- Haskell-Cafe - Haskell.org

Haskell showcase in 5 minutes - follow-up
by Arnaud Bailly 01 Mar '12

01 Mar '12

Hello Cafe, Thanks to all of you who provided ideas and suggestions for introducing Haskell. I finally went the simple way, presenting the "knapsack" problem slightly reframed using tapas (spanish dishes). This problem is small enough to fit in the 5 minutes time-frame and it actually illustrates some of the salient points of the langauge: laziness of course, list-comprehensions, monadic computations (replicateM), first-class and higher-order functions, the REPL, integration with emacs, command-line compiling. Here is the code https://gist.github.com/1948301 I got a lot of positive feedback during the evening as I did some more live coding on simple problems (fizz-buzz, fibonacci) and I ended up showing some code I wrote doing "real stuff", notably the simple music synthesizer I wrote (https://github.com/abailly/haskell-synthesizer) for presenting Haskell to Paris Scala user Group and inspired of course by Paul Hudak's book and Joao Pizani's blog ( http://joaopizani.hopto.org/en/2012/01/haskell-synth/) The meeting ended with an impromptu panel on the disadvantages of the languages presented by each speaker which was really hard for me given the sheer perfection of Haskell. One advantage I mentionned somewhat indirectly which I should stress here is the Haskell community: reactive, vibrant and kind are a few adjectives that spring to my mind. Thanks again for your support, Arnaud Bailly

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ANN: stm-chans 1.3.1
by wren ng thornton 01 Mar '12

01 Mar '12

-------------------------------------------- -- stm-chans 1.3.1 -------------------------------------------- The stm-chans package offers a collection of channel types, similar to Control.Concurrent.STM.TChan but with additional features. -------------------------------------------- -- Changes (since 1.2.0) -------------------------------------------- * (Version 1.3.1) The new stm-2.3 has finally been released with all the new optimized operations on TVars, TMVars, and TChans. I've updated the CPP macros in the Compat modules so that now they will actually switch over to the optimized implementations (instead of waiting for stm-9.0 :) It's highly recommended that all users bump their minimum stm-chans requirement to version 1.3.1. * (Version 1.3.0) Added Control.Concurrent.STM.TMVar.Compat for the stm-2.3 function tryReadTMVar. -------------------------------------------- -- Long description -------------------------------------------- In particular stm-chans offers these types: * Control.Concurrent.STM.TBChan: Bounded FIFO channels. When the channel is full, writers will block/retry. This ensures that the writers do not get too far ahead of the readers, which helps to make sure that memory and cpu resources are used responsibly. * Control.Concurrent.STM.TMChan: Closeable FIFO channels. This is like TChan (Maybe a) but with a monotonicity guarantee that once Nothing is returned all future reads will be Nothing as well. * Control.Concurrent.STM.TBMChan: Bounded Closeable FIFO channels. This combines the capabilities of TBChan and TMChan. In addition, the stm-chans package offers a (partial) compatibility layer for the recent API improvements to the stm package (see ticket #5104 [1]). Among others, these new functions include: * tryReadTChan :: TChan a -> STM (Maybe a) A version of readTChan which does not retry. Instead it returns Nothing if no value is available. * peekTChan :: TChan a -> STM a Get the next value from the TChan without removing it, retrying if the channel is empty. * tryPeekTChan :: TChan a -> STM (Maybe a) A version of peekTChan which does not retry. Instead it returns Nothing if no value is available. [1] http://hackage.haskell.org/trac/ghc/ticket/5104 -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/stm-chans Darcs: http://community.haskell.org/~wren/stm-chans Haddock (Darcs version): http://community.haskell.org/~wren/stm-chans/dist/doc/html/stm-chans -- Live well, ~wren

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Haskell Weekly News: Issue 217
by Daniel Santa Cruz 01 Mar '12

01 Mar '12

Welcome to the 217th issue of the HWN, a newsletter covering developments in the Haskell community during the week of February 19 to 25, 2012. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-217.h… Quotes of the Week * nand`: oh, wait, ANSI is the physical layout * cmccann: some programmers have a problem to solve and think "I know, I need an existential type." now they have another problem, but can't solve it because all they know about it is that the problem exists. * Philippa: The only data type is Nat! (Which succs). * hpc: peano arithmetic SSSSZ life * cmccann: unicode snowman is a valid identifier in Haskell, BTW * Jafet: Haskell is an imperatively functional abstract enterprise programming language template * Cale: Array is immutable boxed Cale: UArray is immutable unboxed Cale: IOArray is mutable boxed elliott: IOUArray is an array of debts. * c_wraith: It [C] frees you to use whatever data structure you want, by providing you none! * Brandon Allbery: If you absolutely insist on shooting yourself in the foot this way in Haskell, it can be done; but there is no reason Haskell should make it *easy* to do so. That's C's superpower. :) Top Reddit Stories * ANNOUNCE: Sodium, a new Haskell FRP (Reactive Programming) system Domain: hackage.haskell.org, Score: 41, Comments: 38 On Reddit: [1] http://goo.gl/IY2a1 Original: [2] http://goo.gl/OOpl1 * Adventures in Parsec - Parsing logs in Haskell Domain: variadic.me, Score: 31, Comments: 24 On Reddit: [3] http://goo.gl/pkHdy Original: [4] http://goo.gl/fDRdE * Wanted: an efficient union algorithm for hash-array mapped tries Domain: blog.johantibell.com, Score: 30, Comments: 13 On Reddit: [5] http://goo.gl/p7n4c Original: [6] http://goo.gl/4XCui * How reliable is Hackage supposed to be? Domain: self.haskell, Score: 29, Comments: 20 On Reddit: [7] http://goo.gl/qfqJ3 Original: [8] http://goo.gl/qfqJ3 * Mathematics in the Age of the Turing Machine -- I recommend starting in section 2 Domain: math.pitt.edu, Score: 28, Comments: 1 On Reddit: [9] http://goo.gl/T4jGx Original: [10] http://goo.gl/9SmzX * Conduits: Simplifying ResourceT Domain: yesodweb.com, Score: 28, Comments: 6 On Reddit: [11] http://goo.gl/PW0C2 Original: [12] http://goo.gl/9a5Dz * Clarifying a mis-understanding about regions (and iteratees) Domain: haskell.org, Score: 27, Comments: 4 On Reddit: [13] http://goo.gl/P3sVE Original: [14] http://goo.gl/WWjUb * Concatenative Haskell Part II: Custom DSL Syntax Using QuasiQuotes Domain: github.com, Score: 25, Comments: 6 On Reddit: [15] http://goo.gl/DP0bY Original: [16] http://goo.gl/Lvlnc * Designing a serialization API Domain: blog.johantibell.com, Score: 24, Comments: 1 On Reddit: [17] http://goo.gl/SFMTx Original: [18] http://goo.gl/vcdsS * Getting Started with Disciple Core Domain: disciple.ouroborus.net, Score: 19, Comments: 1 On Reddit: [19] http://goo.gl/FCPla Original: [20] http://goo.gl/7J8oN * hs-logo: Logo turtle graphics interpreter written in Haskell Domain: deepakjois.github.com, Score: 18, Comments: 5 On Reddit: [21] http://goo.gl/wRIqZ Original: [22] http://goo.gl/JE37a * The Simply-typed Lambda Calculus with Constraints Domain: flippac.wordpress.com, Score: 16, Comments: 3 On Reddit: [23] http://goo.gl/ZNNB1 Original: [24] http://goo.gl/QbT6a * Data Structure Agnostic JSON Serialization Domain: blog.melding-monads.com, Score: 14, Comments: 1 On Reddit: [25] http://goo.gl/9QD9o Original: [26] http://goo.gl/WsPtb Top StackOverflow Questions * Evaluation strategy votes: 19, answers: 4 Read on SO: [27] http://goo.gl/rcJyz * Guards vs. if-then-else vs. cases in Haskell votes: 18, answers: 2 Read on SO: [28] http://goo.gl/BmH0X * Division by zero in Haskell votes: 15, answers: 4 Read on SO: [29] http://goo.gl/a0cER * Memory efficient strings in Haskell votes: 15, answers: 2 Read on SO: [30] http://goo.gl/4kUdm * What prevents an efficient Haskell virtual machine (like JVM)? votes: 12, answers: 4 Read on SO: [31] http://goo.gl/RphqH * Is indexing of Data.Vector.Unboxed.Mutable.MVector really this slow? votes: 11, answers: 2 Read on SO: [32] http://goo.gl/72fhd * Haskell: filtering a heterogenous list by type votes: 10, answers: 2 Read on SO: [33] http://goo.gl/uFm1I * ghci tab-completion in haskell-mode votes: 10, answers: 1 Read on SO: [34] http://goo.gl/QaAo5 * Non-maintainer uploads to Hackage votes: 10, answers: 2 Read on SO: [35] http://goo.gl/lVFHN * withFile vs. openFile votes: 9, answers: 4 Read on SO: [36] http://goo.gl/KQ6Fu Until next time, Daniel Santa Cruz References 1. http://hackage.haskell.org/package/sodium 2. http://www.reddit.com/r/haskell/comments/pyxpx/announce_sodium_a_new_haskel… 3. http://variadic.me/posts/2012-02-24-adventures-in-parsec.html 4. http://www.reddit.com/r/haskell/comments/q41rr/adventures_in_parsec_parsing… 5. http://blog.johantibell.com/2012/02/wanted-efficient-union-algorithm-for.ht… 6. http://www.reddit.com/r/haskell/comments/q0a02/wanted_an_efficient_union_al… 7. http://www.reddit.com/r/haskell/comments/pwjc9/how_reliable_is_hackage_supp… 8. http://www.reddit.com/r/haskell/comments/pwjc9/how_reliable_is_hackage_supp… 9. http://www.math.pitt.edu/~thales/papers/turing.pdf 10. http://www.reddit.com/r/haskell/comments/pwx2b/mathematics_in_the_age_of_th… 11. http://www.yesodweb.com/blog/2012/02/simplifying-resourcet 12. http://www.reddit.com/r/haskell/comments/q37rf/conduits_simplifying_resourc… 13. http://www.haskell.org/pipermail/haskell-cafe/2012-February/099689.html 14. http://www.reddit.com/r/haskell/comments/q293l/clarifying_a_misunderstandin… 15. https://github.com/leonidas/codeblog/blob/master/2012/2012-02-21-concatenat… 16. http://www.reddit.com/r/haskell/comments/pzh53/concatenative_haskell_part_i… 17. http://blog.johantibell.com/2012/02/designing-serialization-api.html 18. http://www.reddit.com/r/haskell/comments/q0kcv/designing_a_serialization_ap… 19. http://disciple.ouroborus.net/wiki/Tutorial/Core 20. http://www.reddit.com/r/haskell/comments/q1url/getting_started_with_discipl… 21. http://deepakjois.github.com/hs-logo/ 22. http://www.reddit.com/r/haskell/comments/pzf9k/hslogo_logo_turtle_graphics_… 23. http://flippac.wordpress.com/2012/02/23/the-simply-typed-lambda-calculus-wi… 24. http://www.reddit.com/r/haskell/comments/q2ga5/the_simplytyped_lambda_calcu… 25. http://blog.melding-monads.com/2012/02/24/data-structure-agnostic-json-seri… 26. http://www.reddit.com/r/haskell/comments/q3t70/data_structure_agnostic_json… 27. http://stackoverflow.com/questions/9439470/evaluation-strategy 28. http://stackoverflow.com/questions/9345589/guards-vs-if-then-else-vs-cases-… 29. http://stackoverflow.com/questions/9354016/division-by-zero-in-haskell 30. http://stackoverflow.com/questions/9398572/memory-efficient-strings-in-hask… 31. http://stackoverflow.com/questions/9352744/what-prevents-an-efficient-haske… 32. http://stackoverflow.com/questions/9409634/is-indexing-of-data-vector-unbox… 33. http://stackoverflow.com/questions/9396023/haskell-filtering-a-heterogenous… 34. http://stackoverflow.com/questions/9431779/ghci-tab-completion-in-haskell-m… 35. http://stackoverflow.com/questions/9435513/non-maintainer-uploads-to-hackage 36. http://stackoverflow.com/questions/9406463/withfile-vs-openfile

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Haskell showcase in 5 minutes
by Arnaud Bailly 29 Feb '12

29 Feb '12

Hello Cafe, I will be (re)presenting Haskell in a "Batlle Language" event Wednesday evening: A fun and interactive contest where various programming language champions try to attract as much followers as possible in 5 minutes. Having successfully experimented the power of live coding in a recent Haskell introduction for the Paris Scala User Group, I would like to do the same but given the time frame I need a simpler example than the music synthesizer program. So I would like to tap in the collective wisdom looking for some concise, eye-opening, mind-shaking and if possible fun example of what one can achieve in Haskell. Things that sprung to my mind are rather dull: prime factors, fibonacci numbers. Thanks in advance, Arnaud

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question about conduit source
by Alexander V Vershilov 29 Feb '12

29 Feb '12

Hello, cafe. Is it possible to read data from different concurrent sources, i.e. read data from source as soon as it become avaliable, e.g. runResourceT $ (source1 stdin $= CL.map Left) >=> (source2 handle $= CL.map Right) $= application $$ sink where >=> - stands for concurrent combining of sources It would be good if it can be sources of different types (handle or STM channel, etc..). Currently I've found no good way to handle with this situation, except of using STM Channels for collecting data source1 ---+ | | sink | output sink +---] Channel [-------> application----->] | source source2 ---+ | From this point of view application takes concurent data, but this implementation requires additional thread per data processing. Also in many cases it will require run additional runResourceT (see later example). So if there any possible simplifications? Or ideas how to make (>=>) operator. Example: So I've got next code in my network-conduit based application: main :: IO () main = do pool <- createDBPool "..." 10 let r = ServerInit pool forkIO $ forever clientConsole --read channel list and send "Left" flip runReaderT r $ runTCPServer (ServerSettings 3500 Nothing) (protoServer) myServer src sink = do ch <- liftIO $ atomically $ newTBMChan 16 initState <- lift $ ask _ <- liftIO $ fork . (flip runReaderT initState) $ runResourceT $ src $= C.sequence decode $= CL.map Right $$ sinkTBMChan ch sourceTBMChan ch $= process $= C.sequence encode $$ sinkHandle stdout But in this situation I don't know if freeing of all resources are guaranteed, because I'm running additional resourceT in main resourceT scope. So can you advice is it possible to make concurrent sources now with currenly implemented library? If it's not possible but worth of implementing, so I can make that functions? Is it correct to runResourceT inside another resourceT? -- Best regards, Alexander V Vershilov

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Use Template Haskell recover inside the IO monad
by Federico Mastellone 28 Feb '12

28 Feb '12

I'm new to Template Haskell and just started playing with it. I have Haskell platform 2011.4.0 and I get this error when I try to run this simple code. main = do code <- runQ $ recover (return []) th putStrLn (pprint code) putStrLn (show code) th :: Q [Dec] th = [d| sum a b = a + b |] Template Haskell error: Can't do `recover' in the IO monad POC.hs: user error (Template Haskell failure) How can I get declarations with the "recover" function so I can print a custom error message using "location"? If I can't do it inside the IO monad, how can I do a program to pretty print Haskell code? Thanks!

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Re: [Haskell-cafe] GSoc-2012: Lock-free Data Structures
by Sergiu Ivanov 28 Feb '12

28 Feb '12

Hello, On Tue, Feb 21, 2012 at 8:11 AM, Ryan Newton <rrnewton(a)gmail.com> wrote: > > Oops, I've attached the PDF. It's a good overview that includes a bunch of > the key ideas that appear in recent work in the area. Very cool article, thank you! I genuinely enjoyed reading it :-) > Absolutely! Feel free to make a pull request. I'm going to play with the existing structures first. However, I can't see a list of issues anywhere. Could you suggest a place where I could look for indications of some issues which I could fix for starters? Sergiu P.S. Sorry for my sluggishness :-(

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Vim plugin for ghc-mod
by Kazu Yamamoto 28 Feb '12

28 Feb '12

Hello, eagletmt implemented a Vim plugin for ghc-mod: https://github.com/eagletmt/ghcmod-vim Happy Haskell programming on Vim! --Kazu

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Haskell showcase in 5 minutes
by Doug McIlroy 28 Feb '12

28 Feb '12

Sorry, a typo in the url for the power-series example. It should have been http://www.cs.dartmouth.edu/~doug/powser.html

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Haskell showcase in 5 minutes
by Doug McIlroy 28 Feb '12

28 Feb '12

Here's an example that fits comfortably in 5 minutes--if your audience knows elementary calculus: http://www.cs.dartmouth.edu/~doug/powswer.html It depends critically on lazy evaluation, which knocks out a lot of competing languages right from the start. The five-minute version would begin with power-series addition--trivial. Then comes multiplication--an eye-opener. No subscripts! No worry about how many terms to carry in intermediate results. That's about all you have time to really derive. Go on to mention that division is about equally easy. Then allude to substitution (also called composition) and its inverse, reversion. Lots of finicky papers have been written about reversion over more than two centuries. Throw that one-liner on the screen side-by-side with Algorithm S (which is only pseudocode!) from Knuth section 4.7. That should convince the most skeptical observer. Doug

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