February 2012 - Haskell-Cafe

ANNOUNCE: system-filepath 0.4.5 and system-fileio 0.3.4
by John Millikin 16 Feb '12

16 Feb '12

Both packages now have much-improved support for non-UTF8 paths on POSIX systems. There are no significant changes to Windows support in this release. system-filepath 0.4.5: Hackage: http://hackage.haskell.org/package/system-filepath-0.4.5 API reference: https://john-millikin.com/software/haskell-filesystem/reference/system-file… system-fileio 0.3.4: Hackage: http://hackage.haskell.org/package/system-fileio-0.3.4 API reference: https://john-millikin.com/software/haskell-filesystem/reference/system-file… ----- In GHC 7.2 and later, file path handling in the platform libraries was changed to treat all paths as text (encoded according to locale). This does not work well on POSIX systems, because POSIX paths are byte sequences. There is no guarantee that any particular path will be valid in the user's locale encoding. system-filepath and system-fileio were modified to partially support this new behavior, but because the underlying libraries were unable to represent certain paths, they were still "broken" when built with GHC 7.2+. The changes in this release mean that they are now fully compatible (to the best of my knowledge) with GHC 7.2 and 7.4. Important changes: * system-filepath has been converted from GHC's escaping rules to its own, more compatible rules. This lets it support file paths that cannot be represented in GHC 7.2's escape format. * The POSIX layer of system-fileio has been completely rewritten to use the FFI, rather than System.Directory. This allows it to work with arbitrary POSIX paths, including those that GHC itself cannot handle. The Windows layer still uses System.Directory, since it seems to work properly. * The POSIX implementation of createTree will no longer recurse into directory symlinks that it does not have permission to remove. This is a change in behavior from the directory package's implementation. See http://www.haskell.org/pipermail/haskell-cafe/2012-January/098911.html for details and the reasoning behind the change. Since Windows does not support symlinks, I have not modified the Windows implementation (which uses removeDirectoryRecursive).

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Transactional memory going mainstream with Intel Haswell
by Ben 16 Feb '12

16 Feb '12

http://arstechnica.com/business/news/2012/02/transactional-memory-going-mai… would any haskell STM expert care to comment on the possibilities of hardware acceleration? best, ben

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GSoc-2012: Lock-free Data Structures
by Sergiu Ivanov 16 Feb '12

16 Feb '12

Hello, On Thu, Feb 16, 2012 at 5:40 AM, Ryan Newton <rrnewton(a)gmail.com> wrote: > I'm interested in mentoring any projects related to concurrent data > structure implementation. Is it too late to propose new projects? > > http://parfunk.blogspot.com/2012/02/potential-gsoc-haskell-lock-free-data.h… This project idea sounds very cool to me :-) Could you please suggest a starting point in approaching it? I cannot view the article [0], since I don't have an active ACM subscription. I've seen the link [1], so, probably, I could contribute some patches to it? I'm not sure whether this is good, but this is not the first project idea that grabs my attention, so, frankly, my principal goal now would be getting the hang of it. Had I had the time, I would have tried to contribute to all Haskell libraries I like (sigh) Sergiu [0] http://cacm.acm.org/magazines/2011/3/105308-data-structures-in-the-multicor… [1] https://github.com/rrnewton/haskell-lockfree-queue

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Behavior of -threaded in GHC 7.4.1?
by Michael Craig 16 Feb '12

16 Feb '12

Hi all, I'm debugging an issue with multithreading and FFI calls in 7.4.1. The code in question is the zeromq3-haskell library, which provides an FFI binding to ZeroMQ. A little background: ZeroMQ gives the programmer "contexts" and "sockets". Contexts are thread-safe and generally used one-per-process, but sockets and not thread-safe so they're generally used one-per-thread. Have a look at the code in this gist: https://gist.github.com/1829190 It's just a server that prints requests and replies with "Hello", and a client that sends the requests ["1", "2", ..., "10"] and prints the responses. Despite the fact that ZMQ sockets are not thread-safe, it was possible to compile and run this code with -threaded under 7.0.4. Using MVars or some other locking mechanism, it was even possible to use the same socket from multiple parallel threads. (As long as the socket was not actually used in parallel by multiple threads, all was well.) In 7.4.1, the code will only run if compiled without -threaded. With -threaded, it croaks with an "operation on non-socket" error, which generally refers to a socket that's either been closed or I've dug through the code in zeromq3-haskell and haven't found anything suspicious looking. Before I go digging into libzmq itself, can somebody assure me that nothing has changed in 7.4.1 with regard to -threaded or the FFI that might cause this breakage? Cheers, Mike Craig

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New GSoC on Concurrent Data Structures -- was: How do I get official feedback (ratings) on my GSoC proposal?
by Ryan Newton 16 Feb '12

16 Feb '12

Neat, thanks Johan! (I had no idea it was so easy to post to Reddit.) I also put the proposed project on the trac: http://hackage.haskell.org/trac/summer-of-code/ticket/1608 So feel free to contact me, interested students! Cheers, -Ryan On Thu, Feb 16, 2012 at 12:51 AM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > On Wed, Feb 15, 2012 at 7:40 PM, Ryan Newton <rrnewton(a)gmail.com> wrote: >> I'm interested in mentoring any projects related to concurrent data >> structure implementation. Is it too late to propose new projects? >> >> >> http://parfunk.blogspot.com/2012/02/potential-gsoc-haskell-lock-free-data.h… > > Not all all. It's quite early in fact (I tried to get people to think > about this early on.) I'd also post it to the Haskell reddit to make > sure it gets a bit more exposure (it's kinda buried here in this > thread.) > > -- Johan

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Undocumented cost-centres (.\) using auto-all, and SCC pragma not being honored
by Dan Maftei 16 Feb '12

16 Feb '12

> > 1 When profiling my code with -auto-all, my .prof file names some sub-expressions with a backslash. Cf. below. What are these? e_step e_step.ewords e_step.\ e_step.\.\ e_step.update_counts e_step.fwords My e_step function binds seven expressions inside a let, then uses them in two ugly nested folds. (Yes, very hackish) As you can see, three such expressions are named explicitly (ewords, fwords, and update_counts). But where are the rest? Further, perhaps the backslashes have something to do with the lambda expressions I am using in the two nested folds? 2. A related question: I tried using the SCC pragma instead of auto-all. I added it to all seven expressions inside the let, and to the nested folds. However, only two showed up in the .prof file! How come? Thanks ninestraycats

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Re: [Haskell-cafe] Haskell-Cafe Tag (was: Optimizations and parallel execution in the IO for a small spellchecker)
by JP Moresmau 16 Feb '12

16 Feb '12

I was not aware that it was something a poster was supposed to do, but now I notice that while the messages I get from the list have [Haskell-Cafe] on their subject, the ones I post don't. The Haskell-Cafe info page (http://www.haskell.org/mailman/listinfo/haskell-cafe) does not mention that you should have that tag in the subject. I'm probably a uncouth Frenchman knowing nothing about netiquette, but surely the list software could add such a tag if it was required? JP On Wed, Feb 15, 2012 at 8:14 PM, Paul Sujkov <psujkov(a)gmail.com> wrote: > Sorry, I've forgotten to add a [Haskell-Cafe] tag for the message. > > > On 15 February 2012 19:33, Paul Sujkov <psujkov(a)gmail.com> wrote: >> >> Hi everyone, >> >> http://hpaste.org/63732 >> >> that's a very simple spellchecker application: it consumes standard Linux >> dictionary, reads a file, and prints out words from that file that are not >> on the dictionary. I have taken it from a little cross-language benchmark >> and used to benchmark existing hash table implementations available on >> Hackage (with one used in code being the best according to timings). The >> question is: can I go further and effectively execute lookup and (maybe) >> output (line 20 or line 26 in the snippet) in parallel? >> >> I'm not good at all with Haskell parallel packages; I've tried >> monad-parallel (naive implementation eats away all the memory available) and >> parallel (haven't tried Par monad yet), but the last dosn't seem to fit well >> with the IO operations. If anyone can give me any suggestions on how can I >> implement parallel execution, or any thoughts on further sequential >> optimizations, it would be deeply appreciated. >> >> -- >> Regards, Paul Sujkov > > > > > -- > Regards, Paul Sujkov > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- JP Moresmau http://jpmoresmau.blogspot.com/

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How do I get official feedback (ratings) on my GSoC proposal?
by Greg Weber 16 Feb '12

16 Feb '12

Other than changing the status myself, how do I get a priority attached to my GSoC proposal?

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Haskell Weekly News: Issue 215
by Daniel Santa Cruz 15 Feb '12

15 Feb '12

Welcome to issue 215 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of February 5 to 11, 2012. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-215.h… Quotes of the Week * nand': haskell has a few advantages in the industry that few other languages can offer. these will become more obvious as time goes on and programming is forced into a functional direction in general. * seo: What i do not realize is actually how you are now not really much more well-liked than you might be right now. You are so intelligent. You realize thus significantly in terms of this subject, produced me in my view consider it from so many numerous angles. Its like men and women are not interested unless it‚s one thing to do with Woman gaga! Your personal stuffs excellent. Always care for it up! * monochrom: Schrodinger's Klein glass bottle: before you look, the cat is inside or outside; after you look, it is inside and outside :) * cmccann: Ruby is a fine language if you're secretly a lisp programmer but want to hide behing perl and smalltalk so nobody catches you * mlh: nice quote -- quick, someone register it for posterity * something: somebody never said, naturally. * irene-knapp: I was going to take a famous quote and put it in lambdabot as my own but I couldn't decide on one * shachaf: getLine :: IO String contains a String in the same way that /bin/ls contains a list of files * edwardk: [unsafePerformIO] sounds like a good idea, its tempting, it makes life easy, for the first 2 lines of code. then you go to write the third and everything goes to shit * cmccann: Idiom brackets are where you consume a mixture of alcohol and Conor McBride papers until your vision gets blurry enough that you can't actually see the Applicative operators anymore. * Philippa: incidentally, if you think about this you're probably starting to image control flow that looks like it's a glyph for summoning cthulhu - and then realising that it's 3d and consists of 'towers'. Make sense? * c_wraith: also, "actual" reals is a funny thing to call a set that consists mostly of things that cannot be described. * NihilistDandy: I feel like a hipster whenever I talk programming with people at my school. I always have to preface discussions with "you've probably never heard of it" :/ * ion: Three lefts make two wrongs * elliott: Explicit recursion should generally be avoided. Also, general recursion should be explicitly avoided! * CodeWeaverX: I expect that Weird Untraceable Suitcases Full Of Bad could result from the program assuming such an operation is pure when it clearly isn't. * elliott: cmccann: the instances list haddock generates is now a thing of majesty edwardk: elliott: welcome to my world * mzero: functional programming has a property that is often lacking in other languages... at dinner last night we were discussing it, and I dubbed it: "The Princess Bride" property --- which is, that a piece of code means what you think it means * sclv: You have a problem and think "I'll use a hash function." Now you have intermittantly colliding problems. * monochrom: data Ph a = Ph -- the phantom monad ksion: Is that a new Star Wars prequel? ;) * user1151874: i already declare hp as int with value 3 now my problem is when i put "hp--", it shows error [...] but if i put "--hp", the result print is 3 not 2. * edwardk: thats my secret. really i don't write anything. i just let others do the work and upload it to hackage behind their backs Top Reddit Stories * Haskell as most mainstream non-mainstream language? The RedMonk Programming Language Rankings: February 2012 Domain: redmonk.com, Score: 45, Comments: 25 On Reddit: [1] http://goo.gl/xWHB8 Original: [2] http://goo.gl/REBXN * The future is parallel, and the future of parallel is declarative (Simon Peyton Jones) [RE: Escape From the Ivory Tower] Domain: yow.eventer.com, Score: 41, Comments: 5 On Reddit: [3] http://goo.gl/PJSlh Original: [4] http://goo.gl/zUKT5 * How Prelude avoids overlapping instances in Show Domain: brandon.si, Score: 32, Comments: 18 On Reddit: [5] http://goo.gl/qGPZK Original: [6] http://goo.gl/rC6qr * InfoQ: Yesod Web Framework (Michael's QCon talk) Domain: infoq.com, Score: 32, Comments: 12 On Reddit: [7] http://goo.gl/93SFv Original: [8] http://goo.gl/Xao7S * Search tree that does not require that items be sorted. Domain: twanvl.nl, Score: 28, Comments: 3 On Reddit: [9] http://goo.gl/R2yLD Original: [10] http://goo.gl/WGfcV * Reverse engineering the ZeroMQ wire protocol for ZMQHS Domain: xrl.tureus.com, Score: 27, Comments: 0 On Reddit: [11] http://goo.gl/UKIun Original: [12] http://goo.gl/oVstA * Announcing Yesod 0.10 - conduits, persistent 0.8, improved Coffeescript integration Domain: yesodweb.com, Score: 25, Comments: 12 On Reddit: [13] http://goo.gl/1RRJH Original: [14] http://goo.gl/KftN5 * Real World Haskell book site -- is it known to be down? If so how long has it been down and will it come back? Domain: book.realworldhaskell.org, Score: 24, Comments: 15 On Reddit: [15] http://goo.gl/GV6z2 Original: [16] http://goo.gl/RCxnT * What do Haskellers think of Rust? It's got lambdas and ADTs with C-style notation. Domain: rust-lang.org, Score: 24, Comments: 40 On Reddit: [17] http://goo.gl/a9bCB Original: [18] http://goo.gl/xAKHx * r/darcs - for discussing darcs and patch theory Domain: reddit.com, Score: 24, Comments: 8 On Reddit: [19] http://goo.gl/37ArD Original: [20] http://goo.gl/qQiaD * Will Haskell be the Smalltalk of functional programming? Domain: self.haskell, Score: 22, Comments: 54 On Reddit: [21] http://goo.gl/ZxmBN Original: [22] http://goo.gl/ZxmBN Top StackOverflow Questions * Abusing the algebra of algebraic data types - why does this work? votes: 53, answers: 4 Read on SO: [23] http://goo.gl/MyQRO * Recommended reading/tutorials to understand reactive-banana FRP library votes: 13, answers: 3 Read on SO: [24] http://goo.gl/hTPv6 * Safe modelling of relational data in Haskell votes: 12, answers: 2 Read on SO: [25] http://goo.gl/4bEXX * What are the definitions for >>= and return for the IO monad? votes: 12, answers: 3 Read on SO: [26] http://goo.gl/9jH2x * Basic Structure of a Haskell Program votes: 11, answers: 1 Read on SO: [27] http://goo.gl/3OcKg * GHC API - How to dynamically load Haskell code from a compiled module using GHC 7.2? votes: 11, answers: 2 Read on SO: [28] http://goo.gl/aYKZz * Lifting a higher order function in Haskell votes: 9, answers: 1 Read on SO: [29] http://goo.gl/JhmjK * How do you check the type of a local variable? votes: 8, answers: 5 Read on SO: [30] http://goo.gl/Ynfgv * Tail optimization guarantee - loop encoding in Haskell votes: 7, answers: 2 Read on SO: [31] http://goo.gl/PKqfq * Writing in pointfree style f x = g x x votes: 7, answers: 1 Read on SO: [32] http://goo.gl/EIFBw Until next time, Daniel Santa Cruz References 1. http://redmonk.com/sogrady/2012/02/08/language-rankings-2-2012/ 2. http://www.reddit.com/r/haskell/comments/ph4uq/haskell_as_most_mainstream_n… 3. http://yow.eventer.com/events/1004/talks/1055 4. http://www.reddit.com/r/haskell/comments/phnd3/the_future_is_parallel_and_t… 5. http://brandon.si/code/how-the-haskell-prelude-avoids-overlapping-types-in-… 6. http://www.reddit.com/r/haskell/comments/pel34/how_prelude_avoids_overlappi… 7. http://www.infoq.com/presentations/Yesod 8. http://www.reddit.com/r/haskell/comments/pjqpr/infoq_yesod_web_framework_mi… 9. http://twanvl.nl/blog/haskell/SemilatticeSearchTree 10. http://www.reddit.com/r/haskell/comments/pj6n7/search_tree_that_does_not_re… 11. http://xrl.tureus.com/black-box-reverse-engineering-zmq 12. http://www.reddit.com/r/haskell/comments/pcojs/reverse_engineering_the_zero… 13. http://www.yesodweb.com/blog/2012/02/release-0-10 14. http://www.reddit.com/r/haskell/comments/phxjo/announcing_yesod_010_conduit… 15. http://book.realworldhaskell.org/read 16. http://www.reddit.com/r/haskell/comments/pd79r/real_world_haskell_book_site… 17. http://www.rust-lang.org/ 18. http://www.reddit.com/r/haskell/comments/pewtc/what_do_haskellers_think_of_… 19. http://reddit.com/r/darcs 20. http://www.reddit.com/r/haskell/comments/pkv8g/rdarcs_for_discussing_darcs_… 21. http://www.reddit.com/r/haskell/comments/pczyz/will_haskell_be_the_smalltal… 22. http://www.reddit.com/r/haskell/comments/pczyz/will_haskell_be_the_smalltal… 23. http://stackoverflow.com/questions/9190352/abusing-the-algebra-of-algebraic… 24. http://stackoverflow.com/questions/9210412/recommended-reading-tutorials-to… 25. http://stackoverflow.com/questions/9234205/safe-modelling-of-relational-dat… 26. http://stackoverflow.com/questions/9244538/what-are-the-definitions-for-and… 27. http://stackoverflow.com/questions/9153574/basic-structure-of-a-haskell-pro… 28. http://stackoverflow.com/questions/9198140/ghc-api-how-to-dynamically-load-… 29. http://stackoverflow.com/questions/9243215/lifting-a-higher-order-function-… 30. http://stackoverflow.com/questions/9210311/how-do-you-check-the-type-of-a-l… 31. http://stackoverflow.com/questions/9149183/tail-optimization-guarantee-loop… 32. http://stackoverflow.com/questions/9169046/writing-in-pointfree-style-f-x-g…

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GHC -WAll, -fwarn-unused-do-bind and Cabal
by JP Moresmau 15 Feb '12

15 Feb '12

I'm confused: I'm using GHC 7.0.2 and Cabal 1.10.1.0 with cabal-install 0.10.2. I use -Wall in my Cabal file. If I build a Haskell file with unused do binds, via the GHC API I get no warning, which is normal, since the doc states: The warnings that are not enabled by -Wall are ..., -fwarn-unused-do-bind .... But if I build my project through cabal it gives me the warning! Why is compiling with Cabal giving that extra warning that GHC on its own with the same flags doesn't give? Using --verbose on cabal build does not give any clue, no suspicious extra flag is passed on. Any pointer appreciated! -- JP Moresmau http://jpmoresmau.blogspot.com/

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