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Haskell Weekly News: Issue 304
by Daniel Santa Cruz 04 Sep '14

04 Sep '14

Welcome to issue 304 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from August 24 to 30, 2014 Quotes of the Week * pjdelport: Haskell: Lazy evaluation, eager debugging. * trap_exit: isn't hoogle awesome? the first time I used it, I was like "for the first time in my life, google sucks" Top Reddit Stories * Hython - a Haskell-powered Python 3 interpreter Domain: github.com, Score: 95, Comments: 22 Original: [1] http://goo.gl/aI16qA On Reddit: [2] http://goo.gl/N67fnX * Using Emacs for Haskell development Domain: github.com, Score: 89, Comments: 47 Original: [3] http://goo.gl/tuPL91 On Reddit: [4] http://goo.gl/RxVWFQ * Ivory Language Domain: ivorylang.org, Score: 70, Comments: 36 Original: [5] http://goo.gl/xJJ68c On Reddit: [6] http://goo.gl/Vz9cTX * A taste of Cabalized Backpack : Inside 206-105 Domain: blog.ezyang.com, Score: 61, Comments: 73 Original: [7] http://goo.gl/8MpD3U On Reddit: [8] http://goo.gl/3kVQOG * Introducing /r/haskellgamedev Domain: self.haskell, Score: 58, Comments: 20 Original: [9] http://goo.gl/B2nzhq On Reddit: [10] http://goo.gl/B2nzhq * Acme.StringlyTyped Domain: hackage.haskell.org, Score: 44, Comments: 11 Original: [11] http://goo.gl/IfK2Uf On Reddit: [12] http://goo.gl/Z0wT5E * A new version of Yampa is out (0.9.6) Domain: keera.co.uk, Score: 43, Comments: 16 Original: [13] http://goo.gl/EHxFdd On Reddit: [14] http://goo.gl/EXGLdX * On CodeWorld and Haskell Domain: cdsmith.wordpress.com, Score: 41, Comments: 23 Original: [15] http://goo.gl/C7tkHk On Reddit: [16] http://goo.gl/qu3l04 * Introduction to Dependent Types: Haskell on Steroids Domain: jozefg.bitbucket.org, Score: 37, Comments: 17 Original: [17] http://goo.gl/fg8Rfo On Reddit: [18] http://goo.gl/YLmHNQ * Evaluation order and state tokens - School of Haskell Domain: fpcomplete.com, Score: 35, Comments: 24 Original: [19] http://goo.gl/v0D1HQ On Reddit: [20] http://goo.gl/jiiLRl * IO Monad and Purity Domain: stackoverflow.com, Score: 34, Comments: 13 Original: [21] http://goo.gl/zkBxl1 On Reddit: [22] http://goo.gl/yVQY4F * [ANN] Haste 0.4, now with GHC 7.8 support and binary packages Domain: groups.google.com, Score: 33, Comments: 5 Original: [23] http://goo.gl/DU4QYT On Reddit: [24] http://goo.gl/xcqxU0 * darcs 2.8.5 released Domain: lists.osuosl.org, Score: 30, Comments: 0 Original: [25] http://goo.gl/FbjpuA On Reddit: [26] http://goo.gl/TM6D7E * Yarr - image processing library and partial Repa clone Domain: hackage.haskell.org, Score: 28, Comments: 8 Original: [27] http://goo.gl/cfHAH8 On Reddit: [28] http://goo.gl/oyK3xF * conduit stream fusion Domain: fpcomplete.com, Score: 28, Comments: 15 Original: [29] http://goo.gl/9e57M2 On Reddit: [30] http://goo.gl/kFL5Pu * New blog post: Dealing with Asynchronous Exceptions during Resource Acquisition Domain: well-typed.com, Score: 28, Comments: 34 Original: [31] http://goo.gl/WS05yN On Reddit: [32] http://goo.gl/5hp59d Top StackOverflow Questions * Zipper Comonads, Generically votes: 43, answers: 3 Read on SO: [33] http://goo.gl/NDzqR1 * Test if a value matches a constructor votes: 25, answers: 3 Read on SO: [34] http://goo.gl/3Sa7hX * What is the difference between a cyclic list and an infinite list in haskell? votes: 16, answers: 2 Read on SO: [35] http://goo.gl/q7f7pu * Powerset Function 1-Liner votes: 13, answers: 1 Read on SO: [36] http://goo.gl/cez03g * How to make a binary tree zipper an instance of Comonad? votes: 13, answers: 1 Read on SO: [37] http://goo.gl/Zc1wr4 * Is there a better performing alternative to read and show in Haskell? votes: 11, answers: 1 Read on SO: [38] http://goo.gl/Y0m0wX * QuickCheck: How to use exhaustiveness checker to prevent forgotten constructors of a sum type votes: 11, answers: 2 Read on SO: [39] http://goo.gl/b8YbtB * Why does FRP consider time as a factor for values? votes: 10, answers: 2 Read on SO: [40] http://goo.gl/Yo65J8 Until next time, [41]+Daniel Santa Cruz References 1. https://github.com/mattgreen/hython 2. http://www.reddit.com/r/haskell/comments/2ejlxm/hython_a_haskellpowered_pyt… 3. https://github.com/serras/emacs-haskell-tutorial/blob/master/tutorial.md 4. http://www.reddit.com/r/haskell/comments/2efpx4/using_emacs_for_haskell_dev… 5. http://ivorylang.org/ 6. http://www.reddit.com/r/haskell/comments/2epdwp/ivory_language/ 7. http://blog.ezyang.com/2014/08/a-taste-of-cabalized-backpack/ 8. http://www.reddit.com/r/haskell/comments/2eoett/a_taste_of_cabalized_backpa… 9. http://www.reddit.com/r/haskell/comments/2f0xm0/introducing_rhaskellgamedev/ 10. http://www.reddit.com/r/haskell/comments/2f0xm0/introducing_rhaskellgamedev/ 11. http://hackage.haskell.org/package/acme-stringly-typed-1.0.0.0/docs/Acme-St… 12. http://www.reddit.com/r/haskell/comments/2f01jl/acmestringlytyped/ 13. http://keera.co.uk/blog/2014/08/29/new-version-yampa-0-9-6/ 14. http://www.reddit.com/r/haskell/comments/2ex7c9/a_new_version_of_yampa_is_o… 15. http://cdsmith.wordpress.com/2014/08/26/on-codeworld-and-haskell/ 16. http://www.reddit.com/r/haskell/comments/2en20t/on_codeworld_and_haskell/ 17. http://jozefg.bitbucket.org/posts/2014-08-25-dep-types-part-1.html 18. http://www.reddit.com/r/haskell/comments/2er003/introduction_to_dependent_t… 19. https://www.fpcomplete.com/user/snoyberg/general-haskell/advanced/evaluatio… 20. http://www.reddit.com/r/haskell/comments/2eiqwv/evaluation_order_and_state_… 21. http://stackoverflow.com/a/25576375/1651941 22. http://www.reddit.com/r/haskell/comments/2f191i/io_monad_and_purity/ 23. https://groups.google.com/forum/#!topic/haste-compiler/mel9aO9Jako 24. http://www.reddit.com/r/haskell/comments/2ev19m/ann_haste_04_now_with_ghc_7… 25. http://lists.osuosl.org/pipermail/darcs-users/2014-August/027041.html 26. http://www.reddit.com/r/haskell/comments/2ekjmx/darcs_285_released/ 27. http://hackage.haskell.org/package/yarr 28. http://www.reddit.com/r/haskell/comments/2eg1nk/yarr_image_processing_libra… 29. https://www.fpcomplete.com/blog/2014/08/conduit-stream-fusion 30. http://www.reddit.com/r/haskell/comments/2es8oj/conduit_stream_fusion/ 31. http://www.well-typed.com/blog/97/ 32. http://www.reddit.com/r/haskell/comments/2ety9f/new_blog_post_dealing_with_… 33. http://stackoverflow.com/questions/25554062/zipper-comonads-generically 34. http://stackoverflow.com/questions/25587501/test-if-a-value-matches-a-const… 35. http://stackoverflow.com/questions/25498431/what-is-the-difference-between-… 36. http://stackoverflow.com/questions/25476248/powerset-function-1-liner 37. http://stackoverflow.com/questions/25519767/how-to-make-a-binary-tree-zippe… 38. http://stackoverflow.com/questions/25496672/is-there-a-better-performing-al… 39. http://stackoverflow.com/questions/25535616/quickcheck-how-to-use-exhaustiv… 40. http://stackoverflow.com/questions/25568712/why-does-frp-consider-time-as-a… 41. https://plus.google.com/105107667630152149014/about

1 0

Race conditions with threadWait(Read/Write) and closeFdWith
by Leon Smith 03 Sep '14

03 Sep '14

I was very tangentially involved with a use-after-close IO descriptor fault recently, and I came to realize that descriptor indexes are typically allocated by the smallest available index, Previously, I had erroneously believed that the indexes were sequentially allocated by an ever-increasing counter, until wrap-around occurs. Obviously, the smallest available index allocation strategy makes use-after-close faults rather more significant, because in a server application that is constantly closing and allocating descriptors, it makes it rather more likely that an erroneous operation will actually have a tangible effect, and not simply return an EINVAL or similar errno. So in my low-level linux-inotify binding, I felt it wise to add protection against use-after-close faults. The approach I'm currently investigating is the obvious one: to (conceptually) represent the inotify socket by a "MVar (Maybe Fd)" value. However, if there's no file system events to read from the socket, we want to call threadWaitRead, So somewhere there's some code that's essentially this: readMVar inotifyfd >>= maybe (throwIO ...) threadWaitRead So the problem is, what happens when the thread executing this snippet yields in between the readMVar and the threadWaitRead? It would then be possible for another thread (or two) to close the inotify descriptor, and then allocate another descriptor with the same index. The first thread would then end up blocking until the new descriptor becomes readable, after which the thread would re-read the MVar and throw an exception that the descriptor has been closed. This is a _relatively_ benign effect, but still, it does prevent the waiting thread from throwing an exception at the earliest possible moment, and there are several particularly unlucky scenarios I can think of where such a thread could be waiting on the wrong descriptor for a very long time. Not to mention that this is a whole family of race conditions, which I haven't really explored the other possible manifestations of which might not be as benign. Somebody did also point me to the new threadWaitReadSTM primitives, but this doesn't cover this use case although the original proposal (properly implemented, of course) would: http://www.haskell.org/ghc/docs/7.8.3/html/libraries/base-4.7.0.1/Control-C… https://ghc.haskell.org/trac/ghc/ticket/7216 In any case, my linux-inotify binding is available on github; the current main branch is unreleased and also has code to attempt to make reading from a descriptor a thread-safe operation. However I'm not too enthusiastic about this unreleased code at the moment, and am considering throwing it out. However, I'd still very much like to add protection against use-after-close faults, and I'd certainly prefer being able to concurrently call both close and read on the descriptor with complete safety. https://github.com/lpsmith/linux-inotify Best, Leon

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"Lower case" infix type operator variables
by Joseph Abrahamson 03 Sep '14

03 Sep '14

I spent a few moments confused by the fact the TypeOperators was insufficient to allow the following type to be parsed constA :: Arrow (~>) => b -> (a ~> b) My current intuition is that since I *can* write things like newtype (~>) a b = A (a -> b) there is clashing in the type operator space for “upper case” and “lower case” identifiers. Is it possible or advisable to mitigate this clash and provide some syntax for “type operator variables”? Joseph

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ANNOUNCE: hemokit 0.6.3 - Now with OpenVibe compatibility
by Niklas Hambüchen 02 Sep '14

02 Sep '14

Heya, out is Hemokit 0.6.3, which can now be connected to the OpenVibe EEG framework. OpenVibe allows fancy stuff like spelling words with your mind: Video: https://www.youtube.com/watch?v=08GNE6OdNcs Docs: http://openvibe.inria.fr/coadapt-p300-stimulator-tutorial There is a new tutorial on how to set up Hemokit+OpenVibe at: https://github.com/nh2/hemokit/wiki/OpenVibe If you set up something cool with that, don't forget to tell me about it! Grab Hemokit: https://hackage.haskell.org/package/hemokit Stand-alone Linux and Windows binaries are also available: https://github.com/nh2/hemokit/releases

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System.Process.readProcess "yes" [] "" >>= return . take 100
by David Fox 02 Sep '14

02 Sep '14

I would expect the expression above to finish - but it doesn't. Is this a bug?

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Third call for papers, IFL 2014
by publicityifl＠gmail.com 02 Sep '14

02 Sep '14

Hello, Please, find below the third call for papers for IFL 2014. The submission page is now open. The submission date has been delayed to Sep. 8 2014 anywhere on the world. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage --- CALL FOR PAPERS 26th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2014 NORTHEASTERN UNIVERSITY/BOSTON, USA OCTOBER 1-3, 2014 http://ifl2014.github.io We are pleased to announce that the 26th edition of the IFL series will be held at Northeastern University in Boston, USA. The symposium will be held from 1st to 3rd of October 2014. Scope ----- The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2014 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2014 will use a post-symposium review process to produce the formal proceedings. All participants of IFL 2014 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings do not count as publication for the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. Submission Details ------------------ Submission deadline draft papers: September 8 Notification of acceptance for presentation: September 10 Early registration deadline: September 11 Late registration deadline: September 17 Submission deadline for pre-symposium proceedings: September 24 26th IFL Symposium: October 1-3 Submission deadline for post-symposium proceedings: December 15 Notification of acceptance for post-symposium proceedings: January 31 2015 Camera-ready version for post-symposium proceedings: March 15 2015 Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must adhere to the standard ACM two columns conference format. For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. A suitable document template for LaTeX can be found at: http://www.acm.org/sigs/sigplan/authorInformation.htm Papers should be submitted online at https://easychair.org/conferences/?conf=ifl2014 Topics ------ IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2014, please contact the PC chair at samth(a)cs.indiana.edu. Topics of interest include, but are not limited to: • language concepts • type systems, type checking, type inferencing • compilation techniques • staged compilation • run-time function specialization • run-time code generation • partial evaluation • (abstract) interpretation • metaprogramming • generic programming • automatic program generation • array processing • concurrent/parallel programming • concurrent/parallel program execution • embedded systems • web applications • (embedded) domain specific languages • security • novel memory management techniques • run-time profiling performance measurements • debugging and tracing • virtual/abstract machine architectures • validation, verification of functional programs • tools and programming techniques • (industrial) applications Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honoured article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee ------------------- Sam Tobin-Hochstadt, Indiana University (Chair) Rinus Plasmeijer, Radboud University Nijmegen (Co-Chair) Atze Dijkstra, Utrecht University Colin Runciman, University of York Graham Hutton, University of Nottingham Mary Sheeran, Chalmers University of Technology Patricia Johann, Appalachian State University Matthew Fluet, Rochester Institute of Technology Josef Svenningsson, Chalmers University of Technology Małgorzata Biernacka, University of Wroclaw Peter Achten, Radboud Univerity Nijmegen Laura Castro, University of A Coruña Hai Paul Liu, Intel Labs Kathryn Gray, Cambridge University Lars Bergstrom, Mozilla Research Lindsey Kuper, Indiana University Nicolas Wu, Oxford T. Stephen Strickland, University of Maryland Xavier Clerc, INRIA Venue ----- The 26th IFL will be held in association with the College of Computer and Information Science at Northeastern University. It can be reached quickly and easily by public transport.

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

01 Sep '14

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer (Functional) at McGraw-Hill Education http://functionaljobs.com/jobs/8737-senior-software-engineer-functional-at-… Cheers, Sean Murphy FunctionalJobs.com

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projects.haskell.org Pipermail not Google-indexed?
by Niklas Hambüchen 01 Sep '14

01 Sep '14

Hi, I noticed that I couldn't find this http://projects.haskell.org/pipermail/quickcheck/2014-July/000158.html on Google. Is the quickcheck list archive, or any of the other sites of projects.haskell.org actually reachable from via any path from some top level web site (like Haskell.org)?

1 0

LFM for Haskell bittorrent effort
by Mateusz Kowalczyk 01 Sep '14

01 Sep '14

Hi, For this or another reason, no bittorrent libraries/clients available today seem to suit my needs: they are either too slow, too memory inefficient, bundled with a client which often can't run in a headless environment or broken in some other way or don't implement all the common BEPs. I came to the conclusion that I'll probably have to write one. Happily, whenever I mentioned the topic in #haskell, there was quite a bit of interest each time so I'm looking to recruit people to join me in this effort. First a brief overlook of what's already available in the Haskell environment: * Combinatorrent[1] – a rather old client, no longer maintained with the last Hackage upload in 2010. Only supports few BEPs and was not able to load in the .torrent file provided by Ubuntu. I have spoken to jlouis who wrote this, you can read the conversation at [2]. He himself says that nowadays you can do better performance wise with available libraries. Additionally, this comes with a client on top rather than just being a library. * conjure – not touched since 2008 * bittorrent – seems to be the closest to what I'd like but it is still incomplete and the library is in limbo. There is a lot of stuff that does not work or is commented out and for various reasons the maintainer is unavailable to help out. I have spoken to the maintainer and he himself would like to redesign the library. That's about it. There seem to be a few related tools for bencoding &c but nothing huge. I think the best way to go about this is to learn from Combinatorrent and bittorrent and start something fresh. I'll outline the goals I personally am looking for even though they might be obvious: * efficient, scalable – the library needs to be able to handle tens of thousands of active (announcing, not necessarily downloading) torrents at once. For reference, libtorrent-rakshasa + rtorrent is able to run about ~10k torrents in about 1GB of memory with low CPU usage. The downside is that the library crashes like no tomorrow and the client itself sucks. A slow library is useless to me but so is a fast but unstable one. * implements all the common BEPs – no use for a library if it doesn't work for a lot of cases. It needs to implement everything from [3] maybe except BEP 8. * sane interface – in the end we need to be able to build clients on top. Fortunately we can leverage Haskell's types to do this whereas the existing C/C++ bittorrent libraries seems to suffer badly here. * resilient – if I wanted a fast library which crashes every ten minutes I'd stick with libtorrent Basically, I'm looking to build a fully usable bittorrent library. Notably, I'm not looking for people to help to write a client (yet!). I plan to start as soon as time allows but it'd be great if there was more than just myself on board. Let me know one way another if you're interested in contributing. If you're just interested in the final product then that's fine too, I could always use more motivation! [1]: https://github.com/jlouis/combinatorrent [2]: http://ircbrowse.net/browse/haskell?id=18701612&timestamp=1406738464#t14067… [3]: http://www.bittorrent.org/beps/bep_0000.html -- Mateusz K.

1 1

Set (Set a) like container - but more efficient?
by Marc Weber 01 Sep '14

01 Sep '14

Does a Set (Set a) like container exist already but which is represented as tree internally, eg A ` - B - C ` - D ` - X - Y which would represent ABC ABD and AXY ? If it does not what would be a nice name to create such container? Marc Weber

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Diony Rosa
by Diony Rosa 01 Sep '14

01 Sep '14

http://www.sarah-aloisi.com/ftpkk/lbqwswuvjc.ocrjttewxqiidnzuutwvyhq

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cabal sdist and auto-generated modules
by Marios Titas 31 Aug '14

31 Aug '14

I am trying to run cabal sdist on a package where one of exposed modules does not exists because it is automatically generated by setup.hs. cabal refuses to do that because it cannot find the file corresponding to that module. Is there a solution to that problem? The package builds just fine.

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RTS options to control name of eventlog file.
by Serguey Zefirov 31 Aug '14

31 Aug '14

Is there any? Because when running and profiling two same programs they will write into same eventlog file. Not a good situation.

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[ANN] igrf-0.2
by Douglas McClean 31 Aug '14

31 Aug '14

igrf is a library that provides an implementation of the International Geomagnetic Reference Field, which is just what it sounds like. I've tested it against NOAA's online calculator, but use it at your own risk. Note that the input is in geocentric and not geodetic coordinates. There is haddock documentation, I'm working on figuring out why it isn't on hackage and how to fix it. -Doug McClean

2 1

Monads vs Linear regions
by Timotej Tomandl 30 Aug '14

30 Aug '14

How do they compare? Can Linear regions handle all uses of monads or does they lack some applications?

1 0

Fwd: Linear regions ("Linear Regions Are All You Need") as the only way of encapsulation of impure state
by Timotej Tomandl 30 Aug '14

30 Aug '14

I was thinking for a while about if other kinds of handling of impure state have any advantages over linear regions. So I thought it will be nice to open the discussion about how does it compare with monads, linear types, uniqueness typing and monadic regions.

1 0

[ANN] codex 0.1.0.4
by Alois Cochard 30 Aug '14

30 Aug '14

Hi Cafe, I just wanted to let you know that I have just released a new version of `codex` on hackage. There is *no new features* in this version, but thanks for the help from Hubert Behaghel I was able to investigate issues with MacOS. So basically this version now work with MacOS, if you had give a try before please delete your "~/.codex" configuration file before installing the new version. Cheers -- *Λ\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

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ICFP pre-party
by Roman Cheplyaka 30 Aug '14

30 Aug '14

We're having a small unofficial ICFP pre-party today in Gothenburg. All haskell-cafe readers that happen to be in Gothenburg are welcome :-) The plan is to meet at The Bishop's Arms (Kungsportsavenyen 36) at 19.00. NOTE: there are three Bishop's Arms in Gothenburg, so mind the address. Feel free to text me at +380662285780. Roman

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Re: [Haskell-cafe] Using 'lens' for "over"-like computation in Monad
by Nikolay Amiantov 30 Aug '14

30 Aug '14

Thank you! I'm still lost in the forest of all lens functions and generality, and haven't expected this signature. Can someone recommend good reading on all the lens-related types and classes, what they represent and how can they be used? (interested not only in "how", but also "why") 30 авг. 2014 г. 11:20 пользователь "Benno Fünfstück" < benno.fuenfstueck(a)gmail.com> написал: > The lens operator that does this is %%~, also called `traverseOf`: > > λ: ("Hello", "World") & _1 %%~ (\a -> putStrLn a >> return a) > Hello > ("Hello","World") > > Also see the documentation: > http://hackage.haskell.org/package/lens-4.4.0.1/docs/Control-Lens-Traversal… > > -- > Benno > > > 2014-08-30 9:44 GMT+02:00 Nikolay Amiantov <nikoamia(a)gmail.com>: > >> Hello Cafe, >> >> I'm trying to wrap my head around 'lens' library. My current exercise is >> to modify something using Lens in monad. Say, >> >> ("Hello", "World") & _1 `myOp` (\a -> putStrLn a >> return a) >> >> in IO, where myOp would be of type: >> >> myOp :: Monad m => Lens s t a b -> (a -> m b) -> s -> m t >> >> Of course I can write it myself, using combination of "view" and "set": >> >> myOp lens f v = f (view lens v) >>= flip (set lens) v >> >> (have not checked this, but something like that should do), but is there >> a more elegant way? >> >> Nikolay. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

1 0

Using 'lens' for "over"-like computation in Monad
by Nikolay Amiantov 30 Aug '14

30 Aug '14

Hello Cafe, I'm trying to wrap my head around 'lens' library. My current exercise is to modify something using Lens in monad. Say, ("Hello", "World") & _1 `myOp` (\a -> putStrLn a >> return a) in IO, where myOp would be of type: myOp :: Monad m => Lens s t a b -> (a -> m b) -> s -> m t Of course I can write it myself, using combination of "view" and "set": myOp lens f v = f (view lens v) >>= flip (set lens) v (have not checked this, but something like that should do), but is there a more elegant way? Nikolay.

1 0

"Hackathon"
by Richard Eisenberg 30 Aug '14

30 Aug '14

Hi Cafe, In my experience, "hackathon" can refer to two very different sorts of events: hacking marathons (such as jacobsHack), where participants tend to work overnight to accomplish something amazing in a limited time; and hacker weekends (such as Hac Phi), where participants work on projects, socialize, and then (presumably) rest at night. Both of these sorts of events have their place in the world, and I'm in no way suggesting one is "better" than the other. But, I do think it be good for all of us to name them differently, so folks know what they are signing up for. In particular, I'm worried that calling hacker weekends "hackathons" may discourage those of us with outside, inflexible commitments (e.g. kids; the need for 8 hours of sleep) from attending. Conversely, folks looking for the higher-energy environment of an all-night marathon might be disappointed to show up at a hacker weekend. What do you think? Is this distinction pointless? Would being consistent about this difference help? Here are some proposed new names for hacker weekends: - Hacker weekend - Hacker meetup - Community Hack - Weekend of Haskell I personally favor reserving the term "hackathon" for the marathon events. Richard

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Problem finding rewrite rules
by Dominik Peteler 30 Aug '14

30 Aug '14

Dear Cafe, I'm currently looking at the optimization GHC is doing and I cannot find the rewrite rules it fires. When I run my test code with ghc -O2 -ddump-simpl-stats -ddump-rule-firings Main.hs GHC shows the rules which are fired: ... Rule fired: Class op + ... Rule fired: +## ... and so on. Nothing new, nothing special. However, where do I find the definitions of these rules ? I grepped[1] the GHC code base and found nothing so far. I didn't find any documentation on it either. Can anyone point me to some place where I can find further information ? Thank you folks and have a nice day Dominik PS.: Since I'm working on numerical stable code with directed rounding I'm only interested in these two particular rules. I suspect them to break parts of my code. [1] http://jamie-wong.com/2013/07/12/grep-test

3 3

Master's thesis topic and mentor
by Konstantin Saveljev 30 Aug '14

30 Aug '14

Hello, *short summary*: I am looking for someone who might have a topic for Master's thesis and will be interested in mentoring me along the way (giving pointers, helping here and there but not doing everything for me) *more detailed*: I have been introduced to Haskell about a year ago. It's been a slow ride, but I am gaining knowledge step by step. I am having some difficulty in finding a topic for my Master's degree and thought it would benefit me to try doing something Haskell related (be it a library, some part of a project or something similar). Unfortunately I do not believe I am able to pick something up by myself. Therefore I am looking for someone out there who might just have something to do they would be interested in delegating to somebody else and helping a bit throughout that work (and it cannot be something too easy, as there are some expectations from the work done for Master's degree) . It might be a long shot but I believe it is worth trying. Thank you for your time. Best regards, Konstantin Saveljev

4 3

Does web-based email harm mailman lists?
by Kim-Ee Yeoh 29 Aug '14

29 Aug '14

Beautiful haskell people, Ever noticed the lacunae on some list threads? Someone hits reply and instead of reflecting via mailman, it goes direct to OP. OP notices absence of To:haskell-cafe and adds it back in in their reply to the reply. End result? The thread looks like OP having a convo with themselves. Unless you look at the quoted parts, which you have to click to reveal in web-based email. The convention, say with a google-groups based mailing list, is that conversations in mailing list are public by default. With some manual C&P, you can email responses in private. For cafe participants using web-based email, the situation is reversed, through no fault of their own. Approx 18 months ago, the haskell-beginners list suffered the same problem [1]. After some digging, it looks like there's a configurable option to Do The Right Thing: http://ccit.mines.edu/Mailman-FAQ#25 I was also privately emailed that there are downsides I wasn't aware of: Reply-To Munging Considered Harmful http://www.unicom.com/pw/reply-to-harmful.html Reply-To Munging Considered Useful http://www.metasystema.org/essays/reply-to-useful.html So what happened in the aftermath? Not much that I can tell. Everything just purred along. Now you might argue that haskell-cafe will just purr along grandly as well given the status quo. But this is a wrinkle that probably needs some awareness at least, hence this email. Would you like this fixed? The list owner would need to be obliged upon. [1] http://www.haskell.org/pipermail/beginners/2013-January/011304.html -- Kim-Ee

4 4

ANN: Yampa 0.9.6
by Ivan Perez 29 Aug '14

29 Aug '14

Hi Café, I'm happy to announce that a new version of the Arrowized Functional Reactive Programming (FRP) library Yampa has been released. This new version includes a lot of haddock documentation, finally addressing the old issue of having to browse a large collection of papers and the code to find information about the API. We would like to encourage users to submit issues on github if they find this documentation insufficient, unclear or otherwise unsuitable in any way. There are no changes to the API itself, so this version remains backwards compatible. Future releases will seek to improve the documentation and introduce new optimisations as mandated by users' needs. As you may have heard, the current version is now working on Android, so we expect new releases to be published relatively soon. Additionally, Henrik Nilsson will give a tutorial on Declarative Game Programming at PPDP 2014, which will take place in Canterbury (UK) on September 8-10 (http://users-cs.au.dk/danvy/ppdp14/) We would like to encourage everyone to attend (see PPDP web page for instructions, venue, etc.). Links and more: * Hackage package: http://hackage.haskell.org/package/Yampa-0.9.6. * Github project page: https://github.com/ivanperez-keera/Yampa * Haskell Wiki page (examples, papers): http://www.haskell.org/haskellwiki/Yampa * Official announcement: http://fplab.bitbucket.org/posts/2014-08-29-yampa-0_9_6-released.html * (Another) Release announcement: http://keera.co.uk/blog/2014/08/29/new-version-yampa-0-9-6. Cheers Ivan -- Facebook: https://www.facebook.com/keerastudios Twitter: https://twitter.com/KeeraStudios Web: http://www.keera.co.uk

1 0

ANN: HacBerlin - Haskell Hackathon in Berlin, 26-28 Sep 2014
by Stefan Wehr 29 Aug '14

29 Aug '14

Hi everyone, this is just a quick reminder: The Haskell Hackathon in Berlin is coming soon and there are still some places left. Please register now: http://goo.gl/aLfnWu The first keynote is also fixed; it will be given by Andres Löh (http://www.andres-loeh.de/) Thanks, Andres! Where: Berlin, Germany When: Fri 26 - Sun 28 September 2014 Meet in Berlin, discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! For all details, visit our wiki page (http://www.haskell.org/haskellwiki/HacBerlin2014) and make sure to register now! Cheers, Stefan

1 0

Conduit+GHC high memory use for simple Sink
by Bryan Vicknair 29 Aug '14

29 Aug '14

Hello Cafe, First I'd like to thank Michael Snoyman and Gabriel Gonzalez for the work they've done on the conduit and pipes stream processing libraries, and all the accompanying tutorial content. I've been having fun converting a text processing app from C to Haskell. I'm seeing unexpectedly high memory usage in a stream-processing program that uses the conduit library. I've created a example that demonstrates the problem. The program accepts gzip files as arguments, and for each one, classifies each line as either Even or Odd depending on the length, then outputs some result depending on the Sink used. For each gzip file: action :: GzipFilePath -> IO () action (GzipFilePath filePath) = do result <- runResourceT $ CB.sourceFile filePath $$ Zlib.ungzip =$ CB.lines =$ token =$ sink2 putStrLn $ show result The problem is the following Sink, which counts how many even/odd Tokens are seen: type SinkState = (Integer, Integer) sink2 :: (Monad m) => SinkState -> Sink Token m SinkState sink2 state@(!evenCount, !oddCount) = do maybeToken <- await case maybeToken of Nothing -> return state (Just Even) -> sink2 (evenCount + 1, oddCount ) (Just Odd ) -> sink2 (evenCount , oddCount + 1) When I give this program a few gzip files, it uses hundreds of megabytes of resident memory. When I give the same files as input, but use the following simple Sink, it only uses about 8Mb of resident memory: sink1 :: MonadIO m => Sink Token m () sink1 = awaitForever (liftIO . putStrLn . show) At first I thought that sink2 performed so poorly because the addition thunks were being placed onto the heap until the end, so I added some bang patterns to make it strict. That didn't help however. I've done profiling, but I'm unable to figure out exactly what is being added to the heap in sink2 but not sink1, or what is being garbage collected in sink1, but not sink2. The full source is here: https://bitbucket.org/bryanvick/conduit-mem/src/HEAD/hsrc/bin/mem.hs Or you can clone the repo, which contains a cabal file for easy building: git clone git@bitbucket.org:bryanvick/conduit-mem.git cd comduit-mem cabal sandbox init cabal install --only-dependencies cabal build mem ./dist/build/mem/mem [GIVE SOME GZIP FILES HERE] You can change which sink is used in the 'action' function to see the different memory usage.

5 14

Haskell Platform 2014 build problems on OS X
by Nicola Gigante 28 Aug '14

28 Aug '14

Hello! I just want to point to your attention this issue of Homebrew: https://github.com/Homebrew/homebrew/issues/31609 It seems that they’re forced to remove the formula for the Haskell Platform because, it seems, the build process has become extremely complicated and they can’t figure out how to automate it. Is there someone that know enough internals of the platform to help them or to give some advice? Thanks, Nicola

4 6

Call to arms for Haskell students
by Ernesto Rodriguez 28 Aug '14

28 Aug '14

Dear Haskellers, My (previous) university is organizing one of the first university hackathons in Europe. I am participating and would like to have a Haskell team for the hackathon. However, I don't know any Haskellers around or who can travel to Bremen. Let me know if anyone is interested to participate and team up with me. If it helps there are quite inexpensive flights to Bremen. The hackathon is only for students though. Quoted is the formal invite. Best regards, Ernesto It is about time to bring hackathons to European universities. A few > students at Jacobs University are therefore organizing a hackathon that > aims to bring people from different countries together to hack. > jacobsHack! is an international student-run hackathon in Bremen, Germany. > Students will get 24 hours to hack on anything they like and have the > chance to win prizes worth up to 2000€. Additionally they can make the > first step in the direction of working with one of our sponsors Google, > SAP, Microsoft, and Entrepreneur First. > > More information at: > https://jacobshack.com/ > > Live long, #hackstrong > -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

1 0

Haskell Weekly News: Issue 303
by Daniel Santa Cruz 28 Aug '14

28 Aug '14

1 0

lightweight web interface for Haskell?
by Richard Eisenberg 27 Aug '14

27 Aug '14

Hi Cafe, Does anyone know of a website where I can write a few lines of Haskell and have them run? Ideally, there would be a place to write a module and then a place to load the module into GHCi. I seem to recall something that fit my needs at paste.hskll.org, but that site is no longer working. I'm asking because I'll be teaching an undergraduate course this fall in introductory Haskell. There are no prerequisites for the course, and my course roster suggests that I'll have several students with no programming background at all. (This is great, actually -- I love facilitating a new programmer's first steps!) It might be nice to get students off the ground quickly before bogging them down with installation details -- hence the website I'm looking for. Thanks! Richard

12 15

[ANN] Wheb 0.3 release
by Kyle Hanson 27 Aug '14

27 Aug '14

Hello all, I have been working on a web framework that I use for my personal projects and I thought I would share my small progress. I started this project several months ago when I hit a big bump in difficulty going from a simple skeleton project to a more intricate project using alternative frameworks. Lately I've added three main features: WebSockets, Cache and Redis. WebSockets use the same WhebT interface as the standard HTTP handlers allowing you to share functions and middlware accross your application. Cache is a new plugin for simplistic Caching. It provides an interface to cache ByteStrings under Text keys. Like the other plugins, it is backend agnostic so you can write your own backend or use the prebuilt Redis cache (this requires you to configure your redis instance for caching). In addition to Redis acting as a Cache, you can also use it as a database and as a backend for the plugins Auth and Sessions. You can mix Redis with other plugins. For one of my projects I use the MongoDB plugin for auth and storage and Redis as a cache for the queries. Also included are typed web-routes, stateful cookie handling, and some other stuff. Feel free to check it out. I tried to include some documentation and examples that I refer to when I use it. Maybe it will help someone. Its far from perfect, so if you find any problems, let me know. https://hackage.haskell.org/package/Wheb https://github.com/hansonkd/Wheb-Framework Cheers, Kyle Hanson

1 0

Re: [Haskell-cafe] Types depending on tuples.
by Mike Izbicki 27 Aug '14

27 Aug '14

This works just fine for me: class Vector a where toLists :: a -> [Double] instance Vector (Double,Double,Double) where toLists (a,b,c)=[a,b,c] On Tue, Aug 26, 2014 at 1:17 AM, Mike Meyer <mwm(a)mired.org> wrote: > It's there, but in the text: > > class Vector a where > toLists :: a -> [Double] > > > On Tue, Aug 26, 2014 at 2:59 AM, Mike Izbicki <mike(a)izbicki.me> wrote: >> >> You never actually provided the Vector type class, so it's difficult >> to tell what exactly you're trying to do. >> >> On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <mwm(a)mired.org> wrote: >> > I'm doing some geometry in 2 and 3 dimensions. To do this, I've got >> > Vector2d >> > and Vector3d classes, which are basically just wrappers around 2 and >> > 3-tuples of doubles: >> > >> > type Vector2d = (Double, Double) >> > type Vector3d = (Double, Double, Double) >> > >> > I've also got a typeclass Vector so I can do polymorphic things on them: >> > >> > type Vector3d = (Double, Double, Double) >> > >> > (Ok, polymorphic thing). >> > >> > I now have a need to convert lists of Vector's to a list of lists of >> > Doubles. I'd like that to be part of the Vector typeclass, but - well, >> > declaring it is interesting. >> > >> > I can't just add a method "toLists :: a -> [Double]", because (according >> > to >> > ghc), there's no match between the expected type 'Double' and the actual >> > type '[t1]'. >> > >> > Can I declare this typeclass? >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > >

4 4

Fwd: Locking of threads in one OS thread
by Nikolay Amiantov 26 Aug '14

26 Aug '14

On Tue, Aug 26, 2014 at 6:59 PM, Sylvain Henry <hsyl20(a)gmail.com> wrote: > AFAIK "errno" is handled explicitly in the RTS: > https://ghc.haskell.org/trac/ghc/browser/ghc/rts/Schedule.c#L484 > > -Sylvain > Oh, thanks for pointing this out! Now this behaviour makes sense.

1 0

Locking of threads in one OS thread
by Nikolay Amiantov 26 Aug '14

26 Aug '14

Hello Cafe, I'm using FFI to interact with a library which calls, when fail, leave the reason in some kind of "errno"-like variable which is retrived via another call. AFAIU, this is not thread-safe in Haskell even if thread-local storage is used inside the library, because Haskell uses its own thread management and the Haskell thread in the same OS thread might be switched between the actual call and the retrival of errno value. This should be somehow handled already in Haskell (errno is widely used with syscalls in Linux, for example), but the source of Foreign.C.Error suggests that this is not handled in any way at all. For example, throwErrnoIf is implemented as such: throwErrno loc = do errno <- getErrno ioError (errnoToIOError loc errno Nothing Nothing) throwErrnoIf pred loc f = do res <- f if pred res then throwErrno loc else return res So, the question is: how is it ensured that this block is "atomic" in sense that at most one Haskell thread computes this whole function at every moment in every OS thread? Thanks for any explanations! (And sorry if my English is poor) Nikolay.

4 5

Types depending on tuples.
by Mike Meyer 26 Aug '14

26 Aug '14

I'm doing some geometry in 2 and 3 dimensions. To do this, I've got Vector2d and Vector3d classes, which are basically just wrappers around 2 and 3-tuples of doubles: type Vector2d = (Double, Double) type Vector3d = (Double, Double, Double) I've also got a typeclass Vector so I can do polymorphic things on them: type Vector3d = (Double, Double, Double) (Ok, polymorphic thing). I now have a need to convert lists of Vector's to a list of lists of Doubles. I'd like that to be part of the Vector typeclass, but - well, declaring it is interesting. I can't just add a method "toLists :: a -> [Double]", because (according to ghc), there's no match between the expected type 'Double' and the actual type '[t1]'. Can I declare this typeclass?

2 1

Undesired email
by felipe zapata 25 Aug '14

25 Aug '14

Dear List, I have a problem with me new smartphone. Sorry for the inconvenient. Best, FelipeZ

1 0

Munich Haskell Meeting
by Heinrich Hördegen 24 Aug '14

24 Aug '14

Dear all, next Tuesday, 26th of August, Munich's Haskell enthusiasts will meet again at Cafe Puck around 19h30. Everyone is wellcome. Please note that we will meet at Cafe Puck. To help us reserve enough tables, please go to http://www.haskell-munich.de/dates and click the button. Heinrich

1 0

ghc-mod v5.0.0
by Kazu Yamamoto 24 Aug '14

24 Aug '14

Hi cafe, We are happy to announce that ghc-mod v5.0.0 has been released. Highlights of this version is as follows: - ghc-mod v4.x.y is suffering from the space leak of GHC 7.8.x(*1). So, ghc-modi consumes huge memory. ghc-modi v5.0.0 consumes much less memory thanks to workaround. - Alejandro Serrano merges his results of Google Summer of Code. He implements case splitting and sophisticated type hole handling. I believe that he will write an article on his results but I also updated the manual of Emacs fronted. (*2) - Daniel Gröber brushed up the internal code by bringing the GhcMod monad. So, the API of ghc-mod drastically changed. If you are users of the ghc-mod library, you should check its document. The main purpose of this release is to show the results of GSoC to everyone. This is a snapshot release and its API would change in the future. I thank two guys above since they worked hard and did good jobs. We hope that everyone enjoys v5.0.0. (*1) https://ghc.haskell.org/trac/ghc/ticket/9314 (*2) http://www.mew.org/~kazu/proj/ghc-mod/en/emacs.html --Kazu

2 2

How to write transform one set of ADTs to another with SYB?
by ducis 23 Aug '14

23 Aug '14

More specifically, I want to transform a tree of SomeExpr to a tree of (SomethingElse, SomeExpr), "prefixing" each node with a default value in the process. The result have essentially the same structure as the input. data ASTAttachment = AA {} deriving (Eq,Read,Show,Ord,Typeable,Data) type family AST a :: * type instance AST () = ASTExpr () type instance AST AA = (AA, ASTExpr AA) data ASTExpr a = ALiteral String | AApplication [AST a] (ASTOp a) | ARef String | ABind (AST a) String | AMany (ASTMany a) data ASTOp a = AOSym String | AOAlpha String | AOMany (ASTMany a) data ASTMany a = AMSimple [AST a] | AMAggeregate [AST a] deriving instance Typeable ASTMany deriving instance Typeable ASTOp deriving instance Typeable ASTExpr deriving instance Data (ASTMany ()) deriving instance Data (ASTOp ()) deriving instance Data (ASTExpr ()) Currently I have values of type (AST ()), and I need to transform them to (AST ASTAttachment) in the obvious way. Now I'm stuck at "derive instance Data (ASTExpr ASTAttachment)", where ghc reports "Multiple declarations of '$cSomeConstructor'". Even if I can get past this, I still don't know how to use SYB to transform between trees of different types.

2 1

Temporarily taking over hdevtools on Hackage.
by Schell Scivally 23 Aug '14

23 Aug '14

Hi all, it seems that Bit Connor (the author of hdevtools - https://github.com/bitc/hdevtools) has been MIA since January. There are a number of pull requests that need to be merged and uploaded to hackage for GHC 7.8. I tried contacting him last week through both emails listed on hackage - one bounced and the other is unanswered. Has anyone seen/heard from the author? I'd like to temporarily hijack hackage responsibilities until we hear from him again in order to merge and upload these changes ( https://github.com/bitc/hdevtools/issues/24#ref-commit-a996411) -- Schell Scivally | github <https://github.com/schell> | twitter <http://twitter.com/schellsan> | blog <http://efnx.com> | wiki <http://wiki.zyghost.com>

3 3

How to add Haddock comment for standalone derived instances?
by Hiromi ISHII 23 Aug '14

23 Aug '14

Hi cafe, Is there any way to add the documentation comment for the instances defined with StandaloneDeriving? I'm currently defining data-type using GADTs and its Typeable instance. Normally, this can be done only StandaloneDeriving and DeriveDataTypeable extensions: > {-# LANGUAGE StandaloneDeriving, GADTs, DeriveDataTypeable #-} > data Ordinal n where > OZ :: Ordinal (S n) > OS :: Ordinal n -> Ordinal (S n) > > deriving instance Typeable Ordinal I added `Typeable` instance for Ordinal recently, so I want to add some comments like "Since 0.2.3.0". But any of the following doesn't work or, even worse, haddock won't compile: * Just before `deriving instance` line using `-- | ` * Just after `deriving` keyword but before `instance` using `-- | ` * Just after the `Typeable Ordinal`, but no newline in-between, with `-- ^ ` * The next line of `deriving` clause with `-- ^ ` Is there any way to add documentation for instances with standalone deriving, or it's just not supported yet? -- Hiromi ISHII konn.jinro(a)gmail.com

2 1

Why this existential type could not work?
by Magicloud Magiclouds 23 Aug '14

23 Aug '14

Hi, For example, code like this: newtype Sealed = Sealed { unSealed :: forall a. SomeClass a => TVar a } mkSealed :: (SomeClass a) => a -> IO Sealed mkSealed = fmap Sealed . newTVarIO When compiling, I would get: Expected type: a -> IO (forall a1. SomeClass a1 => TVar a1) Actual type: a -> IO (TVar a) How to correctly restrict type parameter a here? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

5 5

Vector sort poor performance
by Alexander Pakhomov 22 Aug '14

22 Aug '14

Hi! I've got dramatically low sort performamce: about 2 us of time and 5 kB of allocation for unpacked vector of 4 (four) Double. Code: import Data.List import Control.Monad import Control.Monad.ST import qualified Data.Vector.Unboxed as V import qualified Data.Vector.Unboxed.Mutable as MV import qualified Data.Vector.Algorithms.Intro as Intro import Data.IORef arr = V.fromList ([1,2] :: [Double]) foo x = V.head q where q = runST $ do res <- V.unsafeThaw $ V.concat [ V.map (\e -> e + x) arr , V.map (\e -> e - x) arr] Intro.sort res V.unsafeFreeze res main = do ref <- newIORef 0 forM_ [0..100000] $! \i -> do modifyIORef' ref (+(foo $ fromInteger i)) -- for foo not to optimize out readIORef ref >>= print ghc -O2 sort.hs && time ./sort [1 of 1] Compiling Main ( sort.hs, sort.o ) Linking sort ... -4.999949999e9 real 0m0.189s user 0m0.184s sys 0m0.000s Does anybody know what's going on?

4 7

Re: [Haskell-cafe] Does GHC compare pointers when eval'ing (==)
by Johan Holmquist 21 Aug '14

21 Aug '14

To summarise, the points raised sofar against this optimisation are, roughly, as follows. * More expensive to compare two values which aren't equal. I think the potential gain of the optimisation might outweigh the cost in the general case. (Also, the optimisation would not have to be triggered for simple values like Char, Int and Double.) * Dodgy Eq instances may break the optimisation But may also break other stuff and should be avoided anyway. * _|_ will not necessarily yield _|_ on comparison This seems the biggest problem sofar to me, that (5,_|_) == (5,_|_) would be _|_ if both pairs where at different memory locations and True otherwise. But perhaps not a big deal in practice. * Disrupts other compiler optimisation opportunities My knowledge about compilers is lacking so I cannot comment this. Interesting that this was implemented by hbc. Would be nice to know if it led to problems. 2014-08-20 14:32 GMT+02:00 Jan-Willem Maessen <jmaessen(a)alum.mit.edu>: > On Wed, Aug 20, 2014 at 2:52 AM, Johan Holmquist <holmisen(a)gmail.com> > wrote: > >> Comparing two structures for equality (structurally) can be expensive. >> But if their references are the same they would for sure be equal (unless >> (==) was defined in some funny way). Does GHC perform any such optimization? >> > > The big problem in practice are the Float / Double instances in the > presence of NaN, as noted by other folks. > > Back in the day, hbc had a flag that would do a pointer equality check in > the derived Eq instances it generated before falling back to a traversal. > Interestingly, it does actually change the way you think about programming > when you're working with multiple values that are very likely to share > sub-structure. Suddenly it seems OK to do equality checks as you traverse > (because most of them will succeed), which would be O(n^2) if you weren't > doing the pointer test. > > -Jan-Willem Maessen > > >> (Likely a question for another list but I make a stab here first. ) >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

6 7

Taking over setlocale
by Sven Bartscher 21 Aug '14

21 Aug '14

Greetings, I would like to take over the setlocale package. Following the steps described on http://www.haskell.org/haskellwiki/Taking_over_a_package I need to state my intention on a public forum. The original author (Lukas Mai) published his package in the Public Domain. This would be fine, but the Author seems to be living in Germany. The problem with that is that in Germany an author is not allowed to give up his copyright in this way. This means the licensing of the package defaults to "All rights reserved", making the package undistributable. I already tried to contact the author about this problem, but he didn't respond in a week. This isn't a very long time, but I didn't get an answer on past attempts to contact him (this was at least a month, but probably more, ago). To solve this problem, I rewrote the setlocale binding with the same API under the BSD3-clause. PS: Please keep the CC to Lukas Mai and the debian-haskell list intact. Regards Sven

5 5

Haskell Weekly News: Issue 302
by Daniel Santa Cruz 21 Aug '14

21 Aug '14

Welcome to issue 302 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from August 3 to 16, 2014 Quotes of the Week * alpha123: I don't think I have the gonads for monads. :( * edwardk: There is nothing Natural about a Word. * bernalex: I very often have to deal with people who think I'm pretentious for caring about what programming language I use. "it's just programming". lol. sopvop: I used to think the same sopvop: Then haskell corrupted me sopvop: Haskell is a ghetto Top Reddit Stories * ಠ_ಠ :: String -> a Domain: hackage.haskell.org, Score: 122, Comments: 56 Original: [1] http://goo.gl/S87evt On Reddit: [2] http://goo.gl/vsfwgr * An inspiring Haskell story: the green screen of FP Domain: keera.co.uk, Score: 120, Comments: 13 Original: [3] http://goo.gl/z8Aecw On Reddit: [4] http://goo.gl/fevzzz * Haskell Platform 2014.2.0.0 is released Domain: haskell.org, Score: 117, Comments: 56 Original: [5] http://goo.gl/Wc6TAF On Reddit: [6] http://goo.gl/hMPM7F * criterion 1.0: a major new release Domain: serpentine.com, Score: 110, Comments: 13 Original: [7] http://goo.gl/7Qf3ox On Reddit: [8] http://goo.gl/Pa9fCW * What’s a module system good for anyway? Domain: blog.ezyang.com, Score: 102, Comments: 20 Original: [9] http://goo.gl/qtm97E On Reddit: [10] http://goo.gl/XlWrM2 * Easy Haskell Profiling Domain: danielvelkov.blogspot.com, Score: 83, Comments: 33 Original: [11] http://goo.gl/SWdxdC On Reddit: [12] http://goo.gl/z6clxp * Type Theory Podcast: Peter Dybjer on QuickCheck, verification, and semantics Domain: typetheorypodcast.com, Score: 57, Comments: 21 Original: [13] http://goo.gl/jN12Ak On Reddit: [14] http://goo.gl/tLVPpw * Fantasy World Haskell Domain: self.haskell, Score: 52, Comments: 165 Original: [15] http://goo.gl/UmJLhG On Reddit: [16] http://goo.gl/UmJLhG * Haskell for all: managed-1.0.0: A monad for managed resources Domain: haskellforall.com, Score: 51, Comments: 10 Original: [17] http://goo.gl/tbw8JN On Reddit: [18] http://goo.gl/cKB6xZ * Announcing auto-update Domain: yesodweb.com, Score: 47, Comments: 4 Original: [19] http://goo.gl/xhlw04 On Reddit: [20] http://goo.gl/GO6udg * Codeworld: An open-source educational programming environment using Haskell Domain: github.com, Score: 47, Comments: 8 Original: [21] http://goo.gl/7NTgnF On Reddit: [22] http://goo.gl/wJBgw5 * PureScript By Example Domain: leanpub.com, Score: 46, Comments: 25 Original: [23] http://goo.gl/vzcfP6 On Reddit: [24] http://goo.gl/p3gkEB * Lenses from the ground up Domain: taylor.fausak.me, Score: 45, Comments: 43 Original: [25] http://goo.gl/S31V8w On Reddit: [26] http://goo.gl/DAGBPz * Algebraic Terraforming: Trees from Magma Domain: fpcomplete.com, Score: 42, Comments: 22 Original: [27] http://goo.gl/1nGpnz On Reddit: [28] http://goo.gl/iX8ZST * Denotational Design: from meanings to programs By Conal Elliott at BayHac (Video) Domain: youtube.com, Score: 42, Comments: 5 Original: [29] http://goo.gl/rHeDrK On Reddit: [30] http://goo.gl/SLCZ5D * Just LOOK at the humongous type that Hindley-Milner infers for this tiny program! Domain: spacemanaki.com, Score: 38, Comments: 27 Original: [31] http://goo.gl/cAypnJ On Reddit: [32] http://goo.gl/ztDpK7 * Recommended Reading Material Domain: reinh.com, Score: 37, Comments: 12 Original: [33] http://goo.gl/LwGkEr On Reddit: [34] http://goo.gl/ED5pqI * (Monoidal) transducers are monoid homomorphisms Domain: oleksandrmanzyuk.wordpress.com, Score: 37, Comments: 43 Original: [35] http://goo.gl/VPDl6Y On Reddit: [36] http://goo.gl/5RY9Nf * λ Lessons: Pattern matching, first-class functions, and abstracting over recursion in Haskell Domain: stevekrouse.github.io, Score: 36, Comments: 3 Original: [37] http://goo.gl/NkxhS5 On Reddit: [38] http://goo.gl/4275GH Top StackOverflow Questions * What are the problems with an ADT encoding that associates types with data constructors? (Such as Scala.) votes: 41, answers: 1 Read on SO: [39] http://goo.gl/k4bw4R * Why is Haskell missing “obvious” Typeclasses votes: 26, answers: 7 Read on SO: [40] http://goo.gl/2YElO8 * Is there a way to elegantly represent this pattern in Haskell? votes: 22, answers: 6 Read on SO: [41] http://goo.gl/sQdRMt * Why does Scala not have a return/unit function defined for each monad (in contrast to Haskell)? votes: 16, answers: 4 Read on SO: [42] http://goo.gl/MQlZTi * How did Haskell add Turing-completeness to System F? votes: 15, answers: 1 Read on SO: [43] http://goo.gl/weJo4Z * How to avoid superfluous variables in do notation? votes: 14, answers: 2 Read on SO: [44] http://goo.gl/b3IgC5 Until next time, [45]+Daniel Santa Cruz References 1. https://hackage.haskell.org/package/acme-lookofdisapproval-0.1/docs/Acme-Lo… 2. http://www.reddit.com/r/haskell/comments/2dce3d/%E0%B2%A0_%E0%B2%A0_string_… 3. http://keera.co.uk/blog/2014/08/13/most-inspiring-green-screen-you-will-eve… 4. http://www.reddit.com/r/haskell/comments/2djh3b/an_inspiring_haskell_story_… 5. http://www.haskell.org/platform/#2014.2.0.0 6. http://www.reddit.com/r/haskell/comments/2d3p2c/haskell_platform_2014200_is… 7. http://www.serpentine.com/blog/2014/08/08/criterion-1-0/ 8. http://www.reddit.com/r/haskell/comments/2cyuv4/criterion_10_a_major_new_re… 9. http://blog.ezyang.com/2014/08/whats-a-module-system-good-for-anyway/ 10. http://www.reddit.com/r/haskell/comments/2d3xuh/whats_a_module_system_good_… 11. http://danielvelkov.blogspot.com/2014/08/easy-haskell-profiling.html 12. http://www.reddit.com/r/haskell/comments/2d8ob4/easy_haskell_profiling/ 13. http://typetheorypodcast.com/2014/08/episode-1-peter-dybjer-on-type-theory-… 14. http://www.reddit.com/r/haskell/comments/2df24l/type_theory_podcast_peter_d… 15. http://www.reddit.com/r/haskell/comments/2chb2h/fantasy_world_haskell/ 16. http://www.reddit.com/r/haskell/comments/2chb2h/fantasy_world_haskell/ 17. http://www.haskellforall.com/2014/08/managed-100-monad-for-managed-resource… 18. http://www.reddit.com/r/haskell/comments/2d7109/haskell_for_all_managed100_… 19. http://www.yesodweb.com/blog/2014/08/announcing-auto-update 20. http://www.reddit.com/r/haskell/comments/2crqyg/announcing_autoupdate/ 21. https://github.com/google/codeworld 22. http://www.reddit.com/r/haskell/comments/2drcwx/codeworld_an_opensource_edu… 23. https://leanpub.com/purescript/read 24. http://www.reddit.com/r/haskell/comments/2cswdd/purescript_by_example/ 25. http://taylor.fausak.me/2014/08/03/lenses-from-the-ground-up/ 26. http://www.reddit.com/r/haskell/comments/2cl28r/lenses_from_the_ground_up/ 27. https://www.fpcomplete.com/user/bss/magma-tree 28. http://www.reddit.com/r/haskell/comments/2corq6/algebraic_terraforming_tree… 29. https://www.youtube.com/watch?v=zzCrZEil9iI 30. http://www.reddit.com/r/haskell/comments/2dh0sd/denotational_design_from_me… 31. http://spacemanaki.com/blog/2014/08/04/Just-LOOK-at-the-humongous-type/ 32. http://www.reddit.com/r/haskell/comments/2cka6d/just_look_at_the_humongous_… 33. http://reinh.com/notes/posts/2014-07-25-recommended-reading-material.html 34. http://www.reddit.com/r/haskell/comments/2cpr4z/recommended_reading_materia… 35. http://oleksandrmanzyuk.wordpress.com/2014/08/09/transducers-are-monoid-hom… 36. http://www.reddit.com/r/haskell/comments/2d5ael/monoidal_transducers_are_mo… 37. https://stevekrouse.github.io/hs.js/ 38. http://www.reddit.com/r/haskell/comments/2d1dpg/%CE%BB_lessons_pattern_matc… 39. http://stackoverflow.com/questions/25330359/what-are-the-problems-with-an-a… 40. http://stackoverflow.com/questions/25191659/why-is-haskell-missing-obvious-… 41. http://stackoverflow.com/questions/25107101/is-there-a-way-to-elegantly-rep… 42. http://stackoverflow.com/questions/25238622/why-does-scala-not-have-a-retur… 43. http://stackoverflow.com/questions/25255413/how-did-haskell-add-turing-comp… 44. http://stackoverflow.com/questions/25314793/how-to-avoid-superfluous-variab… 45. https://plus.google.com/105107667630152149014/about

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Re: [Haskell-cafe] Does GHC compare pointers when eval'ing (==)
by Barak A. Pearlmutter 21 Aug '14

21 Aug '14

You don't need to define a new weird datatype in order to violate reflexivity, as an eminent IEEE committee has already designed one for you. Prelude> let x = 0/0 in x == x False

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Does GHC compare pointers when eval'ing (==)
by Johan Holmquist 20 Aug '14

20 Aug '14

Comparing two structures for equality (structurally) can be expensive. But if their references are the same they would for sure be equal (unless (==) was defined in some funny way). Does GHC perform any such optimization? (Likely a question for another list but I make a stab here first. )

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Re: [Haskell-cafe] Reducing CPU load when using Helm
by Kaspar Emanuel 20 Aug '14

20 Aug '14

On 18 August 2014 20:24, Kaspar Emanuel <kaspar.bumke(a)gmail.com> wrote: > Hi, > > are there some simple tricks to reduce the CPU load? This simple > square rendering sits at 20% CPU even when the window size remains > constant. > > import FRP.Helm > import qualified FRP.Helm.Window as Window > > render :: (Int, Int) -> Element > render (w, h) = collage w h [move (100, 100) $ filled red $ square 64] > > main :: IO () > main = do > engine <- startup defaultConfig > run engine $ render <~ Window.dimensions engine > > The binary fractal tree I submitted in another thread on helm-dev > sits at 100% CPU when the mouse isn't moving. It seems to me that > Helm keeps re-drawing and re-computing when it doesn't need to. > > I started digging through the source a bit but it will take me a while > to understand Elerea and how it samples the inputs and causes things > to update. > > Are there any quick fixes? > So I tried stepping the render function using a time-constant. But that just causes the resulting programs to freeze on the first frame. https://github.com/kasbah/helm/commit/9ef9607d8057100b5b7930fa2b5e85c6040d1… I modified the driveNetwork function from here: www.formicite.com/dopage.php?frp/frp.html Bit out of my element with this stuff. I'd just like to _not_ max out the CPU with simple graphical programs.

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