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ANNOUNCE: Win32-services
by Michael Steele 16 Mar '13

16 Mar '13

I uploaded a new packaged named Win32-services. This library is a partial binding to the Win32 System Services API. It's now easier to write Windows service applications in Haskell. The hackage page <http://hackage.haskell.org/package/Win32-services> [1] demonstrates simple usage. There are also 2 examples included with the sources. One is a translation of Microsoft's official example. [1]: http://hackage.haskell.org/package/Win32-services -- Michael Steele

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ANN: Monad.Reader Issue 21
by Edward Z. Yang 16 Mar '13

16 Mar '13

I am pleased to announce that Issue 21 of the Monad Reader is now available. http://themonadreader.files.wordpress.com/2013/03/issue21.pdf Issue 21 consists of the following two articles: * "A Functional Approach to Neural Networks" by Amy de Buitléir, Michael Russell, Mark Daly * "Haskell ab initio: the Hartree-Fock Method in Haskell" by Felipe Zapata, Angel J. Alvarez Feel free to browse the source files. You can check out the entire repository using Git: git clone https://github.com/ezyang/tmr-issue21.git If you’d like to write something for Issue 22, please get in touch! Cheers, Edward

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Re: [Haskell-cafe] attoparsec and backtracking
by wren ng thornton 15 Mar '13

15 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 14 Mar '13

14 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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