February 2012 - Haskell-Cafe

Haskell Weekly News: Issue 213
by Daniel Santa Cruz 01 Feb '12

01 Feb '12

Welcome to issue 213 of the HWN, a newsletter covering quotes, stories, and questions seen around the net concerning Haskell during the week of January 22 to 28, 2012. You can read the HTML version at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-213.h… Quotes of the Week * monochrom: great way to answer a semantics question by commenting on the font ... "what is the semantics of IO?" "the I may be narrower than O in some fonts, and same width in some other fonts" ... "on very old typewriters, it also denotes the number 10 (ten), for those of you looking for a denotation" * elliott: |\/|/-\|-|-|=|\||} is my preferred mappend operator * merijn: rostayob: I used to be a functional programmer like you, but then I took an UML diagram to the knee >.> * mauke: monoids are simple elliott: And as we all know, monads are basically just monoids! monochrom: haskell is basically just ascii * O'Keefe: "Elegance is not optional." * O'Keefe: "If I don't write it, I won't wrong it." * kmc: "monads are like containers, as long as you forget everything you know about containers, and treat it as a synonym for 'monad'" * Enigmagic: this calls for mfix and some tequila Top Reddit Stories * SPJ : Escape From the Ivory Tower: The Haskell Journey, From 1990 to 2011 Domain: yow.eventer.com, Score: 67, Comments: 12 On Reddit: [1] http://goo.gl/JqRUW Original: [2] http://goo.gl/9ILW1 * Make Things Now! - Pragmatic FP With Haskell (slides) Domain: code.haskell.org, Score: 44, Comments: 22 On Reddit: [3] http://goo.gl/WJQnH Original: [4] http://goo.gl/UMd9y * ANNOUNCE: GHC 7.4.1 Release Candidate 2 Domain: haskell.org, Score: 37, Comments: On Reddit: [5] http://goo.gl/D9F37 Original: [6] http://goo.gl/sdD1L * Web application that uses UHC to compile Haskell to JavaScript Domain: alessandrovermeulen.me, Score: 36, Comments: 13 On Reddit: [7] http://goo.gl/A3S0k Original: [8] http://goo.gl/lyX2K * Web app that shows you the changes between releases of packages on hackage Domain: hdiff.luite.com, Score: 33, Comments: 4 On Reddit: [9] http://goo.gl/6ioFh Original: [10] http://goo.gl/aZAyc * Combining Regions and Iteratees Domain: haskell.org, Score: 30, Comments: 10 On Reddit: [11] http://goo.gl/LEPiQ Original: [12] http://goo.gl/k3wpd * MonadBaseControl is unsound : Inside 233 Domain: blog.ezyang.com, Score: 29, Comments: 19 On Reddit: [13] http://goo.gl/ptCF6 Original: [14] http://goo.gl/Y5bbh * Yesod example: wiki, markdown, chat subsite, event source Domain: yesodweb.com, Score: 29, Comments: 1 On Reddit: [15] http://goo.gl/GyfzS Original: [16] http://goo.gl/on96C * inner beauty of tree traversals: pre- and post-order Domain: blog.moertel.com, Score: 29, Comments: On Reddit: [17] http://goo.gl/6fKzq Original: [18] http://goo.gl/NpjrO * Seem like there's a boycott of Elsevier going on. Domain: thecostofknowledge.com, Score: 29, Comments: 1 On Reddit: [19] http://goo.gl/2B106 Original: [20] http://goo.gl/dvxML * Unifying Monoids and Monads with Polymorphic Kinds Domain: jonmsterling.com, Score: 26, Comments: 29 On Reddit: [21] http://goo.gl/kTafx Original: [22] http://goo.gl/A3IrU * Modelling IO: MonadIO and beyond Domain: blog.ezyang.com, Score: 26, Comments: 4 On Reddit: [23] http://goo.gl/gr2M1 Original: [24] http://goo.gl/0n1a9 Top StackOverflow Questions * Is it a good idea to compile a language to C? votes: 26, answers: 7 Read on SO: [25] http://goo.gl/V8sSx * Haskell GHC: what is the time complexity of a pattern match with N constructors? votes: 19, answers: 1 Read on SO: [26] http://goo.gl/SU0l8 * What to use instead of a main loop in Haskell? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/iYasz * Associated types and container elements votes: 12, answers: 2 Read on SO: [28] http://goo.gl/zBNWU * Undefined at the type level votes: 12, answers: 1 Read on SO: [29] http://goo.gl/m9Acn * Is there a good reason why `deleteBy` does not have its most general type? votes: 10, answers: 3 Read on SO: [30] http://goo.gl/dQDrf * Using low bitsize integral types like `Int8` and what they are for votes: 9, answers: 2 Read on SO: [31] http://goo.gl/YaE68 * How is the ghc runtime support for profiling implemented? votes: 9, answers: 2 Read on SO: [32] http://goo.gl/yz7eC * how to achieve “product of two monads” effect? votes: 9, answers: 1 Read on SO: [33] http://goo.gl/pLhVK * Combine two monads when neither has a transformer? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/yw8tr * Why don't type synonyms permit recursion in Haskell? votes: 8, answers: 2 Read on SO: [35] http://goo.gl/hPzVq Until next time, Daniel Santa Cruz References 1. http://yow.eventer.com/events/1004/talks/1054 2. http://www.reddit.com/r/haskell/comments/ovc9s/spj_escape_from_the_ivory_to… 3. http://code.haskell.org/~dons/talks/padl-keynote-2012-01-24.pdf 4. http://www.reddit.com/r/haskell/comments/oxg3c/make_things_now_pragmatic_fp… 5. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-January/021714.… 6. http://www.reddit.com/r/haskell/comments/p0x96/announce_ghc_741_release_can… 7. http://alessandrovermeulen.me/2012/01/26/getting-rid-of-javascript-with-has… 8. http://www.reddit.com/r/haskell/comments/oz27a/web_application_that_uses_uh… 9. http://hdiff.luite.com/ 10. http://www.reddit.com/r/haskell/comments/ozjys/web_app_that_shows_you_the_c… 11. http://www.haskell.org/pipermail/haskell-cafe/2012-January/098704.html 12. http://www.reddit.com/r/haskell/comments/orh4f/combining_regions_and_iterat… 13. http://blog.ezyang.com/2012/01/monadbasecontrol-is-unsound/ 14. http://www.reddit.com/r/haskell/comments/ot5f2/monadbasecontrol_is_unsound_… 15. http://www.yesodweb.com/blog/2012/01/wiki-chat-subsite 16. http://www.reddit.com/r/haskell/comments/oub3p/yesod_example_wiki_markdown_… 17. http://blog.moertel.com/articles/2012/01/26/the-inner-beauty-of-tree-traver… 18. http://www.reddit.com/r/haskell/comments/oyu7d/inner_beauty_of_tree_travers… 19. http://thecostofknowledge.com/ 20. http://www.reddit.com/r/haskell/comments/ozi7i/seem_like_theres_a_boycott_o… 21. http://www.jonmsterling.com/posts/2012-01-12-unifying-monoids-and-monads-wi… 22. http://www.reddit.com/r/haskell/comments/osn5d/unifying_monoids_and_monads_… 23. http://blog.ezyang.com/2012/01/modelling-io/ 24. http://www.reddit.com/r/haskell/comments/ourfu/modelling_io_monadio_and_bey… 25. http://stackoverflow.com/questions/8976423/is-it-a-good-idea-to-compile-a-l… 26. http://stackoverflow.com/questions/9027384/haskell-ghc-what-is-the-time-com… 27. http://stackoverflow.com/questions/9015318/what-to-use-instead-of-a-main-lo… 28. http://stackoverflow.com/questions/9016521/associated-types-and-container-e… 29. http://stackoverflow.com/questions/9036554/undefined-at-the-type-level 30. http://stackoverflow.com/questions/9004937/is-there-a-good-reason-why-delet… 31. http://stackoverflow.com/questions/8963942/using-low-bitsize-integral-types… 32. http://stackoverflow.com/questions/9010808/how-is-the-ghc-runtime-support-f… 33. http://stackoverflow.com/questions/9013329/how-to-achieve-product-of-two-mo… 34. http://stackoverflow.com/questions/9034060/combine-two-monads-when-neither-… 35. http://stackoverflow.com/questions/8963376/why-dont-type-synonyms-permit-re…

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C++ Parser?
by Christopher Brown 01 Feb '12

01 Feb '12

Hi, I have stumbled across language-c on hackage and I was wondering if anyone is aware if there exists a full C++ parser written in Haskell? Many thanks, Chris.

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network-conduit failing to close sockets
by Ertugrul Söylemez 01 Feb '12

01 Feb '12

Hello there, I have tried to implement a simple echo server using the network-conduit library version 0.2.1. This is the code: module Main where import Data.Conduit import Data.Conduit.Network main :: IO () main = runTCPServer (ServerSettings 4000 Nothing) ($$) It works, but at some point it dies with too many open files, even though I never open two connections simultaneously: % ./echo-server echo-server: accept: resource exhausted (Too many open files) Apparently it fails to close the sockets properly. This is the client side code I have used for testing in GHCi: > :m Network Control.Monad System.IO > replicateM_ 512 (connectTo "127.0.0.1" (PortNumber 4000) >>= hClose) I'm running the action once, then wait a few seconds to give the server a chance to close the handles. Then I run it again, causing the server program to die with the above mentioned error message. Am I doing something wrong or is this a bug in network-conduit? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Memory usage with TVar?
by Johan Brinch 01 Feb '12

01 Feb '12

Here's the example program: https://gist.github.com/1cbe113d2c79e2fc9d2b When I run the program (which maintains a list inside an STM TVar), I get the following statistics: > ./Test +RTS -s 176,041,728 bytes allocated in the heap 386,794,976 bytes copied during GC 69,180,224 bytes maximum residency (7 sample(s)) 42,651,080 bytes maximum slop 179 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 336 colls, 0 par 0.44s 0.44s 0.0013s 0.0033s Gen 1 7 colls, 0 par 0.39s 0.40s 0.0570s 0.1968s INIT time 0.00s ( 0.00s elapsed) MUT time 0.23s ( 0.23s elapsed) GC time 0.83s ( 0.84s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.06s ( 1.07s elapsed) %GC time 77.9% (78.3% elapsed) Alloc rate 749,153,093 bytes per MUT second Productivity 22.1% of total user, 22.0% of total elapsed How come this program uses 179 MB of memory? I'm on a 64-bit machine where 2'000'000 integers uses 32 MB. Where does the overhead come from? -- Johan Brinch

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Serialize instance for System.Posix.Files.FileStatus
by Johan Brinch 01 Feb '12

01 Feb '12

Are there plans on implementing an efficient Serialize instance of the FileStatus type? I'm aware that it's just a wrapper around a C stat structure, and I reckon it can be done more efficiently than copying from C to Haskell for every value. -- Johan Brinch

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[ANN] Crypto-API 0.9 Release
by Thomas DuBuisson 31 Jan '12

31 Jan '12

Crypto-API is a generic interface for cryptographic operations, defining classes for hashes, ciphers, and random number generation while also providing convenience functions such as block cipher modes and padding. Maintainers of hash and cipher implementations are encouraged to add instances for the classes defined in Crypto.Classes. Crypto users are similarly encouraged to use the interfaces defined in the Classes module. Release 0.9 Changes: * Crypto.Classes now exports 'Data.Serialize.encode' * AsymCipher now has proper fundeps * cpolysArr is no longer one big line Cheers, Thomas

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Stuck on HXT basics
by Pēteris Paikens 31 Jan '12

31 Jan '12

I'm quite stuck on getting HXT working to do my xml parsing, can you help me to get started? Even in http://www.haskell.org/haskellwiki/HXT#Getting_started:_Hello_world_examples the basic filter example that's given there in section 6: import Text.XML.HXT.Core import Text.XML.HXT.DOM.XmlTreeFilter selectAllText :: ArrowXml a => a XmlTree XmlTree selectAllText = deep isXText fails with the following error message. What could I be doing wrong? Couldn't match expected type `Data.Tree.NTree.TypeDefs.NTree XNode' with actual type `hxt-8.5.4:Data.Tree.NTree.TypeDefs.NTree hxt-8.5.4:Text.XML.HXT.DOM.TypeDefs.XNode' Expected type: a (Data.Tree.NTree.TypeDefs.NTree XNode) XmlTree Actual type: Text.XML.HXT.DOM.XmlTreeTypes.XmlFilter In the first argument of `deep', namely `isXText' In the expression: deep isXText On the other hand, perhaps you can suggest something other than HXT - my needs would be taking "heavy" XML's (a lot of little-neccessary structure, but correctly validated), extracting text tokens from it and outputting them with some light tagsoup annotation. Regards, Peteris

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