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adding the elements of two lists
by TP 29 Mar '12

29 Mar '12

Hello, My primary problem may be reduced to adding elements of two lists: [1,2,3] + [4,5,6] = [5,7,9] My first idea was to declare a list of Int as an instance of Num, and define (+) in the correct way. However, it seems it is not possible to do that: ------------------- instance Num [Int] where l1 + l2 = .... ------------------- Why? It seems it is necessary to do: ------------------ newtype ListOfInt = ListOfInt { getList :: [Int] } deriving (Show, Eq) instance Num ListOfInt where l1 + l2 = ... ------------------- Am I correct? Is it the best way to do that? Now, what is the most usual way to implement l1+l2? I have just read about applicative functors, with which I can do: ------------------- import Control.Applicative let l1 = [1,2,3] let l2 = [4,5,6] print $ getZipList $ (+) <$> ZipList l1 <*> ZipList l2 [5,7,9] ------------------- Is it the correct way to do that? I have tried: ------------------- instance Num ListOfInt where l1 + l2 = ListOfInt $ getZipList $ (+) <$> ZipList (getList l1) <*> ZipList (getList l2) ------------------- Isn't it too much complicated? Thanks TP

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Haskell Weekly News: Issue 220
by Daniel Santa Cruz 29 Mar '12

29 Mar '12

Welcome to issue 220 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of March 18 to 24, 2012. Quotes of the Week * shachaf: Type classes are a scary story that you tell new Haskellers so they can go to sleep at night. * acowley: Parametric polymorphism is the inspiring story you tell them to get them riled up and raring to go. It's like Braveheart for programmers. * monochrom: the purpose of types is to ban more programs * rostayob: I was a type constructor, then I took an arrow in the kind * shachaf: @let otherfoolish = not otherwise * flebron: Monoidal objects in the monoidal category of endofunctors on a category. I took a small detour through tensor products to understand monoidal categories better, with one example of such a product. mauke: I have no idea what you just said * weexplat: starts a proposal to replace launchMissiles with atomicDestroyBiosphere * monochrom: rumour: SPJ will write a thick book "A New Science of Kind" * JonathanShapiro: [on abandoning BitC] "And I *still* don't believe in monads. :-)" Top Reddit Stories * Starting the Mezzo Haskell Book Domain: yesodweb.com, Score: 58, Comments: 28 On Reddit: [1] http://goo.gl/pOSlR Original: [2] http://goo.gl/ecy4O * First patches for iPhone cross compiler against GHC head Domain: haskell.org, Score: 55, Comments: 27 On Reddit: [3] http://goo.gl/OOAai Original: [4] http://goo.gl/p7C3L * Retrospective thoughts on BitC by its author, and why type classes didn't work for him Domain: coyotos.org, Score: 44, Comments: 22 On Reddit: [5] http://goo.gl/0sL80 Original: [6] http://goo.gl/5pCp2 * Google Summer of Code 2012 - Haskell.org is participating Domain: haskell.org, Score: 42, Comments: 11 On Reddit: [7] http://goo.gl/9EwzD Original: [8] http://goo.gl/XqcUJ * Conduits vs. Pipes. Using Void as an Input or Output type can reduce the number of concepts (source, sink, conduit) from 3 to 1 (pipe). Domain: twanvl.nl, Score: 34, Comments: 50 On Reddit: [9] http://goo.gl/bc8RB Original: [10] http://goo.gl/eb7oz * Announcing conduit 0.3 Domain: yesodweb.com, Score: 33, Comments: 9 On Reddit: [11] http://goo.gl/JrgV0 Original: [12] http://goo.gl/OsDl7 * Haskell for embedded domain-specific languages (Functional Programming eXchange 2012) Domain: skillsmatter.com, Score: 31, Comments: 4 On Reddit: [13] http://goo.gl/pSREX Original: [14] http://goo.gl/1azVh * Domain theory course Domain: cs.nott.ac.uk, Score: 31, Comments: 5 On Reddit: [15] http://goo.gl/5VHyG Original: [16] http://goo.gl/o2pRC * ANNOUNCE: options, an easy-to-use command-line option parser Domain: self.haskell, Score: 29, Comments: 21 On Reddit: [17] http://goo.gl/s6ktw Original: [18] http://goo.gl/s6ktw * A Case Study in Creating Rich Internet Apps in Haskell (Silk at FP eXchange 2012) Domain: skillsmatter.com, Score: 22, Comments: 18 On Reddit: [19] http://goo.gl/ZdtDq Original: [20] http://goo.gl/Ta2HQ * It just occurred to me that (lift .) defines a functor... Domain: self.haskell, Score: 22, Comments: 15 On Reddit: [21] http://goo.gl/GMzPt Original: [22] http://goo.gl/GMzPt Top StackOverflow Questions * Haskell platform: nested functions and optimization votes: 15, answers: 2 Read on SO: [23] http://goo.gl/nreIr * newtype with RankNTypes votes: 14, answers: 2 Read on SO: [24] http://goo.gl/UT9DV * Pattern Matching - Prolog Vs Haskell votes: 13, answers: 4 Read on SO: [25] http://goo.gl/3hZV1 * How does this definition of ArrowLoop.loop work? votes: 13, answers: 2 Read on SO: [26] http://goo.gl/ocSCM * Data.Set : does it always know best? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/NUjkB * Building a histogram with haskell, many times slower than with python votes: 11, answers: 3 Read on SO: [28] http://goo.gl/kbh5i * What is the scope of let when used without in? votes: 11, answers: 2 Read on SO: [29] http://goo.gl/bNwna * Strange "half to even" rounding in different languages [closed] votes: 11, answers: 3 Read on SO: [30] http://goo.gl/lnPch * Practical reasons for Сhurch Encoding votes: 10, answers: 1 Read on SO: [31] http://goo.gl/xYoef * Can the FFI deal with arrays? If so, how? votes: 9, answers: 3 Read on SO: [32] http://goo.gl/d4R3H Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/03/start-mezzo-haskell 2. http://www.reddit.com/r/haskell/comments/r9xdq/starting_the_mezzo_haskell_b… 3. http://www.haskell.org/pipermail/iphone/2012-March/000120.html 4. http://www.reddit.com/r/haskell/comments/r3ge7/first_patches_for_iphone_cro… 5. http://www.coyotos.org/pipermail/bitc-dev/2012-March/003300.html 6. http://www.reddit.com/r/haskell/comments/rbk7z/retrospective_thoughts_on_bi… 7. http://www.haskell.org/pipermail/haskell-cafe/2012-March/100247.html 8. http://www.reddit.com/r/haskell/comments/r5h81/google_summer_of_code_2012_h… 9. http://twanvl.nl/blog/haskell/conduits-vs-pipes 10. http://www.reddit.com/r/haskell/comments/rbgvz/conduits_vs_pipes_using_void… 11. http://www.yesodweb.com/blog/2012/03/announcing-conduit-0-3 12. http://www.reddit.com/r/haskell/comments/r6wdh/announcing_conduit_03/ 13. http://skillsmatter.com/podcast/scala/haskell-for-embedded-domain-specific-… 14. http://www.reddit.com/r/haskell/comments/r22d6/haskell_for_embedded_domains… 15. http://www.cs.nott.ac.uk/~gmh/domains.html 16. http://www.reddit.com/r/haskell/comments/r250h/domain_theory_course/ 17. http://www.reddit.com/r/haskell/comments/rbpmb/announce_options_an_easytous… 18. http://www.reddit.com/r/haskell/comments/rbpmb/announce_options_an_easytous… 19. http://skillsmatter.com/podcast/home/silkapp-a-case-study-in-creating-rich-… 20. http://www.reddit.com/r/haskell/comments/r1xey/a_case_study_in_creating_ric… 21. http://www.reddit.com/r/haskell/comments/raxsk/it_just_occurred_to_me_that_… 22. http://www.reddit.com/r/haskell/comments/raxsk/it_just_occurred_to_me_that_… 23. http://stackoverflow.com/questions/9757515/haskell-platform-nested-function… 24. http://stackoverflow.com/questions/9779301/newtype-with-rankntypes 25. http://stackoverflow.com/questions/9780779/pattern-matching-prolog-vs-haske… 26. http://stackoverflow.com/questions/9856342/how-does-this-definition-of-arro… 27. http://stackoverflow.com/questions/9755851/data-set-does-it-always-know-best 28. http://stackoverflow.com/questions/9772098/building-a-histogram-with-haskel… 29. http://stackoverflow.com/questions/9777399/what-is-the-scope-of-let-when-us… 30. http://stackoverflow.com/questions/9829769/strange-half-to-even-rounding-in… 31. http://stackoverflow.com/questions/9806729/practical-reasons-for-hurch-enco… 32. http://stackoverflow.com/questions/9854782/can-the-ffi-deal-with-arrays-if-…

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Can't install snap (problem with syb)
by Michael Iles 28 Mar '12

28 Mar '12

I uninstalled ghc, removed my ~/.cabal directory, reinstalled ghc, then tried to install snap. I get: $ cabal install snap Resolving dependencies... Downloading syb-0.3.6... <command line>: cannot satisfy -package Cabal-1.14.0: Cabal-1.14.0-0338b6f52e6e6a56054371a110e1d79e is unusable due to missing or recursive dependencies: directory-1.1.0.2-8957520ced1fb19160c3a6126dcccfaa process-1.1.0.1-91185c964ab744c1f3cbca1863d2ba45 (use -v for more information) cabal: Error: some packages failed to install: aeson-0.6.0.0 depends on syb-0.3.6 which failed to install. snap-0.8.0.2 depends on syb-0.3.6 which failed to install. syb-0.3.6 failed during the configure step. The exception was: ExitFailure 1 Any ideas? Mike.

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: haskell-src-exts-1.13.0
by dag.odenhall＠gmail.com 28 Mar '12

28 Mar '12

On 28 March 2012 21:05, Jurriaan Hage <J.Hage(a)uu.nl> wrote: > > Our first year students will be very unhappy to hear this. Wait, what?

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ANNOUNCE: haskell-src-exts-1.13.0
by Niklas Broberg 28 Mar '12

28 Mar '12

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.13.0! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts It's been a *very short* while since the last update before this, and another (smallish) major release it is. Sorry about that (especially to Neil who was quick to update). The main update of this release is to add support for DoAndIfThenElse, which means haskell-src-exts at long last is compliant with Haskell2010. This is also the cause for the backwards-incompatible change: default parse mode is now to use Haskell2010 mode. In particular this means that NPlusKPatterns is no longer recognized by default. No old code using haskell-src-exts should fail to compile (unless really unlucky with clashing imports from L.H.E.Extension), but it may now fail to work on some Haskell98-compliant source files. Changelog: 1.12.0 --> 1.13.0 =============== * Add extensions DoAndIfThenElse and NPlusKPatterns to Language.Haskell.Exts.Extensions. * DoAndIfThenElse is now supported, at long last, making HSE compatible with Haskell2010 * Introduce haskell98 and haskell2010 extension groups, exported from Language.Haskell.Exts.Extensions. * Backwards-incompatible change: default parse mode is now to use haskell2010, which means the following features are recognized by default: DoAndIfThenElse, PatternGuards, ForeignFunctionInterface, EmptyDataDecls. NPlusKPatterns is no longer recognized by default. Cheers, /Niklas

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ghci can't load file whose name contains non-ASCII chars?
by Johannes Waldmann 28 Mar '12

28 Mar '12

I must be making some obvious mistake here, but I'm not seeing it. The file name contains O-umlaut, and the OS handles it fine, but ghci does not like it (although it accepts umlauts in the contents of the file (UTF-8) e.g., as a module name) $ cat fÖÖbar.hs main = print $ product [1..100] $ ghci fÖÖbar.hs GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. <no location info>: can't find file: f??bar.hs Failed, modules loaded: none. this is on a standard ubuntu install (11.10 and 11.04) with $ locale LANG=en_US.UTF-8 LANGUAGE= LC_CTYPE="en_US.UTF-8" ... $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 $ uname -a Linux octopus 3.0.0-16-generic #29-Ubuntu SMP Tue Feb 14 12:48:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

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Re: [Haskell-cafe] Mathematics and Statistics libraries
by Vivian McPhail 28 Mar '12

28 Mar '12

Date: Sun, 25 Mar 2012 17:54:11 +0400 > From: Aleksey Khudyakov <alexey.skladnoy(a)gmail.com> > Subject: Re: [Haskell-cafe] Mathematics and Statistics libraries > To: haskell-cafe(a)haskell.org > Message-ID: <4F6F2383.6070404(a)gmail.com> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 25.03.2012 14:52, Tom Doris wrote: > > Hi Heinrich, > > And of course data visualization. Only library I know of is Chart[1] but > I don't like API much. > There is the plot[1] library which provides for updateable plots from GHCi REPL and has a gnuplot-like interface. I wrote it for this very reason, a mathematics/statistics development environment. It uses Data.Vector.Storable, which provides for compatability with both statistics and hmatrix packages (as well as hstatistics). > > I think talking about data frames is a bit pointless unless we specify > what is data frame. Basically there are two representations of tabular > data structure: array of tuples or tuple of arrays. If you want first go > for Data.Vector.Vector YourData. If you want second you'll probably end > up with some HList-like data structure to hold arrays. > > Matrices from hmatrix are easily converted to rows or columns of Data.Vector.Storable and can be sliced and otherwise manipulated. [1] http://hackage.haskell.org/package/plot<%20%20[1]%20http://hackage.haskell.org/package/plot> Vivian

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GSoC project idea: haskell-type-exts
by Niklas Broberg 28 Mar '12

28 Mar '12

See ticket here: http://hackage.haskell.org/trac/summer-of-code/ticket/1620 == What? == The haskell-src-exts library (HSE) is very popular for manipulating Haskell source code in a wide range of scenarios, such as (syntactic) preprocessors (e.g. hsx), syntax highlighting, refactoring and code inspection (e.g. hlint), etc. A commonly sought feature, that would enable a multitude of new and improved scenarios, is the ability to type-check the code, and annotate the AST with type information. Type-driven refactoring, type-aware code inspection, type-based preprocessing, ... Lots of pure awesomeness lies in that direction. Sadly, HSE has no type-checking abilities. For those who seek such, we are forced to point in the direction of the dark cave that is the GHC API. While GHC is an utterly awesome compiler, there are several reasons why this is not an ideal situation. First, GHC was designed and built for optimization, which means that even the early stages of the compilation pipeline contain features that cater to later optimization, but that are (at best) unnecessary for a front-end tool. Second, the API into GHC internals is defined as an afterthought. Some good work has been done to make it somewhat palatable, but it remains a fact that GHC's primary purpose is not to be a library for integration in tools, and this shows. Overall, depending on GHC when all you want is front-end capabilities like parsing and typechecking, is really like using the proverbial sledge hammer. Also, at the meta level, GHC deserves "competition". Right now GHC is de facto the only implementation of Haskell, and as such it more or less defines the progress of Haskell. Too often this leads to overly pragmatic choices, which lead to inconsistencies and headaches for tool programmers. Having a separate, stand-alone front end library would help set higher standards. So - what we need is a type checker that works on the HSE AST: haskell-type-exts! == How? == I now hear you think: "This is too much, an enormous project, surely it can't be done as a GSoC project?" Sure it can! By walking in the footsteps of giants, and following GHC's approach. This can be achieved by closely observing the documentation provided by [1] and [2], with a bit of [3] thrown into the mix. I already have a basic library that provides most of the core framework necessary to cover extensions, but at this time it's mostly just scaffolding. It needs a lot of love, to provide cover for all the various available extensions, but I'm confident it can be done satisfactorily as a GSoC project. Still, it would be reasonable to modularize the task. Certain features are more crucial than others to support, simply because they have a more profound impact on the overall type-checking. Higher-rank types, GADTs and type families (and fundeps) are likely the most important ones to get right. There are also some design issues to get right (e.g. using pure substitution vs efficient mutable variables). I (Niklas Broberg) am more than willing to mentor this project. == Further reading: == [1] S. Peyton Jones, D. Vytiniotis, S. Weirich, and M. Shields. Practical type inference for arbitrary-rank types. [2] D. Vytiniotis, S. Peyton Jones, T. Schrijvers, M. Sulzmann. OutsideIn(X) – Modular type inference with local assumptions [3] M. P. Jones. Typing Haskell in Haskell

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Regarding Haskell FFI
by rajendra prasad 28 Mar '12

28 Mar '12

Hi, I am trying to load the DLL(Wrapper.dll) in my code(Main.hs). When I am placing the dll in local directory, I am able to load it through following command: ghci Main.hs -L. -lWrapper But, I am not able to load it if I am putting it in some other directory(../../bin). I used the following command: ghci Main.hs -L../../bin/ -lWrapper I doubt I am not using the correct way to specify the path of dll in the command. Please correct me if I am wrong. Thanks in advance. Regards, Rajendra

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Happstack routing + boomerang + non-ascii urls
by Semen Trygubenko 27 Mar '12

27 Mar '12

Dear Haskell-Cafe, Last weekend I was test driving Web.Routes.Boomerang. A great package … Is there any way I could make it work with urls containing non-ASCII characters? Happstack.Server routing using "path" handles non-ASCII, as in, for instance, dir "work" $ path $ workHandler , where workHandler gets a correctly decoded path segment passed in as a String. The following, however, sitemap :: Router Sitemap sitemap = ( rWork . anyString ) passes to Work constructor an incorrectly decoded String. All the plumbing re embedding boomerang route into a normal Happstack one, etc. is done as per example from crash course here: http://happstack.com/docs/crashcourse/WebRoutes.html#web-routes-boomerang Many thanks, Sem -- Семен Тригубенко

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