May 2013 - Haskell-Cafe

Haskell Weekly News: Issue 267
by Daniel Santa Cruz 15 May '13

15 May '13

Welcome to issue 267 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 28 to May 11, 2013. Quotes of the Week * ciaranm: a category is just a category in the category of categories * cmccann: the Either monad is like the Maybe monad, except with something instead of Nothing. * sclv: Q: Why are the adjunctions of Galois connections backwards? A: He never got the hang of duals. * acowley: blackdog's experience with IDEs is apparently both more sensual and more culinary than mine * cmccann: shachaf jokes are what the quotes section of HWN is for. cmccann: though lately there have been not as many :[ shachaf: cmccann: Hey, there wasn't a single shachaf quote in the last HWN! shachaf: Leave me alone. shachaf: There were two cmccann quotes. * edwardk: cmccann: i'm NDA'd out of that space for a while ;) elliott: edwardk just collects NDAs so that he can focus. * trapdInIO: in java everything happens elsewhere * edwardk: i never bothered to release the c preprocessor because it was entangled with an old compiler of mind * acowley: I get nervous when I haven't triggered the impossible for more than a month or so :/ * hpc: be careful, excessive consumption of lenses will go straight to your type signature * acowley: The longer I leave this function named "jumanji" the less I want to find a better name * ion: The next step is unsafeCoerce. At that point youâre halfway into implementing lens. * bos: i'm not sure edwardk is a good model to follow. (a) because you won't be able to, and (b) because you won't be able to. Top Reddit Stories * John Carmack interested in doing a moderate-sized project in Haskell Domain: twitter.com, Score: 136, Comments: 30 On Reddit: [1] http://goo.gl/7rJpH Original: [2] http://goo.gl/yL9kV * John Carmack starting port of Wolf 3D in Haskell Domain: twitter.com, Score: 108, Comments: 19 On Reddit: [3] http://goo.gl/MKkiI Original: [4] http://goo.gl/SN7Mp * Haskell for all: Program imperatively using Haskell lenses Domain: haskellforall.com, Score: 103, Comments: 80 On Reddit: [5] http://goo.gl/FIfUd Original: [6] http://goo.gl/zkEHI * unm-hip a Haskell Image Processing Library Domain: self.haskell, Score: 71, Comments: 18 On Reddit: [7] http://goo.gl/erMhx Original: [8] http://goo.gl/erMhx * Crit-bit trees in Haskell: fast, and open to contributors! Domain: github.com, Score: 56, Comments: 16 On Reddit: [9] http://goo.gl/1vQMS Original: [10] http://goo.gl/08byj * Tutorial on HLearn's Markov networks, monoids, and "type level lenses" explained with Futurama Domain: izbicki.me, Score: 53, Comments: 10 On Reddit: [11] http://goo.gl/3mYuD Original: [12] http://goo.gl/4WjnU * Cool mailing list archive I stumbled on: Haskell controlling garbage trucks Domain: haskell.org, Score: 49, Comments: 2 On Reddit: [13] http://goo.gl/h8p4y Original: [14] http://goo.gl/kSGkx * Haskell intuition Domain: self.haskell, Score: 49, Comments: 23 On Reddit: [15] http://goo.gl/F04IM Original: [16] http://goo.gl/F04IM * The Difference between Recursion & Induction Domain: blog.ezyang.com, Score: 47, Comments: 3 On Reddit: [17] http://goo.gl/mZN3T Original: [18] http://goo.gl/z2jZx * Yesod 1.2 released Domain: yesodweb.com, Score: 45, Comments: 8 On Reddit: [19] http://goo.gl/el7dt Original: [20] http://goo.gl/lqBM9 * Haskell 2014 committee has now been formed Domain: thread.gmane.org, Score: 42, Comments: 27 On Reddit: [21] http://goo.gl/okojc Original: [22] http://goo.gl/1g6FG * Web Frameworks Benchmark 4 (includes WAI) Domain: techempower.com, Score: 41, Comments: 60 On Reddit: [23] http://goo.gl/FLGJu Original: [24] http://goo.gl/AJbhV * Haskell for all: pipes-3.3.0: Folds and uniting ListT with Proxy Domain: haskellforall.com, Score: 41, Comments: 14 On Reddit: [25] http://goo.gl/sO1EK Original: [26] http://goo.gl/5rZ0u * First working program using the Repa plugin Domain: disciple-devel.blogspot.com, Score: 39, Comments: 5 On Reddit: [27] http://goo.gl/bQgfV Original: [28] http://goo.gl/9K5Qk * How to use Hoogle from GHCi Domain: youtube.com, Score: 36, Comments: 2 On Reddit: [29] http://goo.gl/9V9o7 Original: [30] http://goo.gl/LU7Zk * 4th Episode of "The Pragmatic Haskeller": HTTP and more Domain: cakesolutions.net, Score: 35, Comments: 3 On Reddit: [31] http://goo.gl/rMLyC Original: [32] http://goo.gl/npjKm * A monadic joke Domain: self.haskell, Score: 34, Comments: 28 On Reddit: [33] http://goo.gl/OmwSp Original: [34] http://goo.gl/OmwSp * A "Natural" Theorem Prover in Haskell — By Tim Gowers! Domain: gowers.wordpress.com, Score: 34, Comments: 5 On Reddit: [35] http://goo.gl/OomyR Original: [36] http://goo.gl/LWNHW * Towards a Haskell Logic Library Domain: spin.atomicobject.com, Score: 30, Comments: 1 On Reddit: [37] http://goo.gl/puVS7 Original: [38] http://goo.gl/7dXA0 * Barabra Liskov - Keynote: The Power of Abstraction Domain: infoq.com, Score: 29, Comments: 25 On Reddit: [39] http://goo.gl/B7AZJ Original: [40] http://goo.gl/SSYSm Top StackOverflow Questions * What constitutes a fold for types other than list? votes: 34, answers: 4 Read on SO: [41] http://goo.gl/AkbgP * Lazy vs Strict implementations of data structures votes: 19, answers: 2 Read on SO: [42] http://goo.gl/De1VO * Compiling very large constants with GHC votes: 17, answers: 1 Read on SO: [43] http://goo.gl/mWOMz * Haskell Tree splitting: Can someone please explain this line? votes: 16, answers: 1 Read on SO: [44] http://goo.gl/ZVaQE * Could cabal notice about unused package in dependencies? votes: 15, answers: 1 Read on SO: [45] http://goo.gl/IUOx2 * How can eta-reduction of a well typed function result in a type error? votes: 14, answers: 2 Read on SO: [46] http://goo.gl/3A4hu * What is so special about Monads? votes: 14, answers: 4 Read on SO: [47] http://goo.gl/CF7BC * Why does ghci desugar type lists and type families? Can this be selectively disabled? votes: 13, answers: 0 Read on SO: [48] http://goo.gl/QBmM7 * How are functors in Haskell and OCaml similar? votes: 13, answers: 1 Read on SO: [49] http://goo.gl/rIYKy * Monitoring GHC activity votes: 10, answers: 1 Read on SO: [50] http://goo.gl/q7WuU * Why isn't the Prelude's words function written more simply? votes: 10, answers: 1 Read on SO: [51] http://goo.gl/W1TqA * How to design a monadic stack? votes: 10, answers: 2 Read on SO: [52] http://goo.gl/LAeJI * How to generate arbitrary instances of a simple type for quickcheck votes: 9, answers: 2 Read on SO: [53] http://goo.gl/oCesk Until next time, +Daniel Santa Cruz References 1. https://twitter.com/ID_AA_Carmack/status/331557216488390656 2. http://www.reddit.com/r/haskell/comments/1dv4ij/john_carmack_interested_in_… 3. https://twitter.com/id_aa_carmack/status/331918309916295168 4. http://www.reddit.com/r/haskell/comments/1dwjup/john_carmack_starting_port_… 5. http://www.haskellforall.com/2013/05/program-imperatively-using-haskell.html 6. http://www.reddit.com/r/haskell/comments/1dpk2b/haskell_for_all_program_imp… 7. http://www.reddit.com/r/haskell/comments/1dzk73/unmhip_a_haskell_image_proc… 8. http://www.reddit.com/r/haskell/comments/1dzk73/unmhip_a_haskell_image_proc… 9. https://github.com/bos/critbit 10. http://www.reddit.com/r/haskell/comments/1e1ywq/critbit_trees_in_haskell_fa… 11. http://izbicki.me/blog/markov-networks-monoids-and-futurama 12. http://www.reddit.com/r/haskell/comments/1e0759/tutorial_on_hlearns_markov_… 13. http://www.haskell.org/pipermail/haskell-cafe/2010-April/075647.html 14. http://www.reddit.com/r/haskell/comments/1df8fy/cool_mailing_list_archive_i… 15. http://www.reddit.com/r/haskell/comments/1e3cl0/haskell_intuition/ 16. http://www.reddit.com/r/haskell/comments/1e3cl0/haskell_intuition/ 17. http://blog.ezyang.com/2013/04/the-difference-between-recursion-induction/ 18. http://www.reddit.com/r/haskell/comments/1dbwp4/the_difference_between_recu… 19. http://www.yesodweb.com/blog/2013/05/yesod-1-2-released 20. http://www.reddit.com/r/haskell/comments/1dko1d/yesod_12_released/ 21. http://thread.gmane.org/gmane.comp.lang.haskell.general/19796 22. http://www.reddit.com/r/haskell/comments/1dhbjr/haskell_2014_committee_has_… 23. http://www.techempower.com/blog/2013/05/02/frameworks-round-4/ 24. http://www.reddit.com/r/haskell/comments/1dk73s/web_frameworks_benchmark_4_… 25. http://www.haskellforall.com/2013/05/pipes-330-folds-and-uniting-listt-with… 26. http://www.reddit.com/r/haskell/comments/1dsuto/haskell_for_all_pipes330_fo… 27. http://disciple-devel.blogspot.com/2013/05/first-working-program-using-repa… 28. http://www.reddit.com/r/haskell/comments/1dm7a6/first_working_program_using… 29. http://www.youtube.com/watch?v=QpDQhGYPqkU 30. http://www.reddit.com/r/haskell/comments/1dxx4m/how_to_use_hoogle_from_ghci/ 31. http://www.cakesolutions.net/teamblogs/2013/04/28/tph-episode-4-http-and-mo… 32. http://www.reddit.com/r/haskell/comments/1d9zou/4th_episode_of_the_pragmati… 33. http://www.reddit.com/r/haskell/comments/1dcfso/a_monadic_joke/ 34. http://www.reddit.com/r/haskell/comments/1dcfso/a_monadic_joke/ 35. http://gowers.wordpress.com/2013/04/14/answers-results-of-polls-and-a-brief… 36. http://www.reddit.com/r/haskell/comments/1dtw7v/a_natural_theorem_prover_in… 37. http://spin.atomicobject.com/2013/05/10/haskell-logic-library/?utm_source=t… 38. http://www.reddit.com/r/haskell/comments/1e2okw/towards_a_haskell_logic_lib… 39. http://www.infoq.com/presentations/programming-abstraction-liskov 40. http://www.reddit.com/r/haskell/comments/1dodfb/barabra_liskov_keynote_the_… 41. http://stackoverflow.com/questions/16426463/what-constitutes-a-fold-for-typ… 42. http://stackoverflow.com/questions/16296571/lazy-vs-strict-implementations-… 43. http://stackoverflow.com/questions/16348340/compiling-very-large-constants-… 44. http://stackoverflow.com/questions/16267497/haskell-tree-splitting-can-some… 45. http://stackoverflow.com/questions/16390273/could-cabal-notice-about-unused… 46. http://stackoverflow.com/questions/16402942/how-can-eta-reduction-of-a-well… 47. http://stackoverflow.com/questions/16439025/what-is-so-special-about-monads 48. http://stackoverflow.com/questions/16346293/why-does-ghci-desugar-type-list… 49. http://stackoverflow.com/questions/16353066/how-are-functors-in-haskell-and… 50. http://stackoverflow.com/questions/16348118/monitoring-ghc-activity 51. http://stackoverflow.com/questions/16446274/why-isnt-the-preludes-words-fun… 52. http://stackoverflow.com/questions/16457111/how-to-design-a-monadic-stack 53. http://stackoverflow.com/questions/16440208/how-to-generate-arbitrary-insta… 54. https://plus.google.com/105107667630152149014/about

1 0

Good to find you on LinkedIn
by Daniel Díaz Casanueva 15 May '13

15 May '13

LinkedIn ------------ Daniel Díaz Casanueva requested to add you as a connection on LinkedIn: ------------------------------------------ Minh Thu, I'd like to add you to my professional network on LinkedIn. - Daniel Accept invitation from Daniel Díaz Casanueva http://www.linkedin.com/e/uc6lxc-hgr1egm6-6q/XvIdBwmueHfd6vFMPXXdLaqreCbl5o… View profile of Daniel Díaz Casanueva http://www.linkedin.com/e/uc6lxc-hgr1egm6-6q/rso/255311896/LIky/name/227245… ------------------------------------------ You are receiving Invitation emails. This email was intended for Minh Thu Vo. Learn why this is included: http://www.linkedin.com/e/uc6lxc-hgr1egm6-6q/plh/http%3A%2F%2Fhelp%2Elinked… (c) 2012, LinkedIn Corporation. 2029 Stierlin Ct, Mountain View, CA 94043, USA.

3 2

Trouble with Debug.Trace when using a library
by hanjoosten 15 May '13

15 May '13

My project is split in two packages, A and B, where B uses A. All together, there are about 100 haskell modules. Currently, there seems to be a loop somewere, that only seems to occur when working with rather large datastructures. I use trace (from Debug.Trace) to see what exactly is going on. As long as I trace functions from B, trace works as expected. However, at some point I need to trace into functions supplied by package A. In package A I have a function like this: (I have simplified the function) fA :: SomeInput -> SomeResult fA a = let x = f a in trace ("Entering fA") x (The function f is defined in package A) In package B, I have a function like this: fB :: SomeInput -> SomeResult fB a = let x = fA a in trace ( "Entering fB") x At runtime, I get the message Entering fB and then nothing more. a lot of cpu is used, but no more output. I have made some wrappers like the ones above. As long as they are in the same package, trace works fine. However, when entering a function from the other package, there is no more debug output. This is very anoying. Anyone some suggestions? Is this a bug? I use the haskell platform 12.4.0.0 on a windows 7 machine. -- View this message in context: http://haskell.1045720.n5.nabble.com/Trouble-with-Debug-Trace-when-using-a-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

whats with cabal and libgmp.so.3
by Rustom Mody 14 May '13

14 May '13

Today cabal suddenly started giving me errors that libgmp.so.3 is not found Moving away my old .cabal makes it work again Any explanations? [I am on debian testing. I think the causing factor was that debian switched major versions recently. There were a lot of updates... Nothing that looked like it was related to ghc]

2 1

ANN: Robot - Simulate keyboard and mouse events under X11
by Chris Wong 14 May '13

14 May '13

I'm pleased to announce the initial release of Robot! Robot lets you send fake keyboard and mouse events, just like its namesake in Java. Only X11 systems are supported right now (via XTest), but Windows and Mac can be added later if anyone cares. ## Features + Simple API (only 9 functions and 3 types) + Few dependencies + Exception safe -- unlike with other libraries, all keys and buttons are released automatically when the robot terminates. `bracket` and `finally` are used where appropriate. ## Links Hackage: http://hackage.haskell.org/package/robot-1.0 Examples: https://github.com/lfairy/robot/tree/master/examples Happy hacking! Chris -- Chris Wong, fixpoint conjurer e: lambda.fairy(a)gmail.com w: http://lfairy.github.io/

4 12

fromIntegral not enough?
by Christopher Howard 13 May '13

13 May '13

This is probably a haskell-beginners sort of question, but I usually get about 4x as many responses from cafe, about 10x as fast. I have code like so: code: -------- data Xy a = Xy a a class Coord2 a where coords2 :: Fractional b => a b -> Xy b data CircAppr a b = CircAppr a b b -- number of points, rotation angle, radius deriving (Show) instance Integral a => Coord2 (CircAppr a) where coords2 (CircAppr divns ang rad) = let dAng = 2 * pi / (fromIntegral divns) in let angles = map (* dAng) [0..divns] in undefined -- To be coded... -------- In the instance definition divns is an integral trying to divide a fractional. I hoped wrapping it in fromIntegral would coerce, but apparently not: quote: -------- Could not deduce (Fractional a) arising from a use of `/' from the context (Integral a) bound by the instance declaration at /scratch/cmhoward/pulse-spin/pulse-spin.hs:34:10-42 or from (Fractional b) bound by the type signature for coords2 :: Fractional b => CircAppr a b -> Xy b at /scratch/cmhoward/pulse-spin/pulse-spin.hs:(36,3)-(39,15) Possible fix: add (Fractional a) to the context of the type signature for coords2 :: Fractional b => CircAppr a b -> Xy b or the instance declaration In the expression: 2 * pi / (fromIntegral divns) In an equation for `dAng': dAng = 2 * pi / (fromIntegral divns) In the expression: let dAng = 2 * pi / (fromIntegral divns) in let angles = map (* dAng) [0 .. divns] in undefined -------- So, I'm wondering how I can do what I'm trying to do here, while still keeping my types as generic as possible. -- frigidcode.com

2 3

Problem with mailman at projects.haskell.org
by Tim Docker 13 May '13

13 May '13

Hi, Has anyone noticed problems with the mailman instance running at projects.haskell.org? As best I can see there are no new posts in any of the hosted list archives since mid April. I know that there have been posts made to chart(a)projects.haskell.org in May, and these have neither be distributed or archived. I've not had a response from mailman(a)projects.haskell.org. Tim

3 4

Release Candidates for Haskell Platform 2013.2
by Mark Lentczner 13 May '13

13 May '13

*Some of the release candidates for Haskell Platform 2013.2 are up.* *These are what I expect to simply "re-brand" as the release, unless anyone uncovers some issues.* *If you decide to test these out, please let me know how it goes.* * * The Mac OS X RC2 installers: *32bit: *Haskell Platform 2013.2.0.0 32bit rc2.signed.pkg<http://www.ozonehouse.com/mark/platform/Haskell%20Platform%202013.2.0.0%203…> *64bit: *Haskell Platform 2013.2.0.0 64bit rc2.signed.pkg<http://www.ozonehouse.com/mark/platform/Haskell%20Platform%202013.2.0.0%206…> The source tarball (RC1): haskell-platform-2013.2.0.0-rc1.tar.gz<http://www.ozonehouse.com/mark/platform/haskell-platform-2013.2.0.0-rc1.tar…> *Notable new things in Haskell Platform 2013.2:* - GHC 7.6.3 - Major update to *OpenGL* & *GLUT* - New packages: - *attoparsec* - *case-insensitive* - *hashable* - *unordered-containers* — Mark "mad releaser" Lentczner

4 5

Set monad
by oleg＠okmij.org 13 May '13

13 May '13

The question of Set monad comes up quite regularly, most recently at http://www.ittc.ku.edu/csdlblog/?p=134 Indeed, we cannot make Data.Set.Set to be the instance of Monad type class -- not immediately, that it. That does not mean that there is no Set Monad, a non-determinism monad that returns the set of answers, rather than a list. I mean genuine *monad*, rather than a restricted, generalized, etc. monad. It is surprising that the question of the Set monad still comes up given how simple it is to implement it. Footnote 4 in the ICFP 2009 paper on ``Purely Functional Lazy Non-deterministic Programming'' described the idea, which is probably folklore. Just in case, here is the elaboration, a Set monad transformer. {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} module SetMonad where import qualified Data.Set as S import Control.Monad.Cont -- Since ContT is a bona fide monad transformer, so is SetMonadT r. type SetMonadT r = ContT (S.Set r) -- These are the only two places the Ord constraint shows up instance (Ord r, Monad m) => MonadPlus (SetMonadT r m) where mzero = ContT $ \k -> return S.empty mplus m1 m2 = ContT $ \k -> liftM2 S.union (runContT m1 k) (runContT m2 k) runSet :: (Monad m, Ord r) => SetMonadT r m r -> m (S.Set r) runSet m = runContT m (return . S.singleton) choose :: MonadPlus m => [a] -> m a choose = msum . map return test1 = print =<< runSet (do n1 <- choose [1..5] n2 <- choose [1..5] let n = n1 + n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] -- Values to choose from might be higher-order or actions test1' = print =<< runSet (do n1 <- choose . map return $ [1..5] n2 <- choose . map return $ [1..5] n <- liftM2 (+) n1 n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] test2 = print =<< runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return (i,j,k)) -- fromList [(3,4,5),(4,3,5),(6,8,10),(8,6,10)] test3 = print =<< runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return k) -- fromList [5,10]

2 3

ANN haskdogs-0.3.2
by Sergey Mironov 13 May '13

13 May '13

Hi. I'm pleased to announce haskdogs-0.3.2, a source navigation helper. Haskdogs is a small HSH-based tool which calls hasktags to create tag file for entire haskell project. It takes into account first-level dependencies by recursively scanning imports and adding matching packages to the final tag list. As a result, programmer can use her text editor supporting tags (vim, for example) to jump directly to definition of any function she uses. Note that haskdogs call Unix shell commands like 'test' or 'mkdir' so this tool will likely fail to work on pure Windows platforms. In return, haskdogs is pretty small - only 125 lines long. Changes: * Generate extended tag file by default (hasktags -c -x) * Add -d command line option which allows user to specify additional source directories * Fix no-sources bug Github: https://github.com/ierton/haskdogs See also Vim-hint at https://github.com/ierton/haskdogs#vim-hint Install: cabal install hasktags haskdogs Usage: haskdogs [-d (FILE|'-')] [FLAGS] FLAGS will be passed to hasktags as-is followed by a list of files. Defaults to -c -x. Basically, just run haskdogs in your Haskell project directory Thanks to yihuang for the contributions! Sergey

1 0