August 2013 - Haskell-Cafe

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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Getting the 'ThreadId' of the calling thread
by Nikita Karetnikov 02 Aug '13

02 Aug '13

(I'm trying to update a package that was written with GHC 6.10.4 in mind.) I feel that a 'myThreadId' action, which is defined in this module [1], is useless, but I'm not sure. I think it will always return the "Map.find: element not in the map" exception because a 'threadMap' contains an empty 'ThreadMap'. Is it right, or am I missing something? [1] https://gitweb.torproject.org/tordnsel.git/blob/HEAD:/src/TorDNSEL/Control/…

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[ANNOUNCE] fficxx : haskell-C++ FFI binding generator
by Ian-Woo Kim 02 Aug '13

02 Aug '13

Dear Haskellers, Hello. I am very happy to announce the first version of fficxx (http://ianwookim.org/fficxx, also, please look at http://github.com/wavewave/fficxx ) fficxx is a haskell Foreign Function Interface (FFI) generator to C++. This tool automatically generates haskell FFI package for a given C++ class structure. It has been used for generating my haskell binding to the ROOT library [1] called HROOT [2]. Now I made it as a separate tool to benefit everyone trying to make a haskell-C++ binding. I am going to explain how to use the library/tool one by one in my blog ( http://ianwookim.org/blog ). Currently, in the fficxx cabal package, I provide a very simple sample in the sample directory. If interested, please try this and give me some comments on it in the discussion mailing list: http://ianwookim.org/fficxx/discuss.html I hope that this is useful to many haskellers. Have fun with it! Thanks, Ian-Woo Kim (wavewave) [1] : http://root.cern.ch [2] : http://ianwookim.org/HROOT

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Hoogle problems?
by Andrew Butterfield 01 Aug '13

01 Aug '13

I've just tried using Hoogle, but either get a 404 not found (http://haskell.org/hoogle/) or else I find I get a ELF 64-bit LSB executable being downloaded If I search using Google and click on the first link (shown as www.haskell.org/hoogle/ I get the following (spaces deliberately added to make it less dangerous) https://www.goo gle.com/url?sa=t&rct=j&q=&esrc=s&sou rce=web&cd=1&cad=rja&ved=0CC8QFjAA&url=http%3A%2F%2Fwww.haskell.org%2Fhoogle%2F&ei=y_njUdZAhY 7sBsGpgNAK&usg=AFQjCNFljq5Oyb4LT7VF-I5MUotq512AQg&sig2 =SVRE_6nF3bhho7-NKqEaCw&bvm=bv.487 05608,d.ZGU Using Mac OS X 10.6, Chrome and Safari Is it me, or is there a wider problem ? -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations & Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ --------------------------------------------------------------------

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Haskell Weekly News: Issue 274
by Daniel Santa Cruz 31 Jul '13

31 Jul '13

Welcome to issue 274 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 14 to 27, 2013. Quotes of the Week * knuth: To help the reader drowning in a sea of abstraction, the following example may serve as a concrete life preserver >From the mail bag * Haskell for schools in Argentina [1] http://goo.gl/EAPHXx Top Reddit Stories * Parallel and Concurrent Programming in Haskell available now in ebook formats, print version can be preordered Domain: shop.oreilly.com, Score: 94, Comments: 14 On Reddit: [2] http://goo.gl/426HB0 Original: [3] http://goo.gl/anqIi * Lenses in pictures Domain: adit.io, Score: 84, Comments: 51 On Reddit: [4] http://goo.gl/e8Bc0N Original: [5] http://goo.gl/1kmqf * Monads made difficult Domain: stephendiehl.com, Score: 70, Comments: 11 On Reddit: [6] http://goo.gl/sQAmVD Original: [7] http://goo.gl/35EDt * Helm - FRP Game Engine Inspired by Elm Domain: github.com, Score: 67, Comments: 19 On Reddit: [8] http://goo.gl/1lJEIB Original: [9] http://goo.gl/4wDcHN * Ian Lynagh leaves GHC HQ Domain: haskell.org, Score: 64, Comments: 4 On Reddit: [10] http://goo.gl/XCnjrA Original: [11] http://goo.gl/Ox5KXW * Yesod in Production Domain: joelt.io, Score: 61, Comments: 13 On Reddit: [12] http://goo.gl/jlhvvm Original: [13] http://goo.gl/7pSCJ * A Comparison Between Perl and Haskell Domain: ocharles.org.uk, Score: 60, Comments: 22 On Reddit: [14] http://goo.gl/cwYXb0 Original: [15] http://goo.gl/P46D4e * Backpack: Retrofitting Haskell with Interfaces (Kilpatrick, Dreyer, SPJ, Marlow) [link to pdf] Domain: mpi-sws.org, Score: 58, Comments: 63 On Reddit: [16] http://goo.gl/sjjAE Original: [17] http://goo.gl/C06Ly * Parallel and Concurrent Programming in Haskell (online version, part of Atlas beta) Domain: chimera.labs.oreilly.com, Score: 56, Comments: 35 On Reddit: [18] http://goo.gl/x5dhf5 Original: [19] http://goo.gl/H7MeFa * Why Haskell at School matters Domain: sawafaso.blogspot.com.ar, Score: 52, Comments: 18 On Reddit: [20] http://goo.gl/2LHlsn Original: [21] http://goo.gl/8Dwy4R * Measuring the Haskell Gap :: PDF Domain: leafpetersen.com, Score: 49, Comments: 13 On Reddit: [22] http://goo.gl/6b4xdx Original: [23] http://goo.gl/nqDJZd * Automatic SIMD Vectorization for Haskell by Leaf Petersen, Dominic Orchard and Neal Glew [ICFP13][PDF] Domain: leafpetersen.com, Score: 47, Comments: 13 On Reddit: [24] http://goo.gl/dPCPio Original: [25] http://goo.gl/gDBajX * IRC Browse Domain: chrisdone.com, Score: 43, Comments: 12 On Reddit: [26] http://goo.gl/OPXVQK Original: [27] http://goo.gl/IOoJgT * How to Help Develop Hackage2 Domain: self.haskell, Score: 40, Comments: 46 On Reddit: [28] http://goo.gl/NQGmY Original: [29] http://goo.gl/NQGmY Top StackOverflow Questions * Can someone explain to me why the app function of ArrowApply makes them as powerful as monads? votes: 13, answers: 2 Read on SO: [30] http://goo.gl/V6I0fK * Haskell newtype with parentheses votes: 12, answers: 2 Read on SO: [31] http://goo.gl/k9A7YU * GHC: insert compilation date votes: 11, answers: 2 Read on SO: [32] http://goo.gl/MbWGqH * How can Haskell function guards operate on other values than the functions parameters? votes: 10, answers: 2 Read on SO: [33] http://goo.gl/sgI9i8 * Can Haskell functions be serialized? votes: 10, answers: 3 Read on SO: [34] http://goo.gl/fVsPvv * Calling Haskell library from C++ votes: 10, answers: 1 Read on SO: [35] http://goo.gl/6V2zqb * Debugging compile time performance issues caused by GHC's constraint solver votes: 10, answers: 1 Read on SO: [36] http://goo.gl/XPAqAy * Haskell performance when using classes and instances votes: 10, answers: 2 Read on SO: [37] http://goo.gl/EZNVMS * Could not deduce (m ~ m1) votes: 9, answers: 1 Read on SO: [38] http://goo.gl/vW0x6D * Running a subexpression once votes: 9, answers: 2 Read on SO: [39] http://goo.gl/MdRgkZ Until next time, [40]+Daniel Santa Cruz References 1. http://sawafaso.blogspot.com.ar/2013/07/why-haskell-at-school-matters.html?… 2. http://shop.oreilly.com/product/0636920026365.do 3. http://www.reddit.com/r/haskell/comments/1ioorw/parallel_and_concurrent_pro… 4. http://adit.io/posts/2013-07-22-lenses-in-pictures.html 5. http://www.reddit.com/r/haskell/comments/1isryj/lenses_in_pictures/ 6. http://www.stephendiehl.com/posts/monads.html 7. http://www.reddit.com/r/haskell/comments/1iafky/monads_made_difficult/ 8. https://github.com/z0w0/helm 9. http://www.reddit.com/r/haskell/comments/1j1fd8/helm_frp_game_engine_inspir… 10. http://www.haskell.org/pipermail/ghc-devs/2013-July/001677.html 11. http://www.reddit.com/r/haskell/comments/1ikc91/ian_lynagh_leaves_ghc_hq/ 12. http://joelt.io/entries/yesod-in-production 13. http://www.reddit.com/r/haskell/comments/1inf8a/yesod_in_production/ 14. http://ocharles.org.uk/blog/posts/2013-07-26-a-comparison-between-perl-and-… 15. http://www.reddit.com/r/haskell/comments/1j3jgd/a_comparison_between_perl_a… 16. http://www.mpi-sws.org/~skilpat/backpack/ 17. http://www.reddit.com/r/haskell/comments/1id0p7/backpack_retrofitting_haske… 18. http://chimera.labs.oreilly.com/books/1230000000929/index.html 19. http://www.reddit.com/r/haskell/comments/1iwr7x/parallel_and_concurrent_pro… 20. http://sawafaso.blogspot.com.ar/2013/07/why-haskell-at-school-matters.html 21. http://www.reddit.com/r/haskell/comments/1j3k19/why_haskell_at_school_matte… 22. http://www.leafpetersen.com/leaf/publications/hs2013/haskell-gap.pdf 23. http://www.reddit.com/r/haskell/comments/1j2k4a/measuring_the_haskell_gap_p… 24. http://www.leafpetersen.com/leaf/publications/icfp2013/vectorization-haskel… 25. http://www.reddit.com/r/haskell/comments/1izyok/automatic_simd_vectorizatio… 26. http://chrisdone.com/posts/ircbrowse 27. http://www.reddit.com/r/haskell/comments/1iy8nv/irc_browse/ 28. http://www.reddit.com/r/haskell/comments/1iki8p/how_to_help_develop_hackage… 29. http://www.reddit.com/r/haskell/comments/1iki8p/how_to_help_develop_hackage… 30. http://stackoverflow.com/questions/17668452/can-someone-explain-to-me-why-t… 31. http://stackoverflow.com/questions/17853232/haskell-newtype-with-parentheses 32. http://stackoverflow.com/questions/17796521/ghc-insert-compilation-date 33. http://stackoverflow.com/questions/17650866/how-can-haskell-function-guards… 34. http://stackoverflow.com/questions/17785916/can-haskell-functions-be-serial… 35. http://stackoverflow.com/questions/17815804/calling-haskell-library-from-c 36. http://stackoverflow.com/questions/17822539/debugging-compile-time-performa… 37. http://stackoverflow.com/questions/17834802/haskell-performance-when-using-… 38. http://stackoverflow.com/questions/17765690/could-not-deduce-m-m1 39. http://stackoverflow.com/questions/17900495/running-a-subexpression-once 40. https://plus.google.com/105107667630152149014/about

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