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Pattern variables in Template Haskell
by Alejandro Serrano Mena 03 Nov '14

03 Nov '14

Dear Haskell-cafe, I'm trying to use Template Haskell to generate patterns in Haskell code, but after a couple of days of trying, I have not come with a solution. I hope somebody could help me. My problem is basically that I want something like: example :: Maybe a -> a example $(j "x") = x to work correctly. The idea of this simple quoter is to generate code similar to: example :: Maybe a -> a example (Just x_123) = x_123 that is, to create the "Just" pattern, and to make "x" bind to the same element. Right now my code reads: j :: String -> Q Pat j s = return (ConP 'Just [VarP (mkName s)]) which generated a new variable "x" each time, thus unable to match with the "x" in the RHS. I have also tried with lookupValueName. Note that my problem is a bit more complicated, with variable names coming from a different source. However, I think that if I solve this problem where I can make variables in a pattern and in an expression match, I should be able to use them in my complete solution. Thanks in advance.

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Nomyx (Beta 7): win 10000 Blings!
by Corentin Dupont 03 Nov '14

03 Nov '14

Join us on Nomyx: www.nomyx.net It's a unique game where you can change the rules of the game, while playing it. Log in, and you'll understand better what the title means (check rule 44 proposed by me in the game)! Cheers, Corentin

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ANN: Released new version of Haskell plugin for IntelliJ
by Rik van der Kleij 03 Nov '14

03 Nov '14

Hi everyone, I'm glad to announce version 0.2. It's available in Jetbrains plugin repository. Besides improvements and minor changes, version 0.2 has support for HLint inspection and quick fixes based on HLint suggestions. For more information, see change notes. Complete feature list is now: - Syntax highlighting (which can be customized); - Error/warning highlighting; - Find Usages of identifiers; - Resolving references of identifiers (also to library code if library source code is added to project and resolves inside import declaration); - Code completion by resolving references; - Renaming variables (which first shows preview so refactoring scope can be adjusted); - View type info from (selected) expression; - View expression info; - View quick documentation; - View quick definition; - Structure view; - Navigate to declaration (called `Class` in IntelliJ menu); - Navigate to identifier (called `Symbol` in IntelliJ menu); - Code completion by looking to import declarations; - Simple form of code formatting; - Inspection by HLint; - Quick fixes for HLint suggestions; - View error, action to view formatted message from ghc-mod. Especially useful in case message consists of multiple lines (Ctrl-F10, Meta-F10 on Mac OSX); See for more information and getting started: https://github.com/rikvdkleij/intellij-haskell Any feedback is welcome! Regards, Rik

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<|> that short-circuits in IO ?
by Semen Trygubenko / Семен Тригубен ко 02 Nov '14

02 Nov '14

Dear Haskell-Cafe, Is there a way to get Control.Applicative.<|> to short-circuit when combining two IO actions? E.g. > let a = putStrLn "A" >> return (Left "hehe") > let b = putStrLn "B" >> return (Right 42) > Prelude> liftA2 (<|>) a b A B Right 42 > liftA2 (<|>) b a B A Right 42 (In the latter case I don't want A in the output…) Many thanks for your help, Semen -- Семен Тригубенко http://trygub.com

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How to use randomized algorithm within the implementation of pure data structures?
by Hiromi ISHII 01 Nov '14

01 Nov '14

Hi cafe, I'm currently implementing the data structure representing algebraic numbers. Implementing algebraic number arithmetic requires polynomial factorisation. Pure algorithm for factoring (Berlekamp's algorithm) is more expensive than the randomized one (Cantor-Zassenhaus algorithm) , so I want to use the latter algorithm. Since C-Z algorithm is a randomized algorithm, we have to have an access for random number generator when calculating algebraic number arithmetics e.g. writing Num instance for algebraic numbers. Here is the problem: how to pass-around random number generator throughout pure computaion? I think one immediate solution is to create global state with `newStdGen` and `unsafePerformIO` like below: ``` randSrc :: IORef StdGen randSrc = unsafePerformIO $ newIORef =<< newStdGen {-# NOINLINE randSrc #-} getRand :: IO Int getRand = atomicModifyIORef' randSrc (swap . next) -- We can probably use the following function to avoid calling -- `unsafePerformIO` whenever calling 'getRand`, -- assuming some optimization flag enabled: getRandUnsafe :: Int getRandUnsafe = unsafePerformIO getRand {-# NOINLINE getRandUnsafe #-} {-# RULES "getRandUnsafe" getRandUnsafe = unsafePerformIO getRand #-} ``` But this hack seems rather dirty and unsafe. Is there any workaround to achieve the same thing? -- Hiromi ISHII konn.jinro(a)gmail.com

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Hackage inconsistent? (pandoc.cabal in 1.13.1)
by Magnus Therning 31 Oct '14

31 Oct '14

It seems pandoc 1.13.1 package on hackage has gotten into an insonsistent state. I just noticed this strange situation with the dependency on http-client: pandoc.cabal at [1] has if flag(https) Build-Depends: http-client >= 0.3.2 && < 0.5, http-client-tls >= 0.2 && < 0.3, http-types >= 0.8 && < 0.9 cpp-options: -DHTTP_CLIENT pandoc.cabal found in [2] has if flag(https) Build-Depends: http-client >= 0.3.2 && < 0.4, http-client-tls >= 0.2 && < 0.3, http-types >= 0.8 && < 0.9 cpp-options: -DHTTP_CLIENT How can something like this even happen? /M [1]: http://hackage.haskell.org/package/pandoc-1.13.1/pandoc.cabal [2]: http://hackage.haskell.org/package/pandoc-1.13.1/src/pandoc.cabal -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Any fool can write code that a computer can understand. Good programmers write code that humans can understand. -- Martin Fowler

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Getting the haddocks back (was: documentation build failing in hackage?)
by Rogan Creswick 30 Oct '14

30 Oct '14

On Sun, Oct 12, 2014 at 4:50 AM, Mateusz Kowalczyk <fuuzetsu(a)fuuzetsu.co.uk> wrote: > > The biggest problem is that there is no official statement as to why the > builds stopped and an estimate of when they are coming back. > I agree! My understanding (which is at /least/ 2nd or 3rd hand) is that the doc builds were turned off intentionally because it was a security issue, and that they are unlikely to come back. Now, assuming that is the case, how can we solve the "there is no documentation" issue? Some ideas to kick-start discussion: - Provide an option to include haddocks in the sdist bundle, and extract them on Hackage for display. - Add a 'cabal uploadHaddock' - Run all haddock builds in a VM/docker container / etc.. that mitigates the security concerns. - ??? --Rogan

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Haskell Weekly News: Issue 311
by Daniel Santa Cruz 30 Oct '14

30 Oct '14

Welcome to issue 311 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from October 19 to 25, 2014 Quotes of the Week * trap_exit: isn't darcs this haskell powered quantum physics simulator that happens to also keep revisions of files? Top Reddit Stories * "GHC uses GMP, and its performance is nothing short of extraordinary. GHC is clearly tuned to call into GMP with extremely low overheads." Domain: wilfred.me.uk, Score: 79, Comments: 26 Original: [1] http://goo.gl/n6tXA3 On Reddit: [2] http://goo.gl/zTH1v5 * Video games in Haskell: Chips Challenge demo Domain: vimeo.com, Score: 73, Comments: 22 Original: [3] http://goo.gl/EyUQ6t On Reddit: [4] http://goo.gl/8Lrj3I * HLint now spots bad unsafePerformIO Domain: neilmitchell.blogspot.it, Score: 56, Comments: 29 Original: [5] http://goo.gl/sxFywP On Reddit: [6] http://goo.gl/nA0rvs * Seemingly impossible functional programs Domain: math.andrej.com, Score: 56, Comments: 15 Original: [7] http://goo.gl/gtXO9 On Reddit: [8] http://goo.gl/XdTGd7 * New blog post: "Quasi-quoting DSLs for free" Domain: well-typed.com, Score: 55, Comments: 2 Original: [9] http://goo.gl/QbblTr On Reddit: [10] http://goo.gl/hv43hU * Flocking behaviour simulation written using helm. Source in comments. Domain: gfycat.com, Score: 46, Comments: 9 Original: [11] http://goo.gl/MegczW On Reddit: [12] http://goo.gl/aKma9A * Finished reading Learn You a Haskell, please recommend me book number 2: Domain: self.haskell, Score: 41, Comments: 24 Original: [13] http://goo.gl/uQiplX On Reddit: [14] http://goo.gl/uQiplX * Simple Haskell IRC client in "two lines of code" Domain: self.haskell, Score: 40, Comments: 62 Original: [15] http://goo.gl/7SQpZL On Reddit: [16] http://goo.gl/7SQpZL * GPU Programming in Haskell (video) Domain: looprecur.com, Score: 36, Comments: 0 Original: [17] http://goo.gl/7N0rgb On Reddit: [18] http://goo.gl/kjuFle * Are there ANY good game engines for haskell? Domain: self.haskell, Score: 33, Comments: 38 Original: [19] http://goo.gl/6ugfSM On Reddit: [20] http://goo.gl/6ugfSM * Darcs turns 12 Domain: hub.darcs.net, Score: 30, Comments: 8 Original: [21] http://goo.gl/tnNsyR On Reddit: [22] http://goo.gl/DXIYyW Top StackOverflow Questions * Dependent Types: How is the dependent pair type analogous to a disjoint union? votes: 20, answers: 6 Read on SO: [23] http://goo.gl/HOSXJO * How should I read this GHC Core “proof”? votes: 11, answers: 0 Read on SO: [24] http://goo.gl/3btTZT * more efficient type-level computations using type families? votes: 11, answers: 0 Read on SO: [25] http://goo.gl/LHYecG * How to make fmap rewrite rules fire? votes: 10, answers: 1 Read on SO: [26] http://goo.gl/JrjnJH * A workaround for the “Template Haskell + C” bug? votes: 8, answers: 1 Read on SO: [27] http://goo.gl/q9sIHZ * Transform a -> a -> Maybe a to take any combination of a's and Maybe a's votes: 8, answers: 2 Read on SO: [28] http://goo.gl/AS4kC0 * Making numeric functions an instance of Num? votes: 7, answers: 2 Read on SO: [29] http://goo.gl/6uNuWB * Inverting a Type Family votes: 7, answers: 2 Read on SO: [30] http://goo.gl/18XHro * FRP frameworks and IO votes: 7, answers: 1 Read on SO: [31] http://goo.gl/BbEWOM Until next time, [32]+Daniel Santa Cruz References 1. http://www.wilfred.me.uk/blog/2014/10/20/the-fastest-bigint-in-the-west/ 2. http://www.reddit.com/r/haskell/comments/2jt37u/ghc_uses_gmp_and_its_perfor… 3. https://vimeo.com/109613639 4. http://www.reddit.com/r/haskell/comments/2jwwlj/video_games_in_haskell_chip… 5. http://neilmitchell.blogspot.it/2014/10/hlint-now-spots-bad-unsafeperformio… 6. http://www.reddit.com/r/haskell/comments/2jsa4c/hlint_now_spots_bad_unsafep… 7. http://math.andrej.com/2007/09/28/seemingly-impossible-functional-programs/ 8. http://www.reddit.com/r/haskell/comments/2k0xg2/seemingly_impossible_functi… 9. http://www.well-typed.com/blog/2014/10/quasi-quoting-dsls/ 10. http://www.reddit.com/r/haskell/comments/2jowjc/new_blog_post_quasiquoting_… 11. http://gfycat.com/WearyAnguishedIggypops 12. http://www.reddit.com/r/haskell/comments/2k51kk/flocking_behaviour_simulati… 13. http://www.reddit.com/r/haskell/comments/2kbnzw/finished_reading_learn_you_… 14. http://www.reddit.com/r/haskell/comments/2kbnzw/finished_reading_learn_you_… 15. http://www.reddit.com/r/haskell/comments/2jvc78/simple_haskell_irc_client_i… 16. http://www.reddit.com/r/haskell/comments/2jvc78/simple_haskell_irc_client_i… 17. http://looprecur.com/blog/gpu-programming-in-haskell/ 18. http://www.reddit.com/r/haskell/comments/2jp9y9/gpu_programming_in_haskell_… 19. http://www.reddit.com/r/haskell/comments/2k93sm/are_there_any_good_game_eng… 20. http://www.reddit.com/r/haskell/comments/2k93sm/are_there_any_good_game_eng… 21. http://hub.darcs.net/darcs/darcs-screened/patch/20021020200105-e9342 22. http://www.reddit.com/r/haskell/comments/2jsvnd/darcs_turns_12/ 23. http://stackoverflow.com/questions/26542350/dependent-types-how-is-the-depe… 24. http://stackoverflow.com/questions/26524223/how-should-i-read-this-ghc-core… 25. http://stackoverflow.com/questions/26538595/more-efficient-type-level-compu… 26. http://stackoverflow.com/questions/26568262/how-to-make-fmap-rewrite-rules-… 27. http://stackoverflow.com/questions/26449154/a-workaround-for-the-template-h… 28. http://stackoverflow.com/questions/26520261/transform-a-a-maybe-a-to-take-a… 29. http://stackoverflow.com/questions/26515102/making-numeric-functions-an-ins… 30. http://stackoverflow.com/questions/26541893/inverting-a-type-family 31. http://stackoverflow.com/questions/26560100/frp-frameworks-and-io 32. https://plus.google.com/105107667630152149014/about

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Polymorphic functions over string libraries
by gonzaw 29 Oct '14

29 Oct '14

Hi. I was wondering what would be the best way to create a polymorphic function over any possible string library (Text, String, Bytestring, etc). For instance, imagine I have to read a file with text, transform this text in some way and output it to another file, or to the console. If I wanted to use String, I'd just do this: / transform :: String -> String main = readFile "input.txt" >>= writeFile "output.txt" . transform / But if I wanted to use Text instead, I'd have to use this: / import qualified Data.Text.IO as T transform :: Text -> Text main = T.readFile "input.txt" >>= T.writeFile "output.txt" . transform / Idem for ByteString. I was wondering if there was a way to create these computations in a generic way, for any kind of string library, something like this: / class StringLibrary s where: sReadFile :: FilePath -> IO s sWriteFile :: FilePath -> s -> IO () ... / So then I'd just have this: / transform :: StringLibrary s => s -> s main = sReadFile "input.txt" >>= sWriteFile "output.txt" . transform / Now I can perform the computation I want without being tied down to a specific library. At times when I create some quick scripts, I find myself using one library (for example using String to get it finished more quickly, since I have less experience with the other ones), but find that it's too slow or has some problem that is solved by using one of the other libraries. Yet swapping from one to the other is more cumbersome than expected at times. In the example above, I could easily swap between them, just by forcing the compiler to typecheck to a specific one (for instance by changing the type of "transform"). Or if I wanted to, I could leave it as it is and export it as a library of my own. Is there a way to do something like this in Haskell, with existing libraries? In terms of using the string datatype as some sort of container of characters, I think there are libraries like Lens and mono-traversable that allow you to do stuff like this. But I'm not too familiar with them (at least using them in this way). -- View this message in context: http://haskell.1045720.n5.nabble.com/Polymorphic-functions-over-string-libr… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Munich Haskell Meeting
by Heinrich Hördegen 29 Oct '14

29 Oct '14

Dear all, tomorrow, the 30th of October, our monthly Munich Haskell Meeting will take place again at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please click the button! Everybody is welcome! Heinrich

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