- Haskell-Cafe - Haskell.org

Haskell Weekly News: Issue 211
by Daniel Santa Cruz 19 Jan '12

19 Jan '12

Welcome to issue 211 of the HWN, a newsletter covering developments in the Haskell community. This release covers top stories found around the net between January 01 to 14, 2012. This is going to be a shorted version, as I have been out of time. I did not want to let more time go by without a heartbeat from your friendly HWN. Happy coding! You can find an HTML version at: http://contemplatecode.blogspot.com/2012/01/haskell-weekly-news-issue-211.h… Quotes of the Week * <ddarius> xtalmath: #math or #not-math would probably be a more appropriate place to ask. <shachaf> ddarius: Those two channels pretty much cover it all. <ddarius> shachaf: You're clearly not an intuitionist. * monochrom: in fact, freedom is the freedom to write 2.2 = 4 * Jafet: caleskell is the standard golf course. * byorgey: Escaped skolem! Authorities mount massive search. News at 11. * BMeph: loves swapping function arguments, he said flippantly * kmc: "Haskell is great, because Curry-Howard! Proving things in the type system. We can prove that, uh, Ints exist, unless they're [bottom]." * Jafet: In Soviet Russia, State executes you * byorgey: that would be known as "low-dimensional linear algebra", where "low-dimensional" is said in as condescending a tone as possible * monochrom: at a crossroad you meet an angel (always tells the truth) or demon (always lies). the crossroad gives you two choices of roads, one to heaven, one to hell, and only the angel or demon knows which is which. if you ask "how would you answer if I asked 'what is a monad?'", what will happen? Top Reddit Stories * Dropbox tech blog » Using the Dropbox API from Haskell Domain: tech.dropbox.com, Score: 68, Comments: 4 On Reddit: [1] http://goo.gl/rsndV Original: [2] http://goo.gl/g3RyF * A new approach to iteratees Domain: self.haskell, Score: 48, Comments: 99 On Reddit: [3] http://goo.gl/ZRbGW Original: [4] http://goo.gl/ZRbGW * Haskell / Repa Real-Time Ray Tracing Demo Domain: youtube.com, Score: 45, Comments: 7 On Reddit: [5] http://goo.gl/rzyZs Original: [6] http://goo.gl/15P2V * Order theory for computer scientists Domain: matt.might.net, Score: 39, Comments: 1 On Reddit: [7] http://goo.gl/nTOfr Original: [8] http://goo.gl/HAVjQ * Generalizing Streams into Coroutines (with an introduction to functors, applicative functors and arrows) Domain: github.com, Score: 38, Comments: 15 On Reddit: [9] http://goo.gl/hUopY Original: [10] http://goo.gl/avTUf * Well-Typed are hiring: Haskell consultant Domain: well-typed.com, Score: 38, Comments: 71 On Reddit: [11] http://goo.gl/Chu7f Original: [12] http://goo.gl/VHPfJ * Why iteratees are hard to understand Domain: blog.ezyang.com, Score: 34, Comments: 11 On Reddit: [13] http://goo.gl/LrEoV Original: [14] http://goo.gl/d1scG * Modern reall-world Haskell? Domain: self.haskell, Score: 33, Comments: 21 On Reddit: [15] http://goo.gl/l0Aud Original: [16] http://goo.gl/l0Aud * FRP: reactive-banana is getting dynamic event switching Domain: apfelmus.nfshost.com, Score: 30, Comments: 3 On Reddit: [17] http://goo.gl/dGblT Original: [18] http://goo.gl/ovRE5 * wxHaskell and wxWidgets 2.9 Domain: wewantarock.wordpress.com, Score: 29, Comments: On Reddit: [19] http://goo.gl/sxiBx Original: [20] http://goo.gl/878Aj * Updated Yesod blog example: i18n, authentication, authorization, and database Domain: yesodweb.com, Score: 28, Comments: 22 On Reddit: [21] http://goo.gl/4eDAT Original: [22] http://goo.gl/duzSG * Experiments in Haskell Packaging Domain: justhaskell.org, Score: 27, Comments: 5 On Reddit: [23] http://goo.gl/y2vHk Original: [24] http://goo.gl/0nADv * http-conduit Domain: yesodweb.com, Score: 26, Comments: 26 On Reddit: [25] http://goo.gl/gisUJ Original: [26] http://goo.gl/mMhaf * Reasoning about Stream Processing with Effects Domain: personal.cis.strath.ac.uk, Score: 26, Comments: 6 On Reddit: [27] http://goo.gl/3Ckzr Original: [28] http://goo.gl/Uw8ZO * Simon Marlow's Parallel and Concurrent Programming Haskell (slides) Domain: self.haskell, Score: 26, Comments: 3 On Reddit: [29] http://goo.gl/Rg0Lz Original: [30] http://goo.gl/Rg0Lz * Who is my Community? -- wxHaskell maintainer's thoughts on C bindings Domain: wewantarock.wordpress.com, Score: 26, Comments: On Reddit: [31] http://goo.gl/tcqZN Original: [32] http://goo.gl/mRwQa Top StackOverflow Questions * Fun with repeated fmap votes: 26, answers: 1 Read on SO: [33] http://goo.gl/T2FPm * Can a compiler automatically detect pure functions without the type information about purity? votes: 23, answers: 2 Read on SO: [34] http://goo.gl/wgYim * What does “floated out” mean? votes: 21, answers: 2 Read on SO: [35] http://goo.gl/8J3no * Haskell projects to study and learn from? votes: 19, answers: 2 Read on SO: [36] http://goo.gl/SHwsZ * How do you identify monadic design patterns? votes: 19, answers: 4 Read on SO: [37] http://goo.gl/dE3EJ * how do I get good (small) shrinks out of QuickCheck? votes: 16, answers: 0 Read on SO: [38] http://goo.gl/X3ZOs * Which applicative functor is used for passing shared parameters? votes: 14, answers: 1 Read on SO: [39] http://goo.gl/UxmZK * Double map in haskell? votes: 12, answers: 2 Read on SO: [40] http://goo.gl/uWpuz * Higher ranked and impredicative types votes: 12, answers: 1 Read on SO: [41] http://goo.gl/zpkXn * Handling events in Haskell votes: 12, answers: 1 Read on SO: [42] http://goo.gl/e7sV9 * Does the term “monadic” in J have anything to do with its Haskell use? votes: 12, answers: 4 Read on SO: [43] http://goo.gl/hTa2j * Any good articles on optimizing Haskell code? votes: 12, answers: 2 Read on SO: [44] http://goo.gl/FzRrf * What is a “free variable”? votes: 11, answers: 2 Read on SO: [45] http://goo.gl/Og5SC * Haskell Thread Communication Pattern Scenario votes: 10, answers: 1 Read on SO: [46] http://goo.gl/3Rzq0 * Defining a function by equations with different number of arguments votes: 10, answers: 1 Read on SO: [47] http://goo.gl/hejqO * What functionality do you get for free with Functors or other type-classes? votes: 10, answers: 5 Read on SO: [48] http://goo.gl/9hSlv * Why are types with no data constructors valid? votes: 10, answers: 1 Read on SO: [49] http://goo.gl/Pp6Ze * Folding flatMap/bind over a list of functions (a.k.a. Name That Combinator!) votes: 9, answers: 2 Read on SO: [50] http://goo.gl/bBtBI Until next time, Daniel Santa Cruz References 1. http://tech.dropbox.com/?p=129 2. http://www.reddit.com/r/haskell/comments/o0j2a/dropbox_tech_blog_using_the_… 3. http://www.reddit.com/r/haskell/comments/ohjg7/a_new_approach_to_iteratees/ 4. http://www.reddit.com/r/haskell/comments/ohjg7/a_new_approach_to_iteratees/ 5. https://www.youtube.com/watch?v=jBd9c1gAqWs 6. http://www.reddit.com/r/haskell/comments/o0im2/haskell_repa_realtime_ray_tr… 7. http://matt.might.net/articles/partial-orders/ 8. http://www.reddit.com/r/haskell/comments/o6nnp/order_theory_for_computer_sc… 9. https://github.com/leonidas/codeblog/blob/master/2012/2012-01-08-streams-co… 10. http://www.reddit.com/r/haskell/comments/o87z5/generalizing_streams_into_co… 11. http://www.well-typed.com/blog/63 12. http://www.reddit.com/r/haskell/comments/odv3t/welltyped_are_hiring_haskell… 13. http://blog.ezyang.com/2012/01/why-iteratees-are-hard-to-understand/ 14. http://www.reddit.com/r/haskell/comments/o2p3p/why_iteratees_are_hard_to_un… 15. http://www.reddit.com/r/haskell/comments/o5iov/modern_reallworld_haskell/ 16. http://www.reddit.com/r/haskell/comments/o5iov/modern_reallworld_haskell/ 17. http://apfelmus.nfshost.com/blog/2012/01/01-frp-api-0-5.html 18. http://www.reddit.com/r/haskell/comments/nz0af/frp_reactivebanana_is_gettin… 19. http://wewantarock.wordpress.com/2012/01/11/wxhaskell-and-wxwidgets-2-9/ 20. http://www.reddit.com/r/haskell/comments/oc8qi/wxhaskell_and_wxwidgets_29/ 21. http://www.yesodweb.com/blog/2012/01/blog-example 22. http://www.reddit.com/r/haskell/comments/of4g9/updated_yesod_blog_example_i… 23. http://justhaskell.org/2012/01/02/experiments-in-haskell-packaging/ 24. http://www.reddit.com/r/haskell/comments/nzv56/experiments_in_haskell_packa… 25. http://www.yesodweb.com/blog/2012/01/http-conduit 26. http://www.reddit.com/r/haskell/comments/o5b4w/httpconduit/ 27. http://personal.cis.strath.ac.uk/~raa/posts/2012-01-06-streams.html 28. http://www.reddit.com/r/haskell/comments/o5ioy/reasoning_about_stream_proce… 29. http://www.reddit.com/r/haskell/comments/oaqsa/simon_marlows_parallel_and_c… 30. http://www.reddit.com/r/haskell/comments/oaqsa/simon_marlows_parallel_and_c… 31. http://wewantarock.wordpress.com/2012/01/12/who-is-my-community/ 32. http://www.reddit.com/r/haskell/comments/ogtos/who_is_my_community_wxhaskel… 33. http://stackoverflow.com/questions/8736995/fun-with-repeated-fmap 34. http://stackoverflow.com/questions/8760956/can-a-compiler-automatically-det… 35. http://stackoverflow.com/questions/8779390/what-does-floated-out-mean 36. http://stackoverflow.com/questions/8703319/haskell-projects-to-study-and-le… 37. http://stackoverflow.com/questions/8777216/how-do-you-identify-monadic-desi… 38. http://stackoverflow.com/questions/8788542/how-do-i-get-good-small-shrinks-… 39. http://stackoverflow.com/questions/8762943/which-applicative-functor-is-use… 40. http://stackoverflow.com/questions/8735072/double-map-in-haskell 41. http://stackoverflow.com/questions/8739741/higher-ranked-and-impredicative-… 42. http://stackoverflow.com/questions/8780183/handling-events-in-haskell 43. http://stackoverflow.com/questions/8807539/does-the-term-monadic-in-j-have-… 44. http://stackoverflow.com/questions/8832624/any-good-articles-on-optimizing-… 45. http://stackoverflow.com/questions/8857038/what-is-a-free-variable 46. http://stackoverflow.com/questions/8705332/haskell-thread-communication-pat… 47. http://stackoverflow.com/questions/8745597/defining-a-function-by-equations… 48. http://stackoverflow.com/questions/8785311/what-functionality-do-you-get-fo… 49. http://stackoverflow.com/questions/8829194/why-are-types-with-no-data-const… 50. http://stackoverflow.com/questions/8716668/folding-flatmap-bind-over-a-list…

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Error in converting List of Lists into PArray ( Parray a )
by mukesh tiwari 19 Jan '12

19 Jan '12

Hello all I am trying to convert List of Lists ( [[(Int , Double )]] ) into PArray ( PArray ( Int , Double )) but getting run time error. This code works fine and print list of PArray ( Int , Double ) but when i put print $ P.fromList ( map P.fromList c ) then i am getting runtime error. It says "Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR". Could some one please tell me how to resolve this issue. Thank you --import ParallelMat import Data.List import System.Environment import Data.Array.Parallel import qualified Data.Array.Parallel.PArray as P processMatrix :: [ [ Double ] ] -> [ [ Double ] ] -> [ ( [ ( Int , Double ) ] , [ ( Int , Double ) ]) ] processMatrix [] [] = [] processMatrix ( x : xs ) ( y : ys ) | ( all ( == 0 ) x ) Prelude.|| ( all ( == 0 ) y ) = processMatrix xs ys | otherwise = ( filter ( \( x , y ) -> y /= 0 ) . zip [ 1..] $ x ,filter ( \( x , y ) -> y /= 0 ) . zip [1..] $ y ) : processMatrix xs ys main = do [ first , second ] <- getArgs a <- readFile first b <- readFile second let a' = transpose . tail . map ( map ( read :: String -> Double ) . words ) . lines $ a b' = tail . map ( map ( read :: String -> Double ) . words ) . lines $ b ( c , d ) = unzip $ processMatrix a' b' print $ ( map P.fromList c ) --print d Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in [fromList<PArray> [(1,1.0),(6,1.0)],fromList<PArray> [(4,11.0),(9,11.0)],fromList<PArray> [(1,4.0),(4,2.0),(6,4.0),(9,2.0)]] Putting print $ P.fromList ( map P.fromList c ) Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR Input file A.in 10 10 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0

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Pattern-matching & substitution for haskell-src-exts?
by Conal Elliott 19 Jan '12

19 Jan '12

Has anyone implemented pattern-matching & substitution for haskell-src-exts? - Conal

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REMINDER: Hac Boston from January 20-22 at MIT
by Edward Z. Yang 19 Jan '12

19 Jan '12

I'd like to remind everyone that Hac Boston, a Haskell hackathon is being held January 20-22, 2012 at MIT (rooms 4-159 and 4-261) in Cambridge, MA. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday with the occasional break for sleep. Everyone is welcome -- you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell skills. If you plan on coming, please officially register<http://haskell.org/haskellwiki/Hac_Boston/Register>, even if you already put your name on the wiki. Registration, travel, some information about lodging and many other details can now be found on the Hac Boston wiki <http://haskell.org/haskellwiki/Hac_Boston>. Edward Kmett tells me we still have space. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on <http://goog_145539007>the wiki <http://haskell.org/haskellwiki/Hac_Boston/Talks>. We look forward to seeing you at MIT! Cheers, Edward

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Encouraging performance with ghc 7.4
by Eugene Kirpichov 19 Jan '12

19 Jan '12

Hi cafe, Just wanted to inform you that I've been benchmarking my compute-intensive stuff on ghc 7.0.4 vs 7.4.0, and 7.4.0 gave a large speedup - one program that took 2.9s on a particular input is now taking 2.2s. This result is repeatable. So I encourage people to try out GHC 7.4.0. Some stuff may stop compiling (I had to do 3 basically one-line fixes in 3 packages before I got mine to compile), but I didn't encounter other problems. Below is +RTS -s of two runs: one compiled with ghc 7.4.0 and another with 7.0.4. I can make a more detailed comparison if it's useful and if someone tells me how - I thought about including +RTS -p, but Simon Marlow wrote recently that it has started giving meaningful results only in 7.4, so comparison with 7.0.4 would be unfair. However in this case I'm sure that the program is heavily compute-bound by the following two functions (because it's always compute-bound by them): 1) https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Plots.hs#L226 2) https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Conf.hs - localToUTC and strptime. jkff@jkff-laptop ~/projects/logs/... $ tplot -if e.trace -dk 'within[.] cumsum 10' -o e.png +RTS -s tplot -if e.trace -dk within[.] cumsum 10 -o e.png +RTS -s 2,751,756,296 bytes allocated in the heap 135,129,112 bytes copied during GC 33,149,720 bytes maximum residency (22 sample(s)) 1,755,868 bytes maximum slop 56 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 4994 collections, 0 parallel, 0.22s, 0.23s elapsed Generation 1: 22 collections, 0 parallel, 0.08s, 0.09s elapsed INIT time 0.01s ( 0.00s elapsed) MUT time 2.61s ( 2.91s elapsed) GC time 0.30s ( 0.32s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.91s ( 3.22s elapsed) %GC time 10.1% (9.8% elapsed) Alloc rate 1,051,262,083 bytes per MUT second Productivity 89.6% of total user, 80.9% of total elapsed jkff@jkff-laptop ~/projects/logs/... $ tplot -if e.trace -dk 'within[.] cumsum 10' -o e.png +RTS -s 2,161,811,620 bytes allocated in the heap 107,589,660 bytes copied during GC 34,799,400 bytes maximum residency (22 sample(s)) 1,721,152 bytes maximum slop 58 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 3899 colls, 0 par 0.13s 0.14s 0.0000s 0.0003s Gen 1 22 colls, 0 par 0.08s 0.09s 0.0043s 0.0482s INIT time 0.00s ( 0.00s elapsed) MUT time 2.03s ( 2.28s elapsed) GC time 0.21s ( 0.23s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.26s ( 2.51s elapsed) %GC time 9.3% (9.1% elapsed) Alloc rate 1,056,397,390 bytes per MUT second Productivity 90.7% of total user, 81.5% of total elapsed -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Lifting IO (IO a) -> IO a to m (m a) -> m a with monad-control
by Edward Z. Yang 18 Jan '12

18 Jan '12

Hello folks, I was curious whether or not it is possible to lift an arbitrary IO (IO a) -> IO a function to MonadBaseControl IO m => m (m a) -> m a. That is, implement a function: liftJoin :: MonadBaseControl mb m => (mb (mb (StM m a)) -> mb (StM m a)) -> m (m a) -> m a The difficulty seems to be that we can't extract the resumable state from the inner base monad action. If this is not possible, is there a suitable strengthening of MonadBaseControl that achieves this effect? Cheers, Edward [1] http://hackage.haskell.org/packages/archive/monad-control/latest/doc/html/C…

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Re: [Haskell-cafe] Need advice: Haskell in Web Client
by dokondr 18 Jan '12

18 Jan '12

On Wed, Jan 18, 2012 at 10:47 AM, John Lenz <lenz(a)math.uic.edu> wrote: > > > I don't see a great need of developing something like GWT for haskell, > since we already have good support for all sorts of existing tools that > span more than just haskell, like extjs, yui, and jqueryui. > Haskell makes my work doable in many areas where other PLs will take enormous efforts and time to achieve the same result. It would be great if I could write Web client code in pure Haskell (no HTML, no DOM, no Ajax, ..) compile it to Javascript and just run it in the browser and do all GUI and backend communication work. Simple as that. Regards, Dmitri.

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strict, lazy, non-strict, eager
by Albert Y. C. Lai 18 Jan '12

18 Jan '12

Most individuals of the Haskell community have long been maintaining a cognitive dissonance; some cases turn into plain hypocrisy. You might excuse it for its ancient and prominent origin: Richard Bird and/or Philip Wadler themselves wrote like "it is too lazy", "make it more strict" 13 years ago and surely more. But perpetuating it is not helping. I have not written this complaint until now because I have been waiting for unmistakable evidence, a smoking gun, a red hand so caught that you cannot explain away, for example you cannot explain that "one sentence is from one person, the other sentence is from a different person". So, on IRC in #haskell, from the same person, speaking on the same topic in the same context, in the same interval of 3 minutes (the first two sentences in the same minute): 1. a function f is strict if f ⊥ = ⊥ 2. ⊥ represents any computation which does not terminate, i.e. an exception or an infinite loop 3. "strict" describes the denotational semantics People, could you please make up your mind already? It has been more than 13 years. Denotational semantics: A. There are no computational steps. There is no termination, and there is no non-termination, since there are no steps to finish, and no steps to keep going. B. ⊥ represents "no information", not "non-termination". There is no "non-termination" to represent. C. fix id = ⊥ because ⊥ is the least fixed point of id, not because fix id non-terminates. There is nothing to terminate or non-terminate. D. You say strict, more strict, less strict; non-strict, more non-strict, less non-strict. You don't say eager, and you don't say lazy. Operational semantics: A. There is no ⊥; it does not appear in any sequence of computational steps, finitely long or infinitely long. B. You say eager, more eager, less eager; lazy, more lazy, less lazy; or speculative, more speculative, less speculative; or any other adjectives for evaluation strategies. You don't say strict, and you don't say non-strict. "Semantics", "semantically speaking": A. Which semantics, which semantically? There are two. B. Actually there are more, but apparently two is already enough to cause all kinds of incoherent statements. If I draw your attention to algebraic semantics, will you start saying "it is too lazy, need to make it more idempotent"?

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implementing a text editor swap file
by Martin DeMello 18 Jan '12

18 Jan '12

I'm writing a gtk2hs-based text editor, and would like to implement continuous (swap-file based) autosave the way vim and emacs do it. Any suggestions for how to implement this in a cross-platform manner? Also, is there a library that returns standard config file locations on a per-platform basis? martin

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haskell tcp server
by Alexander V Vershilov 18 Jan '12

18 Jan '12

Hello. I'm interested if there exists some library like warp but only for tcp. Or maybe some web page with skeleton for such server or with some variants, eg. concurent/single-threaded. I know that each service can have it's specific properties and realization, but there are many common things left. If it matter — my main task is to create server for protobuf protocol, maybe for such a task there are already created solutions. -- Alexander V Vershilov.

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