May 2013 - Haskell-Cafe

Haskell job / adaptive learning
by Carl Baatz 02 May '13

02 May '13

Dear Haskellers, Erudify (http://erudify.com/) is looking for software engineers to join our team. We are developing an autonomous interactive online tutor. We sell our product to enterprise and provide it for free to schools and consumers. We are looking for software engineers who are smart and friendly, have several years of programming experience, and a passion for building practical and high quality software. Haskell is our main language and we are using JavaScript on the client side. The field of automated learning and teaching is still in its infancy and we are thrilled about the work we are doing to improve how people learn. So what are we looking for? We expect you to prefer design principles compatible with Unix and the Web. You make an effort to learn new technologies that could be helpful and you work to master those you already use. You have worked with a broad range of technologies in your career. You get things done and realise that trade-offs are necessary, but compromising the long-term goal is not. You have experience with Haskell, but might not be an expert. Erudify consists of eleven people -- six of us are Engineers, of which four are specialised in Haskell (with another Haskeller joining soon). We are based in Zürich and our working language is English. Please email us at jobs(a)erudify.com if you are curious. Also, two of our engineers, Bas van Dijk and Simon Meier, will be at the Odessa hackathon from 2 May to 6 May -- they'd be delighted to chat with you. Best wishes, Carl Baatz

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Re: [Haskell-cafe] Is it time to start deprecating FunDeps?
by oleg＠okmij.org 02 May '13

02 May '13

> In your class Sum example, > > class Sum x y z | x y -> z, x z -> y > > your own solution has a bunch of helper classes First of all, on the top of other issues, I haven't actually shown an implementation in the message on Haskell'. I posed this as a general issue. In special cases like below > class Sum2 a b c | a b -> c, a c -> b > instance Sum2 Z b b > instance (Sum2 a' b c') => Sum2 (S a') b (S c') > -- note that in the FunDeps, var a is not a target > -- so the instances discriminate on var a I didn't doubt the translation would go through because there is a case analysis on a. But the general case can be more complex. For example, class Sum2 a b c | a b -> c, a c -> b instance Sum2 Z Z Z instance Sum2 O Z O instance Sum2 Z O O instance Sum2 O O Z > In your overlap example you introduce a definition that won't compile! > > > > {- -- correctly prohibited! > > instance x ~ Bool => C1 [Char] x where > > foo = const True > > -} > You expect too much if you think a mechanical translation will 'magic' a > non-compiling program into something that will compile. > I do expect equality constraints to not play well with overlaps. Combining > FunDeps with overlaps is also hazardous. I'm only claiming that EC's will > be at least as good. I don't understand the remark. The code marked `correctly prohibited' is in the comments. There are no claims were made about that code. If you found that comment disturbing, please delete it. It won't affect the the example: the types were improved in t11 but were not improved in t21. Therefore, EC's are not just as good. Functional dependencies seem to do better.

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Haskell Weekly News: Issue 266
by Daniel Santa Cruz 02 May '13

02 May '13

Welcome to issue 266 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 7 to 27, 2013. Quotes of the Week * hpc: the dirty secret of haskell programmers is that their power does not come from their PhD, but their bowtie * tibbe: I think I have implemented a reverse state monad by mistake. The numbers suggest I'm getting the values tunneled back from the future. :/ * SaulGorn: A formalist is one who cannot understand a theory unless it is meaningless. * cmccann: laziness by default is a shortcoming. eagerness by default is a bigger shortcoming. * monochrom: #haskell-blah has brilliant people too. for example me. * limo: OK, guys was nice meeting you and now I'm a proud owner of a GHCi icon on my desktop * otters: lens law #1: the implementation must be shorter than the type * MartinDeMello: Any sufficiently well-commented Lisp program contains an ML program in its comments. * tgeeky: whatever man. Jesus only had twelve or so deciples, lens has 80! * SatnamSingh: I always thought that category theory was something that happened to other people. (#ICFP) * cmccann: see the thing is tabs are good for indentation. haskell doesn't use indentation, it uses alignment. it is not indenting. it is ASCII art. * JohnVonNeumann: In mathematics you don't understand things. You just get used to them. * edwardk: you can view Yoneda f as enforced fmap fusion and Codensity f as enforced bind fusion. Top Reddit Stories * Functors, applicatives, and monads in pictures Domain: adit.io, Score: 136, Comments: 67 On Reddit: [1] http://goo.gl/xRC6K Original: [2] http://goo.gl/QcfqM * What I Wish I Knew When Learning Haskell, Stephen Diehl [39 slides, Left/Rgt-arrow key to back/advance] Domain: dev.stephendiehl.com, Score: 119, Comments: 37 On Reddit: [3] http://goo.gl/T8uzi Original: [4] http://goo.gl/eZTA0 * git clone in Haskell from the bottom up Domain: stefan.saasen.me, Score: 86, Comments: 6 On Reddit: [5] http://goo.gl/Al86J Original: [6] http://goo.gl/nWSXV * Vinyl: Modern Records for Haskell Domain: jonmsterling.com, Score: 56, Comments: 27 On Reddit: [7] http://goo.gl/pSUph Original: [8] http://goo.gl/ra0Jb * Follow-up: Web Woes for Haskell Domain: self.haskell, Score: 49, Comments: 9 On Reddit: [9] http://goo.gl/PJZEA Original: [10] http://goo.gl/PJZEA * What does “coalgebra” mean in the context of programming? (StackOverflow) Domain: stackoverflow.com, Score: 45, Comments: 23 On Reddit: [11] http://goo.gl/nMqBF Original: [12] http://goo.gl/UH2Wl * The Mother of all Monads - School of Haskell Domain: self.haskell, Score: 42, Comments: 27 On Reddit: [13] http://goo.gl/XO7mQ Original: [14] http://goo.gl/XO7mQ * Why do all the Haskell Web Frameworks rely on Template Haskell? Domain: self.haskell, Score: 40, Comments: 59 On Reddit: [15] http://goo.gl/v5p6F Original: [16] http://goo.gl/v5p6F * Job opening for Haskell developer in Nijmegen (NL) Domain: self.haskell, Score: 39, Comments: 6 On Reddit: [17] http://goo.gl/Uzqs8 Original: [18] http://goo.gl/Uzqs8 * Tutorial: Building a Sample Application with Haskell Snap, PostgreSQL, and the PostgreSQL Simple Snaplet Domain: janrain.com, Score: 39, Comments: 17 On Reddit: [19] http://goo.gl/TAM6J Original: [20] http://goo.gl/37Nry * Haskell for all: Defaults Domain: haskellforall.com, Score: 38, Comments: 20 On Reddit: [21] http://goo.gl/7NYex Original: [22] http://goo.gl/FqiuW * Haskell for all: pipes-concurrency-1.0.0: Reactive programming Domain: haskellforall.com, Score: 38, Comments: 23 On Reddit: [23] http://goo.gl/DaSaZ Original: [24] http://goo.gl/eGw3F * Haskell in Ipython Domain: github.com, Score: 38, Comments: 7 On Reddit: [25] http://goo.gl/Y7DEP Original: [26] http://goo.gl/324JX * A taste of Curry (Wolfgang Jeltsch) Domain: jeltsch.wordpress.com, Score: 38, Comments: 0 On Reddit: [27] http://goo.gl/Ijh6Y Original: [28] http://goo.gl/17Ry1 * Complete and Easy Bidirectional Typechecking for Higher-Rank Polymorphism Domain: semantic-domain.blogspot.co.uk, Score: 37, Comments: 21 On Reddit: [29] http://goo.gl/Yv9RY Original: [30] http://goo.gl/qW1Qq * Template Haskell on impossible architectures (x-post from r/programming) Domain: joeyh.name, Score: 37, Comments: 14 On Reddit: [31] http://goo.gl/m1Jvp Original: [32] http://goo.gl/GqiAR * inefficient Ackermann implementation not obvious how to improve Domain: stackoverflow.com, Score: 37, Comments: 20 On Reddit: [33] http://goo.gl/CEaOT Original: [34] http://goo.gl/JploA * Haskell for all: pipes and io-streams Domain: haskellforall.com, Score: 37, Comments: 20 On Reddit: [35] http://goo.gl/fmLhr Original: [36] http://goo.gl/8BoLg * Build applications with Shake build system Domain: lethalman.blogspot.it, Score: 35, Comments: 5 On Reddit: [37] http://goo.gl/MZBLj Original: [38] http://goo.gl/bP7lK * What’s in a number? criterion edition Domain: serpentine.com, Score: 34, Comments: 6 On Reddit: [39] http://goo.gl/cfVMa Original: [40] http://goo.gl/hqBfW * Announcing postgresql-simple 0.3.1 Domain: blog.melding-monads.com, Score: 34, Comments: 5 On Reddit: [41] http://goo.gl/LEuEv Original: [42] http://goo.gl/jrn5v * LGtk: Lens-based Gtk Interface Domain: people.inf.elte.hu, Score: 32, Comments: 64 On Reddit: [43] http://goo.gl/SC7kn Original: [44] http://goo.gl/FfYeE * Introduction to QuickCheck testing (new FP complete SoH tutorial) Domain: fpcomplete.com, Score: 32, Comments: 7 On Reddit: [45] http://goo.gl/2BnS6 Original: [46] http://goo.gl/6DKRG * Introduction to Haskell, Lecture 12 is Live (Category Theory). This is the last lecture in this series. Thank you for your support! Domain: shuklan.com, Score: 31, Comments: 4 On Reddit: [47] http://goo.gl/to9jd Original: [48] http://goo.gl/JZYSv Top StackOverflow Questions * What does “coalgebra” mean in the context of programming? votes: 134, answers: 3 Read on SO: [49] http://goo.gl/UH2Wl * Ackermann very inefficient with Haskell/GHC votes: 39, answers: 7 Read on SO: [50] http://goo.gl/JploA * How to concisely express function iteration? votes: 21, answers: 3 Read on SO: [51] http://goo.gl/OknK3 * Does the GHC garbage collector have any special optimisations for large objects? votes: 16, answers: 1 Read on SO: [52] http://goo.gl/p1zvM * Parallel computations with fast randomness and purity? votes: 16, answers: 4 Read on SO: [53] http://goo.gl/O3FzX * Idiomatic Scala translation of Kiselyov's zippers? votes: 15, answers: 1 Read on SO: [54] http://goo.gl/TCBaL * Singleton types in Haskell votes: 14, answers: 1 Read on SO: [55] http://goo.gl/u3867 * Encoding correctness constraints directly into the Haskell type system? votes: 14, answers: 2 Read on SO: [56] http://goo.gl/in0xv * Monad transformers with identity monad votes: 12, answers: 2 Read on SO: [57] http://goo.gl/hx2Ic * Extensible Haskell Type Classes votes: 12, answers: 4 Read on SO: [58] http://goo.gl/zdYpL * GADT's failed exhaustiveness checking votes: 12, answers: 2 Read on SO: [59] http://goo.gl/GIFDu * QuickCheck: Arbitrary instances of nested data structures that generate balanced specimens votes: 11, answers: 1 Read on SO: [60] http://goo.gl/WH3FV * What is this special functor structure called? votes: 11, answers: 1 Read on SO: [61] http://goo.gl/cKt7o Until next time, [62]+Daniel Santa Cruz References 1. http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictu… 2. http://www.reddit.com/r/haskell/comments/1co0s5/functors_applicatives_and_m… 3. http://dev.stephendiehl.com/hask/ 4. http://www.reddit.com/r/haskell/comments/1cbjqm/what_i_wish_i_knew_when_lea… 5. http://stefan.saasen.me/articles/git-clone-in-haskell-from-the-bottom-up/ 6. http://www.reddit.com/r/haskell/comments/1ch3l2/git_clone_in_haskell_from_t… 7. http://www.jonmsterling.com/posts/2013-04-06-vinyl-modern-records-for-haske… 8. http://www.reddit.com/r/haskell/comments/1buh5y/vinyl_modern_records_for_ha… 9. http://www.reddit.com/r/haskell/comments/1cy58c/followup_web_woes_for_haske… 10. http://www.reddit.com/r/haskell/comments/1cy58c/followup_web_woes_for_haske… 11. http://stackoverflow.com/questions/16015020/what-does-coalgebra-mean-in-the… 12. http://www.reddit.com/r/haskell/comments/1cht4v/what_does_coalgebra_mean_in… 13. http://www.reddit.com/r/haskell/comments/1cyajx/the_mother_of_all_monads_sc… 14. http://www.reddit.com/r/haskell/comments/1cyajx/the_mother_of_all_monads_sc… 15. http://www.reddit.com/r/haskell/comments/1czxvj/why_do_all_the_haskell_web_… 16. http://www.reddit.com/r/haskell/comments/1czxvj/why_do_all_the_haskell_web_… 17. http://www.reddit.com/r/haskell/comments/1clkjb/job_opening_for_haskell_dev… 18. http://www.reddit.com/r/haskell/comments/1clkjb/job_opening_for_haskell_dev… 19. http://janrain.com/blog/tutorial-building-a-sample-application-with-haskell… 20. http://www.reddit.com/r/haskell/comments/1d5c38/tutorial_building_a_sample_… 21. http://www.haskellforall.com/2013/04/defaults.html 22. http://www.reddit.com/r/haskell/comments/1c2g1r/haskell_for_all_defaults/ 23. http://www.haskellforall.com/2013/04/pipes-concurrency-100-reactive.html?m=1 24. http://www.reddit.com/r/haskell/comments/1cc6fe/haskell_for_all_pipesconcur… 25. https://github.com/creswick/ihaskell-notebook 26. http://www.reddit.com/r/haskell/comments/1d0151/haskell_in_ipython/ 27. http://jeltsch.wordpress.com/2013/04/27/a-taste-of-curry/ 28. http://www.reddit.com/r/haskell/comments/1d8n51/a_taste_of_curry_wolfgang_j… 29. http://semantic-domain.blogspot.co.uk/2013/04/thanks-to-olle-fredriksson.ht… 30. http://www.reddit.com/r/haskell/comments/1c50t3/complete_and_easy_bidirecti… 31. http://joeyh.name/blog/entry/Template_Haskell_on_impossible_architectures/ 32. http://www.reddit.com/r/haskell/comments/1cm4n1/template_haskell_on_impossi… 33. http://stackoverflow.com/questions/16115815/ackermann-very-inefficient-with… 34. http://www.reddit.com/r/haskell/comments/1cq3r4/inefficient_ackermann_imple… 35. http://www.haskellforall.com/2013/04/pipes-and-io-streams.html 36. http://www.reddit.com/r/haskell/comments/1ct65e/haskell_for_all_pipes_and_i… 37. http://lethalman.blogspot.it/2013/04/build-vala-applications-with-shake.html 38. http://www.reddit.com/r/haskell/comments/1bv703/build_applications_with_sha… 39. http://www.serpentine.com/blog/2013/04/13/whats-in-a-number-criterion-editi… 40. http://www.reddit.com/r/haskell/comments/1cauc5/whats_in_a_number_criterion… 41. http://blog.melding-monads.com/2013/04/27/announcing-postgresql-simple-0-3-… 42. http://www.reddit.com/r/haskell/comments/1d8mf1/announcing_postgresqlsimple… 43. http://people.inf.elte.hu/divip/LGtk.html 44. http://www.reddit.com/r/haskell/comments/1cemr2/lgtk_lensbased_gtk_interfac… 45. https://www.fpcomplete.com/user/pbv/an-introduction-to-quickcheck-testing 46. http://www.reddit.com/r/haskell/comments/1cus3k/introduction_to_quickcheck_… 47. http://shuklan.com/haskell/lec12.html 48. http://www.reddit.com/r/haskell/comments/1c00tj/introduction_to_haskell_lec… 49. http://stackoverflow.com/questions/16015020/what-does-coalgebra-mean-in-the… 50. http://stackoverflow.com/questions/16115815/ackermann-very-inefficient-with… 51. http://stackoverflow.com/questions/16010778/how-to-concisely-express-functi… 52. http://stackoverflow.com/questions/16186488/does-the-ghc-garbage-collector-… 53. http://stackoverflow.com/questions/16248600/parallel-computations-with-fast… 54. http://stackoverflow.com/questions/15940473/idiomatic-scala-translation-of-… 55. http://stackoverflow.com/questions/16017294/singleton-types-in-haskell 56. http://stackoverflow.com/questions/16207008/encoding-correctness-constraint… 57. http://stackoverflow.com/questions/15946633/monad-transformers-with-identit… 58. http://stackoverflow.com/questions/15967167/extensible-haskell-type-classes 59. http://stackoverflow.com/questions/16225281/gadts-failed-exhaustiveness-che… 60. http://stackoverflow.com/questions/15959357/quickcheck-arbitrary-instances-… 61. http://stackoverflow.com/questions/16123588/what-is-this-special-functor-st… 62. https://plus.google.com/105107667630152149014/about

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Re: [Haskell-cafe] Looking for portable Haskell or Haskell like language
by Christopher Howard 01 May '13

01 May '13

On 05/01/2013 12:21 AM, Gabor Greif wrote: > Am 27. April 2013 um 07:21 schrieb Christopher Howard > <christopher.howard(a)frigidcode.com>: > > > I can feel your pain... Here is a blog post I have written some time ago > http://heisenbug.blogspot.de/2011/09/ghc-704-on-centos.html > about how to bridge the gap. This was actually a RHEL5 system, > but did not want to admit it :-) > > My writeup may be useful to you. > > Cheers, > > Gabor > > For posterity's sake: I actually got this figured out already. First, I downloaded the binary version of GHC 6.8.3 and installed it without issues. Then I used it to build the sources of GHC 6.10.4 and install them. Then I used 6.10.4 to build and install 6.12.3. Then 7.0.4, then 7.2.2, then 7.4.2, and finally 7.6.3. Had no problems doing it that way, except for the optimization flags issue in the 7.* series which I mentioned in my other thread. That was easily fixed by setting the build.mk to the "quick" build profile. -- frigidcode.com

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ANN: hledger 0.20
by Simon Michael 01 May '13

01 May '13

I'm pleased to announce hledger and hledger-web 0.20! hledger is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. In addition to reporting, it can also help you record new transactions, or convert CSV data from your bank. Add-on packages include hledger-web (providing a web interface), hledger-irr and hledger-interest. hledger is inspired by and compatible with John Wiegley's Ledger. For more, see http://hledger.org . Install it: # cabal update; cabal install hledger-web For more installation help, see http://hledger.org/MANUAL.html#installing . Or, sponsor a ready-to-run binary for your platform: http://hledger.org/DOWNLOAD.html . Release notes (http://hledger.org/NEWS.html#hledger-0.20) **Bugs fixed:** * balance: a 0.19 regression which showed wrong total balance with `--flat` has been fixed (#94) * register: when `--date2` is used, the register is now sorted by the secondary date * web: some missing static & template files have been added to the package, fixing cabal-dev and hackage builds (#97, #98) * web: some hardcoded static urls have been fixed * Dependencies and code have been updated to support the latest libraries and GHC versions. For now, hledger requires GHC 7.2+ and hledger-web requires GHC 7.4+. **Journal reading:** - DOS-style line-endings are now also supported in journal and rules files. - `!` is now accepted in the status field as well as `*`, like ledger - The *actual date* and *effective date* terminology has changed to *primary date* and *secondary date*. Use `--date2` to select the secondary date for reports. (`--aux-date` or `--effective` are also accepted for ledger and backwards compatibility). - Per-posting dates are supported, using hledger tags or ledger's posting date syntax - Comment and tag handling has been improved **CSV reading:** - CSV conversion rules have a simpler, more flexible [syntax](MANUAL.html#csv-files). Existing rules files will need to be updated manually: - the filename is now `FILE.csv.rules` instead of `FILE.rules` - `FIELD-field N` is now `FIELD %N+1` (or set them all at once with a `fields` rule) - `base-currency` is now `currency` - `base-account` is now `account1` - account-assigning rules: add `if` before the list of regexps, add indented `account2 ` before the account name - parenthesised amounts are parsed as negative **Querying:** - Use `code:` to match the transaction code (check number) field - Use `amt:` followed by `<`, `=` or `>` and a number N to match amounts by magnitude. Eg `amt:<0` or `amt:=100`. This works only with single-commodity amounts (multi-commodity amounts are always matched). - `tag:` can now match (exact, case sensitive) tag values. Eg `tag:TAG=REGEXP`. **add comand:** - Transaction codes and comments (which may contain tags) can now be entered, following a date or amount respectively. (#45) - The current entry may be restarted by entering `<` at any prompt. (#47) - Entries are displayed and confirmed before they are written to the journal. - Default values may be specified for the first entry by providing them as command line arguments. - Miscellaneous UI cleanups **register command:** - The `--related`/`-r` flag shows the other postings in each transaction, like ledger. - The `--width`/`-w` option increases or sets the output width. **web command:** - The web command now also starts a browser, and auto-exits when unused, by default ("local ui mode"). With `--server`, it keeps running and logs requests to the console ("server mode"). - Bootstrap is now used for styling and layout - A favicon is served - The search field is wider - yesod devel is now supported; it uses `$LEDGER_FILE` or `~/.hledger.journal` - the `blaze_html_0_5` build flag has been reversed and renamed to `blaze_html_0_4` **Add-ons:** - The hledger-interest and hledger-irr commands have been released/updated. - hledger-chart and hledger-vty remain unmaintained and deprecated. **Documentation and infrastructure:** - The hledger docs and website have been reorganised and updated - Manuals for past releases are provided as well as the latest dev version - hledger has moved from darcs and darcs hub to git and github (!) - The bug tracker has moved from google code to github - Feature requests and project planning are now managed on trello - A build bot builds against multiple GHC versions on each commit Release contributors: - Sascha Welter commissioned register enhancements (--related and --width) - David Patrick contributed a bounty for add enhancements - Joachim Breitner added support for ! in status field - Xinruo Sun provided hledger-web build fixes - Peter Simons provided hledger-web build fixes, and a build bot - Marko Kocić provided hledger-web fixes

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What is the process for GSOC?
by Dominic Steinitz 01 May '13

01 May '13

How is it decided which Haskell projects get chosen? Do we discuss them here and take a collective view? Thanks, Dominic. PS I should point out I have an interest in the proposal to port charts to use diagrams (http://www.google-melange.com/gsoc/proposal/review/google/gsoc2013/jbracker…) i.e. I'd really like this as a user.

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Re: [Haskell-cafe] Space leak in hexpat-0.20.3/List-0.5.1
by oleg＠okmij.org 01 May '13

01 May '13

Wren Thornton wrote: > So I'm processing a large XML file which is a database of about 170k > entries, each of which is a reasonable enough size on its own, and I only > need streaming access to the database (basically printing out summary data > for each entry). Excellent, sounds like a job for SAX. Indeed a good job for a SAX-like parser. XMLIter is exactly such parser, and it generates event stream quite like that of Expat. Also you application is somewhat similar to the following http://okmij.org/ftp/Haskell/Iteratee/XMLookup.hs So, it superficially seems XMLIter should be up for the task. Can you explain which elements your are counting? BTW, xml_enum already checks for the well-formedness of XML (including the start-end tag balance, and many more criteria). One can assume that the XMLStream corresponds to the well-formed document and only count the desired start tags (or end tags, for that matter).

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