the binder "Test.sum'" but [1] says that this information is optional so it seems strange that such crucial information would be encoded there. Thanks, Tom [1] http://www.haskell.org/ghc/docs/7.6.2/html/libraries/ghc-7.6.2/IdInfo.html#… % cat Test.hs module Test where sum' :: [Integer] -> Integer -> Integer sum' [] n = n sum' (x:xs) n = n `seq` sum' xs (n + x) % ghc -fforce-recomp -ddump-simpl -O2 Test.hs [1 of 1] Compiling Test ( Test.hs, Test.o ) ==================== Tidy Core ==================== Result size = 14 Rec { Test.sum' [Occ=LoopBreaker] :: [GHC.Integer.Type.Integer] -> GHC.Integer.Type.Integer -> GHC.Integer.Type.Integer [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType SS] Test.sum' = \ (ds_daw :: [GHC.Integer.Type.Integer]) (n_a9J :: GHC.Integer.Type.Integer) -> case ds_daw of _ { [] -> n_a9J; : x_a9K xs_a9L -> Test.sum' xs_a9L (GHC.Integer.Type.plusInteger n_a9J x_a9K) } end Rec } % ghc -fforce-recomp -ddump-prep -O2 Test.hs [1 of 1] Compiling Test ( Test.hs, Test.o ) ==================== CorePrep ==================== Result size = 17 Rec { Test.sum' [Occ=LoopBreaker] :: [GHC.Integer.Type.Integer] -> GHC.Integer.Type.Integer -> GHC.Integer.Type.Integer [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType SS, Unf=OtherCon []] Test.sum' = \ (ds_saQ :: [GHC.Integer.Type.Integer]) (n_saS :: GHC.Integer.Type.Integer) -> case ds_saQ of _ { [] -> n_saS; : x_saW xs_saV -> case GHC.Integer.Type.plusInteger n_saS x_saW of sat_saZ { __DEFAULT -> Test.sum' xs_saV sat_saZ } } end Rec }

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Question about updating GHC on MacOS
by Graham Klyne 14 Mar '13

14 Mar '13

Hi, I have Haskell Platform with GHC[i] 7.4.2 installed on a MacOS system. There's a problem with the handling of certain Markdown constructs in literate Haskell (lines starting with '#') that I understand is fixed in 7.6.2. Therefore, I'd like to be able to update my GHC installation to 7.6.2. But I haven't yet been able to find any instructions about how to upgrade an existing GHC installation. Am I missing something? #g --

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Haskell Weekly News: Issue 262
by Daniel Santa Cruz 14 Mar '13

14 Mar '13

Welcome to issue 262 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of March 03 to 09, 2013. Quotes of the Week * byorgey: The box does not care whether you think it is stupid. It is box. * ksf: duh, we can't run out of energy, as there's a fixed amount of it. * shachaf: getLine :: IO String contains a String in the same way that /bin/ls contains a list of files * macrover: is there a comparison between the various streams/pipes/conduits libraries? Cale: macrover: I am presently resisting the temptation to make a table with the libraries and only one other column which is "maximum number of type parameters to a type constructor". * tac: It's actually really easy to remember all the numeric typeclasses in Haskell. Just think about what makes sense... then make sure whatever your answer is is not that Top Reddit Stories * I can't shake the impression that FRP code is just awful to read. Thoughts? Domain: self.haskell, Score: 63, Comments: 87 On Reddit: [1] http://goo.gl/xKrMm Original: [2] http://goo.gl/xKrMm * Announcing: first release of io-streams Domain: snapframework.com, Score: 58, Comments: 125 On Reddit: [3] http://goo.gl/w53bU Original: [4] http://goo.gl/B1ren * School of Haskell Goes Public -- learning and teaching Haskell Domain: haskell.fpcomplete.com, Score: 50, Comments: 26 On Reddit: [5] http://goo.gl/YPaqA Original: [6] http://goo.gl/4izHq * Announcing http-streams, a new HTTP client library for Haskell using the Snap Framework's io-streams. Domain: blogs.operationaldynamics.com, Score: 43, Comments: 20 On Reddit: [7] http://goo.gl/QKVez Original: [8] http://goo.gl/qygmB * How OCaml type checker works -- or what polymorphism and garbage collection have in common Domain: okmij.org, Score: 43, Comments: 2 On Reddit: [9] http://goo.gl/0CtFH Original: [10] http://goo.gl/7wmWm * Yann Esposito's "Haskell Fast & Hard": A fast-paced tutorial for Haskell beginners Domain: fpcomplete.com, Score: 42, Comments: 16 On Reddit: [11] http://goo.gl/utdiI Original: [12] http://goo.gl/ovldG * hArduino: Control your Arduino board from Haskell Domain: self.haskell, Score: 41, Comments: 3 On Reddit: [13] http://goo.gl/27Ilp Original: [14] http://goo.gl/27Ilp * binary 0.7 Domain: lennartkolmodin.blogspot.ru, Score: 35, Comments: 14 On Reddit: [15] http://goo.gl/VcusD Original: [16] http://goo.gl/LEwBA * IRC Browse: #haskell — first release Domain: ircbrowse.net, Score: 35, Comments: 33 On Reddit: [17] http://goo.gl/3oDKs Original: [18] http://goo.gl/MXygv * Responsive design with Clay Domain: fvisser.nl, Score: 28, Comments: 5 On Reddit: [19] http://goo.gl/0ZLuU Original: [20] http://goo.gl/lQ58q * The 'build' function explained Domain: self.haskell, Score: 28, Comments: 9 On Reddit: [21] http://goo.gl/vlPaF Original: [22] http://goo.gl/vlPaF * How (the syntactic type equality of) GADTs inhibit abstraction Domain: lukepalmer.wordpress.com, Score: 26, Comments: 26 On Reddit: [23] http://goo.gl/fD8j9 Original: [24] http://goo.gl/PY1bj * Caledon 2.0: Consistency & IO. Now based on the calculus of constructions. Domain: github.com, Score: 21, Comments: 7 On Reddit: [25] http://goo.gl/W2qP6 Original: [26] http://goo.gl/rnwSY Top StackOverflow Questions * Are thread pools needed for pure Haskell code? votes: 27, answers: 1 Read on SO: [27] http://goo.gl/Pmrhi * Independent subset of cabal packages set votes: 21, answers: 2 Read on SO: [28] http://goo.gl/qBuws * Should I use a lexer when using a parser combinator library like Parsec? votes: 19, answers: 1 Read on SO: [29] http://goo.gl/6Dfd8 * Current state of Haskell soft real-time votes: 9, answers: 2 Read on SO: [30] http://goo.gl/ueRhX * Difference for ncurses between interpreted and compiled Haskell? votes: 9, answers: 1 Read on SO: [31] http://goo.gl/QnKAQ * More fun with applicative functors votes: 8, answers: 5 Read on SO: [32] http://goo.gl/pykvJ * What's the real benefit of conduit's upstream type parameter? votes: 8, answers: 2 Read on SO: [33] http://goo.gl/RdIUU * What's the benefit of conduit's leftovers? votes: 8, answers: 2 Read on SO: [34] http://goo.gl/oqnty Until next time, +Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/19v4l1/i_cant_shake_the_impression… 2. http://www.reddit.com/r/haskell/comments/19v4l1/i_cant_shake_the_impression… 3. http://snapframework.com/blog/2013/03/05/announcing-io-streams 4. http://www.reddit.com/r/haskell/comments/19qfr6/announcing_first_release_of… 5. https://haskell.fpcomplete.com/blog/2013/03/SoH-Goes-Public 6. http://www.reddit.com/r/haskell/comments/19ntvz/school_of_haskell_goes_publ… 7. http://blogs.operationaldynamics.com/andrew/software/haskell/http-streams-i… 8. http://www.reddit.com/r/haskell/comments/19rlw5/announcing_httpstreams_a_ne… 9. http://okmij.org/ftp/ML/generalization.html 10. http://www.reddit.com/r/haskell/comments/19wogp/how_ocaml_type_checker_work… 11. https://www.fpcomplete.com/user/yogsototh/haskell-fast-hard 12. http://www.reddit.com/r/haskell/comments/19xrhm/yann_espositos_haskell_fast… 13. http://www.reddit.com/r/haskell/comments/19uwf5/harduino_control_your_ardui… 14. http://www.reddit.com/r/haskell/comments/19uwf5/harduino_control_your_ardui… 15. http://lennartkolmodin.blogspot.ru/2013/03/binary-07.html 16. http://www.reddit.com/r/haskell/comments/19kkuc/binary_07/ 17. http://ircbrowse.net/browse/freenode/haskell 18. http://www.reddit.com/r/haskell/comments/19xxuu/irc_browse_haskell_first_re… 19. http://fvisser.nl/post/2013/march/4/responsive-design-with-clay.html 20. http://www.reddit.com/r/haskell/comments/19lpzv/responsive_design_with_clay/ 21. http://www.reddit.com/r/haskell/comments/19p6w3/the_build_function_explaine… 22. http://www.reddit.com/r/haskell/comments/19p6w3/the_build_function_explaine… 23. http://lukepalmer.wordpress.com/2013/03/04/how-gadts-inhibit-abstraction/ 24. http://www.reddit.com/r/haskell/comments/19mwxr/how_the_syntactic_type_equa… 25. https://github.com/mmirman/caledon 26. http://www.reddit.com/r/haskell/comments/19sq5x/caledon_20_consistency_io_n… 27. http://stackoverflow.com/questions/15191649/are-thread-pools-needed-for-pur… 28. http://stackoverflow.com/questions/15186768/independent-subset-of-cabal-pac… 29. http://stackoverflow.com/questions/15216202/should-i-use-a-lexer-when-using… 30. http://stackoverflow.com/questions/15236238/current-state-of-haskell-soft-r… 31. http://stackoverflow.com/questions/15304317/difference-for-ncurses-between-… 32. http://stackoverflow.com/questions/15209516/more-fun-with-applicative-funct… 33. http://stackoverflow.com/questions/15258406/whats-the-real-benefit-of-condu… 34. http://stackoverflow.com/questions/15258587/whats-the-benefit-of-conduits-l… 35. https://plus.google.com/105107667630152149014/about

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Are there any reasons against using ~ when matching one-constructor data types?
by Petr Pudlák 13 Mar '13

13 Mar '13

Dear Haskellers, b) A related suggestion would be the addition of an > irrefutable swap, (swap'?), defined as > "swap ~(a,b) = (b,a)", and its addition to Prelude for > the same reasons. > If I define a function that matches on a single-constructor data type, such as (,), is there any reason against using ~? Like f (a,b) = ... instead of f ~(a,b) = ... ? I've seen that not using ~ can lead to problems sometimes, but not the other way around. (Of course changing the semantics of existing functions such as `swap` is problematic, my question targets the problem in general.) Best regards, Petr Pudlak

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WORLDCOMP Strikes Again for the Last Time
by chrisrogersrogers＠aol.com 13 Mar '13

13 Mar '13

I graduated from University of Florida (UFL) and am currently running a computer firm in Florida. I have attended WORLDCOMP http://www.world-academy-of-science.org in 2010. Except for few keynote speeches and presentations, the conference was very disappointing due to a large number of poor quality papers and cancellation of some sessions. I was instantly suspicious of this conference. Me and my UFL and UGA friends started a study on WORLDCOMP. We submitted a paper to WORLDCOMP 2011 and again (the same paper with a modified title) to WORLDCOMP 2012. This paper had numerous fundamental mistakes. Sample statements from that paper include: (1). Binary logic is fuzzy logic and vice versa (2). Pascal developed fuzzy logic (3). Object oriented languages do not exhibit any polymorphism or inheritance (4). TCP and IP are synonyms and are part of OSI model (5). Distributed systems deal with only one computer (6). Laptop is an example for a super computer (7). Operating system is an example for computer hardware Also, our paper did not express any conceptual meaning. However, it was accepted both the times without any modifications (and without any reviews) and we were invited to submit the final paper and a payment of $500+ fee to present the paper. We decided to use the fee for better purposes than making Prof. Hamid Arabnia (Chairman of WORLDCOMP) rich. After that, we received few reminders from WORLDCOMP to pay the fee but we never responded. We MUST say that you should look at the website http://sites.google.com/site/worlddump1 if you have any thoughts to submit a paper to WORLDCOMP. DBLP and other indexing agencies have stopped indexing WORLDCOMP’s proceedings since 2011 due to its fakeness. The status of your WORLDCOMP papers can be changed from “scientific” to “other” (i.e., junk or non-technical) at anytime. See the comments http://www.mail-archive.com/tccc@lists.cs.columbia.edu/msg05168.html of a respected researcher on this. Better not to have a paper than having it in WORLDCOMP and spoil the resume and peace of mind forever! Our study revealed that WORLDCOMP is a money making business, using UGA mask, for Prof. Hamid Arabnia. He is throwing out a small chunk of that money (around 20 dollars per paper published in WORLDCOMP’s proceedings) to his puppet who publicizes WORLDCOMP and also defends it at various forums, using fake/anonymous names. The puppet uses fake names and defames other conferences/people to divert traffic to WORLDCOMP. That is, the puppet does all his best to get a maximum number of papers published at WORLDCOMP to get more money into his (and Prof. Hamid Arabnia’s) pockets. Monte Carlo Resort (the venue of WORLDCOMP until 2012) has refused to provide the venue for WORLDCOMP’13 because of the fears of their image being tarnished due to WORLDCOMP’s fraudulent activities. WORLDCOMP will not be held after 2013. The paper submission deadline for WORLDCOMP’13 is March 18, 2013 (it will be extended many times, as usual) but still there are no committee members, no reviewers, and there is no conference Chairman. The only contact details available on WORLDCOMP’s website is just an email address! What bothers us the most is that Prof. Hamid Arabnia never posted an apology for the damage he has done to the research community. He is still trying to defend WORLDCOMP. Let us make a direct request to him: publish all reviews for all the papers (after blocking identifiable details) since 2000 conference. Reveal the names and affiliations of all the reviewers (for each year) and how many papers each reviewer had reviewed on average. We also request him to look at the Open Challenge at http://sites.google.com/site/dumpconf We think that it is our professional obligation to spread this message to alert the computer science community. Sorry for posting to multiple lists. Spreading the word is the only way to stop this bogus conference. Please forward this message to other mailing lists and people. We are shocked with Prof. Hamid Arabnia and his puppet’s activities http://worldcomp-fake-bogus.blogspot.com Search Google using the keywords “worldcomp, fake” for additional links. Sincerely, Chris

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ANN: retry 0.1 - retry combinators for monadic actions that often fail
by Ozgun Ataman 13 Mar '13

13 Mar '13

Dear Cafe, I'm happy to announce the availability of the retry package on Hackage[1] and Github[2]. The package provides a few useful combinators for monadic actions that often fail and should be retried in cases of a certain set of exceptions (or failure modes). Such cases are quite common when querying databases (e.g. ResponseTimeout) or uploading files (e.g. socket closed), particularly in busy production systems. The library exposes 'retrying' for failures encoded explicitly in the type and 'recovering' for failures through exceptions, respectively. You can choose either an "exponential back off" series of delays or a simple static delay between retries. In either case, there is a ceiling for the number of total retries. The haddocks provide further documentation and a few examples. This library simply wraps around the given actions and does not really do anything fancy or unexpected. Nevertheless, the widespread applicability of this functionality justified a generic package to address the need centrally. Suggestions and bug reports are most welcome. [1] http://hackage.haskell.org/package/retry [2] https://github.com/Soostone/retry Cheers, Ozgun

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monadic DSL for compile-time parser generator, not possible?
by Jeremy Shaw 13 Mar '13

13 Mar '13

It would be pretty damn cool if you could create a data type for generically describing a monadic parser, and then use template haskell to generate a concrete parser from that data type. That would allow you to create your specification in a generic way and then target different parsers like parsec, attoparsec, etc. There is some code coming up in a few paragraphs that should describe this idea more clearly. I would like to suggest that while it would be cool, it is impossible. As proof, I will attempt to create a simple monadic parser that has only one combinator: anyChar :: ParserSpec Char We need the GADTs extension and some imports: > {-# LANGUAGE GADTs, TemplateHaskell #-} > import Control.Monad (join) > import qualified Text.Parsec.Char as P > import Language.Haskell.TH (ExpQ, appE) > import Language.Haskell.TH.Syntax (Lift(lift)) > import Text.Parsec (parseTest) > import qualified Text.Parsec.Char as P > import Text.Parsec.String (Parser) Next we define a type that has a constructor for each of the different combinators we want to support, plus constructors for the functor and monad methods: > data ParserSpec a where > AnyChar :: ParserSpec Char > Return :: a -> ParserSpec a > Join :: ParserSpec (ParserSpec a) -> ParserSpec a > FMap :: (a -> b) -> ParserSpec a -> ParserSpec b > > instance Lift (ParserSpec a) where > lift _ = error "not defined because we are screwed later anyway." In theory, we would extend that type with things like `Many`, `Some`, `Choice`, etc. In Haskell, we are used to seeing a `Monad` defined in terms of `return` and `>>=`. But, we can also define a monad in terms of `fmap`, `return` and `join`. We will do that in `ParserSpec`, because it makes the fatal flaw more obvious. Now we can define the `Functor` and `Monad` instances: > instance Functor ParserSpec where > fmap f p = FMap f p > instance Monad ParserSpec where > return a = Return a > m >>= f = Join ((FMap f) m) and the `anyChar` combinator: > anyChar :: ParserSpec Char > anyChar = AnyChar And now we can define a simple parser that parses two characters and returns them: > charPair :: ParserSpec (Char, Char) > charPair = > do a <- anyChar > b <- anyChar > return (a, b) Now, we just need to define a template haskell function that generates a `Parser` from a `ParserSpec`: > genParsec :: (Lift a) => ParserSpec a -> ExpQ > genParsec AnyChar = [| anyChar |] > genParsec (Return a) = [| return a |] > genParsec (Join p) = genParsec p > -- genParsec (FMap f p) = appE [| f |] (genParsec p) -- uh-oh Looking at the `FMap` case we see the fatal flaw. In order to generate the parser we would need some way to transform any arbitrary Haskell function of type `a -> b` into Template Haskell. Obviously, that is impossible (for some definition of obvious). Therefore, we can assume that it is not possible to use Template Haskell to generate a monadic parser from a monadic specification. We can also assume that `Applicative` is not available either. Seems likely that `Category` based parsers would also be out. Now, we can, of course, do the transformation at runtime: > interpretParsec :: ParserSpec a -> Parser a > interpretParsec AnyChar = P.anyChar > interpretParsec (Return a) = return a > interpretParsec (FMap f a) = fmap f (interpretParsec a) > interpretParsec (Join mm) = join (fmap interpretParsec (interpretParsec mm)) > test = parseTest (interpretParsec charPair) "ab" My fear is that doing that will result in added runtime overhead. One reason for wanting to create a compile-time parser generator is to have the opportunity to generate very fast parsing code. It seems like here we can only be slower than the parser we are targeting? Though.. perhaps not? Perhaps the parser returned by `interpretParsec` has all the interpret stuff removed and is as fast as if we have constructed it by hand? I don't have an intuitive feel for it.. I guess criterion would know.. Any thoughts? - jeremy

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Re: [Haskell-cafe] monadic DSL for compile-time parser generator, not possible?
by oleg＠okmij.org 13 Mar '13

13 Mar '13

Jeremy Shaw wrote: > It would be pretty damn cool if you could create a data type for > generically describing a monadic parser, and then use template haskell > to generate a concrete parser from that data type. That would allow > you to create your specification in a generic way and then target > different parsers like parsec, attoparsec, etc. There is some code > coming up in a few paragraphs that should describe this idea more > clearly. After rather mild and practical restrictions, the problem is solvable. (BTW, even the problem of lifting arbitrary functional values, let alone handles and other stuff, is solvable in realistic settings -- or even completely, although less practically.) Rather than starting from the top -- implementing monadic or applicative parser, let's start from the bottom and figure out what we really need. It seems that many real-life parsers aren't using the full power of Applicative, let alone monad. So why to pay, a whole lot, for what we don't use. Any parser combinator library has to be able to combine parsers. It seems the applicative rule <*> :: Parser (a->b) -> Parser a -> Parser b is very popular. It is indeed very useful -- although not the only thing possible. One can come up with a set of combinators that are used for realistic parsing. For example, *> :: Parser a -> Parser b -> Parser b for sequential composition, although expressible via <*>, could be defined as primitive. Many other such combinators can be defined as primitives. In other words: the great advantage of Applicative parser combinators is letting the user supply semantic actions, and executing those actions as parsing progresses. There is also a traditional approach: the parser produces an AST or a stream of parsing events, which the user consumes and semantically processes any way they wish. Think of XML parsing: often people parse XML and get a DOM tree, and process it afterwards. An XML parser can be incremental: SAX. Parsers that produce AST need only a small fixed set of combinators. We never need to lift arbitrary functions since those parsers don't accept arbitrary semantic actions from the user. For that reason, these parsers are also much easy to analyze. Let's take the high road however, applicative parsers. The <*> rule is not problematic: it neatly maps to code. Consider newtype Code a = Code Exp which is the type-annotated TH Code. We can easily define app_code :: Code (a->b) -> Code a -> Code b app_code (Code f) (Code x) = Code $ AppE f x So, Code is almost applicative. Almost -- because we only have a restricted pure: pureR :: Lift a => a -> Code a with a Lift constraint. Alas, this is not sufficient for realistic parsers, because often we have to lift functions, as in the example of parsing a pair of characters: pure (\x y -> (x,y)) <*> anyChar <*> anyChar But aren't functions really unliftable? They are unliftable by value, but we can also lift by reference. Here is an example, using tagless final framework, since it is extensible. We define the basic minor Applicative > class Sym repr where > pureR :: Lift a => a -> repr a > app :: repr (a->b) -> repr a -> repr b > > infixl 4 `app` And a primitive parser, with only one primitive parser. > class Sym repr => Parser repr where > anychar :: repr Char For our example, parsing two characters and returning them as a pair, we need pairs. So, we extend our parser with three higher-order _constants_. > class Sym repr => Pair repr where > pair :: repr (a -> b -> (a,b)) > prj1 :: repr ((a,b) -> a) > prj2 :: repr ((a,b) -> b) And here is the example. > test1 = pair `app` anychar `app` anychar One interpretation of Sym is to generate code (another one could analyze the parsers) > data C a = C{unC :: Q Exp} Most interesting is the instance of pairs. Actually, it is not that interesting: we just lift functions by reference. > pair0 x y = (x,y) > > instance Pair C where > pair = C [e| pure pair0 |] > prj1 = C [e| pure fst |] > prj2 = C [e| pure snd |] Because tagless-final is so extensible, any time we need a new functional constant, we can easily introduce it and define its code, either by building a code expression or by referring to a global name that is bound to the desired value. The latter is `lift by reference' (which is what dynamic linking does). The obvious limitation of this approach is that all functions to lift must be named -- because we lift by reference. We can also build anonymous functions, if we just add lambda to our language. If we go this way we obtain something like http://okmij.org/ftp/meta-programming/index.html#meta-haskell (which has lam, let, arrays, loops, etc.) Sample code, for reference {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE NoMonomorphismRestriction #-} module P where import Language.Haskell.TH import Language.Haskell.TH.Syntax import Language.Haskell.TH.Ppr import Control.Applicative import Text.ParserCombinators.ReadP class Sym repr where pureR :: Lift a => a -> repr a app :: repr (a->b) -> repr a -> repr b infixl 4 `app` class Sym repr => Parser repr where anychar :: repr Char -- Higher-order constants class Sym repr => Pair repr where pair :: repr (a -> b -> (a,b)) prj1 :: repr ((a,b) -> a) prj2 :: repr ((a,b) -> b) -- parse two characters and return them as a pair test1 = pair `app` anychar `app` anychar -- Implementations -- we don't need Q monad actually, neither here -- nor anywhere! -- It's a bummer that lift has the signature t -> Q Exp -- rather than t -> Exp! data C a = C{unC :: Q Exp} instance Sym C where pureR = C . lift app f x = C $ appE (appE (varE '(Control.Applicative.<*>)) (unC f)) (unC x) instance Parser C where anychar = C . varE $ 'get pair0 x y = (x,y) instance Pair C where pair = C [e| pure pair0 |] prj1 = C [e| pure fst |] prj2 = C [e| pure snd |] printC :: C a -> IO String printC m = runQ (fmap pprint $ unC m ) test1C = printC test1 {- "(Control.Applicative.<*>) ((Control.Applicative.<*>) (Control.Applicative.pure P.pair0) Text.ParserCombinators.ReadP.get) Text.ParserCombinators.ReadP.get" -}

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Open-source projects for beginning Haskell students?
by Michel Kuhlmann 13 Mar '13

13 Mar '13

Hi Brent, I am myself a haskell-beginner. Too, trying to come to terms with Applicative and Monad. Nevertheless, I forked [1] @tomahawkin's `devsurf` library, with the aim of extending it to triangular-mesh and point-cloud manipulation library. There are lot's of things I still miss. For example: * Point-cloud/mesh simplification * Slicing of meshes/point-clouds (taking cross-sections) * Interpolation from point-cloud to mesh * Merging different meshes * Difference between two meshes, resulting in a diff-mesh * ... Related to that I created a repository for associated executables [3]. The main-program there is `mshConv`, which should convert meshes into and from different file-formats, analog to fabulous `pandoc`. There are several things to be done: * Proper command-line interface * Using Parsec in Readers * More Readers * More Writers Currently mesh-conversion could be done in `meshlab` [4]. On the homepage is also a list with potential Readers and Writers. I am willing to exchange email, but I do not have the competence of mentoring or guidance. Best regards, Michel [1] https://www.github.com/tomahawkins/devsurf [2] https://www.github.com/michelk/devsurf [3] https://www.github.com/michelk/devsurf-bin [4] http://www.meshlab.org

1 0

cabal install oddities
by Tycho Andersen 13 Mar '13

13 Mar '13

Hi all, I'm having some strange issues with cabal install. Some packages installed via `cabal install $foo` are failing for strange (and seemingly unrelated) reasons, but install just fine when I do something like: cabal unpack network cd network cabal configure cabal install Below is some sample output from a failing package: ps168825:~/playground$ cabal install network Resolving dependencies... Configuring network-2.4.1.2... configure: WARNING: unrecognized options: --with-compiler, --with-gcc checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking for gcc... gcc checking whether the C compiler works... yes checking for C compiler default output file name... a.out checking for suffix of executables... checking whether we are cross compiling... configure: error: in `/tmp/network-2.4.1.2-28534/network-2.4.1.2': configure: error: cannot run C compiled programs. If you meant to cross compile, use `--host'. See `config.log' for more details Failed to install network-2.4.1.2 cabal: Error: some packages failed to install: network-2.4.1.2 failed during the configure step. The exception was: ExitFailure 1 ps168825:~/playground 1$ cabal --version cabal-install version 1.16.0.2 using version 1.16.0 of the Cabal library ps168825:~/playground$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.2 /tmp/network-* doesn't exist (which is why I tried unpack, but unfortunately that succeeds). Any thoughts on how I can debug this? Thanks, \t

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Overloading
by Peter Caspers 12 Mar '13

12 Mar '13

Hi, I just started playing around a bit with Haskell, so sorry in advance for very basic (and maybe stupid) questions. Coming from the C++ world one thing I would like to do is overloading operators. For example I want to write (Date 6 6 1973) + (Period 2 Months) for some self defined types Date and Period. Another example would be (Period 1 Years) + (Period 3 Months). Just defining the operator (+) does not work because it collides with Prelude.+. I assume using fully qualified names would work, but that is not what I want. So maybe make the types instances of typeclasses? This would be Num for (+) I guess. For the first example above it will not work however, alone for it is not of type a -> a -> a. Also the second example does not fit, because I would have to make Period an instance of Num, which does not make sense, because I can not multiply Periods (for example). Am I missing something or is that what I am trying here just impossible by the language design (and then probably for a good reason) ? A second question concerns the constructors in own datatypes like Date above. Is it possible to restrict the construction of objects to sensible inputs, i.e. reject something like Date 50 23 2013 ? My workaround would be to provide a function say date :: Int->Int->Int->Date checking the input and returning a Date object or throw an error if the input does not correspond to a real date. I could then hide the Date constructor itself (by not exporting it). However this seems not really elegant. Also again, taking this way I can not provide several constructors taking inputs of different types, can I ? Thanks a lot Peter

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newtype a Constraint?
by Roman Cheplyaka 12 Mar '13

12 Mar '13

Is there a way to newtype a constraint? Imagine a type class parameterised over constraints. What do I do if I want multiple instances for (essentially) the same constraint? Roman

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ANNOUNCE xml-conduit-generic
by Dmitry Olshansky 12 Mar '13

12 Mar '13

Hello, cafe! I made a package xml-conduit-generic<https://github.com/odr/xml-conduit-generic> to provide conversion from ADT to xml and vice versa. Conversion works as Conduit (ToXml) or Consumer (FromXml). Example: data T4 = T4 {v4 :: Int, n4 :: Maybe T4} deriving (Eq, Show, Generic) instance ToXml T4 instance FromXml T4 > runToXml $ T4 5 $ Just $ T4 6 Nothing "<T4 v4=\"5\"><n4 v4=\"6\"/></T4>" > runFromXml $ "<T4 v4=\"5\"><n4 v4=\"6\"/></T4>" :: IO (Either String T4) Right $ T4 5 $ Just $ T4 6 Nothing It would be great if someone look to the version on github and give me some notes. Then I am going to put it on hackage. Unfortunately, I didn't find a way to avoid OverlappingInstances (for GFromXml class). Any ideas are welcome. Best regards, Dmitry

1 0

QuickCheck: Why are these instances of Testable?
by Paul Brenman 11 Mar '13

11 Mar '13

Why does QuickCheck (2.6) list the following as instances of class Testable? Testable Result Testable Prop Testable prop => Testable (Gen prop) The Hughes/Claessen paper "QuickCheck: A Lightweight Tool for Random Testing of Haskell Programs" mentions nesting property combinators on page 6, but I didn't understand this reference.

1 0

File I/O benchmark help (conduit, io-streams and Handle)
by Michael Snoyman 09 Mar '13

09 Mar '13

Hi all, I'm turning to the community for some help understanding some benchmark results[1]. I was curious to see how the new io-streams would work with conduit, as it looks like a far saner low-level approach than Handles. In fact, the API is so simple that the entire wrapper is just a few lines of code[2]. I then added in some basic file copy benchmarks, comparing conduit+Handle (with ResourceT or bracket), conduit+io-streams, straight io-streams, and lazy I/O. All approaches fell into the same ballpark, with conduit+bracket and conduit+io-streams taking a slight lead. (I haven't analyzed that enough to know if it means anything, however.) Then I decided to pull up the NoHandle code I wrote a while ago for conduit. This code was written initially for Windows only, to work around the fact that System.IO.openFile does some file locking. To avoid using Handles, I wrote a simple FFI wrapper exposing open, read, and close system calls, ported it to POSIX, and hid it behind a Cabal flag. Out of curiosity, I decided to expose it and include it in the benchmark. The results are extreme. I've confirmed multiple times that the copy algorithm is in fact copying the file, so I don't think the test itself is cheating somehow. But I don't know how to explain the massive gap. I've run this on two different systems. The results you see linked are from my local machine. On an EC2 instance, the gap was a bit smaller, but the NoHandle code was still 75% faster than the others. My initial guess is that I'm not properly tying into the IO manager, but I wanted to see if the community had any thoughts. The relevant pieces of code are [3][4][5]. Michael [1] http://static.snoyman.com/streams.html [2] https://github.com/snoyberg/conduit/blob/streams/io-streams-conduit/Data/Co… [3] https://github.com/snoyberg/conduit/blob/streams/conduit/System/PosixFile.h… [4] https://github.com/snoyberg/conduit/blob/streams/conduit/Data/Conduit/Binar… [5] https://github.com/snoyberg/conduit/blob/streams/conduit/Data/Conduit/Binar…

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Re: [Haskell-cafe] Shake, Shelly, & FilePath
by Greg Weber 09 Mar '13

09 Mar '13

Shelly is using system-filepath which was created as an improvement over using a simple String. For a build system in which you name all your files you may not care about the upside. If you want a version of Shelly that uses String you can try Shellish, its predecessor. Otherwise the shim should be written for Shake to use system-filepath. Greg Weber

1 0

ANNOUNCE: hF2-0.2
by Marcel Fourné 09 Mar '13

09 Mar '13

Hi, this is the second release of hF2, a F(2^e) backend for cryptographic code, to be found at http://hackage.haskell.org/package/hF2 (or simply by "cabal install hF2") This library is used in hecc for elliptic cryptography on binary field curves and came into existence during my master thesis. Since the code from back then some speedups and changes to data representation were made which lead to an increase in speed from the first correct protoype to this release by the factor 10^86 on my main development machine. Sadly, this is still slower than pure C or Assembler versions, but a lot more portable, (arguably) easier to read and easier to parallelize. The code does automatic bit slicing and uses mainly the vector library as a fast backend. Feats of this release: - Speed (256 bit curve point multiplication in hecc is now at about a second in time) - LINEAR speedup in threaded execution with the number of cores (up to the number of bits divided by the wordsize) - Manually tested - Prelude-like Interface (more in progress) - Mostly timing attack resistant (also in progress) Next up: Testing and fixing hecc for this release. Have a nice weekend, Marcel -- Marcel Fourné OpenPGP-Key-ID: 4991 8AA4 202F 12AC 41F7 6C77 CA83 BDF0 7454 5C72 I am a programmer. I want languages and libraries, not just huge tools.

1 0

ACCAT 2013: final call for participation and invitation to discussion
by Ulrike Golas 09 Mar '13

09 Mar '13

[Apologies if you receive more than one copy of the following announcement] FINAL CALL FOR PARTICIPATION AND INVITATION TO DISCUSSION ========== 8th International Workshop on Applied and Computational Category Theory ACCAT 2013 http://accat2013.zib.de/ Satellite Event of ETAPS 2013, Rome, March 17 2013 ========== Deadline for online registration: March 8 *Attendees are warmly invited to join the closing discussion. Please communicate your intention to deliver a position statement to the organizers.* ========== Since the 1960s, the use of category theory in computer science has been a fruitful one, ranging form automata theory to algebraic specification to programming languages. 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. Despite ACCAT success, we believe that the formula should be revised. Indeed, we have the feeling that a 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). This year, we would like to use the ACCAT forum to raise this issue and to discuss it within a larger audience. Thus, we invited 8 top researched in the area of application of category theory. The list of speakers are Samson Abramsky Robin B. Cockett Barbara Koenig Ugo Montanari Till Mossakowski Dusko Pavlovic Andrzej Tarlecki Glynn Winskel We do hope that the meeting will be fruitful, providing a good exchange of ideas and planting the seed for future events. Indeed, one of the outcome of the meeting is to decide whether to push an high-level workshop/conference on the application of category theory to computer science, or at least to verify the viability of a Daghstul meeting on the issue. Therefore, after the presentations, the workshop will end up with a general discussion among the attendees. -- 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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Simple way to do something like ArrowChoice.right on a Conduit? (version 1.0.0)
by Joey Adams 09 Mar '13

09 Mar '13

Can I transform a conduit so some values are passed through unchanged, but others go through the conduit? For example: right :: Conduit i m o -> Conduit (Either x i) m (Either x o) This is named after the Control.Arrow combinator of the same name: right :: ArrowChoice a => a b c -> a (Either d b) (Either d c) Here's my use case (simplified): I want to compress data with zlib-conduit, which provides: compress :: Conduit (Flush ByteString) m (Flush ByteString) The Flush<http://hackage.haskell.org/packages/archive/conduit/latest/doc/html/Data-Co…>wrapper lets me flush the compressor so it will yield cached data right away (though hurting compression a little). But before compressing the data, I want to encode it, using this conduit: encode :: Conduit Entry m ByteString I want to combine these, so that if I send a 'Flush', it bypasses 'encode' and feeds to 'compress': compressEncode :: Conduit (Flush Entry) m (Flush ByteString) Thus, I need a variant of 'encode' that passes 'Flush' along: encode' :: Conduit (Flush Entry) m (Flush ByteString) In my actual program, I don't use Flush, so providing a Conduit combinator just for Flush would not help me. Is something like 'right' possible to implement with Conduit's public API? Here's an implementation using Data.Conduit.Internal (untested): import Control.Monad (liftM) import Data.Conduit.Internal (Pipe(..), ConduitM(..), Conduit) right :: Monad m => Conduit i m o -> Conduit (Either x i) m (Either x o) right = ConduitM . rightPipe . unConduitM rightPipe :: Monad m => Pipe i i o () m () -> Pipe (Either x i) (Either x i) (Either x o) () m () rightPipe p0 = case p0 of HaveOutput p c o -> HaveOutput (rightPipe p) c (Right o) NeedInput p c -> NeedInput p' (rightPipe . c) where p' (Left x) = HaveOutput (rightPipe p0) (return ()) (Left x) p' (Right i) = rightPipe $ p i Done r -> Done r PipeM mp -> PipeM $ liftM rightPipe mp Leftover p i -> Leftover (rightPipe p) (Right i) I'm wondering if we could have a Data.Conduit.Arrow module, which provides a newtype variant of Conduit that implements Arrow, ArrowChoice, etc.: import qualified Data.Conduit as C newtype Conduit m i o = Conduit (C.Conduit i m o) -- May need Monad constraints for these instance Category (Conduit m) instance Arrow (Conduit m) instance ArrowChoice (Conduit m) Does 'Conduit' follow Category, Monad, MonadTrans laws* these days? I'm not talking about Pipe in general, just the special case of it represented by the 'Conduit' type alias: Conduit i m o = ConduitM i o m () = Pipe i i o () m () Or are there some thorny issues (e.g. leftovers) that make following these laws impossible in some cases? Thanks for the input, -Joey * Assume functions that use Data.Conduit.Internal do so correctly.

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Shake, Shelly, & FilePath
by Eli Frey 09 Mar '13

09 Mar '13

I began converting an unwieldy Makefile into a Haskell program via Shake, hoping that I could increase both its readability and modularity. The modularity has increased greatly, and I have found it exhilarating how much more I can express about my dependencies. However, readability has suffered. I quickly found that heavy shell interaction became unwieldy and I came out with code that was much more difficult to scan with my eyes then what I had in my Makefile. I attempted to fix this by using the Shelly library for my shell interactions, and was well pleased until I attempted to compile and discovered Shelly.FilePath is NOT Prelude.FilePath. Now I am left sprinkling coercions all over the place and am again shaking my head at how difficult to scan my code has become. I have been considering writing a shim over Shelly, but the prospect makes me uneasy. Has anyone else walked down this path before, and if so what did you bring away from the experience? I find this situation such a shame, as all my other experiences with both libraries have been quite wonderful. - Eli

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ANNOUNCE: grid-4.0 (tile maps for board games or maths)
by Amy de Buitléir 08 Mar '13

08 Mar '13

I'm happy to announce a new major release of the grid package: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) WHAT'S NEW: * The package now uses associated (type) synonyms instead of multi-parameter type classes with functional dependencies. * Added two new classes: FiniteGrid and BoundedGrid. Moved some functions from the Grid class to the other classes, as appropriate. For example, only finite grids need the "size" function, and only bounded grids need the "boundary" function. * Added Unbounded grids for square, hexagonal, and triangular tiles. * Updated the userguide to reflect the changes. ABOUT GRID: Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, and hexagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome.

1 0

Parsing email id
by mukesh tiwari 08 Mar '13

08 Mar '13

Hello Everyone I am trying to learn parsec. I wrote this code to solve the problem ( http://www.spoj.com/problems/MAIN12C/ ) but getting time limit exceed so how can I improve this code ? In case of indentation, I have pasted the code at ideone ( http://ideone.com/AmxvZq ). import Data.List import qualified Text.Parsec.ByteString as PB import Text.Parsec.Prim import Text.Parsec.Char import Text.Parsec.Combinator import qualified Data.ByteString.Char8 as BS import Control.Applicative hiding ( ( <|> ) , many ) validChars :: PB.Parser Char validChars = alphaNum <|> oneOf "._" dontCare :: PB.Parser Char dontCare = oneOf "~!@#$%^&*()<>?,." {-- emailAddress :: PB.Parser String emailAddress = do _ <- many dontCare fi <- alphaNum se <- validChars th <- validChars fo <- validChars ft <- validChars restAddr <- many validChars let addr = fi : se : th : fo : ft : restAddr char '@' dom <- many1 alphaNum rest <- try ( string ".com" <|> string ".org" <|> string ".edu" ) <|> try ( string ".co.in" ) _ <- many dontCare return $ addr ++ ( '@': dom ++ rest ) --} emailAddress :: PB.Parser String emailAddress = conCatfun <$> ( many dontCare *> alphaNum ) <*> validChars <*> validChars <*> validChars <*> validChars <*> many alphaNum <*> ( char '@' *> many1 alphaNum ) <*> ( try ( string ".com" <|> string ".org" <|> string ".edu" ) <|> try ( string ".co.in" ) <* many dontCare ) where conCatfun a b c d e f dom rest = ( a : b : c : d : e : f ) ++ ( '@' : dom ++ rest ) collectEmail :: BS.ByteString -> String collectEmail email = case parse emailAddress "" email of Right addr -> addr Left err -> "" process :: ( Int , [ String ] ) -> BS.ByteString process ( k , xs ) = ( BS.pack "Case " ) `BS.append` ( BS.pack . show $ k ) `BS.append` ( BS.pack ": " ) `BS.append` ( BS.pack . show $ k ) `BS.append` ( BS.pack "\n" ) `BS.append` ( BS.pack ( unlines . filter ( not . null ) $ xs ) ) main = BS.interact $ BS.concat . map process . zip [ 1.. ] . map ( map collectEmail . BS.words ) . tail . BS.lines Regards Mukesh Tiwari

1 0

Country names and language names
by Obscaenvs 08 Mar '13

08 Mar '13

Hi! I am working on an app in Yesod that craves I18N. A problem I've come across there is giving correct country and language names based on currently selected language. So far, I've used the Yesod I18N message approach, but a lot of hand coding is involved. The iso3166-country-codes [1] package at Hackage by Jon Fairbairn provides a start in the right direction, but an obvious improvement upon it would be to have a function or map that takes an ISO 639 code and an ISO 3166 code and gives the correct human-readable name for the country as per the chosen target language (the ISO 639 code), and another function/map for languages. It would alleviate coding those pesky country and language switchers a *lot*, among other things. Jon Fairbarn that coded the iso3166-country-codes package said in private correspondence that it seemed worthwhile doing, but he couldn't do it in his spare time, which is understandable. I am willing to do some of the stuff involved (I know Swedish, French and some Turkish in addition to the ubiquitous English), but obviously it's too big a project for one man to handle (what with all the c'n'p involved :) ). I feel that this should be done, since it seems it isn't yet. I am inexperienced in coordinating such endeavours, though, so I would like to share that task at least to begin with, if possible. Any thoughts? [1] http://hackage.haskell.org/package/iso3166-country-codes-0.20111111.4 /Fredrik

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[Announcement] New release of hArduino: Control your Arduino board from Haskell
by Levent Erkok 07 Mar '13

07 Mar '13

A new release (v0.5) of hArduno is out, now supporting servo-motors, shift-registers, seven-segment displays, and distance sensors. Comes with a bunch of examples to play around with: http://leventerkok.github.com/hArduino/ On Hackage: http://hackage.haskell.org/package/hArduino -Levent.

2 1

GHC 7.6.2 introduces <<loop>> in UUAGC
by Jeroen Bransen 07 Mar '13

07 Mar '13

Hi all, The uuagc package [1] works fine with GHC versions 7.4.* and 7.6.1. However, with GHC 7.6.2 the binary still compiles but runs into an infinite loop at runtime. To reproduce, do: cabal install uuagc && touch tmp.ag && uuagc tmp.ag With GHC versions < 7.6.2 this succeeds and creates a (rather boring) file tmp.hs, while with GHC 7.6.2 this displays: uuagc: <<loop>> I suspect that this is related to strictness annotations, which exist in many places in the UUAGC source code (which itself is also preprocessed by the UUAGC system). In the release notes I cannot find anything related to strictness which could introduce this, so would anyone have a clue on where to look? Has there been a longstanding bug in the UUAGC source code which only now surfaces? Or is it a bug introduced in GHC 7.6.2 where something is a bit too strict? Regards, Jeroen Bransen [1] http://hackage.haskell.org/package/uuagc

1 1

Re: [Haskell-cafe] Announce: Leksah 0.13.2.0 (still a bit experimental)
by Hamish Mackenzie 07 Mar '13

07 Mar '13

I am not able to reproduce this on my Ubuntu 12.10 VM. What version of GHC are you using? Do you have anything in your ~/.ghci file? What command are you running? Does putStrLn "Hello World" work? What is the active .cabal project? The GHCi interface is sensitive to changes in the way GHCi formats it output, so it is a bit fragile. Some things that will currently break it are... * Anything that tries to read input on stdin * :set prompt (Leksah uses the prompt to detect when the output finishes) I added some tests a while back to try to catch issues running GHCi. Can you please try this... cd leksah/vendor/leksah-server cabal install --enable-tests --force-reinstall You should see this near the end... Running 1 test suites... Test suite test-tool: RUNNING... Test suite test-tool: PASS Test suite logged to: dist/test/leksah-server-0.13.0.0-test-tool.log 1 of 1 test suites (1 of 1 test cases) passed. Thanks, Hamish On 7 Mar 2013, at 08:48, Alejandro Toribio Bello Ruiz <alejandro.bello67(a)gmail.com> wrote: > Leksah 0.13.2 compiled successfully on Ubuntu 12.10 following the new instructions on .travis.yml, but arise the old problem where the GHCi integration break when the first command set is from the debug scratch pane. This bug was fixed on Leksah 0.12.1.3. What happened? This issue is very import for my students. Would be very grateful if you fix this bug. > > Regards > > El viernes, 1 de marzo de 2013 23:04:59 UTC-5, Hamish escribió: > 12.10 uses webkit 1.10 so the -fwebkit1.8 was probably tripping it up. > > I have updated webkit so that webkit 1.8 is detected automatically and > updated the .travis.yml file. > > Please try again. > > On 23 Feb 2013, at 06:20, alejandr...(a)gmail.com wrote: > >> I compiled Leksah 0.13.2 using Ubuntu 12.04 (I followed the instructions on .travis.yml). It was OK, but when I tried to compile with Ubuntu 12.10, I got the following error: >> >> Building webkit-0.12.5... >> Preprocessing library webkit-0.12.5... >> gtk2hsC2hs: Errors during expansion of binding hooks: >> >> webkit1.8/Graphics/UI/Gtk/WebKit/DOM/File.chs:17: (column 14) [ERROR] >> >>> Unknown identifier! >> Cannot find a definition for `webkit_dom_file_get_file_name' in the header file. >> webkit1.8/Graphics/UI/Gtk/WebKit/DOM/File.chs:23: (column 16) [ERROR] >> >>> Unknown identifier! >> Cannot find a definition for `webkit_dom_file_get_file_size' in the header file. >> >> Can you help me, please? >> >> Alejandro T. Bello Ruiz >> >> >> -- >> You received this message because you are subscribed to the Google Groups "leksah" group. >> To unsubscribe from this group and stop receiving emails from it, send an email to leksah+un...(a)googlegroups.com. >> For more options, visit https://groups.google.com/groups/opt_out. >> >> > > > -- > You received this message because you are subscribed to the Google Groups "leksah" group. > To unsubscribe from this group and stop receiving emails from it, send an email to leksah+unsubscribe(a)googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > >

1 0

GSOC application level
by Mateusz Kowalczyk 07 Mar '13

07 Mar '13

Greetings, It seems that the Haskell community consistently participates in the Google Summer of Code project. I (and probably many others) am interested in taking part in one of such projects but I have a question in regards to the expertise. I know that this year's projects aren't up yet but by looking at the past results, the expertise required seems to vary pretty widely. Can someone that has been around for a bit longer comment on what level of experience with Haskell and underlying concepts is usually expected from candidates? Are applications discarded simply based on the applicant not having much previous experience in the target area? What is the level of the competition for places on the projects? It's not my first week of meddling with Haskell (and studying the ideas behind it) but I can't say that I would be able to confidently take on any project that might be put out. I do however realise that the project is open to students so I don't imagine the requirements specify something like couple of years with type theory research either. Any insight about the topic is appreciated. I'd much rather flip bits than do labour this summer and if it can be done using the language I'm interested in, even better! -- Mateusz K.

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ANNOUNCE: Start Ajhc project with forking jhc.
by Kiwamu Okabe 07 Mar '13

07 Mar '13

Hi all. I am a user of jhc Haskell compiler. Jhc can compile Haskell code to micro arch such as Cortex-M3. I have written LED blinking demo for Cortex-M3 with jhc. Very fun! https://github.com/ajhc/demo-cortex-m3 http://www.youtube.com/watch?v=3R9sogReVHg And I created many patches for jhc. But...I think that the upstream author of jhc, John Meacham, can't pull the contribution speedy, because he is too busy. It's difficult that maintain many patches without any repositories, for me. Then, I have decided to fork jhc, named Ajhc. # pain full... http://ajhc.github.com/ I will feedback Ajhc's big changes to jhc mailing list. Or I am so happy if John joins Ajhc project. Regards, -- Kiwamu Okabe

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Haskell Weekly News: Issue 261
by Daniel Santa Cruz 07 Mar '13

07 Mar '13

Welcome to issue 261 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of February 24 to March 02, 2013. Quotes of the Week * EvilMachine: I always picture some poor mutant guy whose every body part looks alike in the deepest depths of the dark ages going to the doc, and being told he has ‚monomorphism‚. * pmk: the last time i asked GHC to compile a function with forty (.)'s in a row, it took over an hour Top Reddit Stories * Some draft chapters of "Parallel and Concurrent Programming in Haskell" available online for comment Domain: self.haskell, Score: 79, Comments: 40 On Reddit: [1] http://goo.gl/emvnI Original: [2] http://goo.gl/emvnI * Hulk: A Haskell IRC server - Nice example of real world monad without IO Domain: chrisdone.com, Score: 62, Comments: 18 On Reddit: [3] http://goo.gl/CR986 Original: [4] http://goo.gl/RhjG6 * Hole-driven Haskell Domain: matthew.brecknell.net, Score: 59, Comments: 52 On Reddit: [5] http://goo.gl/5ctnJ Original: [6] http://goo.gl/NqJVi * Chasing a Space Leak in Shake: Profiling, reduction, digging thru stacks Domain: neilmitchell.blogspot.com, Score: 46, Comments: 11 On Reddit: [7] http://goo.gl/cO86j Original: [8] http://goo.gl/v09x6 * Haskell patterns ad nauseam Domain: yaxu.org, Score: 39, Comments: 2 On Reddit: [9] http://goo.gl/dapAz Original: [10] http://goo.gl/hPW6y * Cross Compiling for iOS with GHC Domain: hackage.haskell.org, Score: 35, Comments: 18 On Reddit: [11] http://goo.gl/3uzjR Original: [12] http://goo.gl/Kw0Pe * From Set Theory to Type Theory Domain: golem.ph.utexas.edu, Score: 35, Comments: 8 On Reddit: [13] http://goo.gl/HaPN4 Original: [14] http://goo.gl/LUi5f * EclipseFP 2.5.0 released: cabal-dev support, import cleaning, and more Domain: jpmoresmau.blogspot.fr, Score: 31, Comments: 0 On Reddit: [15] http://goo.gl/lR6Tc Original: [16] http://goo.gl/JJzBz * Michael Snoyman's overview of the new conduit 1.0, on School of Haskell Domain: haskell.fpcomplete.com, Score: 31, Comments: 11 On Reddit: [17] http://goo.gl/aFbw1 Original: [18] http://goo.gl/ObEHA * How is Haskell tackling the multicore problem? - Simon Marlow Domain: youtube.com, Score: 28, Comments: 5 On Reddit: [19] http://goo.gl/qOKlJ Original: [20] http://goo.gl/HVddu * Generics and Protocol Buffers... now on Hackage Domain: breaks.for.alienz.org, Score: 28, Comments: 5 On Reddit: [21] http://goo.gl/WqIyn Original: [22] http://goo.gl/J9NKB * Demonstrating tidal - haskell music patterns in action Domain: blog.lurk.org, Score: 24, Comments: 5 On Reddit: [23] http://goo.gl/4ddqI Original: [24] http://goo.gl/Ift8y * GHC iOS Packages Domain: github.com, Score: 23, Comments: 5 On Reddit: [25] http://goo.gl/PONjI Original: [26] http://goo.gl/6D3Gz * SoH tutorial: a regular expression matcher using continuations Domain: haskell.fpcomplete.com, Score: 22, Comments: 4 On Reddit: [27] http://goo.gl/toQpy Original: [28] http://goo.gl/BZ1xI * [nyc-haskell] Greg Wright - A Purely Symbolic Effort in Mathematics Domain: vimeo.com, Score: 22, Comments: 6 On Reddit: [29] http://goo.gl/jNybI Original: [30] http://goo.gl/Q8pq8 * ANN: clckwrks 0.16 Haskell blog/CMS platform released. Includes a screencast demonstrating clckwrks features. Domain: clckwrks.com, Score: 20, Comments: 2 On Reddit: [31] http://goo.gl/Tp92p Original: [32] http://goo.gl/zV7Sj Top StackOverflow Questions * Do Hask or Agda have equalisers? votes: 23, answers: 2 Read on SO: [33] http://goo.gl/k74C9 * Prove idempotency of type-level disjunction votes: 17, answers: 2 Read on SO: [34] http://goo.gl/wsFUL * Internal representation of Haskell lists? votes: 16, answers: 2 Read on SO: [35] http://goo.gl/TCGDk * GHC Generating Redundant Core Operations votes: 15, answers: 1 Read on SO: [36] http://goo.gl/d9zWP * eliminating repetition in cabal files votes: 14, answers: 0 Read on SO: [37] http://goo.gl/2StJ9 * Translate from monad to applicative votes: 12, answers: 3 Read on SO: [38] http://goo.gl/IbSUi * Memoizing multiplication votes: 10, answers: 2 Read on SO: [39] http://goo.gl/EXvUk * Most efficient way to seek around in a large file votes: 9, answers: 1 Read on SO: [40] http://goo.gl/C9FGp * Does Haskell optimizer utilize memoization for repeated function calls in a scope? votes: 9, answers: 3 Read on SO: [41] http://goo.gl/3qDPQ * Existential type in higher order function votes: 9, answers: 1 Read on SO: [42] http://goo.gl/YfalJ * Haskell `ncurses` library votes: 9, answers: 2 Read on SO: [43] http://goo.gl/w7vKf Until next time, +Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/195app/some_draft_chapters_of_para… 2. http://www.reddit.com/r/haskell/comments/195app/some_draft_chapters_of_para… 3. http://chrisdone.com/posts/hulk-haskell-irc-server 4. http://www.reddit.com/r/haskell/comments/194ngx/hulk_a_haskell_irc_server_n… 5. http://matthew.brecknell.net/posts/2013/02/26/hole-driven-haskell/ 6. http://www.reddit.com/r/haskell/comments/19aj9t/holedriven_haskell/ 7. http://neilmitchell.blogspot.com/2013/02/chasing-space-leak-in-shake.html 8. http://www.reddit.com/r/haskell/comments/197ax4/chasing_a_space_leak_in_sha… 9. http://yaxu.org/haskell-patterns-ad-nauseam/ 10. http://www.reddit.com/r/haskell/comments/19e2ln/haskell_patterns_ad_nauseam/ 11. http://hackage.haskell.org/trac/ghc/wiki/Building/CrossCompiling/iOS 12. http://www.reddit.com/r/haskell/comments/19cq83/cross_compiling_for_ios_wit… 13. http://golem.ph.utexas.edu/category/2013/01/from_set_theory_to_type_theory.… 14. http://www.reddit.com/r/haskell/comments/19igl8/from_set_theory_to_type_the… 15. http://jpmoresmau.blogspot.fr/2013/02/eclipsefp-250-released.html 16. http://www.reddit.com/r/haskell/comments/199smc/eclipsefp_250_released_caba… 17. https://haskell.fpcomplete.com/school/pick-of-the-week/conduit-overview 18. http://www.reddit.com/r/haskell/comments/19akp3/michael_snoymans_overview_o… 19. https://www.youtube.com/watch?v=Mu6JjZsIdu8 20. http://www.reddit.com/r/haskell/comments/19eh7e/how_is_haskell_tackling_the… 21. http://breaks.for.alienz.org/blog/2013/02/28/generics-and-protocol-buffers-… 22. http://www.reddit.com/r/haskell/comments/19et3i/generics_and_protocol_buffe… 23. http://blog.lurk.org/demonstrating-tidal/ 24. http://www.reddit.com/r/haskell/comments/19jq16/demonstrating_tidal_haskell… 25. https://github.com/kaoskorobase/ghc-ios-cabal-scripts 26. http://www.reddit.com/r/haskell/comments/19gufd/ghc_ios_packages/ 27. https://haskell.fpcomplete.com/user/pbv/a-regular-expression-matcher 28. http://www.reddit.com/r/haskell/comments/199523/soh_tutorial_a_regular_expr… 29. https://vimeo.com/60737699 30. http://www.reddit.com/r/haskell/comments/19fp85/nychaskell_greg_wright_a_pu… 31. http://clckwrks.com/page/view-page-slug/10/ann--clckwrks-0-16---now-built-w… 32. http://www.reddit.com/r/haskell/comments/19a393/ann_clckwrks_016_haskell_bl… 33. http://stackoverflow.com/questions/15047775/do-hask-or-agda-have-equalisers 34. http://stackoverflow.com/questions/15180557/prove-idempotency-of-type-level… 35. http://stackoverflow.com/questions/15063115/internal-representation-of-hask… 36. http://stackoverflow.com/questions/15095902/ghc-generating-redundant-core-o… 37. http://stackoverflow.com/questions/15179316/eliminating-repetition-in-cabal… 38. http://stackoverflow.com/questions/15123552/translate-from-monad-to-applica… 39. http://stackoverflow.com/questions/15080399/memoizing-multiplication 40. http://stackoverflow.com/questions/15052905/most-efficient-way-to-seek-arou… 41. http://stackoverflow.com/questions/15084162/does-haskell-optimizer-utilize-… 42. http://stackoverflow.com/questions/15125522/existential-type-in-higher-orde… 43. http://stackoverflow.com/questions/15176015/haskell-ncurses-library 44. https://plus.google.com/105107667630152149014/about

1 0

Commercial Users of Functional Programming: Call for tutorials
by Simon Thompson 06 Mar '13

06 Mar '13

Commercial Users of Functional Programming Call for tutorials Commercial Users of Functional Programming (CUFP) is an annual meeting co-located with the International Conference on Functional Programming which this year will take place in Boston, MA, USA on 22-24 September 2013. CUFP aims to bridge the gap between academia and users applying functional programming in practice. CUFP provides high-quality practical tutorials covering state-of-the-art techniques and tools for functional programming. We are seeking proposals for half-day tutorials to be presented during the first two days of the meeting, 22 and 23 September, with the main CUFP session on 24 September. Among the suggested topics for tutorials are: - Introductions to functional programming languages: in the past we have had introductions to Clojure, Erlang, F#, Haskell, ML, OCaml, Scala, Scheme and others. - Applying functional programming in particular areas, including the web, high-performance computing, finance. - Tools and techniques supporting state of the art functional programming. Tutorial proposals should address the following points - Title - Abstract (about 100 words) - Goals: by the end of this tutorial you will be able to … - Intended audience: e.g. beginners, those with a working knowledge of X, … - Infrastructure required: For example, - will participants need access to a particular system? - can they be expected to have this on a laptop, or does it need to be provided by the meeting? and should be sent by email to - Francesco Cesarini: francesco(a)erlang-solutions.com - Simon Thompson: S.J.Thompson(a)kent.ac.uk by 31 March 2013. Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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ANNOUNCE: http-streams 0.3.1.0
by Andrew Cowie 06 Mar '13

06 Mar '13

Hey, I'd like to announce the initial release of http-streams, an HTTP client library using the Snap Framework's io-streams library to handle the streaming I/O. I blogged about it the background and API design here: http://blogs.operationaldynamics.com/andrew/software/haskell/http-streams-i… The SHA256 sum of http-streams-0.3.1.0.tar.gz as uploaded is: e0671f8eac83e9a4b092bdd7b03e69b80ac0417818b8dbc51d8604b769179bd3 Comments and feedback welcome. Cheers, AfC Sydney

1 0

ANNOUNCE: io-streams 1.0.0.0
by Gregory Collins 06 Mar '13

06 Mar '13

Hi all, After more than eight months of careful design and development, The Snap Framework team is happy to announce the first version of io-streams, a simple and easy-to-use library for doing streaming I/O in Haskell. The io-streams library is based around two basic types, InputStream a and OutputStream a, and three fundamental I/O primitives: -- read an item from an input stream read :: InputStream a -> IO (Maybe a) -- push an item back to an input stream unRead :: a -> InputStream a -> IO () -- write to an output stream write :: Maybe a -> OutputStream a -> IO () Streams can be transformed by composition and hooked together (using Kleisli composition) with a large set of provided combinators. Simple types and operations in the IO monad mean straightforward and simple exception handling and resource cleanup using Haskell standard library functions like bracket. The io-streams library comes with: - functions to use files, handles, concurrent channels, sockets, lists, vectors, and more as streams. - a plethora of combinators for wrapping and transforming streams, including compression and decompression using zlib, controlling precisely how many bytes are read from or written to a stream, buffering output using bytestring builders, folds, maps, filters, zips, etc. - support for parsing from streams using attoparsec. For first-time users, io-streams comes with an included tutorial in theSystem.IO.Streams.Tutorial module, written by Gabriel Gonzalez of “pipes” fame. The io-streams library is tested on GHC 7.0, 7.2, 7.4, and 7.6, and includes an extensive test suite with 100% function, statement, and branch coverage. We would like to acknowledge the financial assistance of Erudify AG, who graciously funded some of the documentation and development of the io-streams library. Many thanks are also due to Gabriel Gonzalez, Andrew Cowie, Johan Tibell, and Bas van Dijk for their helpful discussions, contributions, and review. For further details, you can read our release announcement here: http://snapframework.com/blog/2013/03/05/announcing-io-streams Cheers, G -- Gregory Collins <greg(a)gregorycollins.net>

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OdHac: second call for participation
by Roman Cheplyaka 06 Mar '13

06 Mar '13

------------------------------------------------------------------------ OdHac International Haskell Hackathon May 3-5, 2013 Odessa, Ukraine http://haskell.org/haskellwiki/OdHac Co-organized with Provectus IT ------------------------------------------------------------------------ The international Haskell hackathon in Odessa is just two months away! To attend, please register at http://bit.ly/OdHacReg. ## Projects A big idea of this Hackathon is to concentrate the effort around a few important and established projects. Although participants are free to work on their own projects, it's more fun to work together! So far we've got two great projects: Hakyll and Fay http://www.haskell.org/haskellwiki/OdHac/Projects#Hakyll http://www.haskell.org/haskellwiki/OdHac/Projects#Fay Feel free to add new projects to the wiki! One project we'd love to see represented, because of its enormous value to the community, is Cabal. So far several core Cabal hackers have expressed willingness to offer help via IRC, but it would be great to have a Cabal expert on site. If you can coordinate the effort around Cabal and answer questions, and you're planning to come — please get in touch with me. ## Accommodation The Hackathon dates overlap with a series of holidays in Ukraine and neighbour countries, and Odessa is a popular tourist destination. So book early! We offer an accommodation program to make it simple for you: http://www.haskell.org/haskellwiki/OdHac/Accommodation

1 0

Library API design: functional objects VS type classes
by Rob Stewart 05 Mar '13

05 Mar '13

Hi, I have a question about API design for Haskell libraries. It is a simple one: functional object data structures encapsulating mutable state VS type classes encapsulating mutable state Here is a simple example. I present an API: using a type class `FooC`, and aso as a data structure `FooT`. Both are stateful, in the form of an MVar holding an Integer, with an operation `incrFoo` to increment this value by one, and another `readFoo` to read the Integer value. ----- import Control.Concurrent -- API approach 1: Using type classes class FooC a where mkFooC :: IO a readFooC :: a -> IO Int incrFooC :: a -> IO () newtype Bar = Bar (MVar Int) instance FooC Bar where mkFooC = newMVar 0 >>= \i -> return $ Bar i readFooC (Bar mv) = readMVar mv incrFooC (Bar mv) = modifyMVar_ mv $ \i -> return (i+1) -- API approach 2: Using direct field records data FooT a = FooT { readFooT :: IO a , incrFooT :: IO () } mkBar :: IO (FooT Int) mkBar = do mv <- newMVar 0 return FooT { readFooT = readMVar mv , incrFooT = modifyMVar_ mv $ \i -> return (i+1) } -- Tests the type class API testTypeClass :: IO () testTypeClass = do bar <- mkBar incrFooT bar incrFooT bar i <- readFooT bar print i -- prints 2 -- Tests the direct data structure API testDataStruct :: IO () testDataStruct = do bar <- (mkFooC :: IO Bar) incrFooC bar incrFooC bar i <- readFooC bar print i -- prints 2 ---- With that, I now ask: which is more common? Which is the better API design for a library? The APIs are almost identical. Under what conditions is the type classes preferred over the "mutable object" style data structure? There are two related resources that provides additional context here, that favour the functional objects approach: - Section 3.4 "Mutable Objects" in "Haskell's Overlooked Object System" http://goo.gl/gnZXL - A similar question (data structures vs type classes) in "Haskell Antipattern: Existential Typeclass" http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-… Thanks! -- Rob

4 3

"translating" recursively defined sequence
by Christopher Howard 05 Mar '13

05 Mar '13

Hi. My Haskell is (sadly) getting a bit rusty. I was wondering what would be the most straightforward and easily followed "procedure" for translating a recursively defined sequence into a Haskell function. For example, this one from a homework assignment. quote: -------- a_1 = 10 a_(k+1) = (1/5) * (a_k)**2 -------- (The underscore is meant to represent subscripting what follows it.) -- frigidcode.com

4 4

Concurrency performance problem
by Łukasz Dąbek 05 Mar '13

05 Mar '13

Hello Cafe! I have a problem with following code: http://hpaste.org/83460. It is a simple Monte Carlo integration. The problem is that when I run my program with +RTS -N1 I get: Multi 693204.039020917 8.620632s Single 693204.039020917 8.574839s End And with +RTS -N4 (I have four CPU cores): Multi 693204.0390209169 11.877143s Single 693204.039020917 11.399888s End I have two questions: 1) Why performance decreases when I add more cores for my program? 2) Why performance of single threaded integration also changes with number of cores? Thanks for all answers, Łukasz Dąbek.

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Haskell + RankNTypes + (forall p. p Char -> p Bool) sound?
by Shachaf Ben-Kiki 05 Mar '13

05 Mar '13

I was trying to figure out a way to write absurd :: (forall p. p Char -> p Bool) -> Void using only rank-n types. Someone suggested that Haskell with RankNTypes and a magic primitive of type (forall p. p Char -> p Bool) might be sound (disregarding the normal ways to get ⊥, of course). Is that true? Given either TypeFamilies or GADTs, you can write absurd. But it doesn't seem like you can write it with just RankNTypes. (This is related to GeneralizedNewtypeDeriving, which is more or less a version of that magic primitive.) This seems like something that GADTs (/TypeFamilies) give you over Leibniz equality: You can write data Foo a where FooA :: Foo Char FooB :: Void -> Foo Bool foo :: Foo Bool -> Void foo (FooB x) = x Without any warnings. On the other hand data Bar a = BarA (Is a Char) | BarB (Is a Bool) Void bar :: Bar Bool -> Void bar (BarB _ x) = x bar (BarA w) = -- ??? Doesn't seem possible. If it's indeed impossible, what's the minimal extension you would need to add on top of RankNTypes to make it work? GADTs seems way too big. Shachaf

2 1

What pattern is this (Something.T -> IO a) in Sound.ALSA.Sequencer
by Martin Drautzburg 05 Mar '13

05 Mar '13

Hello all, this was previously posted on Haskell Beginners, but only partially answered. In Sound.ALSA.Sequencer, there are a number of functions which together set up a midi environement (client, port, queue). They all have a type, where the last argument has a type like: (something.T -> IO a) i.e. *Main> :t SndSeq.withDefault SndSeq.withDefault :: SndSeq.OpenMode mode => SndSeq.BlockMode -> (SndSeq.T mode -> IO a) -> IO a *Main> :t Port.withSimple Port.withSimple :: SndSeq.T mode -> String -> Port.Cap -> Port.Type -> (Port.T -> IO a) -> IO a *Main> :t Queue.with Queue.with :: SndSeq.T mode -> (Queue.T -> IO a) -> IO a There is example code, where a full setup is created by a number of nested "do" blocks. The repeating pattern there is: something1 $ \x -> do something2 $ \y -> do something3 $ \z -> do What is this all about? I particularly would like to understand, when this parttern is needed and what determines the the number of nested "do" blocks. -- Martin

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simple parsec question
by Immanuel Normann 05 Mar '13

05 Mar '13

Hi, I am trying to parse a semi structured text with parsec that basically should identify sections. Each section starts with a headline and has an unstructured content - that's all. For instance, consider the following example text (inside the dashed lines): --------------------------- top 1: some text ... bla top 2: more text ... bla bla --------------------------- This should be parsed into a structure like this: [Section (Top 1) (Content "some text ... bla"), Section (Top 1) (Content "more text ... bla")] Say, I have a parser "headline", but the content after a headline could be anything that is different from what "headline" parses. How could the "section" parser making use of "headline" look like? My idea would be to use the "manyTill" combinator, but I don"t find an easy solution. Many thanks for any hint Immanuel

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Haskell syntax/indentation for vim
by Tristan Ravitch 05 Mar '13

05 Mar '13

Cafe, I've recently been playing with vim and wasn't quite satisfied with the existing syntax highlighting and indentation, so I thought I'd try my hand at a new Haskell mode: https://github.com/travitch/hasksyn It is minimal in that it doesn't provide support for running external commands over code or anything fancy. It just does syntax highlighting and reasonably-smart indentation. There is no support for literate Haskell since supporting both with one mode is very tricky. It might be useful to some people. Comments, bug reports, and suggestions welcome.

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LambdaJam 2013
by Tony Morris 05 Mar '13

05 Mar '13

Hey haskell guys, LambdaJam 2013 is a functional programming conference to be held in Brisbane in early May. Call for submissions ends this Friday 08 March. If you are planning to make a submission, please make sure you do soon! If you are stuck somehow, I'd love to be able to help you get to the goal. So feel free to send me an email or I am on IRC (dibblego), lurking around the #haskell channel or privmsg if you like. Personally, I would love see more haskell submissions :) LambdaJam2013 call for papers: http://www.yowconference.com.au/lambdajam/Call.html -- Tony Morris http://tmorris.net/

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Re: [Haskell-cafe] Trouble installing and using Chart/cairo on windows 7
by Brent Yorgey 05 Mar '13

05 Mar '13

On Mon, Mar 04, 2013 at 02:30:55PM -0800, briand(a)aracnet.com wrote: > On Mon, 4 Mar 2013 17:27:29 -0500 > Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: > > > On Sun, Mar 03, 2013 at 05:38:02PM -0800, briand(a)aracnet.com wrote: > > > On Sun, 3 Mar 2013 19:58:37 -0500 > > > Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: > > > > > > > > > > > Good access to fonts and font metrics is the kicker. Otherwise I'd > > > > say to switch to using diagrams as a backend, hence getting a whole > > > > bunch of actual backends for free. I would love to see development of > > > > some good Haskell font packages -- maybe it would even make a good > > > > GSoC project? Unfortunately I don't know enough about it to even know > > > > what would be involved, or how much work it would be. > > > > > > > > > > I assume that to use diagram the font package would have to be a vector font system, or could bit-mapped fonts be used ? > > > > A vector font system would be ideal, as then you could go crazy > > turning glyphs into paths and doing whatever the heck you want with > > them using the diagrams framework. However, bit-mapped fonts could be > > used too (as long as there is a backend to support them), it would > > just be less useful. > > > > yes - seems to me that vector fonts are definitely the way to go. > > it seems like this problem has to already have a solution. maybe all that's needed is a font importer which understands some, already established, vector font representation. > Well, there is the SVGFonts package: http://hackage.haskell.org/package/SVGFonts I don't have a good sense of how widely supported this representation is, or how easy it is to convert between it and other font representations. -Brent

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Trouble installing and using Chart/cairo on windows 7
by Arnaud Bailly 04 Mar '13

04 Mar '13

Hello, I am trying to install timeplot and splot on a windows 7 host, using Haskell platform 2012.2.0.0, and I have troubles installing HSChart. First difficulties were installing cairo, which requires Gtk+ libraries and headers. I installed those using a Gtk+-bundle, first in c:\Program Files\. I had to move to d:\soft (eg. a directory without spaces) to be able to install cairo. Then I tried to install Chart and at first failed with a cryptic 'Exit Failure 1' error from cabal. I succeeded when I redirected cabal output to a file, eg: cabal install > out 2<&1 whereas a plain 'cabal install' failed. Then I managed to install splot and timeplot. I then tried to use Chart to draw a simple chart, following http://hackage.haskell.org/packages/archive/Chart/0.16/doc/html/Graphics-Re… it hang forever with a CPU at 100%. Suggestions are more than welcomed. Regards, Arnaud

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ANN: pipes-network-0.1.0 - Stream your network sockets using the pipes and pipes-safe libraries
by Renzo Carbonara 04 Mar '13

04 Mar '13

I'm happy to announce the release of pipes-network 0.1.0. The pipes-network package allows using network sockets together with the pipes and pipes-safe libraries, for streaming purposes. Currently, just TCP sockets are supported. http://hackage.haskell.org/package/pipes-network-0.1.0 The package is split into two halves: one “Safe” half that can be used if proxies need to safely and timely acquire and release new sockets within a pipeline, using the facilities provided by the pipes-safe package; and another simpler and faster half that doesn't depend on pipes-safe, but it also doesn't provide any means for safely acquiring and releasing new sockets within a pipeline itself. There's plenty of documentation and some examples in the Haddocks. Many thanks to Gabriel Gonzalez, author of pipes, for discussing the design of the library with me; and to Paolo Capriotti, author of a previous pipes-network version, for his original work and for handing me the maintenance of this package. ¡Enjoy your TCP streams! Regards, Renzo Carbonara.

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building darcs failed (Unable to link against the iconv library)
by Dmitry Malikov 04 Mar '13

04 Mar '13

Trying to install darcs with cabal-install: Linking /tmp/darcs-2.8.4-12051/darcs-2.8.4/dist/setup/setup ... Configuring darcs-2.8.4... checking whether to use -liconv... setup: Unable to link against the iconv library. Failed to install darcs-2.8.4 cabal: Error: some packages failed to install: What is actually going on here? Iconv libraries already installed with libc6 package. Running ubuntu, cabal-install-1.16.0.2. -- Best regards, dmitry malikov !

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ANN: Nomyx 0.1 beta, the game where you can change the rules
by Corentin Dupont 04 Mar '13

04 Mar '13

Hello everybody! I am very happy to announce the beta release [1] of Nomyx, the only game where You can change the rules. This is an implementation of a Nomic [2] game in Haskell (I believe the first complete implementation). In a Nomyx game you can change the rules of the game itself while playing it. The players can submit new rules or modify existing ones, thus completely changing the behaviour of the game through time. The rules are managed and interpreted by the computer. They must be written in the Nomyx language, which is a subset of Haskell. At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Here is a video introduction and first tutorial of the game: http://vimeo.com/58265498 The game is running here: www.nomyx.net:8000/Nomyx I have set up a forum where players can learn about Nomyx and discuss the rules they intend to propose: www.nomyx.net/forum As this is the first beta release of the game, I'm looking for beta testers :) Although I tested it quite a lot, I'm sure a lot of bugs remains, especially in multiplayer. So if you are interested in testing Nomyx, please go to this forum thread and we'll set up a small team to start a match! http://www.nomyx.net/forum/viewtopic.php?p=5 Comments/contributions are very highly welcome! There is still a lot to do. As for now, the game is not completely securised. It is easy to break it by submitting rules containing malicious code. I'm working on it. If you'd like to do security testing, please do it locally on your own machine and send me a bug report :). Cheers, Corentin [1] http://hackage.haskell.org/package/Nomyx [2] www.nomic.net

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