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Haskell Weekly News: Issue 234
by Daniel Santa Cruz 06 Sep '12

06 Sep '12

Welcome to issue 243 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 26 to September 01, 2012. Quotes of the Week * companion_cube: "I don't understand this article at all" edwardk: "Well, when you finish reading it, here's a follow-up article" Top Reddit Stories * Parsing with Haskell and Attoparsec Domain: newartisans.com, Score: 64, Comments: 8 On Reddit: [1] http://goo.gl/rrlQG Original: [2] http://goo.gl/aHlTJ * FRP - Release of reactive-banana-0.7. Finally features dynamic event switching! Domain: apfelmus.nfshost.com, Score: 48, Comments: 21 On Reddit: [3] http://goo.gl/5QnBl Original: [4] http://goo.gl/8dnLg * Combinatorrent - Writing Haskell Code for Fun and Profit [InfoQ] Domain: infoq.com, Score: 46, Comments: 14 On Reddit: [5] http://goo.gl/FE4OF Original: [6] http://goo.gl/feU7U * Putting it all together -- Example project that pulls together Snap, Blaze, OpenId, IxSet, AcidState, and Sessions. Domain: self.haskell, Score: 34, Comments: 3 On Reddit: [7] http://goo.gl/huFaI Original: [8] http://goo.gl/huFaI * Invert the Inversion of Control Domain: thev.net, Score: 32, Comments: 30 On Reddit: [9] http://goo.gl/1uO5a Original: [10] http://goo.gl/89UnF * A follow up on my initial experience and why there's still something immensely attractive about Haskell Domain: binarysculpting.com, Score: 27, Comments: 19 On Reddit: [11] http://goo.gl/3RLNO Original: [12] http://goo.gl/81YF2 * Haskim -- a convenient Haskell VIM distribution Domain: github.com, Score: 25, Comments: 19 On Reddit: [13] http://goo.gl/9sJ3O Original: [14] http://goo.gl/Wq0Rq * Introducing Perdure: persistence for acyclic immutable data Domain: hackage.haskell.org, Score: 23, Comments: 12 On Reddit: [15] http://goo.gl/ACWDi Original: [16] http://goo.gl/CSi57 * Existential types and data abstraction Domain: iveselov.info, Score: 22, Comments: On Reddit: [17] http://goo.gl/K6JGl Original: [18] http://goo.gl/GBYlW * I just realized that running monads is applying a left monad action Domain: self.haskell, Score: 20, Comments: 63 On Reddit: [19] http://goo.gl/Geus4 Original: [20] http://goo.gl/Geus4 Top StackOverflow Questions * How can I recover sharing in a GADT? votes: 14, answers: 2 Read on SO: [21] http://goo.gl/I3cuW * Constructing efficient monad instances on `Set` (and other containers with constraints) using the continuation monad votes: 13, answers: 2 Read on SO: [22] http://goo.gl/Ys4Uy * Numbers as multiplicative functions (weird but entertaining) votes: 12, answers: 1 Read on SO: [23] http://goo.gl/yN8KN * Game server in Haskell votes: 10, answers: 3 Read on SO: [24] http://goo.gl/QV9de * Why doesn't sortBy take (a -> a -> Bool)? votes: 10, answers: 5 Read on SO: [25] http://goo.gl/Y7EzD * Default type instances referring to each other votes: 9, answers: 1 Read on SO: [26] http://goo.gl/6cUG7 * Applying a fixed-length-vector-function to the inital part of a longer fixed-length-vector votes: 9, answers: 2 Read on SO: [27] http://goo.gl/03KcE * Haskell: example of function of type a -> a, besides the identity votes: 9, answers: 5 Read on SO: [28] http://goo.gl/q2UDW * Stack overflow when using Haskell sort function votes: 8, answers: 2 Read on SO: [29] http://goo.gl/D31Ku * How to navigate up inside a HUET Zipper votes: 7, answers: 2 Read on SO: [30] http://goo.gl/X7dE3 * Haskell : reference to previously updated elements of list within the update function votes: 7, answers: 6 Read on SO: [31] http://goo.gl/0yRIo * Is there any way to make parsec report “shift-reduce” conflicts? votes: 7, answers: 1 Read on SO: [32] http://goo.gl/LQeDG * Haskell: how to write a monadic variadic function, with parameters using the monadic context votes: 7, answers: 1 Read on SO: [33] http://goo.gl/ThniY * Haskell parMap and parallelism votes: 7, answers: 1 Read on SO: [34] http://goo.gl/3qQHu Until next time, Daniel Santa Cruz References 1. http://newartisans.com/2012/08/parsing-with-haskell-and-attoparsec/ 2. http://www.reddit.com/r/haskell/comments/z2lir/parsing_with_haskell_and_att… 3. http://apfelmus.nfshost.com/blog/2012/08/26-frp-banana-0-7.html 4. http://www.reddit.com/r/haskell/comments/yv6ku/frp_release_of_reactivebanan… 5. http://www.infoq.com/presentations/Combinatorrent-Haskell-casestudy 6. http://www.reddit.com/r/haskell/comments/yzdyr/combinatorrent_writing_haske… 7. http://www.reddit.com/r/haskell/comments/z4iay/putting_it_all_together_exam… 8. http://www.reddit.com/r/haskell/comments/z4iay/putting_it_all_together_exam… 9. http://www.thev.net/PaulLiu/invert-inversion.html 10. http://www.reddit.com/r/haskell/comments/z4inb/invert_the_inversion_of_cont… 11. http://binarysculpting.com/2012/09/01/theres-something-about-haskell/ 12. http://www.reddit.com/r/haskell/comments/z6idz/a_follow_up_on_my_initial_ex… 13. https://github.com/jejansse/haskim 14. http://www.reddit.com/r/haskell/comments/z3orc/haskim_a_convenient_haskell_… 15. http://hackage.haskell.org/package/perdure-0.1.2 16. http://www.reddit.com/r/haskell/comments/z2sc6/introducing_perdure_persiste… 17. http://iveselov.info/posts/2012-08-30-existential-types.html 18. http://www.reddit.com/r/haskell/comments/z1ojj/existential_types_and_data_a… 19. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 20. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 21. http://stackoverflow.com/questions/12230088/how-can-i-recover-sharing-in-a-… 22. http://stackoverflow.com/questions/12183656/constructing-efficient-monad-in… 23. http://stackoverflow.com/questions/12133932/numbers-as-multiplicative-funct… 24. http://stackoverflow.com/questions/12134097/game-server-in-haskell 25. http://stackoverflow.com/questions/12152561/why-doesnt-sortby-take-a-a-bool 26. http://stackoverflow.com/questions/12199324/default-type-instances-referrin… 27. http://stackoverflow.com/questions/12221080/applying-a-fixed-length-vector-… 28. http://stackoverflow.com/questions/12230820/haskell-example-of-function-of-… 29. http://stackoverflow.com/questions/12203380/stack-overflow-when-using-haske… 30. http://stackoverflow.com/questions/12129300/how-to-navigate-up-inside-a-hue… 31. http://stackoverflow.com/questions/12144973/haskell-reference-to-previously… 32. http://stackoverflow.com/questions/12170780/is-there-any-way-to-make-parsec… 33. http://stackoverflow.com/questions/12175912/haskell-how-to-write-a-monadic-… 34. http://stackoverflow.com/questions/12224842/haskell-parmap-and-parallelism

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Monospace code in Haddock
by Niklas Hambüchen 05 Sep '12

05 Sep '12

Is it possible to use inlined monospaced font in Haddock that does *not* replace < and > to links and does *not* link to Haskell functions (like @ and ' and " and ` do)? Like: -- | This function gives you @<span a1="v1">str</span>@ something :: String The @ above of course makes everything unreadable (as it thinks span... is a link). The only thing I've found to work so far is: -- | This function gives you -- -- >@<span a1="v1">str</span>@ something :: String Which is not inline.

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Haskell seems setup for iterative numerics; i.e. a standard example is Newton's method where lazy evaluation ...
by KC 05 Sep '12

05 Sep '12

separates control from computation. It seems as if Haskell would be better for iterative matrix methods rather than direct calculation. -- -- Regards, KC

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[CUFP 2012] Birds of a Feather sessions
by Ashish Agarwal 05 Sep '12

05 Sep '12

========================================================= Birds of a Feather sessions (BOFs) Commercial Users of Functional Programming Workshop (CUFP 2012) http://cufp.org/bofs-2012 Copenhagen, Denmark, September 13 - 15 ========================================================= CUFP 2012 will again include Birds of a Feather sessions as in previous years. BoFs provide a place for our community to gather informally and reach consensus on matters of importance. Any CUFP attendee can propose a BoF session, and grab one of the spare rooms in the evening slots below. Attendance in the evening is open to all. BoF sessions facilitate ad-hoc discussions and provide a place to gather and start off the chat, before moving on during the evening to a local restaurant or pub. There is no deadline for proposing a session---you can even request a room on the day---but we encourage you to do so at the earliest possible time to help us with planning and to publicize your proposed discussion. Attendees are also free to show up even if you have not confirmed, but again earlier confirmation will help us with planning. Please see http://cufp.org/bofs-2012 for details on how to propose a BoF session or confirm attendance at one of them. General information on CUFP 2012 can be found at http://cufp.org/conference/2012. The conference is co-located with ICFP. You can register for CUFP at the main ICFP website http://www.icfpconference.org/icfp2012.

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Research Position, Leeds: Parallel functional visualization
by David Duke 05 Sep '12

05 Sep '12

I have an open position in the Visualization group at Leeds for a researcher to work on one of two projects, the second of which is specific to Haskell: We are seeking to appoint an ambitious researcher within the Visualization and VR group at Leeds to work on one of two projects, (i) time-varying and higher-dimensional graphs, or (ii) high-performance visualization in Haskell. The post is available immediately, and lasts until 31 July 2016. 1. Time-varying and higher-dimensional graphs. We are developing powerful tools for the topological analysis of (time-varying) volumetric datasets. These produce large graphs capturing combinatorial structures and events, and visual analysis of these graphs provides significant insight into the data. Similar structures may be found in graphs resulting from optimisation problems in multi-parameter spaces. The research will investigate (i) the link between layout aesthetics and specific tasks over time-varying data; (ii) layout techniques that respect these aesthetics, for example embedding of the graph into higher-dimensional manifolds; (iii) techniques for eliding structural detail, and linking visual abstractions to other models of the data; and (iv) efficient implementation of layout algorithms, using established toolkits (e.g. VTK), and parallelisation on cluster-based hardware and/or many-core GPU architecture. You will be working with researchers on the "Multifield Extension of Topological Analysis" project, funded recently by the EPSRC.  2. Parallel functional visualization: Scientific visualization involves the application of compute-intensive algorithms to large volumes of data, often larger than can fit into core memory. Systems are still largely based on the "pipeline" model, extended to exploit different processing technologies (multi-core CPUs, GPUs, clusters), and strategies (streaming, tiling, out-of-core processing, etc). This research concerns an innovative approach that exploits laziness and parallelism in the pure functional language Haskell. Scivis "applications" will be constructed from expressions in a declarative domain-specific language, and transformed into imagery via lower-level DSLs and strategies that coordinate use of heterogeneous resources. Both projects involve fundamental computer science and interdisciplinary work alongside experts from other disciplines providing data and contributing to its analysis. The appointee will be expected to contribute to the programme described above through all stages of the research process, including basic research, programming, testing, evaluation, publication of results at the highest level, and assistance in developing proposals for funding. Work will involve collaboration with other senior members of the Leeds visualization group (Dr. Hamish Carr and Dr. Roy Ruddle), and there will be opportunities to assist with academic supervision of undergraduate and postgraduate students. CLOSING DATE: 21st September. For further information including person specification, and/or to submit an application, please see the following URL: http://jobs.leeds.ac.uk/fe/tpl_universityofleeds01.asp?s=FoQnTYvIgXJoLlXgd&… For informal inquiries, please feel free to contact me directly. David Duke Head, School of Computing, and Reader in Visualization E: D.J.Duke(a)leeds.ac.uk W: www.comp.leeds.ac.uk/djd/

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SYB with class: Bug in Derive.hs module
by Roman Cheplyaka 05 Sep '12

05 Sep '12

There's a bug in syb-with-class reported by Alexey Rodriguez Yakushev in 2008 [1]. I can confirm that the bug is still there (syb-with-class-0.6.1.3, ghc 7.4.1). [1]: http://www.haskell.org/pipermail/haskell-cafe/2008-March/041179.html Here's an even simpler test case: {-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, TemplateHaskell, OverlappingInstances, DeriveDataTypeable #-} import Data.Generics.SYB.WithClass.Basics import Data.Generics.SYB.WithClass.Derive data Foo = Foo Foo | Bar deriving (Typeable, Show) deriveData [''Foo] f :: (Data NoCtx ast, Typeable ast) => ast -> TypeRep f = typeOf main = print $ f $ Foo Bar The cause of this bug is a self-referencing instance created by deriveData: instance (Data ctx Foo, Sat (ctx Foo)) => Data ctx Foo where ... What's the proper way to fix it? -- Roman I. Cheplyaka :: http://ro-che.info/

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How to waitForProcess?
by Magicloud Magiclouds 05 Sep '12

05 Sep '12

Hi, I have code like this and it leaves lots of zombies of flow-export. Then I add waitForProcess.... Well I do not know where to put it. Before or after 'hGetContents' both make the program hung. exportCSV :: FilePath -> IO [String] exportCSV file = do csv_ <- withBinaryFile file ReadMode $ \i -> do (_, Just o, _, h) <- createProcess $ CreateProcess (RawCommand "/usr/bin/flow-export" ["-f2"]) Nothing Nothing (UseHandle i) CreatePipe Inherit True False hGetContents o return $ tail $ lines csv_ -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Over general types are too easy to make.
by timothyhobbs＠seznam.cz 05 Sep '12

05 Sep '12

I'd have to say that there is one(and only one) issue in Haskell that bugs me to the point where I start to think it's a design flaw: It's much easier to type things over generally than it is to type things correctly. Say we have a >data BadFoo = > BadBar{ > badFoo::Int} | > BadFrog{ > badFrog::String, > badChicken::Int} This is fine, until we want to write a function that acts on Frogs but not on Bars. The best we can do is throw a runtime error when passed a Bar and not a Foo: >deBadFrog :: BadFoo -> String >deBadFrog (BadFrog s _) = s >deBadFrog BadBar{} = error "Error: This is not a frog." We cannot type our function such that it only takes Frogs and not Bars. This makes what should be a trivial compile time error into a nasty runtime one :( The only solution I have found to this is a rather ugly one: >data Foo = Bar BarT | Frog FrogT If I then create new types for each data constructor. >data FrogT = FrogT{ > frog::String, > chicken::Int} >data BarT = BarT{ > foo :: Int} Then I can type deFrog correctly. >deFrog :: FrogT -> String >deFrog (FrogT s _) = s But it costs us much more code to do it correctly. I've never seen it done correctly. It's just too ugly to do it right :/ and for no good reason. It seems to me, that it was a design mistake to make data constructors and types as different entities and this is not something I know how to fix easily with the number of lines of Haskell code in existence today :/ >main = do > frog <- return (Frog (FrogT "Frog" 42)) > print $ > case frog of > (Frog myFrog) -> deFrog myFrog > badFrog <- return (BadBar 4) > print $ > case badFrog of > (notAFrog@BadBar{}) -> deBadFrog notAFrog The ease with which we make bad design choices and write bad code(in this particular case) is tragically astounding. Any suggestions on how the right way could be written more cleanly are very welcome! Timothy Hobbs

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Re: [Haskell-cafe] From monads to monoids in a small category
by Alberto G. Corona 04 Sep '12

04 Sep '12

Not to mention the ugly formatting ;) 2012/9/5 Richard O'Keefe <ok(a)cs.otago.ac.nz>: > > On 4/09/2012, at 10:39 PM, Alberto G. Corona wrote: > >> "Monads are monoids in the category of endofunctors" >> >> This Monoid instance for the endofunctors of the set of all elements >> of (m a) typematch in Haskell with FlexibleInstances: >> >> instance Monad m => Monoid (a -> m a) where >> mappend = (>=>) -- kleisly operator >> mempty = return >> >> The article can be found here: >> >> http://haskell-web.blogspot.com.es/2012/07/from-monads-to-monoids-in-small.… >> >> I would appreciate some comments. > > s/kleisly/Kleisli/ > In the article, > s/Lets/Let's/ > /Here 'm b' as/ s/as/is/ > s/this_are/this are/ > s/first, is/first is/ > s/haskell/Haskell/ > s/polimorphic/polymorphic/ > s/x=/x =/ > s/let's/Let's/ > s/condition, associativity/condition, associativity,/ > /if not where that way, .* guess/ > I *think* you mean to say something like > (If it were not so, it would be impossible to > define the denotational semantics of imperative > languages in terms of monads, I guess.) > Generally, it's "according TO", not "according WITH", > and "associated WITH", not "associated TO". > > instance Functor a > doesn't seem to be legal Haskell. > > At this point I stopped reading. > >

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Asking for advice on programming style and also performance
by Harald Bögeholz 04 Sep '12

04 Sep '12

Dear Haskell Cafe, I am working on a solver for Slitherlink puzzles in Haskell. Today I am completely puzzled that a small change I made had a huge performance impact. I thought the change to be merely cosmetic to make a function a little more elegant and the code more readable. But this change slowed down my program by a factor of 100! I would greatly appreciate advice from experienced Haskellers why this has happened. The program is hosted on github; this is the direct link to the change with unexpected outcome: https://github.com/ctbo/slitherlink/commit/b6f58699258ef68ddee21a1346bd1844… The program itself isn't commented at all, but I have written a sketch of how it works in the wiki: https://github.com/ctbo/slitherlink/wiki/How-it-works While I am at it: I would also appreciate any advice you might have on the programming style in general. I am relatively new to Haskell and this is my first nontrivial program. So any suggestions for improvements are welcome! Thanks for your time, -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

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