- Haskell-Cafe - Haskell.org

Standard output of exec
by Ernesto Rodriguez 13 Dec '14

13 Dec '14

Dear All, I have a program which invokes an external program via exec. I wish to capture the standard output of the program without using external (temporary) files. Is it possible to create an in memory handle which I can tell exec to use as standard output of the invoked program. It would be great if it can be done without any extra dependencies. Best, -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

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Having a very annoying problem related with (I think) lazy IO
by Ömer Sinan Ağacan 12 Dec '14

12 Dec '14

Hi all, I'm having a horrible bug in my program: a print statement that doesn't force any values(e.g. it just prints a static string) effecting the behavior of the program. I tried hard to produce a minimal example, but couldn't manage it. So instead I'm just going to point to the source code of my program: https://github.com/osa1/rho-torrent/blob/master/src/Rho/Listener.hs#L125 If I remove this print statement everything goes weird. I think when I remove this statement some effects are not happening, or happening in wrong times. Here's how I'm testing the different behavior: This is a network application and I'm creating an environment in my system and making this program talk to another program. I run my program 10 times without the print statement, and run 10 times with it. Both groups produce same output in every time, but outputs of first group and second group are different from each other. (apart from extra prints, of course) So this print statement effects some other things, and expected behavior is the one happening with print statement. I know the program is big for anyone to figure things in short amount of time, I'm just looking for tips and pitfalls that I may be falling. Does anyone have any ideas what I may be doing wrong? Note that the program doesn't use any unsafe stuff. Thanks in advance.

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Typesafe multi-table PostgreSQL queries in Yesod with schema names?
by Manuel Gómez 12 Dec '14

12 Dec '14

Hi list, I’m looking for a rather specific point in the design space for database access libraries in Haskell and I’m having some trouble figuring out whether any existing library covers my use cases. Specifically, I need to 1. get a Yesod application to access a PostgreSQL database without having to reinvent the universe to get them decently integrated, 2. write queries in an EDSL with a good approach to composability and decent static verification against some declared form of the database objects, 3. be able to perform joins 4. be able to specify the schema name for each database object used in queries These are the closest solutions I’ve found: * HaskellDB: seems like the best approach to the EDSL issue and it’s certainly capable of performing joins, and I haven’t looked into schema names, but it seems possible — but it seems to be poorly maintained and somewhat abandoned, and comfortable Yesod integration doesn’t sound like a lot of fun. * Persistent: it’s perfectly integrated into Yesod, and together with Esqueleto, it provides a nice, typesafe and fully expressive EDSL — but, as far as I can tell, there is no support for specifying schema names, which is essential for my use case. * Groundhog: Yesod integration is reasonable since Persistent borrowed a good bit of its design, and after a few weeks of experimenting, I’ve got it pretty much worked out by now, but while it certainly takes an interesting and solid approach to the EDSL issue, it seems completely incapable of performing joins, and hence its usability is very heavily limited — I don’t fancy creating SQL views for each and every join I need. * postgresql-simple: minimal Yesod integration shouldn’t be too uncomfortable, since Persistent uses it internally anyway, and it can certainly do joins and use whatever fancy SQL magic I could ever want to write, certainly including schema names — but it doesn’t really do anything for statically checking queries against a schema specification or for composability and code reuse, and I might as well just use Persistent to do raw SQL. I’ve breifly looked at modifying Persistent to add support for schema names, but I got lost in the source code very quickly, and I worry about the fact that it requires serial primary keys — Groundhog is *very* nice in that it has a great approach to DBMS-specific syntax and other things I’ll probably need (non-numeric and composite keys), but it’s really a shame that it can’t do joins, and the workaround with views sort of defeats the purpose of SQL abstraction. I’m not sure what to do about this. Is there some grand unified relational database access library I just haven’t found, or is a pull request the only solution?

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Using Docker to ease the pain of building GHC
by Sean Seefried 12 Dec '14

12 Dec '14

Hi all, I wrote a blog post recently on my experience of using Docker to relieve the pain of developing a GHC cross compiler build script. I thought it might be helpful. http://lambdalog.seanseefried.com/posts/2014-12-12-docker-build-scripts.html Sean

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Linker troubles with cabal sandbox
by Zach Moazeni 12 Dec '14

12 Dec '14

Hello folks, I'm getting some linker trouble in the form of: "Undefined symbols for architecture x86_64: ... ld: symbol(s) not found for architecture x86_64" When trying to compile a simple hashtables example https://hackage.haskell.org/package/hashtables-1.2.0.0 I came across a previous thread with a solution https://www.haskell.org/pipermail/haskell-cafe/2014-October/116469.html but it's not working for me. Here are the steps I'm taking and the verbose errors: https://gist.github.com/zmoazeni/b68c66f5e15654948946 Can someone give me a hand? Thanks! Zach

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Re: [Haskell-cafe] Standard output of exec
by Michael Snoyman 11 Dec '14

11 Dec '14

That's not going to be sufficient; you could end up in a deadlock by not flushing the output buffer of the child process. Have a look at the implementation of readProcess to see how to deal with some of these tricky issues in the lazy I/O world: http://hackage.haskell.org/package/process-1.2.0.0/docs/src/System-Process.… On Thu Dec 11 2014 at 6:40:54 PM Derek McLoughlin < derek.mcloughlin(a)gmail.com> wrote: > Something like this would work: > > import System.Process > import System.IO > import System.Exit > > readExecOutput :: String -> [String] -> IO () > readExecOutput cmdName cmdOptions = do > (_, Just hOut, _, jHandle) <- > createProcess (proc cmdName cmdOptions) > { cwd = Just "." > , std_out = CreatePipe > } > exitCode <- waitForProcess jHandle > exec_output <- hGetContents hOut > case exitCode of > ExitSuccess -> putStr $ "Success!\n" ++ exec_output > _ -> putStrLn "Command failed" > > > Derek. > > On Thursday, 11 December 2014 13:29:34 UTC, Ernesto Rodriguez wrote: > >> Hi Michael, >> >> Thank you for your answer. Conduits certainly are the best approach, but >> the output I am going to read is very short and the external program will >> only be called once or twice per execution so I think Lazy IO won't be >> problematic here. >> >> Best regards, >> >> Ernesto >> > On Thu, Dec 11, 2014 at 10:18 AM, Michael Snoyman <mic...(a)snoyman.com> >> wrote: >> > You can use readProcess[1], but it has some downsides: >>> >>> 1. Can't specify all options to CreateProcess, e.g., working directory >>> and environment variables. >>> 2. Relies on lazy I/O >>> >>> I use Data.Conduit.Process[2] for this kind of thing, which has a full >>> tutorial[3]. However, it *does* involve non-base dependencies. >>> >>> [1] http://www.stackage.org/haddock/2014-12-10-ghc78-exc/ >>> process-1.2.0.0/System-Process.html#v:readProcess >>> [2] http://www.stackage.org/haddock/2014-12-10-ghc78-exc/ >>> conduit-extra-1.1.4.2/Data-Conduit-Process.html >>> [3] https://www.fpcomplete.com/user/snoyberg/library- >>> documentation/data-conduit-process?show=tutorials >>> >>> On Thu Dec 11 2014 at 11:11:12 AM Ernesto Rodriguez <ne...(a)netowork.me> >>> wrote: >>> >> Dear All, >>>> >>>> I have a program which invokes an external program via exec. I wish to >>>> capture the standard output of the program without using external >>>> (temporary) files. Is it possible to create an in memory handle which I can >>>> tell exec to use as standard output of the invoked program. It would be >>>> great if it can be done without any extra dependencies. >>>> >>>> Best, >>>> >>>> -- >>>> Ernesto Rodriguez >>>> >>>> Masters Student >>>> Computer Science >>>> Utrecht University >>>> >>>> www.netowork.me >>>> github.com/netogallo >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> >>> Haskel...(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>> >> >> >> -- >> Ernesto Rodriguez >> >> Masters Student >> Computer Science >> Utrecht University >> >> www.netowork.me >> github.com/netogallo >> >> >>

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Haskell Weekly News: Issue 313
by Daniel Santa Cruz 11 Dec '14

11 Dec '14

Welcome to issue 313 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from November 16 to December 6, 2014 Quotes of the Week * Dijkstra: How do we convince people that in programming simplicity and clarity (in short: what mathematicians call "elegance") are not a dispensable luxury, but a crucial matter that decides between success and failure? * Dijkstra: Simplicity is a great virtue but it requires hard work to achieve it and education to appreciate it. And to make matters worse: complexity sells better. * monochrom: those who have learned from history are bound to helplessly watch other people repeat it. Top Reddit Stories * 24 Days of GHC Extensions: Welcome! Domain: ocharles.org.uk, Score: 140, Comments: 24 Original: [1] http://goo.gl/HvjuvJ On Reddit: [2] http://goo.gl/4Skf5Z * How I Start: Haskell Domain: howistart.org, Score: 120, Comments: 11 Original: [3] http://goo.gl/T2OCwj On Reddit: [4] http://goo.gl/1bGl7I * Lucid: templating DSL for HTML Domain: chrisdone.com, Score: 105, Comments: 59 Original: [5] http://goo.gl/JJsJlT On Reddit: [6] http://goo.gl/jyvkHu * Erik Meijer planning to make Category theory for programmers MOOC Domain: imgur.com, Score: 105, Comments: 45 Original: [7] http://goo.gl/ZnCi6Z On Reddit: [8] http://goo.gl/8vOFP0 * An opportunity in Singapore Domain: self.haskell, Score: 98, Comments: 31 Original: [9] http://goo.gl/X0OBAE On Reddit: [10] http://goo.gl/X0OBAE * Dan Piponi working on an audio synthesis tool written in Haskell Domain: plus.google.com, Score: 90, Comments: 1 Original: [11] http://goo.gl/J83NRS On Reddit: [12] http://goo.gl/Cbhx8x * GHC and LLVM: The State of the Union Domain: smart-cactus.org, Score: 86, Comments: 13 Original: [13] http://goo.gl/CA7R4C On Reddit: [14] http://goo.gl/l3L8H7 * Learning Haskell as a Nonprogrammer Domain: superginbaby.wordpress.com, Score: 84, Comments: 77 Original: [15] http://goo.gl/9Q1ufS On Reddit: [16] http://goo.gl/Yqni7Y * Announcing Opaleye: SQL-generating embedded domain specific language for Postgres Domain: self.haskell, Score: 84, Comments: 85 Original: [17] http://goo.gl/HoMXjj On Reddit: [18] http://goo.gl/HoMXjj * Deploy any Haskell application. Instantly. Domain: halcyon.sh, Score: 79, Comments: 27 Original: [19] http://goo.gl/sxQWKk On Reddit: [20] http://goo.gl/5V2dMj * Haskell for all: How to build library-agnostic streaming sources Domain: haskellforall.com, Score: 75, Comments: 26 Original: [21] http://goo.gl/1QcyRf On Reddit: [22] http://goo.gl/eYwlBp * Image Processing with Comonads Domain: jaspervdj.be, Score: 74, Comments: 33 Original: [23] http://goo.gl/GZu2oc On Reddit: [24] http://goo.gl/Xsh0z9 * 24 Days of GHC Extensions: View Patterns Domain: ocharles.org.uk, Score: 69, Comments: 39 Original: [25] http://goo.gl/uN3PLf On Reddit: [26] http://goo.gl/2YZ1r0 * Well-Typed are hiring! Domain: well-typed.com, Score: 68, Comments: 36 Original: [27] http://goo.gl/DxYgrT On Reddit: [28] http://goo.gl/2imRKu * 24 Days of GHC Extensions: Pattern Synonyms Domain: ocharles.org.uk, Score: 65, Comments: 20 Original: [29] http://goo.gl/j75DP8 On Reddit: [30] http://goo.gl/VTvCof * How lazy evaluation works Domain: hackhands.com, Score: 63, Comments: 30 Original: [31] http://goo.gl/YPOFaM On Reddit: [32] http://goo.gl/w2uKGJ * Learn You an Agda Domain: williamdemeo.github.io, Score: 59, Comments: 61 Original: [33] http://goo.gl/hQ5DaT On Reddit: [34] http://goo.gl/2I00Zh * 24 Days of GHC Extensions: Record Wildcards Domain: ocharles.org.uk, Score: 59, Comments: 38 Original: [35] http://goo.gl/BnXqjA On Reddit: [36] http://goo.gl/yBzMoY * Functional Programming Patterns (BuildStuff '14) Domain: slideshare.net, Score: 55, Comments: 15 Original: [37] http://goo.gl/fzo56I On Reddit: [38] http://goo.gl/XCy482 * The GHCJS guys are working on TH cross compiling Domain: thread.gmane.org, Score: 54, Comments: 15 Original: [39] http://goo.gl/XyA4FS On Reddit: [40] http://goo.gl/eRULJH * Silk is looking for a Haskell engineer Domain: self.haskell, Score: 53, Comments: 21 Original: [41] http://goo.gl/eB10Wa On Reddit: [42] http://goo.gl/eB10Wa * Keera studios: Haskell games & adventure engine for Android, beta testers wanted! Domain: keera.co.uk, Score: 53, Comments: 8 Original: [43] http://goo.gl/rk7Wrx On Reddit: [44] http://goo.gl/ciCbYp * 24 Days of GHC Extensions: Bang Patterns Domain: ocharles.org.uk, Score: 51, Comments: 34 Original: [45] http://goo.gl/X4Huvh On Reddit: [46] http://goo.gl/eHmymQ * Let's Build a Browser Engine in Haskell: part 6 Domain: hrothen.github.io, Score: 49, Comments: 4 Original: [47] http://goo.gl/mDnmpX On Reddit: [48] http://goo.gl/96aLMl * A Docker build script for GHC 7.8.3 cross-compiler targeting Android Domain: github.com, Score: 49, Comments: 21 Original: [49] http://goo.gl/uCsMNK On Reddit: [50] http://goo.gl/KvY3JX * amazonka - Comprehensive Amazon Web Services SDK Domain: self.haskell, Score: 46, Comments: 17 Original: [51] http://goo.gl/Yi18LS On Reddit: [52] http://goo.gl/Yi18LS * 24 Days of GHC Extensions: Rebindable Syntax (guest post by Benjamin Kovach) Domain: ocharles.org.uk, Score: 46, Comments: 14 Original: [53] http://goo.gl/7e3J1I On Reddit: [54] http://goo.gl/jJJONL * Haskell Communities and Activities Report — November 2014 Domain: haskell.org, Score: 45, Comments: 6 Original: [55] http://goo.gl/7dfgQk On Reddit: [56] http://goo.gl/fME8UF * A taste of State: parsers are easy Domain: nikita-volkov.github.io, Score: 44, Comments: 4 Original: [57] http://goo.gl/HF4R3O On Reddit: [58] http://goo.gl/d7bLLV * Hybrid server/client Haskell web apps / Michael Snoyman Domain: youtube.com, Score: 42, Comments: 31 Original: [59] http://goo.gl/IbxCkI On Reddit: [60] http://goo.gl/Yjx8wj * Nix: The Cabal Purgatory HOWTO Domain: cse.chalmers.se, Score: 41, Comments: 45 Original: [61] http://goo.gl/pxdnYA On Reddit: [62] http://goo.gl/2uE8ya * Ranjit Jhala talks about Liquid Haskell Domain: youtu.be, Score: 41, Comments: 14 Original: [63] http://goo.gl/EAubrl On Reddit: [64] http://goo.gl/f1D8iz * A quick and easy Emacs configuration for Haskell Domain: github.com, Score: 39, Comments: 16 Original: [65] http://goo.gl/8akoJo On Reddit: [66] http://goo.gl/Ma8iDO Top StackOverflow Questions * Turning A => M[B] into M[A => B] votes: 22, answers: 2 Read on SO: [67] http://goo.gl/P4AXC2 * When are type signatures necessary in Haskell? votes: 19, answers: 3 Read on SO: [68] http://goo.gl/9YcgHo * Boilerplate-free annotation of ASTs in Haskell? votes: 18, answers: 4 Read on SO: [69] http://goo.gl/dNv5kU * Can fusion see through newtype wrappers? votes: 17, answers: 1 Read on SO: [70] http://goo.gl/00ONhp * Haskell: Why use Proxy? votes: 17, answers: 2 Read on SO: [71] http://goo.gl/bWRvVN * Is Haskell's laziness an elegant alternative to Python's generators? votes: 16, answers: 3 Read on SO: [72] http://goo.gl/oblTSW * Type inference interferes with referential transparency votes: 16, answers: 7 Read on SO: [73] http://goo.gl/2nIW1x * What exactly is the kind “*” in Haskell? votes: 15, answers: 2 Read on SO: [74] http://goo.gl/sOXwgi * What's the difference between partial evaluation and function inlining in a functional language? votes: 14, answers: 1 Read on SO: [75] http://goo.gl/AOZuCH Until next time, [76]+Daniel Santa Cruz References 1. https://ocharles.org.uk/blog/posts/2014-12-01-24-days-of-ghc-extensions.html 2. http://www.reddit.com/r/haskell/comments/2nyy5z/24_days_of_ghc_extensions_w… 3. http://howistart.org/posts/haskell/1 4. http://www.reddit.com/r/haskell/comments/2mkbay/how_i_start_haskell/ 5. http://chrisdone.com/posts/lucid 6. http://www.reddit.com/r/haskell/comments/2my5bc/lucid_templating_dsl_for_ht… 7. http://imgur.com/OQbeouW 8. http://www.reddit.com/r/haskell/comments/2o5m95/erik_meijer_planning_to_mak… 9. http://www.reddit.com/r/haskell/comments/2nkjdq/an_opportunity_in_singapore/ 10. http://www.reddit.com/r/haskell/comments/2nkjdq/an_opportunity_in_singapore/ 11. https://plus.google.com/107913314994758123748/posts/9EQP1xqsQZ6 12. http://www.reddit.com/r/haskell/comments/2nsdkq/dan_piponi_working_on_an_au… 13. http://smart-cactus.org/~ben/posts/2014-11-28-state-of-llvm-backend.html 14. http://www.reddit.com/r/haskell/comments/2nphz5/ghc_and_llvm_the_state_of_t… 15. http://superginbaby.wordpress.com/2014/11/18/learning-haskell-as-a-nonprogr… 16. http://www.reddit.com/r/haskell/comments/2mp3fb/learning_haskell_as_a_nonpr… 17. http://www.reddit.com/r/haskell/comments/2nxx7n/announcing_opaleye_sqlgener… 18. http://www.reddit.com/r/haskell/comments/2nxx7n/announcing_opaleye_sqlgener… 19. https://halcyon.sh/ 20. http://www.reddit.com/r/haskell/comments/2ogoot/deploy_any_haskell_applicat… 21. http://www.haskellforall.com/2014/11/how-to-build-library-agnostic-streamin… 22. http://www.reddit.com/r/haskell/comments/2n8xo4/haskell_for_all_how_to_buil… 23. http://jaspervdj.be/posts/2014-11-27-comonads-image-processing.html 24. http://www.reddit.com/r/haskell/comments/2nmf5l/image_processing_with_comon… 25. https://ocharles.org.uk/blog/posts/2014-12-02-view-patterns.html 26. http://www.reddit.com/r/haskell/comments/2o341e/24_days_of_ghc_extensions_v… 27. http://www.well-typed.com/blog/101/ 28. http://www.reddit.com/r/haskell/comments/2nlbei/welltyped_are_hiring/ 29. https://ocharles.org.uk/blog/posts/2014-12-03-pattern-synonyms.html 30. http://www.reddit.com/r/haskell/comments/2o6bq5/24_days_of_ghc_extensions_p… 31. https://hackhands.com/lazy-evaluation-works-haskell/ 32. http://www.reddit.com/r/haskell/comments/2n0991/how_lazy_evaluation_works/ 33. http://williamdemeo.github.io/2014/02/27/learn-you-an-agda/ 34. http://www.reddit.com/r/haskell/comments/2o49aw/learn_you_an_agda/ 35. https://ocharles.org.uk/blog/posts/2014-12-04-record-wildcards.html 36. http://www.reddit.com/r/haskell/comments/2oaj7e/24_days_of_ghc_extensions_r… 37. http://www.slideshare.net/ScottWlaschin/fp-patterns-buildstufflt 38. http://www.reddit.com/r/haskell/comments/2ntyqz/functional_programming_patt… 39. http://thread.gmane.org/gmane.comp.lang.haskell.ghc.devel/7299 40. http://www.reddit.com/r/haskell/comments/2o9fez/the_ghcjs_guys_are_working_… 41. http://www.reddit.com/r/haskell/comments/2mok2d/silk_is_looking_for_a_haske… 42. http://www.reddit.com/r/haskell/comments/2mok2d/silk_is_looking_for_a_haske… 43. http://keera.co.uk/blog/2014/11/24/haskell-android-games-adventure-engine-b… 44. http://www.reddit.com/r/haskell/comments/2nesiz/keera_studios_haskell_games… 45. https://ocharles.org.uk/blog/posts/2014-12-05-bang-patterns.html 46. http://www.reddit.com/r/haskell/comments/2oe1kk/24_days_of_ghc_extensions_b… 47. http://hrothen.github.io/2014/11/19/lets-build-a-browser-engine-in-haskell-… 48. http://www.reddit.com/r/haskell/comments/2mthmd/lets_build_a_browser_engine… 49. https://github.com/sseefried/docker-build-ghc-android 50. http://www.reddit.com/r/haskell/comments/2n5njy/a_docker_build_script_for_g… 51. http://www.reddit.com/r/haskell/comments/2n2sgc/amazonka_comprehensive_amaz… 52. http://www.reddit.com/r/haskell/comments/2n2sgc/amazonka_comprehensive_amaz… 53. https://ocharles.org.uk/blog/guest-posts/2014-12-06-rebindable-syntax.html 54. http://www.reddit.com/r/haskell/comments/2ohm6p/24_days_of_ghc_extensions_r… 55. https://www.haskell.org/communities/11-2014/report.pdf 56. http://www.reddit.com/r/haskell/comments/2mirc9/haskell_communities_and_act… 57. http://nikita-volkov.github.io/a-taste-of-state-parsers-are-easy/ 58. http://www.reddit.com/r/haskell/comments/2mk0ww/a_taste_of_state_parsers_ar… 59. https://www.youtube.com/watch?v=XfINRj5OzGw 60. http://www.reddit.com/r/haskell/comments/2mhzj3/hybrid_serverclient_haskell… 61. http://www.cse.chalmers.se/~bernardy/nix.html 62. http://www.reddit.com/r/haskell/comments/2mv3vo/nix_the_cabal_purgatory_how… 63. http://youtu.be/vYh27zz9530 64. http://www.reddit.com/r/haskell/comments/2nc9w6/ranjit_jhala_talks_about_li… 65. https://github.com/chrisdone/emacs-haskell-config 66. http://www.reddit.com/r/haskell/comments/2oib5x/a_quick_and_easy_emacs_conf… 67. http://stackoverflow.com/questions/27267848/turning-a-mb-into-ma-b 68. http://stackoverflow.com/questions/27067905/when-are-type-signatures-necess… 69. http://stackoverflow.com/questions/27157717/boilerplate-free-annotation-of-… 70. http://stackoverflow.com/questions/27014235/can-fusion-see-through-newtype-… 71. http://stackoverflow.com/questions/27044209/haskell-why-use-proxy 72. http://stackoverflow.com/questions/26952366/is-haskells-laziness-an-elegant… 73. http://stackoverflow.com/questions/27019906/type-inference-interferes-with-… 74. http://stackoverflow.com/questions/27095011/what-exactly-is-the-kind-in-has… 75. http://stackoverflow.com/questions/26996110/whats-the-difference-between-pa… 76. https://plus.google.com/105107667630152149014/about

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Polykinded promoted types
by Gautier DI FOLCO 10 Dec '14

10 Dec '14

Hi all, Is there a way to get a Polykinded promoted type. For example with *Maybe* I get: data *Maybe* a = Just a | Nothing Prelude> :k *Maybe* *Maybe* :: ** -> ** Is it possible to get a *k -> **? If so, how can I do that? Thanks in advance for your help, Regards.

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Cabal builds file at top level incorrectly
by Tom Ellis 10 Dec '14

10 Dec '14

Cabal's behaviour is strange to me. My example cabal file is as follows: name: break version: 0.1 cabal-version: >= 1.2 build-type: Simple library hs-source-dirs: src build-depends: base >= 4 && < 5 exposed-modules: Break and I have a module `src/Break.hs` that I want it to compile. That would work fine. But if I have another file at the top-level caled `Break.hs` it compiles that one instead. It also seems to want it to be an executable. What's going on? Full reproduction below. You can see it is trying to compile `Break.hs` not `src/Break.hs`. % git clone https://github.com/tomjaguarpaw/break.git Cloning into 'break'... [...] % cd break % cabal configure && cabal test Resolving dependencies... Configuring break-0.1... Re-configuring with test suites enabled. If this fails, please run configure manually. Resolving dependencies... Configuring break-0.1... Building break-0.1... Preprocessing library break-0.1... [1 of 1] Compiling Main ( Break.hs, dist/build/Main.o ) Break.hs:1:1: The function `main' is not defined in module `main:Main'

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Why doesn't text-icu work on 64 bit Windows?
by Yitzchak Gale 10 Dec '14

10 Dec '14

The text-icu library works fine with a 32 bit build of GHC on Windows. But with a 64 bit build of GHC, any use of a function from this library causes a segfault. See the issue on github for simple instructions for how to reproduce the problem: https://github.com/bos/text-icu/issues/12 As far as we know now, this could be caused by any of the following: 1. Some difference in the way ICU builds their DLLs for 64 bit Windows. 2. Some difference in the FFI bindings that is needed specifically for 64 bit Windows. 3. A problem with GHC's linking on 64 bit Windows. 4. A problem with Cabal, e.g., some option or flag that needs to be passed by default on 64 bit Windows. This is an important (and extremely high quality, thanks to Bryan O'Sullivan) library for the community, and very important for us, so I'm posting about the issue here to raise visibility. Any ideas about possible approaches to investigate or eliminate any of the above possibilities would be highly appreciated. Thanks, Yitz

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[ANN] servant 0.2 - trying to rethink webservices and APIs
by Alp Mestanogullari 10 Dec '14

10 Dec '14

Hello -cafe, These past few weeks, with Sönke Hahn and Julian Arni, we've been working on a complete rewrite of servant, a library whose aim is to assist as much as possible in writing webservices. We came up with a (type-level) "API combinator" based design, that lets us declare webservice APIs, implement handlers that serve those APIs, derive client functions in Haskell and Javascript to query those endpoints and generate documentation. This is all very composable, you can write your own combinators or write your own interpretation of an API's type, or just use the "standard" ones already provided by the core library. Here's an announcement post with an example: http://alpmestan.com/posts/2014-12-09-rethinking-webservices-apis-haskell.h… Getting started guide: http://haskell-servant.github.io/getting-started/ The reddit discussion (in case you prefer reddit to here for such discussions): http://www.reddit.com/r/haskell/comments/2orb8t/rethinking_webservice_and_a… All 4 libraries are installable with a simple 'cabal install', have a decent amount of documentation and accompanying examples. We would be glad to hear any kind of feedback or ideas -- we're already deploying servant webservices in our respective companies so it suits our use cases, but we would gladly discuss new combinators and what not. -- Alp Mestanogullari

2 2

Cabal 1.8 ignores bounds in test-suite sections?
by Tom Ellis 09 Dec '14

09 Dec '14

I have a cabal file with two test suites https://github.com/tomjaguarpaw/haskell-opaleye/blob/master/opaleye.cabal If I change the dependency in "test-suite tutorial", say, from `profunctors` to `profunctors >= 0.6` then the bound is completely ignored and cabal will attempt to compile and run the test suite even without a suitable version present. Is this expected behaviour? Tom

3 5

Typed communications in (Safe) Haskell
by Giacomo Tesio 09 Dec '14

09 Dec '14

Hi, I'm Giacomo and I'm new to this list. I'm looking for advices to develop an haskell p2p system where the nodes comunicate over an untrusted channel (the internet). I'd like to use TLS to authenticate each node with each other. I'm wondering if exists any library/framework that already suits my needs: 1. Enable typed communications (make it easy to go from ByteString to our ADT for messages) 2. Support TLS 3. Is written in plain (possibly Safe) Haskell, because I plan to make it runnable on HaLVM/HaLNS (in the long run) 4. Acceptably easy to use since most of nodes will simply receive messages, run a simple (often pure) function on them and return a response to the caller I've seen a lot of interesting libraries on hackage (courier, daemons and more...) but AFAIK none support TLS. Any suggestion? Thanks, Giacomo

2 1

External representation of GHC bytecode
by Sean Seefried 09 Dec '14

09 Dec '14

Friends, I'd like to ask a possible quite ignorant question. Can GHC produce an external representation of GHC byte code, and load that byte code back up? If it's just an internal representation is there any good reason for this? It seems to me that this would be useful for at least a few reasons: 1. This could solve the Template Haskell on cross compiler's dilemma. Currently, the problem is that a cross compiler will produce object code for the target machine which cannot be run on the host machine. But if this were bytecode the problem would be trivial to solve. 2. You could transmit Haskell programs over the network to run on arbitrary machines. 3. As it stands GHC sometimes has long standing bugs where it produces incorrect machine code on some architectures for various (fairly boring) reasons. Interpreting bytecode might be less efficient but would at least ensure that you could, in principle, run GHC on a variety of different platforms until these bugs are finally squashed. Sean

3 5

Music made with Haskell
by Anton Kholomiov 08 Dec '14

08 Dec '14

I wrote two tracks completely with Haskell. You can listen to them on the soundcloud: https://soundcloud.com/anton-kho/celtic https://soundcloud.com/anton-kho/invisible-ocean The music is based on samples but a single track uses no more than 6 samples. The code is under 100 lines of code. I'm using my libs csound-sampler and csound-expression. You can check out the source code at: http://ge.tt/3MjwF852/v/0 Please turn off the AdBlock for this page to download the code. Happy haskelling! Anton

4 7

New Functional Programming Job Opportunities
by Functional Jobs 08 Dec '14

08 Dec '14

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8771-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

1 0

[Haskell] ANNOUNCE: Applied Functional Programming (AFP) Summerschool 6-17 July 2015, Utrecht, Netherlands
by Atze Dijkstra 08 Dec '14

08 Dec '14

=========== AFP Summerschool 2015 =========== Applied Functional Programming (AFP) Summerschool July 6-17, 2015 Utrecht University, Department of Information and Computing Sciences Utrecht, The Netherlands Summerschool & registration website: http://www.utrechtsummerschool.nl/courses/science/applied-functional-progra… AFP website : http://www.cs.uu.nl/wiki/USCS contact : Uscs-afp(a)lists.science.uu.nl *** The 2015 edition of the Applied Functional Programming (AFP) Summerschool in Utrecht, Netherlands will be held from 6-17 July 2015. The summerschool teaches Haskell on both beginners and advanced levels via lectures and lab exercises. More info can be found via the references above, included here is a summary from the summerschool info: ``Typed functional programming in Haskell allows for the development of compact programs in minimal time and with maximal guarantees about robustness and correctness. The course introduces Haskell as well as its theoretical underpinnings such as typed lambda calculus, and Damas-Milner type inference. There is ample opportunity to put this all in practice during lab sessions. Typed functional programming languages allow for the development of robust, concise programs in a short amount of time. The key advantages are higher-order functions as an abstraction mechanism, and an advanced type system for safety and reusability. This course introduces Haskell, a state-of-the-art functional programming language, together with some of its theoretical background, such as typed lambda calculi, referential transparency, Damas-Milner type inference, type level programming, and functional design patterns. We will combine this with applications of functional programming, concentrating on topics such as language processing, building graphical user interfaces, networking, databases, and programming for the web. The goal of the course is not just to teach the programming language and underlying theory, but also to learn about the Haskell community and to get hands-on experience by doing lab exercises or a Haskell project of your own.'' *** This year is somewhat special in that the Tour de France starts in Utrecht the weekend before the start of the summerschool. It is an opportunity for enjoying the related festivities. It also implies that housing may be more difficult the weekend before the summerschool, something to keep in mind when you wait with registration until the latest. regards, - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)uu.nl ............... / |___\ _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

1 0

Announce: Leksah 0.14.3
by Hamish Mackenzie 08 Dec '14

08 Dec '14

Download it here https://github.com/leksah/leksah/wiki/download If you have not tried Leksah recently here are some things that are new(ish). If you have GHCJS installed you can build and run your applications in Leksah's output pane. https://www.youtube.com/watch?v=zQnExdDL63c There is also a WebKit "Web Inspect" pane built in (Right click in the Output pane and select Inspect Element). Windows and OS X binaries now use Gtk 3.14. Windows installer includes all the Gtk+-3.0 files you need to build Gtk2Hs applications (including Leksah).

1 0

Haskell CAD DSL
by Ovidiu Deac 07 Dec '14

07 Dec '14

Is there any Haskell-based CAD DSL? I found ImplicitCAD (http://www.implicitcad.org/) which has an Haskell API but the project seems dead as there is no activity on their github repository for the last 2 years. Are there any other options? Thanks!

2 2

No warning for unused import on first line
by Tom Ellis 06 Dec '14

06 Dec '14

Using GHC 7.6.3, if I have ``` import Data.List main :: IO () main = return () ``` then I get a warning as expected. ``` % ghc -fforce-recomp -Wall test1.hs [1 of 1] Compiling Main ( test1.hs, test1.o ) test1.hs:2:1: Warning: The import of `Data.List' is redundant except perhaps to import instances from `Data.List' To import instances alone, use: import Data.List() Linking test1 ... ``` However, if `import Data.List` appears on the first line, i.e. with no blank line at the top of the file ``` import Data.List main :: IO () main = return () ``` then I get no warning. ``` % ghc -fforce-recomp -Wall test1.hs [1 of 1] Compiling Main ( test1.hs, test1.o ) Linking test1 ... ``` Can anyone explain this? Tom

2 1

HaskellWiki
by Henk-Jan van Tuyl 05 Dec '14

05 Dec '14

L.S., Regarding the HaskellWiki: I am missing the colorization of differences in the "Difference between revisions" pages for some time, and now I see that the number of page views is not displayed anymore. Can we get those features back? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

1 0

Reminder: Dutch functional programming day (NL-FP 2015)
by Christiaan Baaij 05 Dec '14

05 Dec '14

Hi everyone, This is a reminder that the next annual Dutch Functional Programming day will take place on: Friday, January 9th, 2015 at the the University of Twente, Enschede, The Netherlands. You are all invited to participate, and of course, to give a presentation. The day will end with a joint dinner. Some additional information: - Although it is called the dutch functional programming day, the program will be in english. - Participation for the day itself is free of charge; we expect dinner to cost around 25 euro. In order to make the necessary reservations, we would like to know as soon as possible how many people we may expect. More information, and the online registration form, can be found at: http://wwwhome.cs.utwente.nl/~jankuper/fp-dag/ Hope to meet you all in Twente at the next FP day! Cheers, Christiaan Baaij

1 0

2D symmetry groups implementation in Haskell?
by Stuart Hungerford 05 Dec '14

05 Dec '14

Hi, As per the subject line--does anyone know of any implementations of 2D (planar) symmetry groups in Haskell? (i.e. "rosette", "frieze" and "wallpaper") groups? I've looked in Hackage but as a complete Haskell novice may not being looking in the right places or communities. Many thanks, Stu

1 0

Looking for help finishing off Data.Sequence <*> optimization
by David Feuer 04 Dec '14

04 Dec '14

With help from Carter Schonwald, I've put together a framework to optimize both the zip functions and the (<*>) method for sequences, as shown in https://github.com/haskell/containers/pull/84 The zip optimization is complete, but I'm getting lost in modular arithmetic in the optimization of fs<*>xs. The remaining challenge is to come up with an efficiently splittable representation of fmap (\f -> fmap f xs) fs and a way to split it. The first part, coming up with a representation that (theoretically) does the job, is easy: data CP a b = SingleCP a (Seq b) | FullCP Int Int (Seq a) (Seq b) The SingleCP constructor represents [f] * xs; the FullCP one holds fs, xs, and the start and end points within the original sequence (this representation was half-heartedly suggested by Cale Gibbard, who thinks probably no one uses the Applicative instance for Seq anyway). The trouble is that actually calculating things properly with these numbers is rather confusing for me; I keep going off by one in one way or another, and producing unreadable code. Can anyone help? David Feuer

1 0

Hackage documentation for executables
by Bardur Arantsson 04 Dec '14

04 Dec '14

Hi all, I'm trying to upload documentation for an executable-only package to Hackage, but I can't get Haddock syntax to work in the description field. Here's the previous upload: http://hackage.haskell.org/package/pg-harness-0.1.0 As you can see, the "@" bits haven't been respected, nor have the ">" sections. Is there something I can do to fix this? (I guess I might upload the documentation myself, but I'd obviously prefer if I could avoid that.) (I'm trying to upload a new version, but I'd like to know what to first to avoid this issue.) Regards,

1 0

Array creation in constant time
by Wolfgang Jeltsch 04 Dec '14

04 Dec '14

Hi, I want to create an array without initializing its elements, so that array creation takes constant time (modulo heap management). What are possible approaches for this? I thought about using newArray_ with the IOUArray type. Is this guaranteed to not initialize any elements and run in O(1)? Are there other ways? All the best, Wolfgang

1 0

Stackage survey
by Michael Snoyman 04 Dec '14

04 Dec '14

Please fill out the Stackage survey, targeted at both Haskell library authors and users. It's just a few multiple choice questions, and shouldn't take you more than 1-2 minutes to complete. http://goo.gl/forms/J1xaWV9BOR

1 0

Haskell and SCADA
by Vasili I. Galchin 03 Dec '14

03 Dec '14

http://en.m.wikipedia.org/wiki/SCADA ,,,, this post is in particular for people who are somewhat familliar with SCADA. Question: are SCADA hardware processors running anything like an operating system that would allow software to be written in Haskell?? Vasili

4 3

Re: [Haskell-cafe] Getting Haskell in the top 20
by Henk-Jan van Tuyl 02 Dec '14

02 Dec '14

Using the words "Haskell" and "programming" in emails probably helps getting Haskell a few places up, but I think there is much more needed to get it in the top twenty. Things like a GHC backend that produces Java bytecode and/or Common Intermediate Language, commercial support, or Microsoft delivering Haskell in a Visual Studio environment. These are things that will raise much more interest in Haskell in the commercial world, and therefore would also interest more programmers. Regards, Henk-Jan van Tuyl On Sun, 30 Nov 2014 18:37:34 +0100, Jacques Carette <carette(a)mcmaster.ca> wrote: > Make sure the actual words "Haskell programming" occur in as many places > as possible [posts here, on reddit, on stackoverflow, etc]. Not > variants of those words, but those exact words. Like you did in your > .sig (although that may not be indexed, so it may need to be in the > actual text). > > Get it into people's phrasing that they always speak of "Haskell > programming", not Haskell code, coding, etc, etc. > > Jacques > > On 2014-11-30 11:29 AM, Henk-Jan van Tuyl wrote: >> >> L.S., >> >> In the Tiobe Index[0], F# jumped from position 43 in June to 13 in July >> and is since than in the >> top twenty. What happened in July and how can we get Haskell in the top >> 20? >> >> Regards, >> Henk-Jan van Tuyl >> >> >> [0] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

9 9

Use distributivity proof in a simple example
by Marco Vassena 02 Dec '14

02 Dec '14

My GADT data type is indexed over type level list. data DF (xs :: [ * ]) where Empty :: DF [] Single :: a -> DF '[ a ] Cons :: DF xs -> DF ys -> DF (Append xs ys) Append is a type family that append two type level lists: type family Append (xs :: [ * ]) (ys :: [ * ]) :: [ * ] where Append '[] ys = ys Append (x ': xs) ys = x ': Append xs ys Map is a type level map: type family Map (f :: * -> *) (xs :: [ * ]) :: [ * ] where Map f '[] = '[] Map f (x ': xs) = f x ': Map f xs Now I want to write a function that wraps all the component in Single in a list: wrap :: DF xs -> DF (Map [] xs) wrap Empty = Empty wrap (Single a) = Single [ a ] wrap (Cons x y) = Cons (wrap x) (wrap y) -- Type Error The type-checker doesn't know anything about distributivity: Could not deduce (Append (Map [] xs) (Map [] ys) ~ Map [] zs) from the context (zs ~ Append xs ys) I managed to prove this property by induction on the first type level list. class MapDist xs where mapDist :: (zs ~ Append xs ys) => Proxy xs -> Proxy ys -> Append (Map [] xs) (Map [] ys) :~: Map [] zs instance MapDist '[] where mapDist Proxy Proxy = Refl tailP :: Proxy (x ': xs) -> Proxy xs tailP _ = Proxy instance MapDist xs => MapDist (x ': xs) where mapDist p1 p2 = case mapDist (tailP p1) p2 of Refl -> Refl I have adjusted wrap as follows: toProxy :: DataFormat xs -> Proxy xs toProxy _ = Proxy wrap (Cons x y) = let p1 = toProxy x p2 = toProxy y in case mapDist p1 p2 of Refl -> Cons (wrap x) (wrap y) The problem now is that MapDist xs is not in the context. Adding it to the Cons constructor in the GADT definition does not solve the problem: Could not deduce (MapDist (Map [] xs)) ... How can I solve this issue? Clearly the two instances of MapDist cover all possible cases, and therefore it should be possible to use it for any list xs. I tried avoiding the type class MapDist with a normal function: proof :: forall xs ys zs . Append xs ys ~ zs => Proxy xs -> Proxy ys -> Proxy zs -> Append (Map [] xs) (Map [] ys) :~: Map [] zs However here I have no mean to distinguish the base case ('[]) from the inductive case (x ': xs), thus I cannot complete the proof. Do you have any idea, suggestion or workaround for this kind of situations? Cheers, Marco

3 4

Julia and Charts on Mac
by Dominic Steinitz 02 Dec '14

02 Dec '14

I just had an interesting experience installing the Julia charts package (Gadfly based on ggplot) on my Mac which I thought I would share as I know I and other folk have had trouble getting diagrams / Cairo to work on Macs. It installs homebrew and then all the required packages from its own repo (not sure what the homebrew terminology is for this - possibly a brewery?) so presumably guaranteeing that the installation is consistent and “just works”. I must say I was a bit surprised (I think I would have liked it to tell me what it was about to do) but it was entirely painless and I drew my first chart after less than 5 minutes. Something to bear in mind for making the barrier to entry to Haskell a bit lower. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com PS I am not sure that haskell-cafe is the right list to share this on but as we don’t have a mailing list for numerical stuff or mac stuff...

4 4

Haskell 2014
by Дмитрий 02 Dec '14

02 Dec '14

4 3

10 PhD studentships in Nottingham
by Graham Hutton 02 Dec '14

02 Dec '14

Dear all, The School of Computer Science in Nottingham is advertising 10 fully-funded PhD studentships. Applicants in the area of the Functional Programming lab (fp.cs.nott.ac.uk) are encouraged! If you are interested in applying, please contact a potential supervisor in the FP lab prior to submitting your application: Thorsten Altenkirch - constructive logic, proof assistants, homotopy type theory, category theory, lambda calculus. Venanzio Capretta - type theory, mathematical logic, corecursive structures, proof assistants, dependently-typed programming. Graham Hutton - not taking on any new students this year. Henrik Nilsson - functional reactive programming, modelling and simulation, domain-specific languages, probabilistic languages. Best wishes, Graham +-----------------------------------------------------------+ 10 Fully-Funded PhD Studentships School of Computer Science University of Nottingham, UK http://tinyurl.com/ten-phds Applications are invited for up to ten fully-funded PhD studentships in the School of Computer Science at the University of Nottingham, starting on 1st October 2015. The topics for the studentships are open, but should relate to the interests of one of the School’s research groups: Agents Lab; Automated Scheduling, Optimisation and Planning; Computer Vision Lab; Functional Programming Lab; Intelligent Modelling and Analysis; Mixed Reality Lab; Networked Systems. The studentships are for three years and include a stipend of £13,863 per year and tuition fees, and are available to students of any nationality. Applicants are normally expected to have a first-class Masters or Bachelors degree in Computer Science or a related discipline, and must obtain the support of a potential supervisor in the School prior to submitting their application. Initial contact with supervisors should be made at least two weeks prior to the closing date for applications. Informal enquiries may be addressed to <Christine.Fletcher(a)nottingham.ac.uk>. To apply, please submit the following items by email to the above address: (1) a brief covering letter that describes your reasons for wishing to pursue a PhD, your proposed research area and topic, and the name of a potential supervisor; (2) a copy of your CV, including your actual or expected degree class(es), and results of all University examinations; (3) an example of your technical writing, such as a project report or dissertation; (4) contact details for two academic referees. Closing date for applications: 14th January 2015 +-----------------------------------------------------------+ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

Using ghc-mod with FFI
by Dan Piponi 01 Dec '14

01 Dec '14

In my code I have this line: foreign import ccall "set_num_states" setNumStates :: Int -> IO () I can build my project using cabal and have an appropriate "c-sources:" line. But when I use ghc-mod check (eg. from " ghc-mod-5.2.0.0", and launched from Syntastic) it complains: "ghc-mod: ByteCodeLink: can't find label During interactive linking, GHCi couldn't find the following symbol: set_num_states" How should I be checking my code? -- Dan

2 1

New Functional Programming Job Opportunities
by Functional Jobs 01 Dec '14

01 Dec '14

Here are some functional programming job opportunities that were posted recently: Scala Developer at Mediative http://functionaljobs.com/jobs/8767-scala-developer-at-mediative Cheers, Sean Murphy FunctionalJobs.com

1 0

Re: [Haskell-cafe] Haskell 2014
by Roman Cheplyaka 01 Dec '14

01 Dec '14

The language («GHC Haskell») is evolving quite rapidly, it's just no-one is really interested in maintaining the standard anymore. I don't think it should disappoint you, unless you're a language researcher or compiler writer. On 30/11/14 14:29, Dmitry wrote: > Thanks for the answer, > > it's sad. > Even C++ now gets new releases every several years. > > Does anybody feel like the language is surpassed by other languages, for > example F#? > > > 30.11.2014, 17:46, "Roman Cheplyaka" <roma(a)ro-che.info>: >> >> On 30/11/14 04:03, Дмитрий wrote: >> >> Hi everyone, >> >> I couldn't find when haskell 2014 standard will come out. Is >> there any >> information about it? >> Somewhere I read that a new standard should be released each year >> but it >> seems not to be true nowadays >> >> There's no-one working on the Haskell standard, to the best of my >> knowledge. So no, it probably won't come out. >> >> Roman >>

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ANN: Opaleye -- SQL-generating embedded domain specific language for Postgres
by Tom Ellis 01 Dec '14

01 Dec '14

I am pleased to announce the public release of Opaleye, now available on Hackage and at GitHub: http://hackage.haskell.org/package/opaleye https://github.com/tomjaguarpaw/haskell-opaleye Opaleye is a Haskell library which provides an SQL-generating embedded domain specific language for targeting Postgres. You need Opaleye if you want to use Haskell to write typesafe and composable code to query a Postgres database. Opaleye allows you to define your database tables and write queries against them in Haskell code, and aims to be typesafe in the sense that if your code compiles then the generated SQL query will not fail at runtime. A wide range of SQL functionality is supported including inner and outer joins, restriction, aggregation, distinct, sorting and limiting, unions and differences. Facilities to insert to, update and delete from tables are also provided. Code written using Opaleye is composable at a very fine level of granularity, promoting code reuse and high levels of abstraction. For further information please refer to the README and tutorials: https://github.com/tomjaguarpaw/haskell-opaleye/blob/master/README.md https://github.com/tomjaguarpaw/haskell-opaleye/blob/master/Doc/Tutorial/Tu… https://github.com/tomjaguarpaw/haskell-opaleye/blob/master/Doc/Tutorial/Tu… I would be happy to answer any questions anyone might have about this library. Tom

1 0

Passionate About Using Haskell?
by jonathan.erwin 01 Dec '14

01 Dec '14

Dear Haskellers, I am looking for developers passionate about using Haskell on the job to join my engineering team based in Norwich, Norfolk. About Us: · Inchora Ltd (http://www.inchora.com) is privately funded with a clear growth plan for the next 5 years. Our business model revolves around developing products and distributing them with our own marketing assets. · Our products are diverse and serve multiple industries including utilities, local government, online marketing and finance. You will be working across them all. The Role: · We're looking for experienced developers who have previously worked on commercial software projects (even if they were written in C++ or Java...). · Work from the Norwich office on a permanent basis developing applications using Haskell, MongoDB, Heroku and other goodies. · Help document and subsequently unify the code base of at least 5 different product suites all suffering from delirious imperative code. · Salary of £45,000-£60,000 (plus bonus) depending on experience. The Perks · Norfolk is a real ale Mecca. Seriously there is so much good beer we have a festival every October. · We love FP! We want to show the world what applications written in Haskell can do. · Build your own products or tinker with them in our lab (coming 2015). If it's a great idea, we'll back it with real money as part of our accelerator programme. Any questions? Ready to join? My number is below. Give me a ring! Sincerely, Jonathan Erwin Chief Technical Officer M: 07833 254186 | DDI: 0845 6432371 [inchora-logo-email]<http://www.inchora.com/> Inchora Limited - Quatro House, Lyon Way, Frimley, Surrey, GU16 7ER www.inchora.com<http://www.inchora.com/> | 01276 804400<tel:+441276804400> This email and any attachments may be legally privileged and are confidential, protected by copyright and for the addressee only. We have taken reasonable steps to reduce risks against viruses but cannot accept liability for any damages sustained as a result of this transmission. If you are not the intended receiver, any reading, copying or transmission of this email is prohibited. Please tell us by replying to this email and then delete the email from your system. Thank you.

1 0

installation problems: wx, phooey, checkers
by Jeffrey Brown 01 Dec '14

01 Dec '14

I believe I have wxWidgets installed, because when I go to the demos/ or samples/ subfolders of ~/wxWidgets-3.0.2/build-cocoa-debug/, I can run the apps. Now I want to install the phooey library for Haskell. When I try, I am warned that some things will be broken, so I unregister them and then cabal install them all at once. (The complete output from my session with cabal appears below.) I run cabal from ~/wxWidgets, because that folder contains the files wx-config-inplace.in wx-config.in and I know cabal wants wx-config. The two errors I get: wx-config is not found checkers does not install I am left stumped. Thanks for your help. -------------------- jefferys-mbp:~ jbb$ cd wxWidgets-3.0.2/ jefferys-mbp:wxWidgets-3.0.2 jbb$ cabal install phooey Resolving dependencies... In order, the following would be installed: mtl-1.1.1.1 (latest: 2.2.1) (new version) parsec-3.1.7 (reinstall) changes: mtl-2.2.1 -> 1.1.1.1 reactive-0.5.0.1 (latest: 0.11.5) (new package) wxdirect-0.91.0.0 (reinstall) wxc-0.91.0.0 (new package) wxcore-0.91.0.0 (new package) wx-0.91.0.0 (new package) phooey-2.0.0.1 (new package) cabal: The following packages are likely to be broken by the reinstalls: tidal-0.4.21 network-uri-2.6.0.1 threepenny-gui-0.5.0.0 hmt-0.15 Use --force-reinstalls if you want to install anyway. jefferys-mbp:wxWidgets-3.0.2 jbb$ ghc-pkg unregister tidal jefferys-mbp:wxWidgets-3.0.2 jbb$ ghc-pkg unregister threepenny-gui jefferys-mbp:wxWidgets-3.0.2 jbb$ ghc-pkg unregister network-uri jefferys-mbp:wxWidgets-3.0.2 jbb$ ghc-pkg unregister hmt jefferys-mbp:wxWidgets-3.0.2 jbb$ cabal install tidal network-uri threepenny-gui hmt phooey Resolving dependencies... Configuring checkers-0.1.4... Configuring network-uri-2.6.0.1... Configuring hmt-0.15... Configuring wxc-0.91.0.0... Building network-uri-2.6.0.1... Building checkers-0.1.4... Building hmt-0.15... Failed to install checkers-0.1.4 Last 10 lines of the build log ( /Users/jbb/.cabal/logs/checkers-0.1.4.log ): at src/Test/QuickCheck/Instances/Num.hs:23:12-49 Possible fix: add (Eq a) to the context of the type signature for nonZero :: (Num a, Arbitrary a) => Gen a -> Gen a In the first argument of ‘satisfiesM’, namely ‘(/= 0)’ In the expression: satisfiesM (/= 0) (if (s == 0) then (resize 1 g) else g) In the first argument of ‘sized’, namely ‘(\ s -> satisfiesM (/= 0) (if (s == 0) then (resize 1 g) else g))’ Failed to install wxc-0.91.0.0 Last 10 lines of the build log ( /Users/jbb/.cabal/logs/wxc-0.91.0.0.log ): [1 of 1] Compiling Main ( /var/folders/q1/2_cf13z53tg__sqdk7v8ycgm0000gn/T/wxc-0.91.0.0-41510/wxc-0.91.0.0/Setup.hs, /var/folders/q1/2_cf13z53tg__sqdk7v8ycgm0000gn/T/wxc-0.91.0.0-41510/wxc-0.91.0.0/dist/setup/Main.o ) Linking /var/folders/q1/2_cf13z53tg__sqdk7v8ycgm0000gn/T/wxc-0.91.0.0-41510/wxc-0.91.0.0/dist/setup/setup ... Error: wx-config not found, please install wx-config before installing wxc Installed network-uri-2.6.0.1 Configuring threepenny-gui-0.5.0.0... Building threepenny-gui-0.5.0.0... Installed threepenny-gui-0.5.0.0 Installed hmt-0.15 Configuring tidal-0.4.21... Building tidal-0.4.21... Installed tidal-0.4.21 Updating documentation index /Users/jbb/Library/Haskell/share/doc/index.html cabal: Error: some packages failed to install: checkers-0.1.4 failed during the building phase. The exception was: ExitFailure 1 phooey-2.0 depends on wxc-0.91.0.0 which failed to install. reactive-0.9.3 depends on checkers-0.1.4 which failed to install. wx-0.91.0.0 depends on wxc-0.91.0.0 which failed to install. wxc-0.91.0.0 failed during the configure step. The exception was: ExitFailure 1 wxcore-0.91.0.0 depends on wxc-0.91.0.0 which failed to install.

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Haskell in the "Enterprise"
by gonzaw 01 Dec '14

01 Dec '14

Hi. I've been wondering if there are any plans or projects to bring Haskell to the "enterprise" ecosystem An by this I mean allowing applications made in Haskell (and other languages) to be integrated in a distributed fashion, easily being able to integrate different systems (DBMSs, web services, etc), etc. For example, creating an ESB in Haskell to allow easier integration of distributed applications. We all worked (or know someone who worked) with huge Java/IBM/etc enterprise systems, and know how clunky the experience was. Being able to include Haskell in this ecosystem would be so refreshing. I was also wondering if there is any research going on on how to apply the mathematical concepts and FP principles behind Haskell to these enterprise constructs. For instance, how to apply FP and category theory to Enterprise Application Integration to make scalable, robust and composable systems that are actually easy to use and discover. For example, I always had this idea of having "pure Web Services" and "effectful Web Services" as different protocols that can be used in different ways depending on the service being provided. Or maybe allowing functions to be passed as parameters in web services, by defining a small typed lambda calculus that can be serialized to XML and passed in a SOAP message (with the appropriate interpreters in the client+server). The ideas never went beyond that, but they sparked my interest in this sort of thing. To start somewhere, are there any Haskell libraries with Web Service support? Like a WS client tools (SOAP messaging DSL + WSDL import tool)? Or a haskell library that allows one to provide web services (and maybe provide the WS-* extensions)? -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-in-the-Enterprise-tp5761538.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Kind product
by Gautier DI FOLCO 01 Dec '14

01 Dec '14

Hello all, I have the following code: {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} data Branch = Left | Right type family Product (v :: Branch) (a :: k) (b :: k) :: k where Product Left l r = l Product Right l r = r My goal is to have a thing like that: type family Product (v :: Branch) (a :: k1) (b :: k2) :: (Either k1 k2) where Is it possible to do such a thing? If not I can find a solution with Kind families/Kind pattern matching, but I don't think that exists, am I wrong? Thanks in advance for your help, Regards.

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How to update Cabal manually?
by Jeffrey Brown 01 Dec '14

01 Dec '14

"cabal install cabal-install" is not working for me. I am not the first to find this problem; it is mentioned here <https://www.haskell.org/haskellwiki/Cabal/Survival>: "Notice, that in some cases (OS X, especially) the new cabal-install might not be installed on top of the previous one, and the old cabal-install must be replaced manually" I don't know how to manually replace Cabal. The closest thing I could find to instructions for that were here <https://www.haskell.org/pipermail/haskell-cafe/2010-February/073526.html>, but I don't understand them. The Haskell Platform installer is what installed Cabal for me. I have not found how to undo that, nor how to install Cabal in any other fashion. Running the Haskell Platform installer again seems dangerous, potentially conflicting with its earlier work. A likely unnecessary shell transcript demonstrating the problem appears below. I am running OS X 10.9 on a MacBook Pro, of which the decision to purchase I regret more daily. Thank you. ----- When I try to update cabal, it looks like it works: jefferys-mbp:~ jbb$ cabal install cabal-install Resolving dependencies... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 Updating documentation index /Users/jbb/Library/Haskell/share/doc/index.html But the version number exhibits no change: jefferys-mbp:~ jbb$ cabal --version cabal-install version 1.18.0.5 using version 1.18.1.4 of the Cabal library

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Typeclass Show Question
by Yasuaki Kudo 01 Dec '14

01 Dec '14

Hi, When I launch GHCI with the option -XGADTs and load the following program, f = \x y -> (1, x,y) data S a b where S :: (Num a, Num b) => a -> b -> S a b I encounter this dialog: *Main> S 1 1 <interactive>:3:1: No instance for (Show (S a0 b0)) arising from a use of `print' Possible fix: add an instance declaration for (Show (S a0 b0)) In a stmt of an interactive GHCi command: print it I tried to follow the recommendation but still could not figure out how. For example, after adding: instance Show (S a b) where show (S x y) = show(x) I still get: conmath.hs:10:24: Could not deduce (Show a) arising from a use of `show' from the context (Num a, Num b) bound by a pattern with constructor S :: forall a b. (Num a, Num b) => a -> b -> S a b, in an equation for `show' at conmath.hs:10:15-19 Possible fix: add (Show a) to the context of the data constructor `S' or the instance declaration In the expression: show (x) In an equation for `show': show (S x y) = show (x) In the instance declaration for `Show (S a b)' Failed, modules loaded: none. What should I do to show S 1 1 ? Regards, Yasu

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Hosting a static site with Yesod
by Yifan Yu 30 Nov '14

30 Nov '14

Hi all, I'm experimenting with Yesod and I've created a simple scaffolding site with yesod. I've downloaded a bootstrap template site and wish to simply host this site with yesod. The template site has an index.html and a bunch of css and js files. This seemly simple task has baffled me. By my understanding, the site should be placed under the 'static' directory, I tried to use sendFile to send the index.html file in getHomeR, but only the content of the that file is displayed, without the css and js. Should I do this with a Subsite? Thank you

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multiple declarations error in Haskell - should it be relaxed?
by Andrew Butterfield 30 Nov '14

30 Nov '14

Condsider the following Haskell program (fragment): f 0 = “zero” g 0 = “NULL” f n = “non-zero” g n = “PRESENT” This will result in two “Multiple Declaration” errors. There is a good motivation for this - disallowing such an interleaving of declarations makes it easy for the compiler to capture a common typo, namely errors of the following form - here an attempt to define a single function called myFun. myFun 0 = “zero” myfun 1 = “one” myFun n = “too big!” However I have use-cases where it would be nice to interleave as per the first example above - with markedly different function names. It invokes a large case analysis, where I have other auxiliary functions associated with each case, but which I’d like to (1) have at the top-level for testability (2) keep textually local to the case with which they are associated. I don’t think there is a language extension to disable the multiple declaration check - but would such a feature we possible. I’d see it as one which still performs the check, but issues a warning rather than an error - particularly if it notices that the interleaved names are very similar. Is this a reasonable suggestion, or are there other reasons for not doing this that I’ve missed? Maybe there is a better way to satisfy (1) and (2) above? Regards, Andrew -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations & Methods Research Group 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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[Announce] Music Suite v1.8
by Hans Höglund 30 Nov '14

30 Nov '14

I am happy to announce the release of Music Suite version 1.8. This release in short: Cleaner API for dealing with Voices, Notes and time structures in general. New equal-temperament pitch representation. Improved handling of diatonic vs. chromatic transposition in the Common pitch representation. Better support for clef and staff line positions. Better support for handling instrument ambitus etc. See full notes at https://github.com/music-suite/music-docs/blob/master/releases/Notes.md#18 The Music Suite is a language for description, analysis, composition and manipulation of music embedded in Haskell. For more information see http://music-suite.github.io or my talk at FARM2014 http://functional-art.org/2014/. Regards, Hans Hoeglund - https://twitter.com/hanshogl https://soundcloud.com/hanshoglund http://github.com/hanshoglund

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Getting Haskell in the top 20
by Henk-Jan van Tuyl 30 Nov '14

30 Nov '14

L.S., In the Tiobe Index[0], F# jumped from position 43 in June to 13 in July and is since than in the top twenty. What happened in July and how can we get Haskell in the top 20? Regards, Henk-Jan van Tuyl [0] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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looking for ongoing Haskell projects ..
by Vasili I. Galchin 30 Nov '14

30 Nov '14

Hi Fellow Haskellers, I was just reading at http://www.haskell.org. I am looking for a list of currently active Haskell projects needing help .. :-) Vasili

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Data migration and software versioning
by Corentin Dupont 30 Nov '14

30 Nov '14

Hi the list, I have some question relative to data migration. Say you have a software in version A which save data in a file with format FA. Later, you update your software to version B, with data format FB. Now, if you want your version B of the software to be able to read data saved by version A, you're obliged to include FA in version B, together with some functions to translate from FA to FB. This is what I don't find elegant: you're obliged to keep old code (data formats) in your software. -> do you know of any way to avoid keeping old data formats (for retro-compatiblilty) in your code? In practice I use acid-state with safecopy: http://acid-state.seize.it/safecopy In this example you can see the problem: the author is obliged to keep old code (data structures) to maintain compatibility. Even worth, you might be obliged to suffix your data structure with a version number: data MyType_V1 = MyType_V1 Intdata MyType_V2 = MyType_V2 Integer Instead, I'm thinking of a process using GIT, or Cabal as a back-end. The idea would be to have an additional program (or library) specialized in the data migration of your main software. It would extract both version A and B from the repo, and then would compile an application capable of handling migrations from FA to FB. Does something like this exists (even outside of Haskell)? Cheers, Corentin

1 0

Kind signatures and closed type families syntax
by Gautier DI FOLCO 30 Nov '14

30 Nov '14

Hello all, I have the following code: {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} data Branch = Left | Right type family Product (v :: Branch) (a :: k) (b :: k) :: k where Product Left l r = l Product Right l r = r He is doing what I expect it to do and Product has the right Kind: *Main> :kind Product Product :: Branch -> k -> k -> k But when I change the Kind signature syntax, its Kind changes: type family Product v a b :: Branch -> k -> k -> k where Its Kind become: *Main> :kind Product Product :: Branch -> (Branch -> k -> k -> k) -> (Branch -> k1 -> k1 -> k1) -> Branch -> k2 -> k2 -> k2 It's even worse with this syntax: type family Product :: Branch -> k -> k -> k where Produces: *Main> :r [1 of 1] Compiling Main ( product-highkind.hs, interpreted ) product-highkind.hs:13:3: Number of parameters must match family declaration; expected 0 In the equations for closed type family ‘Product’ In the type family declaration for ‘Product’ Failed, modules loaded: none. I don't know if I'm tired or not but in the documentation [1] these syntaxes should be equivalent. If not, I'll take any explanations. Thanks in advance for your help. [1] https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/kind-polymor…

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