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Haskell Weekly News: Issue 275
by Daniel Santa Cruz 08 Aug '13

08 Aug '13

Welcome to issue 275 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 28 to August 3, 2013. Quotes of the Week * littlecalculist: combinatorics. n. math's way of punishing you for failing to use abstraction. * dmwit: ?let instance Num Bool where (+) = (/=); (*) = (&&); negate = id; abs = id; signum = id; fromInteger = odd * edwardk: clearly it should be \x -> x <$ do f . g $ x. do is a magical () killing machine * dmwit: Type families are not injective. Your first three inscrutable error messages with type families in them will be because you don't understand this rule. * elliott: halting problem, n. the property which makes stating anything about programs impossible, according to at least one person in any room * carmack: Everything that is syntactically legal that the compiler will accept will eventually wind up in your codebase. Top Reddit Stories * John Carmack talking at length about Haskell in his QuakeCon Keynote speech (live) Domain: twitch.tv, Score: 105, Comments: 47 On Reddit: [1] http://goo.gl/1g1zqD Original: [2] http://goo.gl/C17ci8 * SPJ: GHC's Glorious future Domain: haskell.org, Score: 79, Comments: 16 On Reddit: [3] http://goo.gl/97DGuc Original: [4] http://goo.gl/YAY6wC * There's a massive gap between the "average" and "expert" Haskeller. How do I bridge that gap? Domain: self.haskell, Score: 73, Comments: 53 On Reddit: [5] http://goo.gl/Y3jEoa Original: [6] http://goo.gl/Y3jEoa * Getting Started With GHC Hacking Domain: dagit.github.io, Score: 64, Comments: 2 On Reddit: [7] http://goo.gl/ZwHVpm Original: [8] http://goo.gl/ljhyff * Extensible Effects: An Alternative to Monad Transformers Domain: cs.indiana.edu, Score: 63, Comments: 47 On Reddit: [9] http://goo.gl/2JqImi Original: [10] http://goo.gl/zq5klw * The Intel Labs Haskell Research Compiler [pdf] Domain: leafpetersen.com, Score: 55, Comments: 19 On Reddit: [11] http://goo.gl/wRQs7E Original: [12] http://goo.gl/2eZhEX * Goto in Haskell Domain: self.haskell, Score: 36, Comments: 34 On Reddit: [13] http://goo.gl/pelEJo Original: [14] http://goo.gl/pelEJo * The Algebra of Algebraic Data Type Domain: chris-taylor.github.io, Score: 35, Comments: 3 On Reddit: [15] http://goo.gl/PPi0M1 Original: [16] http://goo.gl/xE78Ko * Functors and monads for analyzing data Domain: izbicki.me, Score: 32, Comments: 16 On Reddit: [17] http://goo.gl/42ux1T Original: [18] http://goo.gl/LAJ7ti * Why use _ <-? Domain: self.haskell, Score: 31, Comments: 24 On Reddit: [19] http://goo.gl/QpGPP1 Original: [20] http://goo.gl/QpGPP1 * CIS 194: Introduction to Haskell (Spring 2013) Domain: cis.upenn.edu, Score: 30, Comments: 3 On Reddit: [21] http://goo.gl/9zHuXb Original: [22] http://goo.gl/cNyEOT * The Commutative monad Domain: gelisam.blogspot.ca, Score: 30, Comments: 41 On Reddit: [23] http://goo.gl/kDVTrl Original: [24] http://goo.gl/mTCHL0 * FRP GUI - reactive-banana + threepenny-gui = awesome Domain: apfelmus.nfshost.com, Score: 30, Comments: 11 On Reddit: [25] http://goo.gl/xu91UA Original: [26] http://goo.gl/9Vu6ng Top StackOverflow Questions * Monad Stack Penetration Classes with Free/Operational Monad Transformers? votes: 16, answers: 1 Read on SO: [27] http://goo.gl/t4F6kd * Haskell reexport everything votes: 12, answers: 1 Read on SO: [28] http://goo.gl/7HQCVC * What are the main differences between Scala and Frege (in programming paradigms)? votes: 11, answers: 2 Read on SO: [29] http://goo.gl/TRhjDB * If a thunk results in an exception, is the exception kept as the result of the thunk? votes: 10, answers: 1 Read on SO: [30] http://goo.gl/rPSd6J * Check if import is redundant votes: 9, answers: 1 Read on SO: [31] http://goo.gl/jEqBnt * Haskell Parser Combinators votes: 9, answers: 1 Read on SO: [32] http://goo.gl/YCrsX3 * A library implementation of a recursion scheme votes: 8, answers: 1 Read on SO: [33] http://goo.gl/DdYPtw Until next time, [34]+Daniel Santa Cruz References 1. http://www.twitch.tv/bethesda?utm_campaign=live_embed_click&utm_source=www.… 2. http://www.reddit.com/r/haskell/comments/1jj40p/john_carmack_talking_at_len… 3. http://www.haskell.org/pipermail/ghc-devs/2013-August/001818.html 4. http://www.reddit.com/r/haskell/comments/1jky60/spj_ghcs_glorious_future/ 5. http://www.reddit.com/r/haskell/comments/1jeo0p/theres_a_massive_gap_betwee… 6. http://www.reddit.com/r/haskell/comments/1jeo0p/theres_a_massive_gap_betwee… 7. http://dagit.github.io/posts/2013-08-03-getting-started-with-ghc-hacking.ht… 8. http://www.reddit.com/r/haskell/comments/1jnaz5/getting_started_with_ghc_ha… 9. http://www.cs.indiana.edu/~sabry/papers/exteff.pdf 10. http://www.reddit.com/r/haskell/comments/1j9n5y/extensible_effects_an_alter… 11. http://www.leafpetersen.com/leaf/publications/hs2013/hrc-paper.pdf 12. http://www.reddit.com/r/haskell/comments/1jh4iv/the_intel_labs_haskell_rese… 13. http://www.reddit.com/r/haskell/comments/1jk06q/goto_in_haskell/ 14. http://www.reddit.com/r/haskell/comments/1jk06q/goto_in_haskell/ 15. http://chris-taylor.github.io/blog/2013/02/13/the-algebra-of-algebraic-data… 16. http://www.reddit.com/r/haskell/comments/1jhq7k/the_algebra_of_algebraic_da… 17. http://izbicki.me/blog/functors-and-monads-for-analyzing-data 18. http://www.reddit.com/r/haskell/comments/1ja3js/functors_and_monads_for_ana… 19. http://www.reddit.com/r/haskell/comments/1jcls5/why_use/ 20. http://www.reddit.com/r/haskell/comments/1jcls5/why_use/ 21. http://www.cis.upenn.edu/~cis194/ 22. http://www.reddit.com/r/haskell/comments/1j9m5e/cis_194_introduction_to_has… 23. http://gelisam.blogspot.ca/2013/07/the-commutative-monad.html 24. http://www.reddit.com/r/haskell/comments/1jegxb/the_commutative_monad/ 25. http://apfelmus.nfshost.com/blog/2013/08/02-reactive-banana-threepenny.html 26. http://www.reddit.com/r/haskell/comments/1jk16t/frp_gui_reactivebanana_thre… 27. http://stackoverflow.com/questions/17936900/monad-stack-penetration-classes… 28. http://stackoverflow.com/questions/18035458/haskell-reexport-everything 29. http://stackoverflow.com/questions/17905322/what-are-the-main-differences-b… 30. http://stackoverflow.com/questions/18018254/if-a-thunk-results-in-an-except… 31. http://stackoverflow.com/questions/17904265/check-if-import-is-redundant 32. http://stackoverflow.com/questions/18028220/haskell-parser-combinators 33. http://stackoverflow.com/questions/18013547/a-library-implementation-of-a-r… 34. https://plus.google.com/105107667630152149014/about

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Hackage upload problem
by Thomas Hallgren 07 Aug '13

07 Aug '13

Hi, I get the following error when I try to upload gf-3.5.tar.gz [1] to Hackage. 400 Error in upload could not extract gf-3.5 directory from gf-3.5.tar.gz I get the same error when I try to Check the previous version, gf-3.4.tar.gz [2], which was uploaded without problems 6 months ago [3], so it seems that something has changed on the server. Does anyone know what it could be that is going wrong? Is there a way to get a more informative error message? Thomas H [1] http://www.grammaticalframework.org/download/gf-3.5.tar.gz [2] http://www.grammaticalframework.org/download/gf-3.4.tar.gz [3] http://hackage.haskell.org/package/gf-3.4

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ScopedTypeVariables
by Evan Laforge 06 Aug '13

06 Aug '13

Occasionally I have to explicitly add a type annotation, either for clarity or to help choose a typeclass instance. Usually top-level type annotations take care of this, but sometimes it's convenient to only annotate a certain value, e.g. one argument of a lambda. I've noticed that while vanilla haskell is happy to allow me to put type annotations on variables where they are used (e.g. '\x -> f (x :: T)'), if I put it on the variable where it is bound (e.g. '\(x :: T) -> f x'), it wants me to turn on ScopedTypeVariables. I think ScopedTypeVariables is a nice extension and I'm sure it comes from a perfectly respectable family and all, but it feels like annotations on arguments comes in as a side-effect. Would it make sense to split it into a separate extension like TypesOnArguments so I can more accurately express my deviation from haskell2010 orthodoxy? Or is there some deeper tie between scoped type variables and annotations on arguments? Now that I think of it, it seems inconsistent that 'x :: A -> B; x a = ...' is valid, but 'x = \(a :: A) -> (...) :: B' is not. Doesn't the former desugar to the latter? And what about getting ScopedTypeVariables into haskell prime? As far as I know everyone loves it, or at least no one actually hates it :)

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Haddock GSOC project progress
by Mateusz Kowalczyk 05 Aug '13

05 Aug '13

Greetings cafe, As some of you might know, I'm hacking on Haddock as part of Google Summer of Code. I was recently advised to create a blog and document some of what I have been doing recently. You can find the blog at [1] if you're interested. The first post goes over the work from the last month or so. Future posts should be shorter and on more specific topics. There's an overview of what has happened/changed/will change at the bottom of the post if you're short on time. Thanks. [1] - http://fuuzetsu.co.uk/blog -- Mateusz K.

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Call For Papers, FLOPS 2014
by unknown＠example.com 04 Aug '13

04 Aug '13

Dear Colleagues, Please find below the CFP for FLOPS 2014. Apologies for possible cross posting. Program Committee Co-Chairs, Michael Codish and Eijiro Sumii ---------------------------------------------------------------------- Call For Papers =============== Twelfth International Symposium on Functional and Logic Programming (FLOPS 2014) June 4-6, 2014 Kanazawa, Japan http://www.jaist.ac.jp/flops2014/ (to be published) FLOPS is a forum for research on all issues concerning declarative programming, including functional programming and logic programming, and aims to promote cross-fertilization and integration between the two paradigms. Previous FLOPS meetings were held at Fuji Susono (1995), Shonan Village (1996), Kyoto (1998), Tsukuba (1999), Tokyo (2001), Aizu (2002), Nara (2004), Fuji Susono (2006), Ise (2008), Sendai (2010), and Kobe (2012). Topics ====== FLOPS solicits original papers in all areas of functional and logic programming, including (but not limited to): - Language issues: language design and constructs, programming methodology, integration of paradigms, interfacing with other languages, type systems, constraints, concurrency and distributed computing. - Foundations: logic and semantics, rewrite systems and narrowing, type theory, proof systems. - Implementation issues: compilation techniques, memory management, program analysis and transformation, partial evaluation, parallelism. - Applications: case studies, real-world applications, graphical user interfaces, Internet applications, XML, databases, formal methods and model checking. The proceedings will be published as an LNCS volume. The proceedings of the previous meetings (FLOPS 1999, 2001, 2002, 2004, 2006, 2008, 2010, and 2012) were published as LNCS 1722, 2024, 2441, 2998, 3945, 4989, 6009, and 7294. PC Co-Chairs ============ Michael Codish (Ben-Gurion University of the Negev) Eijiro Sumii (Tohoku University) PC Members ========== Lars Birkedal (Aarhus University) Michael Codish (Ben-Gurion University of the Negev) [co-chair] Marina De Vos (University of Bath) Moreno Falaschi (Universita degli studi di Udine) Carsten Fuhs (University College London) John Gallagher (Roskilde Universitet / IMDEA Software Institute) Samir Genaim (Universidad Complutense de Madrid) Laura Giordano (Universita del Piemonte Orientale) Ichiro Hasuo (University of Tokyo) Fritz Henglein (University of Copenhagen) Andy King (University of Kent) Oleg Kiselyov Vitaly Lagoon (MathWorks) Shin-Cheng Mu (Academia Sinica) Keiko Nakata (Institute of Cybernetics at Tallinn University of Technology) Luke Ong (University of Oxford) Peter Schachte (University of Melbourne) Takehide Soh (Kobe University) Eijiro Sumii (Tohoku University) [co-chair] Tachio Terauchi (Nagoya University) Joost Vennekens (KU Leuven) Janis Voigtlaender (Universitaet Bonn) Stephanie Weirich (University of Pennsylvania) Local Chair =========== Yuki Chiba (JAIST) Submission ========== Submissions must be unpublished and not submitted for publication elsewhere. Work that already appeared in unpublished or informally published workshops proceedings may be submitted. See also ACM SIGPLAN Republication Policy: http://www.sigplan.org/Resources/Policies/Republication Submissions should fall into one of the following categories: - Regular research papers: they should describe new results and will be judged on originality, correctness, and significance. - System descriptions: they should contain a link to a working system and will be judged on originality, usefulness, and design. - Declarative pearls: new and excellent declarative programs or theories with illustrative applications. System descriptions and declarative pearls must be explicitly marked as such in the title. Submissions must be written in English and can be up to 15 pages long including references, though pearls are typically shorter. Authors are required to use LaTeX2e and the Springer llncs class file, available at: http://www.springer.de/comp/lncs/authors.html Regular research papers should be supported by proofs and/or experimental results. In case of lack of space, this supporting information should be made accessible otherwise (e.g., a link to a Web page, or an appendix). Papers should be submitted electronically at: https://www.easychair.org/conferences/?conf=flops2014 Important Dates =============== Submission deadline: December 13, 2013 Author notification: February 10, 2014 Camera-ready copy: March 7, 2014 Venue ===== Main Hall, Ishikawa Prefectural Museum of Art, 2-1 Dewa-machi, Kanazawa, Ishikawa 920-0963 JAPAN. Some Previous FLOPS =================== FLOPS 2012, Kobe: http://www.org.kobe-u.ac.jp/flops2012/ FLOPS 2010, Sendai: http://www.kb.ecei.tohoku.ac.jp/flops2010/ FLOPS 2008, Ise: http://www.math.nagoya-u.ac.jp/~garrigue/FLOPS2008/ Sponsor ======= Japan Society for Software Science and Technology (JSSST), Special Interest Group on Programming and Programming Languages (SIG-PPL) In Cooperation With =================== ACM SIGPLAN Asian Association for Foundation of Software (AAFS) Association for Logic Programming (ALP)

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Why GHC is written in Happy and not a monadic parser library?
by blackbox.dev.ml 04 Aug '13

04 Aug '13

Hi! Is there any specific reason why GHC is written in a parser GENERATOR (Happy) and not in MONADIC PARSER COMBINATOR (like parsec)? Is Happy faster / handles better errors / hase some great features or anything else? thank you :)

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Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
by Henk-Jan van Tuyl 03 Aug '13

03 Aug '13

On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > > I am willing to do administrator tasks. > >> 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external >> links' - which is all of the spam. > > This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with "buy" o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, "Nike Air Jordan") Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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ICFP Programming Contest (starts in < 1 week)
by Nikhil Swamy 02 Aug '13

02 Aug '13

Update: This year's ICFP programming contest is just 1 week away. You will have to register to participate 72 hours in advance. Do it now! Update: For updates about the contest, follow us on twitter at ICFPContest2013 If you fancy yourself a discriminating hacker, or just want to prove that your programming language of choice is clearly the best, you should plan to participate in the contest now! -- The contest will start at: 1700 PDT on August 8, 2013 (0000 UTC on August 9, 2013) -- The contest will end at: 1700 PDT on August 11, 2013 (0000 UTC August 12, 2013). -- You need to pre-register for the contest, 72 hours prior to the contest at the latest A hint for this year: The programming task will involve an element of program synthesis. If you have friends who use languages that are not too shabby, or are just extremely cool hackers, give them a heads up too! For more information, visit: http://research.microsoft.com/en-us/events/icfpcontest2013/ ----- The organizers, Peli de Halleux, Michal Moskal, Nikhil Swamy, and Nikolai Tillmann, are all members of the Research in Software Engineering (RiSE) group at MSR Redmond.

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Learning GHC API
by blackbox.dev.ml 02 Aug '13

02 Aug '13

HI! Could you please give me hints where to find materials to learn GHC API? I know there is http://www.haskell.org/haskellwiki/GHC/As_a_library, but this contains only 2 examples and I would to learn how to create datatypes, classes, functions etc using GHC API, how to compile them later or generate Haskell code out of them (if its possible). I would be thnakful for any hint! :) Thank you, Wojtek

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Proposal: Non-recursive let
by oleg＠okmij.org 02 Aug '13

02 Aug '13

Here is a snippet from a real code that could benefit from non-recursive let. The example is notable because it incrementally constructs not one but two structures (both maps), ast and headers. The maps are constructed in a bit interleaved fashion, and stuffing them into the same State would be ungainly. In my real code -- Add an update record to a buffer file do_update :: String -> Handle -> Parsed -> [Parsed] -> IO () do_update "TAF" h ast asts@(_:_) = do rv <- reflect $ get_trange "TRange" ast headers0 <- return . M.fromList =<< sequence (map (\ (n,fld) -> reflect (fld ast) >>= \v -> return (n,v)) fields_header) let headers = M.insert "_report_ranges" (format_two_tstamps rv) headers0 foldM write_period (rv,headers,(snd rv,snd rv)) asts return () where write_period (rv,headers,mv) ast = do pv@(p_valid_from,p_valid_until) <- reflect $ get_trange "TRange" ast check_inside pv rv let prevailing = M.lookup "PREVAILING" ast (mv,pv) <- case prevailing of Just _ -> return (pv,pv) -- set the major valid period -- Make sure each VAR period occurs later than the prevailing -- period. If exactly at the same time add 1 min Nothing -> case () of _ | fst mv < p_valid_from -> return (mv,pv) _ | fst mv == p_valid_from -> return (mv,(p_valid_from + 60, p_valid_until)) _ -> gthrow . InvalidData . unwords $ [ "VAR period begins before prevailing:", show ast, "; prevailing TRange", show mv] let token = maybe (M.findWithDefault "" "VAR" ast) id prevailing let ast1 = M.insert "_token" token . M.insert "_period_valid" (format_two_tstamps pv) . M.unionWith (\_ x -> x) headers $ ast let title = M.member "Title" ast let headers1 = if title then headers else M.delete "_limit_to " . M.delete "_limit_recd" $ headers write_fields h ast1 fields return (rv,headers1,mv)

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