- Haskell-Cafe - Haskell.org

haskell/clojure job opprtunity
by Vagif Verdi 11 Nov '11

11 Nov '11

Hello. My company is looking to hire a haskell developer. South California (San Dimas), full time job, local only (no telecommute) We use yesod (haskell web framework) for internal web application and web services, and compojure (clojure web framework) for customer facing web site. All development will be ether with haskell (if libraries permit) or clojure on JVM. If interested please contact me. Regards, Vagif Verdi

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howto best use emacs + tiling WM (Xmonad,DWM)
by kaffeepause73 10 Nov '11

10 Nov '11

Hello, I'm using dwm which I really love (ev. consider switching to xmonad). However when I'm working with emacs (programming haskell) und dwm I feel, I'm not as effecient as I eventually could be. -- I can have the shell in one window (to execute the compiled program), but most work happens in emacs (in one screen only). And I have up to buffers in emacs which I find hard to switch between. Even on Xinerama this doesn't really change. When I create new frames for emacs with strg-x 5-2 then I'm sort getting closer to where I would like to be, but then I find myself having to windows on which I switch buffers and I get "confused" again. What I would like is to tab through the buffers, as I walk through the screens in dwm/xmonad and see the list of buffers as id do so. -- And can directly jump to a specific buffer via e.g. mod-4. I'm thinking that each buffer in emacs gets one frame and occupies one screen and xmonad than gets dynamically 20 or more screens (like tabs). If there is an good solution within emacs and the emacs mode than I'm of course also more than open for it. (my experience with emacs so far: -- only 10 buffers are listed in menue -- speedbar is very nice but works for files not for buffers -- using "list all buffers" is sort of cluncy as it uses half the screen (my screen splits horizontally Thanks, Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/howto-best-use-emacs-tiling-WM-Xmonad-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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"Bounds checking" pragma?
by Artyom Kazak 10 Nov '11

10 Nov '11

Hello! The (!) operator is short and nice. Unfortunately, when doing heavy computing, we have to use "unsafeAt" instead. It looks ugly and it is ugly, also. Some compilers for imperative languages like Free Pascal have an option to turn on/off bounds checking for arrays. Wouldn't it be nice to have such option in GHC? Is it possible? There is a problem: Haskell has a lot of array libraries. The only solution I see is a new FLAG pragma: (!) :: Array i a -> i -> a --definition {-# FLAG "boundsCheck" (!) = unsafeAt #-} It is similar to RULES pragma, but only fires when flag is set. To set the flag you need to complile with option -flags="boundsCheck". Also, the mantainers of vector library, bytestring library, repa library and so on will have to include such pragmas in their code. I don't know about C++ preprocessor, though. Maybe this is already solvable with #define's... Anyway, I have to say it once again: unsafeAt is ugly and Haskell is beautiful. Why high-performance code should be ugly?

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Tool to brute-force test against hackage libraries to determine lower bounds?
by Ryan Newton 10 Nov '11

10 Nov '11

I don't know about you, but I personally haven't found the time to cast back in time for each of my package's dependencies to find a true lower bound version. Do we have any tools that would do the following? - ask Hackage for the available versions of package foo - use cabal-dev to build your package against foo-X.Y.Z forall {X,Y,Z} (but leaving other packages unconstrained) - report successes and failures, including last failure before the present version (and therefore lower bound, exclusive) Johan, would it make any sense to extend your Jenkins setup to do this? -Ryan

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Haskell Weekly News: Issue 207
by Daniel Santa Cruz 10 Nov '11

10 Nov '11

Welcome to issue 207 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of October 30 to November 05, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/11/haskell-weekly-news-issue-207.h… New and Updated Projects * hierarchical-clustering-diagrams (Felipe Almeida Lessa; 0.1) Library to draw diagrams of dendrograms. [1] http://goo.gl/7deyG * websockets (Jasper Van der Jeugt; 0.4.0.0) Major re-write to the library with support for multiple versions of the protocol. [2] http://goo.gl/i6bVr * cereal (Trevor Elliott; 0.3.4.0) Provides support for IEEE754 encoded Float/Double values. [3] http://goo.gl/GwZk8 Quotes of the Week * wyfrn: whats broken? my brain or ghci? * dons: Being really mean to your software, and following up when it breaks, makes it a lot better. But you have to be really, really mean. * hpc: and when you want to get really ridiculous, agda has universe polymorphism, which is when you don't know how high up the tower of turtles you have managed to climb Top Reddit Stories * Quick and dirty reinversion of control. Domain: blog.sigfpe.com, Score: 59, Comments: 7 On Reddit: [4] http://goo.gl/J2oon Original: [5] http://goo.gl/pQYGA * Running a Startup on Haskell [Presentation] Domain: infoq.com, Score: 57, Comments: 12 On Reddit: [6] http://goo.gl/uqfqN Original: [7] http://goo.gl/QMu5c * LLVM patch is merged for inclusion in LLVM 3.0 release | GHC on ARM Domain: ghcarm.wordpress.com, Score: 48, Comments: 2 On Reddit: [8] http://goo.gl/UNRN6 Original: [9] http://goo.gl/h6q7I * Thunks and lazy blackholes: an introduction to GHC at runtime Domain: mainisusuallyafunction.blogspot.com, Score: 39, Comments: 0 On Reddit: [10] http://goo.gl/nQN3U Original: [11] http://goo.gl/HqKaL * GHC feature request: warn about unused constraints -- "I think this could make for a nice project for someone wanting to dig into hacking on GHC" Domain: byorgey.wordpress.com, Score: 34, Comments: 16 On Reddit: [12] http://goo.gl/MIMCN Original: [13] http://goo.gl/GPt0B * Haskell Implementors Workshop 2011 Videos Online! Domain: neilmitchell.blogspot.com, Score: 31, Comments: 3 On Reddit: [14] http://goo.gl/MqszQ Original: [15] http://goo.gl/LUsFe * What Constraints Entail, Part 1 Domain: comonad.com, Score: 29, Comments: 0 On Reddit: [16] http://goo.gl/Tt41V Original: [17] http://goo.gl/SdGdm * [Newbie]: Haskell code is really slow compared to same code in Python. Can anyone help me find what the problem is ? Domain: self.haskell, Score: 29, Comments: 51 On Reddit: [18] http://goo.gl/15bb8 Original: [19] http://goo.gl/15bb8 * Please help me gather information about Haskell installations on Macs Domain: gist.github.com, Score: 26, Comments: 14 On Reddit: [20] http://goo.gl/Yfc32 Original: [21] http://goo.gl/xQACw * Why learning Functional Programming (and Haskell in particular) can be hard Domain: harishankar.org, Score: 25, Comments: 46 On Reddit: [22] http://goo.gl/8Xb3V Original: [23] http://goo.gl/FKlcj * How do you write nontrivial tests in QuickCheck? Domain: self.haskell, Score: 23, Comments: 15 On Reddit: [24] http://goo.gl/FL6jh Original: [25] http://goo.gl/FL6jh Top StackOverflow Questions * Calling Haskell from Java with C in between votes: 14, answers: 1 Read on SO: [26] http://goo.gl/RyN00 * Is “List” specially handled in Haskell's pattern matching? votes: 14, answers: 5 Read on SO: [27] http://goo.gl/y3hdi * Why does this simple use of the State monad cause a stack overflow? votes: 12, answers: 1 Read on SO: [28] http://goo.gl/bLlnX * Why does this Haskell code not terminate? votes: 10, answers: 4 Read on SO: [29] http://goo.gl/LStfr * Is it possible to make a type an instance of a class if its type parameters are in the wrong order? votes: 10, answers: 4 Read on SO: [30] http://goo.gl/aRh0b * How to make a type with restrictions votes: 9, answers: 3 Read on SO: [31] http://goo.gl/SRTfY * Desugaring do-notation for Monads votes: 9, answers: 1 Read on SO: [32] http://goo.gl/CqoQF * Equation from “Programming Pearls” - can somebody explain me? votes: 8, answers: 1 Read on SO: [33] http://goo.gl/kUr67 * ADT names. What is `Left a`, and then what is `a`, in Haskell? votes: 8, answers: 2 Read on SO: [34] http://goo.gl/IVoN0 * Why are boxed vectors so slow? votes: 8, answers: 1 Read on SO: [35] http://goo.gl/4RrQX Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.bio/156 2. http://permalink.gmane.org/gmane.comp.lang.haskell.web/2204 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/93644 4. http://blog.sigfpe.com/2011/10/quick-and-dirty-reinversion-of-control.html 5. http://www.reddit.com/r/haskell/comments/luyf4/quick_and_dirty_reinversion_… 6. http://www.infoq.com/presentations/Running-a-Startup-on-Haskell 7. http://www.reddit.com/r/haskell/comments/lv9ge/running_a_startup_on_haskell… 8. https://ghcarm.wordpress.com/2011/11/03/llvm-patch-is-merged-for-inclusion-… 9. http://www.reddit.com/r/haskell/comments/lzaa7/llvm_patch_is_merged_for_inc… 10. http://mainisusuallyafunction.blogspot.com/2011/10/thunks-and-lazy-blackhol… 11. http://www.reddit.com/r/haskell/comments/luxzj/thunks_and_lazy_blackholes_a… 12. https://byorgey.wordpress.com/2011/11/05/wanted-ghc-feature-warn-about-unus… 13. http://www.reddit.com/r/haskell/comments/m23h2/ghc_feature_request_warn_abo… 14. http://neilmitchell.blogspot.com/2011/11/haskell-implementors-workshop-2011… 15. http://www.reddit.com/r/haskell/comments/lwpzf/haskell_implementors_worksho… 16. http://comonad.com/reader/2011/what-constraints-entail-part-1/ 17. http://www.reddit.com/r/haskell/comments/lyqnx/what_constraints_entail_part… 18. http://www.reddit.com/r/haskell/comments/m03p9/newbie_haskell_code_is_reall… 19. http://www.reddit.com/r/haskell/comments/m03p9/newbie_haskell_code_is_reall… 20. https://gist.github.com/f8f0464fb76e9fa2cfa1 21. http://www.reddit.com/r/haskell/comments/lz26e/please_help_me_gather_inform… 22. http://harishankar.org/blog/entry.php/why-learning-functional-programming-a… 23. http://www.reddit.com/r/haskell/comments/lv84d/why_learning_functional_prog… 24. http://www.reddit.com/r/haskell/comments/lvupa/how_do_you_write_nontrivial_… 25. http://www.reddit.com/r/haskell/comments/lvupa/how_do_you_write_nontrivial_… 26. http://stackoverflow.com/questions/7961335/calling-haskell-from-java-with-c… 27. http://stackoverflow.com/questions/7992524/is-list-specially-handled-in-has… 28. http://stackoverflow.com/questions/7998458/why-does-this-simple-use-of-the-… 29. http://stackoverflow.com/questions/7960543/why-does-this-haskell-code-not-t… 30. http://stackoverflow.com/questions/8005372/is-it-possible-to-make-a-type-an… 31. http://stackoverflow.com/questions/7978191/how-to-make-a-type-with-restrict… 32. http://stackoverflow.com/questions/8019670/desugaring-do-notation-for-monads 33. http://stackoverflow.com/questions/7967337/equation-from-programming-pearls… 34. http://stackoverflow.com/questions/7969499/adt-names-what-is-left-a-and-the… 35. http://stackoverflow.com/questions/7978130/why-are-boxed-vectors-so-slow

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Symbol collisions with -fhpc
by Michael Orlitzky 10 Nov '11

10 Nov '11

Does anyone know what causes these? This used to work, like, a month ago. It builds fine without the -fhpc. $ ghc -fhpc -O2 -odir /tmp -hidir /tmp --make -o bin/spline3 src/*.hs [ 1 of 17] Compiling ScaleFactor ( src/ScaleFactor.hs, /tmp/ScaleFactor.o ) ... [13 of 17] Compiling Tetrahedron ( src/Tetrahedron.hs, /tmp/Tetrahedron.o ) /tmp/ghc29431_0/ghc29431_0.s: Assembler messages: /tmp/ghc29431_0/ghc29431_0.s:48147:0: Error: symbol `d2ln_info' is already defined /tmp/ghc29431_0/ghc29431_0.s:48158:0: Error: symbol `d2ll_info' is already defined /tmp/ghc29431_0/ghc29431_0.s:48169:0: Error: symbol `d2lh_info' is already defined

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Haskell Platform for OSX ?
by dokondr 10 Nov '11

10 Nov '11

Hi, I am running GHC 6.12.3 at Mac OSX and have numerous problems with 'cabal install' of different packages. For example: ~>cabal install mongoDB Resolving dependencies... Configuring mongoDB-1.1.0... Preprocessing library mongoDB-1.1.0... Building mongoDB-1.1.0... ... Control/Monad/MVar.hs:16:34: Module `Control.Exception.Control' does not export `mask' cabal: Error: some packages failed to install: mongoDB-1.1.0 failed during the building phase. The exception was: ExitFailure 1 Questions about current Haskell Platform for OSX 2011.2.0.1. : (http://hackage.haskell.org/platform/mac.html) 1) Where can I find release notes to understand what version of GHC Haskell Platform for OSX 2011.2.0.1 includes? 2) Will I be able to install mongoDB-1.1.0 on this release? 3) How to install it into a separate location so it would not ruin my current platform? Thanks! Dmitri

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Depth first search
by mukesh tiwari 09 Nov '11

09 Nov '11

Hello all I am trying to implement depth search in Haskell. My issue here is kind of backtracking. Haskell code for depth first search import Data.List import Data.Array import Control.Monad type Node = Int type Graph = Array Int [ Node ] dfs' :: Graph -> Node -> [ Node ] -> [ Node ] dfs' g v vis = dfsfun lst where lst = g ! v dfsfun [] = vis dfsfun ( x : xs ) | elem x vis = dfsfun xs | otherwise = dfs' g y ( v : vis ) --set the flag true if the graph is direct otherwise false buildGraph :: ( Int , Int ) -> [ ( Node , Node ) ] -> Bool -> Graph buildGraph bnds xs f | f = accumArray ( flip (:) ) [] bnds xs --direct graph a->b | otherwise = accumArray ( flip (:) ) [] bnds xss where --indirect a -> b and b -> a xss = foldr ( \ ( x , y ) b -> ( y , x ) : b ) xs xs dfsSearch :: Graph -> Int -> [ Node ] dfsSearch g v = dfs' g v [] Lets create a indirect graph ghci>let g = buildGraph ( 0 , 5 ) [ ( 0 , 1 ) , ( 0 , 2 ) , ( 0 , 3 ) , ( 0 , 4 ) ] False ghci>g array (0,5) [(0,[4,3,2,1]),(1,[0]),(2,[0]),(3,[0]),(4,[0]),(5,[])] ghci>dfsSearch g 0 [0] Here I am getting only 0 but it should return [ 0 , 1 , 2 , 3 , 4 ] . What is happening here when i am passing 0 as root node to function , at first level i get lst = [ 4 , 3, 2, 1 ] . First element of this list is 4 so 0 is added to vis list and now 4 is passed to dfs' as parent. When it goes down , we get lst = [0] and since 0 is member of vis list so it returns the vis as result . When we write dfs in c/c++ then it returns to calling function and again loops through remaining element which i am not able visualize in Haskell. Could some one please help me how to solve this issue . Regards Mukesh Tiwari

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A Brazilian community for Haskell
by Giovanni Tirloni 09 Nov '11

09 Nov '11

Hello, To gather all Haskellers in Brazil, a new mailing list has been created: http://projects.haskell.org/cgi-bin/mailman/listinfo/haskell-br There is also the #haskell-br channel on Freenode as well as Twitter and Google+ feeds to keep everybody updated on recent activities. More information at: http://www.haskell-br.org/ []'s -- Giovanni

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Re: [Haskell-cafe] memorize function with number parameterized types in GHC
by oleg＠okmij.org 09 Nov '11

09 Nov '11

Bin Jin wrote: > Here is a function that will be called everytime by `(*)` in `Num` typeclass > > montgKeys :: (PostiveN p, Integral a, Bits a) => p -> a > > as you can imagine, I always pass (undefined :: p) as parameter to > `montgKeys`, so if it's handled well, it should be memorized for > future usage. But tracing shows that both `p2num` and `montgKeys` are > evaluated every time being called. First of all, let us get rid of the argument p. Let's define > newtype W p a = W{unW:: a} then we can easily re-write montgKeys to give it the following signature: > > montgKeys :: (PostiveN p, Integral a, Bits a) => W p a You can use ScopedTypevariables to set the needed 'p' from the context. So, montgKeys becomes a polymorphic constant, quite like minBound. Now, the hope is that when the types p and a are determined, GHC could specialize montgKeys and turn it into a real constant. Perhaps some RULE or specialize pragmas may help...

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