September 2011 - Haskell-Cafe

regex-applicative library needs your help! (algorithmic challenge)
by Roman Cheplyaka 23 Sep '11

23 Sep '11

Please help make the regex-based parsing library efficient! https://github.com/feuerbach/regex-applicative/wiki/Call-For-Help -- Roman I. Cheplyaka :: http://ro-che.info/

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Quick check finite data generation
by mukesh tiwari 23 Sep '11

23 Sep '11

Hello all I have a data type data Index = Index {indexSize::Float, indexIds::[Int], indexDown::(IntMap.IntMap Index)} | IndexLeaf {indexSize::Float, indexIds::[Int]} | IndexEmpty {indexSize::Float} deriving ( Show , Eq ) I derived some thing like this for QuickCheck testing instance Arbitrary a => Arbitrary ( IntMap a ) where arbitrary = liftM IntMap.fromList arbitrary instance Arbitrary Index where arbitrary = oneof [ liftM3 Index arbitrary arbitrary arbitrary , liftM2 IndexLeaf arbitrary arbitrary , liftM IndexEmpty arbitrary ] but its generating infinite list of data. After reading the paper QuickCheck: A Lightweight Tool for Random Testing of Haskell Programs<http://www.eecs.northwestern.edu/%7Erobby/courses/395-495-2009-fall/quick.p…> <http://www.eecs.northwestern.edu/%7Erobby/courses/395-495-2009-fall/quick.p…> i tried to write above one using sized but i am not sure how to write this . instance Arbitrary a => Arbitrary ( IntMap a ) where arbitrary = liftM IntMap.fromList arbitrary instance Arbitrary Index where arbitrary = sized arbIndex arbIndex 0 = liftM IndexEmpty arbitrary arbIndex 1 = liftM2 IndexLeaf arbitrary arbitrary arbIndex n = frequency [ ( 1 , liftM IndexEmpty arbitrary ) , ( 1 , liftM2 IndexLeaf arbitrary arbitrary ) , ( 2 , liftM2 Index arbitrary arbitrary ( arbIndex $ div n 2 ) ) ] but from this type i can see arbIndex $ div n 2 will return Gen Index not Gen ( IntMap Index ) and i am getting compiler error . Could some one please tell me , how to write arbIndex n in terms of smaller n like in the paper arbTree 0 = liftM Leaf arbitrary arbTree n = frequency[ ( 1 , liftM Leaf arbitrary ) , ( 4 , liftM2 Branch ( arbTree $ div n 2 ) ( arbTree $ div n 2 ) ) ] Thank You Mukesh Tiwari

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ANN: New releases in the lightweight monadic regions family
by Bas van Dijk 22 Sep '11

22 Sep '11

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problem with cabal install MissingH-1.1.1.0
by Mariano Cortesi 22 Sep '11

22 Sep '11

Hi, My name is mariano and I'm quite new to haskell. I trying to setup up my environment and use the eclipse plugin for haskell. But is not working property because, it tries to compile the scion browser and this depends on MissingH which fails on build. I'm with mac OS X lion, ghc version 7.2.1 and when a i try to install MissingH version 1.1.1.0 it fails with [24 of 47] Compiling Data.BinPacking ( src/Data/BinPacking.hs, dist/build/Data/BinPacking.o ) src/Data/BinPacking.hs:45:6: Illegal datatype context (use -XDatatypeContexts): (Num size, Ord size, Show size, Show obj) => cabal: Error: some packages failed to install: MissingH-1.1.1.0 failed during the building phase. The exception was: ExitFailure 1 Any ideas? I don't know what to do :S best, Mariano

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Re: [Haskell-cafe] Quickcheck, Arbitrary and Overlapping instances
by Valentin Robert 22 Sep '11

22 Sep '11

I think it is one case where you should use newtype instead of type, so that it is not just a simple type synonym, but an actual "other" type. With type, you're just making an alias, but Haskell sees the same type. A newtype wrapper will hide this, at the price of having to define a (unique) constructor... So yes, you'll have to change some code.

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Proposal: Subcomputations in arrow notation
by Ertugrul Soeylemez 22 Sep '11

22 Sep '11

Hello fellow Haskellers, this is a proposal to extend the arrow notation (-XArrows). I find myself writing the following very often: system :: Wire IO () String system = proc _ -> do botAddPeriod <- succ ^<< noise -< () botAddSpeed <- noise1 -< () botAddStart <- noise1 -< () botMsg <- event addBot -< (botAddPeriod, botAddSpeed, botAddStart) bots <- manager -< ((), maybe MgrNop id botMsg) let botStr = concatMap (printf "%8.2") . M.elems $ bots :: String identity -< printf "Bot positions: %s" botStr where addBot :: Wire IO (Double, Double, Double) (MgrMsg Int IO () Double) addBot = proc (addPeriod, addSpeed, addStart) -> do periodically -< addPeriod botId <- identifier -< () identity -< MgrAdd botId (constant addSpeed >>> integral addStart) The relevant part is the first paragraph of the first arrow computation: botAddPeriod <- succ ^<< noise -< () botAddSpeed <- noise1 -< () botAddStart <- noise1 -< () botMsg <- event addBot -< (botAddPeriod, botAddSpeed, botAddStart) This line should generate a message for the bot manager at random intervals. The actual event generator is in the second arrow computation 'addBot'. I would like to be able to write this more in line with the rest of the code. The following is possible: system :: Wire IO () String system = proc _ -> do botAddPeriod <- succ ^<< noise -< () botAddSpeed <- noise1 -< () botAddStart <- noise1 -< () botMsg <- event (proc (addPeriod, addSpeed, addStart) -> do periodically -< addPeriod botId <- identifier -< () identity -< MgrAdd botId (constant addSpeed >>> integral addStart)) -< (botAddPeriod, botAddSpeed, botAddStart) bots <- manager -< ((), maybe MgrNop id botMsg) let botStr = concatMap (printf "%8.2") . M.elems $ bots :: String identity -< printf "Bot positions: %s" botStr This is probably not a big improvement. It's more concise, but also harder to understand. My proposal is to add syntax to allow the following notation: system :: Wire IO () String system = proc _ -> do botAddPeriod <- succ ^<< noise -< () botAddSpeed <- noise1 -< () botAddStart <- noise1 -< () botMsg <- event $ do periodically -< addPeriod botId <- identifier -< () identity -< MgrAdd botId (constant addSpeed >>> integral addStart) bots <- manager -< ((), maybe MgrNop id botMsg) let botStr = concatMap (printf "%8.2") . M.elems $ bots :: String identity -< printf "Bot positions: %s" botStr Again the relevant part is the event generator in the middle. In this hypothetical syntax, the compiler would figure out from the inner computation, which variables from the outer scope are used and pass them automatically in an appropriate tuple. You wouldn't need any explicit passing anymore. If others like the idea, too, and there is nobody to implement it, then I would be willing to get in touch with the GHC code and implement this as a patch myself. What do you think? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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New: A French translation of Learn You A Haskell for Great Good!
by Valentin ROBERT 22 Sep '11

22 Sep '11

Hello everyone, I've decided to translate the famous Learn You a Haskell for Great Good in French a few weeks ago, and I'm pleased to share it with you all today! Well, especially with the French-speaking community, but I guess everyone should be aware of its existence, in case you ever encounter a French newbie and want to redirect him to some French resources. So here it is: http://lyah.haskell.fr You will also notice that I did register a domain name for this, and will try to provide other information on Haskell in French at the same place. For instance, I created a wiki which contains basic information, available here: http://haskell.fr All French-speaking Haskell enthusiasts (Haskelletors? :D) are welcome to participate in this wiki in any way they can think of. I doubt this will be as useful as the HaskellWiki, but if we can have a few introductions to some subjects in French, pointing to more comprehensive resources in English, I guess it'll be good enough. Oh, I also created the less-inspired logo ever out of the usual Haskell logo! :) Any feedback on whether these are good ideas or not, and what could be done better, would be greatly appreciated. Big thanks to Miran Lipovača for writing the original LYAH (well, except for the jokes that were hard to translate/adapt!), and to the few people on #haskell-fr for somehow deciding me to actually do this. Also, thanks to my anonymous proof-reader, who did a great job I believe! And since Brent allowed me to, French monads are now officially like crêpes! - Valentin "Ptival" Robert

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Re: [Haskell-cafe] Evaluating type expressions in GHCi
by Brandon Moore 22 Sep '11

22 Sep '11

>From: Sean Leather <leather(a)cs.uu.nl> > >I would like to ask GHCi for the type that a type expression will evaluate to, once all definitions of type synonyms and (when possible) type families have been inlined. > > >It appears that I can do some part of this for type T by using ":t undefined :: T": ... >undefined :: F (Int, Bool) :: (Bool, Int) > >I also get what I expect here: > >ghci> :t undefined :: F (a, Bool) >undefined :: F (a, Bool) :: (F a, Int) > >Of course, this doesn't work on types of kinds other than *. In the absence of interpreter support, you can work around that by making appropriate type constructors. {-# LANGUAGE TypeFamilies, KindSignatures, EmptyDataDecls #-} type family G a :: * -> * type instance G Int = Maybe type instance G Bool = [] data Wrap1 (t :: * -> *) ghci> :t undefined :: Wrap1 (G Int) undefined :: Wrap1 (G Int) :: Wrap1 Maybe The development version of ghci seems to support type declarations, which would make this easier. https://github.com/ghc/ghc/commit/3db757241ce7fb99c096c30481aefa86bb9855a1 Brandon

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Haskell Weekly News: Issue 200
by Daniel Santa Cruz 22 Sep '11

22 Sep '11

Welcome to issue 200 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of September 11 to 17, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/09/haskell-weekly-news-issue-200.h… Announcements Chris Smith is exploring the interest of holding a Virtual Hackathon, using Google+ and a Wiki. Come check it out if you would be interested in being part of it! [1] http://goo.gl/lWnea Valentin Robert has completed Apprendre Haskell vous fera le plus grand bien!, a translation of Learn You a Haskell for Great Good! into French, and has also created a wiki page with basic information about Haskell for the French speaking world. Kudos to Valentin for his work on this project! [2] http://goo.gl/ixiXW [3] http://goo.gl/1CG7j New and Updated Projects * netwire (Ertugrul Soeylemez; 1.2.5) Multiple major and minor changes. [4] http://goo.gl/DS4QR * webwire (Ertugrul Soeylemez; 0.1.0) An experimental web framework based on the functional reactive programming library netwire. [5] http://goo.gl/DS4QR * numerals (Roel van Dijk) Contains functions to convert numbers to words in a number of languages. [6] http://goo.gl/4YfhK * splaytree (Jehn Lato; 0.1) Provides splay tree based implementations of Sets, Seqs, and RangeSets. [7] http://goo.gl/aQfX5 * qrcode (Chris Yuen) Intended to be a full featured QR Code library with encoding and decoding and other advanced features. [8] http://goo.gl/pymtr * haskdogs (Sergey Mironov; 0.1) Project-level ctag file generator. [9] http://goo.gl/SA6cx Quotes of the Week * Jafet: Can oleg create a term so complicated that even he could not type-check it? * ion: let es = ["vim", "emacs", "nano"] in map (\e -> intercalate " and " (delete e es) ++ " suck, learn " ++ e) es * cmccann: [on using SomeException] just catch all exceptions, then ignore them. it's the industry-proven way * kmc: une monade est comme une crepe. una monada es como un burrito. eine Monade ist wie ein Strudel. * dylukes: I learned PBMMPPULCDADMABRRRBBBLLTAAMMBBC. It's the operator precedence of C. * benmachine: ozataman: does it have a main, or a thing with type IO thing which looks sort of mainish? Top Reddit Stories * Categorized Weaknesses from the State of Haskell 2011 Survey Domain: nickknowlson.com, Score: 72, Comments: 42 On Reddit: [10] http://goo.gl/0h0K5 Original: [11] http://goo.gl/DZB5V * Haskell B. Curry, born Sept. 12, 1900 Domain: en.wikipedia.org, Score: 69, Comments: 17 On Reddit: [12] http://goo.gl/FYP0F Original: [13] http://goo.gl/S520L * The Records problem in Haskell - help build a consensus about how to fix it, finally Domain: hackage.haskell.org, Score: 59, Comments: 132 On Reddit: [14] http://goo.gl/8cZnI Original: [15] http://goo.gl/VuVoQ * Wrriten in Haskell: The Dynamic Scheduling System for the Green Bank Telescope Domain: self.haskell, Score: 51, Comments: 4 On Reddit: [16] http://goo.gl/vaD3C Original: [17] http://goo.gl/vaD3C * Announcing: Sirkel, a Haskell implementation of the Chord DHT with fault tolerance and replication! Domain: mortenlysgaard.com, Score: 36, Comments: 1 On Reddit: [18] http://goo.gl/VNpSv Original: [19] http://goo.gl/05NIn * Not going to ICFP? Here's an excuse for you Domain: ro-che.info, Score: 30, Comments: 5 On Reddit: [20] http://goo.gl/ABvsq Original: [21] http://goo.gl/uI2Mj * Announce: Yesod 0.9.2 - Everything Just Works Domain: yesodweb.com, Score: 24, Comments: 2 On Reddit: [22] http://goo.gl/izCme Original: [23] http://goo.gl/H0bLr * A play in one act Domain: hpaste.org, Score: 24, Comments: 17 On Reddit: [24] http://goo.gl/OLncN Original: [25] http://goo.gl/lcssv * HaTeX 3 - New version of the Haskell LaTeX library. Domain: deltadiaz.blogspot.com, Score: 20, Comments: 6 On Reddit: [26] http://goo.gl/AgAOp Original: [27] http://goo.gl/4z7nJ * Open Quark: Another haskell like language on JVM. Much more older and mature than Frege. Yet unknown and forgotten. Domain: openquark.org, Score: 19, Comments: 6 On Reddit: [28] http://goo.gl/6Lfx6 Original: [29] http://goo.gl/BI0mC Top StackOverflow Questions * Proving “no corruption” in Haskell votes: 17, answers: 3 Read on SO: [30] http://goo.gl/Xclnk * How to handle feature requests that add new package dependencies votes: 13, answers: 3 Read on SO: [31] http://goo.gl/Gw6HQ * Left and Right Folding over an Infinite list votes: 12, answers: 4 Read on SO: [32] http://goo.gl/YKHlB * Shorter way to write this code votes: 11, answers: 6 Read on SO: [33] http://goo.gl/7ZmBZ * Can't link OpenCL on Windows with GHC votes: 10, answers: 1 Read on SO: [34] http://goo.gl/AnvJo * What's the point of 'const' in the Haskell Prelude? votes: 10, answers: 4 Read on SO: [35] http://goo.gl/g04xE * Haskell functional dependency conflict votes: 9, answers: 2 Read on SO: [36] http://goo.gl/ksmM0 * DatatypeContexts Deprecated in Latest GHC: Why? votes: 9, answers: 2 Read on SO: [37] http://goo.gl/R03pa * Clojure equivalent of Haskell's “Scrap Your Boilerplate” (SYB) votes: 8, answers: 1 Read on SO: [38] http://goo.gl/R6NoS * Why using functions defined in the same module faster than the same function defined in another? votes: 8, answers: 2 Read on SO: [39] http://goo.gl/qAjCk * repeatedly applying a function until the result is stable votes: 8, answers: 4 Read on SO: [40] http://goo.gl/NlP4u About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92438 2. http://lyah.haskell.fr/ 3. http://haskell.fr/ 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92414 5. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92414 6. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92426 7. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18934 8. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92433 9. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92353 10. http://nickknowlson.com/blog/2011/09/12/haskell-survey-categorized-weakness… 11. http://www.reddit.com/r/haskell/comments/kd2ky/categorized_weaknesses_from_… 12. http://en.wikipedia.org/wiki/Haskell_Curry 13. http://www.reddit.com/r/haskell/comments/kcq8m/haskell_b_curry_born_sept_12… 14. http://hackage.haskell.org/trac/ghc/wiki/Records 15. http://www.reddit.com/r/haskell/comments/kgd4g/the_records_problem_in_haske… 16. http://www.reddit.com/r/haskell/comments/ke3ho/wrriten_in_haskell_the_dynam… 17. http://www.reddit.com/r/haskell/comments/ke3ho/wrriten_in_haskell_the_dynam… 18. http://mortenlysgaard.com/?p=35 19. http://www.reddit.com/r/haskell/comments/kilx4/announcing_sirkel_a_haskell_… 20. http://ro-che.info/ccc/13.html 21. http://www.reddit.com/r/haskell/comments/kc4aa/not_going_to_icfp_heres_an_e… 22. http://www.yesodweb.com/blog/2011/09/yesod-0.9.2 23. http://www.reddit.com/r/haskell/comments/kc6h2/announce_yesod_092_everythin… 24. http://hpaste.org/51474 25. http://www.reddit.com/r/haskell/comments/kixek/a_play_in_one_act/ 26. http://deltadiaz.blogspot.com/2011/09/hatex-3-previous-notes.html 27. http://www.reddit.com/r/haskell/comments/kflnp/hatex_3_new_version_of_the_h… 28. http://openquark.org/Welcome.html 29. http://www.reddit.com/r/haskell/comments/kgtxm/open_quark_another_haskell_l… 30. http://stackoverflow.com/questions/7435363/proving-no-corruption-in-haskell 31. http://stackoverflow.com/questions/7436939/how-to-handle-feature-requests-t… 32. http://stackoverflow.com/questions/7396978/left-and-right-folding-over-an-i… 33. http://stackoverflow.com/questions/7453744/shorter-way-to-write-this-code 34. http://stackoverflow.com/questions/7391634/cant-link-opencl-on-windows-with… 35. http://stackoverflow.com/questions/7402528/whats-the-point-of-const-in-the-… 36. http://stackoverflow.com/questions/7436109/haskell-functional-dependency-co… 37. http://stackoverflow.com/questions/7438600/datatypecontexts-deprecated-in-l… 38. http://stackoverflow.com/questions/7376572/clojure-equivalent-of-haskells-s… 39. http://stackoverflow.com/questions/7401724/why-using-functions-defined-in-t… 40. http://stackoverflow.com/questions/7442892/repeatedly-applying-a-function-u…

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Monad Laws and Do Notation
by diazepan 22 Sep '11

22 Sep '11

I've got this expression expression = do w <- "hello" y <- "to you" return w I wanna know how can I reduce it using monad laws -- View this message in context: http://haskell.1045720.n5.nabble.com/Monad-Laws-and-Do-Notation-tp4828598p4… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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