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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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ghc 7.0.3 view patterns and exhaustiveness
by Richard Cobbe 22 Sep '11

22 Sep '11

I'm starting to play around with GHC's support for view patterns, and I'm running into what appears to be an annoying limitation of the implementation. GHC 7.0.3 (32-bit), MacOS 10.6.8. First module; defines an abstract type & provides a (trivial) view for it. module Term(Term, TermView(..), view) where data Term = TVar String | TApp Term Term | TLam String Term data TermView = Var String | App Term Term | Lam String Term view :: Term -> TermView view (TVar x) = Var x view (TApp rator rand) = App rator rand view (TLam x body) = Lam x body Second module tries to use the view in a trivial function: {-# LANGUAGE ViewPatterns #-} module Client where import Term numVarRefs :: Term -> Integer numVarRefs (view -> Var _) = 1 numVarRefs (view -> App rator rand) = numVarRefs rator + numVarRefs rand numVarRefs (view -> Lam _ body) = numVarRefs body -- numVarRefs (view -> _) = error "bogus" f :: TermView -> Integer f (Var _) = 1 f (App rator rand) = f (view rator) + f (view rand) f (Lam _ body) = f (view body) GHCI complains when trying to load this second module: Client.hs:8:1: Warning: Pattern match(es) are non-exhaustive In an equation for `numVarRefs': Patterns not matched: _ (I have ":set -fwarn-incomplete-patterns" in my .ghci.) I wrote 'f' to make sure that my patterns for TermView are indeed exhaustive, and GHC doesn't complain about it all. If I uncomment the last definition for numVarRefs, the warning goes away. I did some searching around on the web, in the mailing list archives, and in the GHC bug database, and I see that early on, views had trouble giving useful diagnostics for overlapping or non-exhaustive patterns, but most of those problems seem to have been fixed. I also couldn't find a bug report for precisely this situation -- #4439 is the closest, but I'm not using existential types here at all. Should I file a bug, or am I overlooking something simple? Or is this a known limitation of the current implementation? Thanks, Richard

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stack overflow pain
by Tim Docker 21 Sep '11

21 Sep '11

I'm getting a stack overflow exception in code like this: -- applyAction :: A -> IO [B] .... vs <- fmap concat $ mapM applyAction sas return vs I don't get it if I change the code to this: -- applyAction :: A -> IO [B] .... mapM_ applyAction sas return [] But of course, I need the results from the actions. I know that the returned list contains approximately 1 million items. Any suggestions on how I should rewrite the first code snippet to not blow the stack? I do find debugging stack overflow errors quite difficult - with little information from the runtime I'm often left guessing which parts of a large codebase might be causing them. Note that there's plenty of heap space available, it's the evaluation stack that is being used up. I could run with -K to increase the size of the stack, but if possible I'd rather solve this in the code. Thanks for any pointers. Tim

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hackage library info
by kioto mitsubisi 21 Sep '11

21 Sep '11

Hi, I want to ask a question about hackage which contains additional libraries to Haskell. Is there a mechanism that will help developer to choose a library among the others? I mean, there are several packages that may do the same thing in hackage and to choose one among the others, developer may want to have more information about the library such as other developers' ratings and comments, activity of development of the library, etc so that one can choose the better alternative. Is there a mechanism for that or is there a plan to add these to the hackage? Thanks

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poles/residues -> FIR tap weights ?
by Captain Freako 21 Sep '11

21 Sep '11

Anyone know of a Haskell package containing a function for converting a list of pole/residue pairs into FIR filter tap weights? Thanks, -db

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