March 2011 - Haskell-Cafe

Blocking problem reading a handle
by Daniel Díaz 24 Mar '11

24 Mar '11

Hi, cafe, all I want is to read the unread portion of a handle, without blocking waiting for an EOF or a newline. My first attempt was to try with hGetBufNonBlocking from System.IO, and other functions that use it in its definition (directly or indirectly). But, unfortunately: hGetBufNonBlocking doesn't work on Windows: http://hackage.haskell.org/trac/ghc/ticket/806 This was a bad surprise. How can achieve my purpose without hGetBufNonBlocking? Thanks in advance, Daniel Díaz

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Haskell Weekly News: Issue 174
by Daniel Santa Cruz 23 Mar '11

23 Mar '11

Welcome to issue 174 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 13-19. You can find the HTML version at: http://bit.ly/hLu36R Announcements JP Moresmau [2]announced a new release of EclipseFP with support for ghc 7! S. Doaitse Swierstra [3]announced the release of version 2.7.0 to the uu-parsinglib library to hackage. Vincent Gerard [4]informed us that he has taken over maintainership of hsmagick, which is "a FFI bindings library to ImageMagick." He is pleased to announce the 0.5 release of the library. Malte Harder [5]announce the first release of the Craftwerk graphics library for 2d vector graphics. Corey O'Connor [6]annouced a new release of his VTY library. Brent Yorgey [7]announced the release of the special Poetry and Fiction Edition of The Monad.Reader. Thanks for putting this together Brent! Gregory D. Weber [8]announced version 1.2.4 of his Sifflet and sifflet-lib. "This is a compatibility release for HXT >= 9.1." Edward Kmett [9]remminded us that the Google Summer of Code is upon us. Follow the link to see the sign up information. Quotes of the Week * ddarius: *ddarius is more of a glass SUM_n (-1)^n full person. * monochrom: some kind of lazy evaluation is already known to highschool kids. teachers tell you that in a*(b+c), "evaluate b+c first", right? well, I challenge you to take 0*(389238493+97283748) and find one single student who faithfully evaluate 389238493+97283748 first. * hpc: in soviet russia, yoda speaks flawless english * lambdabot: trololololo * hpc: jokes are subject to an incompleteness theorem; if you can prove they are funny, they were never good jokes to begin with * hpc: when all you have is Haskell, everything looks like a research paper * Edward Kmett:Clearly we need some sort of xbox live-like achievement system for these. Achievement Unlocked: Stumped Oleg! Top Reddit Stories * Haskell.org has been accepted for the 2011 Google Summer of Code From (twitter.com) scored 48 with 1 comments. Read on [10]reddit. Read the [11]original post. * Haskell's Niche: Hard Problems From (cdsmith.wordpress.com) scored 47 with 17 comments. Read on [12]reddit. Read the [13]original post. * A little care and feeding can go a long way From (serpentine.com) scored 39 with 6 comments. Read on [14]reddit. Read the [15]original post. * The reception of monads circa 1995... From (self.haskell), scored 35 with 1 comments. Read on [16]reddit. * BlazeHtml: a blazingly fast HTML combinator library From (skillsmatter.com) scored 29 with 18 comments. Read on [17]reddit. Read the [18]original post. * Simon Peyton Jones: Managing parallelism: embrace diversity, but control side effects From (skillsmatter.com) scored 28 with 12 comments. Read on [19]reddit. Read the [20]original post. * EZYang: Mutation is frequently not what people expect ; mutation is not modular, and mutation is complicated. From (blog.ezyang.com) scored 26 with 0 comments. Read on [21]reddit. Read the [22]original post. * Functor is to Lens as Applicative is to Biplate: Introducing Multiplate From (front.math.ucdavis.edu) scored 25 with 19 comments. Read on [23]reddit. Read the [24]original post. * Monad.Reader special poetry and fiction edition From (themonadreader.wordpress.com) scored 23 with 1 comments. Read on [25]reddit. Read the [26]original post. * Functional flocks : modelling and visualizing flocking behavior in Haskell From (syntacticsalt.com) scored 22 with 0 comments. Read on [27]reddit. Read the [28]original post. Top StackOverflow Answers * [29]Why does application of `sequence` on List of Lists lead to computation of its Cartesian Product? votes: 12 This works because using lists as monads in Haskell makes them model indeterminism. Consider: sequence [[1,2],[3,4]] By definition this is the same as: do x <- [1,2] y <- [3,4] return [x,y] Just read it as "First a choice between 1 and 2, then a choice between 3 and 4". The list monad will now accumulate all possible outcomes - hence the ... * [30]In GHCi, why does the kind of the function arrow `:kind (->)` include question marks `(->) :: ?? -&ht; ? -> *`? votes: 11 The ? and ?? kinds refer to GHC extensions, specifically unboxed types. http://hackage.haskell.org/trac/ghc/wiki/IntermediateTypes has a diagram showing relationships between the extended kinds ? (all possible types), # (unboxed types), ?? (boxed or normal unboxed types â "least upper bound of # and *"), (#) (unboxed tuples, which can only be used in a ... * [31]How to use (->) instances of Monad and confusion about (->) votes: 11 For a given type r, the function of type r -> a can be thought of as a computation delivering an a using an environment typed r. Given two functions r -> a and a -> (r -> b), it's easy to imagine that one can compose these when given an environment (again, of type r). But wait! That's exactly what monads are about! So we can create an instance ... * [32]How and why is ap defined as liftM2 id in Haskell votes: 10 The type variable a from id can be instantiated at any type, and in this case that type is a -> b. So we are instantiating id at (a -> b) -> (a -> b). Now the type variable a1 from liftM2 is being instantiated at (a -> b), a2 is being instantiated at a, and r is being instantiated at b. Putting it all together, liftM2 is instantiated at ... * [33]What "reify" and "reification" means in the context of (functional?) programming? votes: 8 So I read up on this, and it is pretty much what it means: taking an abstract concept and making it concrete. Or, there is a proxy that represents the abstract concept. For example, in Lisp, the concept of procedure abstraction and application is reified when you use lambdas. Reification by itself is a broad concept and not just applicable to functional ... Top StackOverflow Questions * [34]What "reify" and "reification" means in the context of (functional?) programming? votes: 17, answers: 6 * [35]How to use (->) instances of Monad and confusion about (->) votes: 12, answers: 2 * [36]In GHCi, why does the kind of the function arrow `:kind (->)` include question marks `(->) :: ?? -> ? -> *`? votes: 12, answers: 1 * [37]Haskell-like type system in C votes: 11, answers: 2 * [38]Write a Maximum Monoid using Maybe in Haskell votes: 9, answers: 1 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://jpmoresmau.blogspot.com/2011/03/eclipsefp-204-released-supports-ghc-… 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87205 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18564 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18568 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18574 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18572 8. http://article.gmane.org/gmane.comp.lang.haskell.general/18584 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87399 10. http://www.reddit.com/r/haskell/comments/g6nwe/haskellorg_has_been_accepted… 11. http://twitter.com/haskellorg/status/48838580029755392 12. http://www.reddit.com/r/haskell/comments/g382d/haskells_niche_hard_problems/ 13. http://cdsmith.wordpress.com/2011/03/13/haskells-niche-hard-problems/ 14. http://www.reddit.com/r/haskell/comments/g6a01/a_little_care_and_feeding_ca… 15. http://www.serpentine.com/blog/2011/03/18/a-little-care-and-feeding-can-go-… 16. http://www.reddit.com/r/haskell/comments/g4f1m/the_reception_of_monads_circ… 17. http://www.reddit.com/r/haskell/comments/g7akc/blazehtml_a_blazingly_fast_h… 18. http://skillsmatter.com/podcast/scala/blazehtml-a-blazingly-fast-html-combi… 19. http://www.reddit.com/r/haskell/comments/g7ajj/simon_peyton_jones_managing_… 20. http://skillsmatter.com/podcast/scala/talk-by-haskell-expert-simon-peyton-j… 21. http://www.reddit.com/r/haskell/comments/g3dsw/ezyang_mutation_is_frequentl… 22. http://blog.ezyang.com/2011/03/killer-mutants-attack-mutation-gone-wrong/ 23. http://www.reddit.com/r/haskell/comments/g56xw/functor_is_to_lens_as_applic… 24. http://front.math.ucdavis.edu/1103.2841 25. http://www.reddit.com/r/haskell/comments/g4r6e/monadreader_special_poetry_a… 26. http://themonadreader.wordpress.com/2011/03/15/special-poetry-and-fiction-e… 27. http://www.reddit.com/r/haskell/comments/g2v75/functional_flocks_modelling_… 28. http://syntacticsalt.com/2011/03/10/functional-flocks/ 29. http://stackoverflow.com/questions/5299295/why-does-application-of-sequence… 30. http://stackoverflow.com/questions/5359707/in-ghci-why-does-the-kind-of-the… 31. http://stackoverflow.com/questions/5310203/how-to-use-instances-of-monad-an… 32. http://stackoverflow.com/questions/5358868/how-and-why-is-ap-defined-as-lif… 33. http://stackoverflow.com/questions/5314884/what-reify-and-reification-means… 34. http://stackoverflow.com/questions/5314884/what-reify-and-reification-means… 35. http://stackoverflow.com/questions/5310203/how-to-use-instances-of-monad-an… 36. http://stackoverflow.com/questions/5359707/in-ghci-why-does-the-kind-of-the… 37. http://stackoverflow.com/questions/5290408/haskell-like-type-system-in-c 38. http://stackoverflow.com/questions/5344164/write-a-maximum-monoid-using-may…

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How to use roots package?
by Artyom Kazak 23 Mar '11

23 Mar '11

Hi Café! roots (http://hackage.haskell.org/package/roots) is a package to solve equations like "f(x)==0". In RootFinder class there is an 'defaultNSteps' value, which is used as maximal count of iterations functions like findRoot and traceRoot can make. By default it is 250, but sometimes it's not enough. How can I use another value instead of 250? Should I write my own RootFinder instance, or findRoot function? Thanks in advance. — Artyom.

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a finite free category
by Vasili I. Galchin 23 Mar '11

23 Mar '11

Hello, Does there exist Haskell to generate a finite free category from a finite multipath graph with loops? Thank you, Vasilki

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Installation of Haskell Platform on CentOS 5.5
by frode k 23 Mar '11

23 Mar '11

Hi. I've spent some time trying to setup the Haskell Platform on CentOS 5.5: # cat /proc/version Linux version 2.6.18-028stab070.14 (root@rhel5-build-x64) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)) #1 SMP Thu Nov 18 16:04:02 MSK 2010 I've compiled from source, similar to what is done here: http://www.haskell.org/pipermail/haskell-cafe/2010-April/076489.html Installation of GHC 7.0.2 failed: # ./configure --prefix=/usr/haskell-platform-2011.2.0.0/ checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I had to use GHS 6.12.3 which compiled without errors. However when I tried installing the Haskell Platform I ran into problems. Briefly, this is what I did and what happened trying to "make" it: wget http://hackage.haskell.org/platform/2010.2.0.0/haskell-platform-2010.2.0.0.… unpack... ./configure --prefix=/usr/haskell-platform-2010.2.0.0/ make # make scripts/build.sh ************************************************** Scanning system for any installed Haskell Platform components... Found:None. New packages to install: None! All done. ************************************************** Building "/usr/haskell-platform-2010.2.0.0/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.8.0.6" Linking Setup ... collect2: ld terminated with signal 15 [Terminated] Error: Compiling the Setup script failed make: *** [build.stamp] Error 2 I first thought it was related to the fact that Cabal was using the /tmp directory for executables, but the alias trick (alias cabal="TMPDIR=/foo cabal") did not help. Anyone have got any ideas on what might be causing this? Can it me any missing dependencies? I've gone through the list described at http://www.vex.net/~trebla/haskell/haskell-platform.xhtml and used "yum install" to get the software mentioned. Regards, Frode

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Haskell internship in the south of France
by David Waern 23 Mar '11

23 Mar '11

To all undergrads on this list: Are you a student looking for an internship? Do you want to work with Haskell in the south of France? My company, Amadeus, is able to offer an internship on the subject of property-based testing of XML schema based web service APIs. The project goal is to write an XML test case generation tool in Haskell. The exact length of the intership can be discussed. It may be six months or a full year. XML based web services may not be the most fashionable technology, but this is a chance to apply Haskell to real world problems! Also, you get to work in Sophia-Antipolis, a beautiful technology park close to Nice on the French riviera. Contact me if you're interested! David

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Looking for feedback on my attempt at a tree construction edsl
by C K Kashyap 23 Mar '11

23 Mar '11

Hi, With my "edsl", one can describe a tree like this - import TreeEdsl import Data.Tree createTree :: TreeContext String () createTree = do insertSubTree "Fruits" $ do insertLeaf "Apple" insertLeaf "Mango" insertSubTree "Arbitrary" $ do insertSubTree "Numbers" $ do insertLeaf "1" insertLeaf "2" insertLeaf "3" insertSubTree "Letters" $ do insertLeaf "A" insertLeaf "B" insertLeaf "C" return () main = do tree <- process "root" createTree putStrLn (drawTree (fmap show tree)) return () and get a tree like this - "root" | +- "Arbitrary" | | | +- "Letters" | | | | | +- "C" | | | | | +- "B" | | | | | `- "A" | | | `- "Numbers" | | | +- "3" | | | +- "2" | | | `- "1" | `- "Fruits" | +- "Mango" | `- "Apple" My code is here https://github.com/ckkashyap/LearningPrograms/blob/master/Haskell/edsl/Tree… I'd appreciate your feedback on this. Does this qualify to be a edsl? Regards, Kashyap

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Help optimising a Haskell program
by David MacIver 23 Mar '11

23 Mar '11

Hi, I have a Haskell program I'm trying to optimise, and could use some assistance. It's for rank aggregation - taking a bunch of partial rankings of some items from users and turning them into an overall ranking (aka "That thing that Hammer Principle does"). The code is here: https://github.com/DRMacIver/hs-rank-aggregation. The interesting code lives in the Algorithms.RankAggregation module. The sample data I'm running it on is samples/electornot (voting data from http://electornot.org.uk/) Here's a profile: https://gist.github.com/1ee9356f45330e9f8caa Here's a recent profile: https://gist.github.com/1ee9356f45330e9f8caa It's doing pretty well on this data, in that it's taking 8 seconds for 180k records (down from about 80 seconds earlier today) on my computer, but I'd like it to be faster - ideally under a second. Admittedly this is an arbitrary figure and I will just move the goal posts if I achieve it, but it would still be nice. :-) I suspect that it may be hitting the limits of what can be improved without actually changing the algorithm (which I'm open to doing, but not if it makes the results significantly worse), but the last 3 or 4 times I've thought that it was shortly followed by an optimisation which halved the runtime or better. In particular the most likely culprit for improvement is to deal with the fact that it allocates nearly 5GB of data (that appears to be mostly transient - watching the process as it runs the memory usage never seems to raise above 150MB, which is large but unproblematic). The biggest culprit for this is divideAndConquer which is probably generating a silly number of intermediate lists, but it's not obvious how to fix that - maybe by making the tree structure in the recursion more explicit? Don't know. Suggestions very welcome. Regards, David

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Computing the multiplication table of a group using the GHC inliner ; )
by Daniel Schüssler 22 Mar '11

22 Mar '11

Hello, turns out that you can define the group operation of the symmetric group on 3 elements in this abstract way (via the isomorphism to the group of bijective functions from a three-element type to itself): s3mult g2 g1 = fromFun (toFun g2 . toFun g1) and convince GHC to compile it down to a nested case statement. It even somehow made the left multiplication with the identity non-strict. Just thought it's neat ;) $ ghc-core S3.hs -funfolding-use-threshold=64 ... -- identifiers manually un-qualified for readability s3mult = \ (g2_ahA :: S3) (g1_ahB :: S3) -> case g2_ahA of _ { S3abc -> g1_ahB; S3bca -> case g1_ahB of _ { S3abc -> S3bca; S3bca -> S3cab; S3cab -> S3abc; S3acb -> S3bac; S3bac -> S3cba; S3cba -> S3acb }; S3cab -> case g1_ahB of _ { S3abc -> S3cab; S3bca -> S3abc; S3cab -> S3bca; S3acb -> S3cba; S3bac -> S3acb; S3cba -> S3bac }; S3acb -> case g1_ahB of _ { S3abc -> S3acb; S3bca -> S3cba; S3cab -> S3bac; S3acb -> S3abc; S3bac -> S3cab; S3cba -> S3bca }; S3bac -> case g1_ahB of _ { S3abc -> S3bac; S3bca -> S3acb; S3cab -> S3cba; S3acb -> S3bca; S3bac -> S3abc; S3cba -> S3cab }; S3cba -> case g1_ahB of _ { S3abc -> S3cba; S3bca -> S3bac; S3cab -> S3acb; S3acb -> S3cab; S3bac -> S3bca; S3cba -> S3abc } } -- inverse s3inv = \ (g_ahC :: S3) -> case g_ahC of _ { S3abc -> S3abc; S3bca -> S3cab; S3cab -> S3bca; S3acb -> S3acb; S3bac -> S3bac; S3cba -> S3cba } --- end core --- --- source --- module S3 where -- | Symmetric group / permutation group on 3 elements data S3 = S3abc | S3bca | S3cab | S3acb | S3bac | S3cba deriving(Eq) -- | Returns an element of S3 satisfying the given predicate s3the :: (S3 -> Bool) -> S3 s3the p | p S3abc = S3abc | p S3acb = S3acb | p S3bac = S3bac | p S3bca = S3bca | p S3cba = S3cba | p S3cab = S3cab | otherwise = error "s3the: no element satisfies the predicate" data ABC = A | B | C deriving(Eq) toFun :: S3 -> ABC -> ABC toFun g = case g of S3abc -> mkFun A B C S3bca -> mkFun B C A S3cab -> mkFun C A B S3acb -> mkFun A C B S3bac -> mkFun B A C S3cba -> mkFun C B A where mkFun imA _ _ A = imA mkFun _ imB _ B = imB mkFun _ _ imC _ = imC fromFun :: (ABC -> ABC) -> S3 fromFun f = s3the (\g -> toFun g A == f A && toFun g B == f B) s3mult :: S3 -> S3 -> S3 s3mult g2 g1 = fromFun (toFun g2 . toFun g1) s3inv g = s3the (\g' -> s3mult g' g == S3abc)

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Install GTK on Windows
by Daniel Díaz 22 Mar '11

22 Mar '11

Hi, cafe, I know this has been treated many times, but I've tried install GTK so hard without any success that I decided to ask for help. Firstly, I followed the instructions listed in HaskellWiki [1]. No results. Then, I followed the instruction from the Mark Shroyer blog [2]. No results. I did some mixing of the two guides, too, but my error was always the same: Configuring cairo-0.12.0... [...] setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: z, cairo Configuring glib-0.12.0... [...] setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: gobject-2.0, gthread-2.0, glib-2.0, intl I've searched for these libraries, and, with --extra-include-dirs=, I've included them. But the same error was thrown. I'm sure I'm doing something wrong or, perhaps, I'm missing something. Any help is welcomed! Thanks in advance, Daniel Díaz References: [1] http://www.haskell.org/haskellwiki/Gtk2Hs [2] http://markshroyer.com/2010/10/gtk2hs-on-windows/

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