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Haskell Weekly News: Issue 206
by Daniel Santa Cruz 03 Nov '11

03 Nov '11

Welcome to issue 206 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of October 23 to 29, 2011. A HTML version of this issue can be found at: http://contemplatecode.blogspot.com/2011/11/haskell-weekly-news-issue-206.h… Announcements Edward Kmett extended an invitation to the Hac Boston, to be held between Jan 20 to 22, 2012 at MIT in Cambridge, MA. [1] http://goo.gl/cZpvu Brent Yorgey has released the lastest issue of the Monad.Reader. This is a special issue on parallelism and concurrency. [2] http://goo.gl/JqIpx Ganesh Sittampalam shared the haskell.org committee's first-year report. [3] http://goo.gl/O1vNQ New and Updated Projects * diagrams (Brent Yorgey; 0.4) There are "quite a few changes and improvements" since the 0.1 release. [4] http://goo.gl/4nCE7 * HasGP (Sean Holden; 0.1) A library for Gaussian process inference. [5] http://goo.gl/JAI8u * OpenGL (Jason Dagit) Updates to the following packages: OpenGL, OpenGLRaw, GLURaw, and GLUT. [6] http://goo.gl/mhTek Quotes of the Week * DanBurton: are you using -XAbsurdInstances again? * ritchie: C has the power of assembly language and the convenience of ... assembly language. * RaptorRarr: Analogy puzzles are to intelligence as __________ is to elephants * Eduard_Munteanu: Haskell Ain't Some Kludgy Enterprisey Lame Language * byorgey: Monads are not supposed to be anything. They just are. * kmc: 4:20 smoke MagicHash every day Top Reddit Stories * Giving Haskell a Promotion (typed type-level programming/new kind system paper) Domain: research.microsoft.com, Score: 76, Comments: 29 On Reddit: [7] http://goo.gl/W5Jbb Original: [8] http://goo.gl/1p4BU * Frameworks drive language adoption - Yesod Blog Domain: yesodweb.com, Score: 57, Comments: 15 On Reddit: [9] http://goo.gl/BAOKH Original: [10] http://goo.gl/KEZU9 * Snap 0.6 Released Domain: snapframework.com, Score: 57, Comments: 8 On Reddit: [11] http://goo.gl/YdfTn Original: [12] http://goo.gl/3wTbZ * Code that writes code and conversation about conversations Domain: yesodweb.com, Score: 49, Comments: 23 On Reddit: [13] http://goo.gl/3cjxL Original: [14] http://goo.gl/701ar * The Monad.Reader Issue 19: Parallelism and Concurrency Domain: themonadreader.files.wordpress.com, Score: 49, Comments: 8 On Reddit: [15] http://goo.gl/D8AcV Original: [16] http://goo.gl/virTX * Zero-Analogy Monad Tutorial Domain: unknownparallel.com, Score: 48, Comments: 57 On Reddit: [17] http://goo.gl/kwf8k Original: [18] http://goo.gl/RbAWG * Dear Redditor Vim-loving Haskeller(s): what cool vim plugins or features do you use? Domain: self.haskell, Score: 46, Comments: 35 On Reddit: [19] http://goo.gl/oL6vH Original: [20] http://goo.gl/oL6vH * Rendering animated quasicrystals in parallel, with Haskell and Repa Domain: mainisusuallyafunction.blogspot.com, Score: 43, Comments: 12 On Reddit: [21] http://goo.gl/aJzFN Original: [22] http://goo.gl/rEs6X * Why is there no "cabal uninstall" option? Domain: self.haskell, Score: 39, Comments: 26 On Reddit: [23] http://goo.gl/2zy6P Original: [24] http://goo.gl/2zy6P * Quasicrystals rendered in real time using a pre-release or Repa3 [video]. Domain: youtube.com, Score: 36, Comments: 0 On Reddit: [25] http://goo.gl/SA7Zt Original: [26] http://goo.gl/YaJN8 Top StackOverflow Questions * How to make my Haskell program faster? Comparison with C votes: 28, answers: 4 Read on SO: [27] http://goo.gl/s01Nc * Why is it faster to sum a Data.Sequence by divide-and-conquer, even with no parallelism? votes: 25, answers: 2 Read on SO: [28] http://goo.gl/nR2eC * ST Monad == code smell? votes: 19, answers: 3 Read on SO: [29] http://goo.gl/sQeYv * Algorithm for type checking ML-like pattern matching? votes: 19, answers: 3 Read on SO: [30] http://goo.gl/Sl6Zy * Why does GHC think that this type variable is not injective? votes: 16, answers: 2 Read on SO: [31] http://goo.gl/smdFX * Expression evaluation mode in haskell for scripting votes: 13, answers: 3 Read on SO: [32] http://goo.gl/1UIAq * PCRE in Haskell - what, where, how? votes: 10, answers: 4 Read on SO: [33] http://goo.gl/43Dtu * Non-Trivial Lazy Evaluation votes: 9, answers: 2 Read on SO: [34] http://goo.gl/2BlZ4 * How to get variable name in haskell votes: 7, answers: 2 Read on SO: [35] http://goo.gl/we5N7 * Is it possible to define type with callable objects in Haskell? votes: 7, answers: 3 Read on SO: [36] http://goo.gl/qnNjb Until next time, Daniel Santa Cruz References 1. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/93405 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19000 3. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19002 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/93323 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18999 6. http://comments.gmane.org/gmane.comp.lang.haskell.hopengl/858 7. http://research.microsoft.com/en-us/people/dimitris/fc-kind-poly.pdf 8. http://www.reddit.com/r/haskell/comments/lpw4a/giving_haskell_a_promotion_t… 9. http://www.yesodweb.com/blog/2011/10/frameworks-drive 10. http://www.reddit.com/r/haskell/comments/ln8hi/frameworks_drive_language_ad… 11. http://snapframework.com/blog/2011/10/28/snap-0.6-released 12. http://www.reddit.com/r/haskell/comments/lrwdi/snap_06_released/ 13. http://www.yesodweb.com/blog/2011/10/code-generation-conversation 14. http://www.reddit.com/r/haskell/comments/lp4ya/code_that_writes_code_and_co… 15. http://themonadreader.files.wordpress.com/2011/10/issue19.pdf 16. http://www.reddit.com/r/haskell/comments/lqmqh/the_monadreader_issue_19_par… 17. http://unknownparallel.com/monads.php 18. http://www.reddit.com/r/haskell/comments/lskew/zeroanalogy_monad_tutorial/ 19. http://www.reddit.com/r/haskell/comments/lmapn/dear_redditor_vimloving_hask… 20. http://www.reddit.com/r/haskell/comments/lmapn/dear_redditor_vimloving_hask… 21. http://mainisusuallyafunction.blogspot.com/2011/10/quasicrystals-as-sums-of… 22. http://www.reddit.com/r/haskell/comments/lnxe7/rendering_animated_quasicrys… 23. http://www.reddit.com/r/haskell/comments/lrc39/why_is_there_no_cabal_uninst… 24. http://www.reddit.com/r/haskell/comments/lrc39/why_is_there_no_cabal_uninst… 25. http://www.youtube.com/watch?v=v_0Yyl19fiI 26. http://www.reddit.com/r/haskell/comments/lq845/quasicrystals_rendered_in_re… 27. http://stackoverflow.com/questions/7893786/how-to-make-my-haskell-program-f… 28. http://stackoverflow.com/questions/7869359/why-is-it-faster-to-sum-a-data-s… 29. http://stackoverflow.com/questions/7880555/st-monad-code-smell 30. http://stackoverflow.com/questions/7883023/algorithm-for-type-checking-ml-l… 31. http://stackoverflow.com/questions/7866375/why-does-ghc-think-that-this-typ… 32. http://stackoverflow.com/questions/7888632/expression-evaluation-mode-in-ha… 33. http://stackoverflow.com/questions/7882512/pcre-in-haskell-what-where-how 34. http://stackoverflow.com/questions/7868507/non-trivial-lazy-evaluation 35. http://stackoverflow.com/questions/7896928/how-to-get-variable-name-in-hask… 36. http://stackoverflow.com/questions/7902520/is-it-possible-to-define-type-wi…

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Working with the code For "Typing Haskell In Haskell"
by Patrick LeBoutillier 03 Nov '11

03 Nov '11

Hi all, I'm working with the code that accompanies this paper (http://web.cecs.pdx.edu/~mpj/thih/) and I'm trying to use it but I can't figure out how to get started. I have the following code but it is not giving me the expected result: import TypingHaskellInHaskell mapt = "map" :>: Forall [Star, Star] ([] :=> ((TGen 0 `fn` TGen 1) `fn` TAp tList (TGen 0) `fn` TAp tList (TGen 1))) idt = "id" :>: Forall [Star] ([] :=> (TGen 0 `fn` TGen 0)) exprt = Ap (Const mapt) (Const idt) test = runTI $ tiExpr initialEnv [] exprt When I execute the test function above in ghci I get: ([],TVar (Tyvar "v3" Star)). I was expecting someting like below for the type part: TAp tList (TGen 0) `fn` TAp tList (TGen 0) What I want is the library to compute for me the type of "map id". What is the proper way to achieve this? Has anybody on the list worked with this code before? Thanks as lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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[ANNOUNCE] hierarchical-clustering-diagrams 0.1: diagrams of dendrograms
by Felipe Almeida Lessa 03 Nov '11

03 Nov '11

Hello! I'm pleased to announce the very first version of hierarchical-clustering-diagrams [1]. This library builds upon Brent Yorgey's great diagrams library to draw diagrams of dendrograms. You can see an example diagram at [2]. This library has already been tested with a Real World Big Data dendrogram and has a pretty nice performance. It generates a 20,000 x 5,000 px PNG in less than 15s on a 2 years-old Core 2 laptop. Note that while this library depends on the hierarchical-clustering library [3], nothing stops you from using any other method of creating your dendrogram. You only need to convert your dendrogram to the data type provided by hierarchical-clustering, which should be straightforward. However, I'd love to hear if someone finds both libraries useful =). Cheers, =D [1] http://hackage.haskell.org/package/hierarchical-clustering-diagrams [2] https://patch-tag.com/r/felipe/hierarchical-clustering-diagrams/snapshot/cu… [3] http://hackage.haskell.org/package/hierarchical-clustering-diagrams -- Felipe.

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Tell cabal-install to generate only shared libraries
by Yves Parès 03 Nov '11

03 Nov '11

Hello, When I tell cabal-install to build shared libraries (with "Shared: True" in .cabal/config), it doubles the libraries installation time since it compiles them twice, since it seems that is what GHC's flag "-shared" does. Is there a way to generate only the .so and not the the .a libs? Furthermore, I have the impression that the executables compiled by cabal-install are always linked statically... Can I tell cabal-install to activate the GHC's flag "-dynamic" for executables? Thanks.

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Haskell hackers send an idea to the "off the beaten track" workshop at POPL
by David Walker 02 Nov '11

02 Nov '11

Haskell is an amazing language for implementing Domain-Specific Languages. If you have an idea about a new domain-specific language you'd like to implement, a new problem that needs to be solved or any other new and unusual idea concerning programming language research, submit it to OBT: "Off the Beaten Track: Under-represented Problems for Programming Language Researchers." This is a new, informal, lively workshop associated with POPL in Philadelphia, Saturday January 28. It is easy to submit something, just write up a 1-page or 2-page PDF on your idea and submit it here: http://www.cs.princeton.edu/~dpw/obt/ We are eager to have lots of 5-minute talks as well as longer talks interleaved with discussion. Don't be shy: submit your off-the-beaten-track problems and ideas. The deadline is Nov 14. See the web site or below for more info Thanks, David Walker Program Chair ------ Dates: ------ Paper submission Monday November 14, 2011 (11:59PM US EST) Author notification Friday December 9, 2011 Conference Saturday January 28, 2012 ------ Scope: ------ Programming language researchers have the principles, tools, algorithms and abstractions to solve all kinds of problems, in all areas of computer science. However, identifying and evaluating new problems, particularly those that lie outside the typical core PL problems we all know and love, can be a significant challenge. Hence, the goal of this workshop is to identify and discuss problems that do not often show up in our top conferences, but where programming language researchers can make a substantial impact. The hope is that by holding such a forum and associating it directly with a top conference like POPL, we can slowly start to increase the diversity of problems that are studied by PL researchers and that by doing so we will increase the impact that our community has on the world. While many workshops associated with POPL have become more like mini-conferences themselves, this is not the goal for Off the Beaten Track. The workshop will be informal and structured to encourage discussion. It will also be centered around problems and problem areas as opposed to fully-formed solutions. A good submission is one that outlines a new problem or an interesting, underrepresented problem domain. Good submissions may also remind the PL community of problems that were once in vogue but have not recently been seen in top PL conferences. Good submissions do not need to propose complete or even partial solutions, though there should be some reason to believe that programming languages researchers have the tools necessary to search for solutions in the area at hand. Submissions that seem likely to stimulate discussion about the direction of programming language research are encouraged. Possible topics include any of the following. Biology, chemistry, or other natural sciences Art, music, graphics and animation Networking, cloud computing, systems programming Linguistics Economics, law, politics or other social sciences Web programming, social computing Algorithms and complexity Mathematics, statistics Machine learning or artificial intelligence Education Unusual compilers; underrepresented programming languages Surprise us We certainly hope to see submissions on topics not mentioned above. The goal of the workshop is to be inclusive, not exclusive. Submissions are evaluated on the basis of creativity, novelty, clarity, possible impact and potential for stimulating discussion. ------------------ Submission Details ------------------ See the web site: http://www.cs.princeton.edu/~dpw/obt/ ----------------- Program Committee ----------------- Program Chair: David Walker (Princeton University) Program Committee: Thomas Ball (Microsoft Research, Redmond) Trevor Jim (AT&T) Julia Lawall (DIKU) Boon Thau Loo (University of Pennsylvania) Geoff Mainland (Microsoft Research, Cambridge) Chung-chieh Shan (Cornell)

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Problem with TemplateHaskell
by Magicloud Magiclouds 02 Nov '11

02 Nov '11

Hi, I have code as following, to make a toDocument function (using Data.Bsin.=:) for a data structure. bson :: DecsQ -> DecsQ bson decsq = do decs <- decsq let datad = head decs DataD _ _ _ cons _ = datad to = mkName "toDocument" from = mkName "fromDocument" fund <- mapM (\con -> case con of RecC n types -> do let nvs = map (\(nv, _, _) -> nv ) types funD to [clause [conP n $ map varP nvs] (normalB $ listE $ map (\nv -> infixE (Just $ litE $ stringL $ show nv) (varE $ mkName "=:") $ Just $ appE (varE $ mkName "val") $ varE nv ) nvs) []] ) cons return (datad : fund) Testing code is as: data T = T { a :: Int , b :: Char } *TH> runQ (bson [d|data T = T {a :: Int, b :: Char}|]) [DataD [] T_0 [] [RecC T_1 [(a_2,NotStrict,ConT GHC.Types.Int),(b_3,NotStrict,ConT GHC.Types.Char)]] [],FunD toDocument [Clause [ConP T_1 [VarP a_2,VarP b_3]] (NormalB (ListE [InfixE (Just (LitE (StringL "a_2"))) (VarE =:) (Just (AppE (VarE val) (VarE a_2))),InfixE (Just (LitE (StringL "b_3"))) (VarE =:) (Just (AppE (VarE val) (VarE b_3)))])) []]] So you see that, it changed the name from T/a/b to T_0/T_1/a_2/b_3. Why is that? I did not have code to modify original data declaration. -- 竹密岂妨流水过山高哪阻野云飞

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HDBC SQLite error
by Jurriën Stutterheim 02 Nov '11

02 Nov '11

Hi all, I have recently switched one of my web applications to SQLite via HDBC. I use it to store some user credentials and data. Initially it seemed to work fine, until I tried logging in from two different browsers. That's when I got the following error when trying to log in: SqlError {seState = "", seNativeError = 5, seErrorMsg = "step: database is locked"} My application only uses two functions (query and query', see [1]) directly, and the authentication code[2] uses HDBC directly. I'm not sure why I'm getting this error, because as far as I can see, I'm not keeping any transactions open longer than I have to in the auth code. Does anyone have an idea what might be wrong and how to fix it? Cheers, Jurriën [1] https://github.com/norm2782/snaplet-hdbc/blob/master/src/Snap/Snaplet/Hdbc.… [2] https://github.com/norm2782/snaplet-hdbc/blob/master/src/Snap/Snaplet/Auth/…

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Hackage feature request: E-mail author when a package breaks
by Gregory Crosswhite 02 Nov '11

02 Nov '11

Hey everyone, I have uploaded a number of small packages to Hackage that I no longer actively use so that I don't find out immediately when a new version of GHC has broken them. Since Hackage is going to the trouble of finding out when a package no longer builds anyway, could it have a feature where when a working package breaks with a new version of GHC the author is automatically e-mailed? This would make me (and probably others) a lot more likely to notice and proactively fix broken packages. (Heck, I wouldn't even necessarily mind being nagged about it from time to time. :-) ) Cheers, Greg

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Persistent Concurrent Data Structures
by dokondr 02 Nov '11

02 Nov '11

Hi, Please comment on the idea and advise on steps to implement it. Real world applications need persistent data, that can be accessed and modified concurrently by several clients, in a way that preserves "happen-before" relationship. Idea: Design and implement Persistent Concurrent Data Types in Haskell. These data types should mirror existing Data.List , Data.Map and similar types but provide persistency and support consistent concurrent access and modification (or simply - "concurrency"). Persistency and concurrency should be configurable through these type interfaces. Configuration should include: 1) Media to persist data, such as file, DBMS, external key-value store (for example Amazon SimpleDB, CouchDB, MongoDB, Redis, etc) 2) Caching policy - when (on what events) and how much data to read/write from/to persistent media. Media reads / writes can be done asynchronously in separate threads. 3) Concurrency configuration: optimistic or pessimistic data locking. One may ask why encapsulate persistency and concurrency in the data type instead of using "native" storage API, such as for example key-value / row-column API that NoSQL databases provide? The answer is simple: APIs that your code use greatly influence the code itself. Using low-level storage API directly in your code results in bloated obscure code, or you need to encapsulate this low-level API in clear and powerful abstractions. So why not to do this encapsulation once and for all for such powerful types as Data.Map, for example, and forget all Cassandra and SimpleDB low-level access method details? When the right time comes and you will need to move your application to the next new "shiny_super_cloud", you will just write the implementation of NData.Map backed by Data.Map in terms of low-level API of this super-cloud. (Side note: I really need such a NData.Map type. I was requested to move my code that heavily uses Data.Map and simple text file persistence into Amazon AWS cloud. Looking at SimpleDB API, I realized that I will have to rewrite 90% of code. This rewrite will greatly bloat my code and will make it very unreadable. In case I had NData.Map I would just switch implementation from 'file' to SimpleDB persistency inside my NData.Map type.) Implementation: To start playing with this idea, NData.Map persisted in a regular file will do, no concurrency yet. Next step - NData.Map persisted in SimpleDB or Cassandra or Redis, with concurrent access supported. So it looks like NData.Map should be a monad ... Any ideas on implementation and similar work? Thanks! Dmitri --- http://sites.google.com/site/dokondr/welcome

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Re: [Haskell-cafe] How to implement digital filters using Arrows
by Captain Freako 02 Nov '11

02 Nov '11

Hi John, Thanks for all your help. I've been studying your suggested code: > type FilterAu b c = Automaton (->) b c > liftAu :: ((x,FilterState s)->(y,FilterState s)) -> FilterState s -> FilterAu x y > liftAu f s0 = proc x -> do > rec (y,s') <- arr f -< (x,s) > s <- delay s0 -< s' > returnA -< y runAutomaton is a bit cumbersome, so define a custom run function that takes a list > runAuto a [] = [] > runAuto (Automaton f) (x:xs) = let > (y,a) = f xt > in y:runAuto a xs as well as the various instance definitions for Automaton. I think I understand how the `returnA' in the last line of your `liftAu' function is getting translated by those instance definitions into: c where c = Automaton ( arr id &&& arr (const c) ) and, furthermore, how that is passing the supplied `y' into the first element of the resulting couple. However, I don't understand how the recursively defined `c' is capturing the modified filter state and preserving it for the next call. It seems like the Automaton being referred to by `c' is a newly constructed entity, which knows nothing about the current state of the running Automaton. Any help in understanding this would be greatly appreciated. Thanks! -db

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