- Haskell-Cafe - Haskell.org

haskell/clojure job opprtunity
by Vagif Verdi 10 Nov '11

10 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 09 Nov '11

09 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 09 Nov '11

09 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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capri cabal-dev virtualenv cab
by Rustom Mody 08 Nov '11

08 Nov '11

Does anyone give me a little comparison of these? What would all my requirements be? Not sure... these seem important for me 1. Need to sandbox not just haskell-projects but ghc (different compilations/versions) itself 2. Stability of the (sandboxing) tool 3. Correcting the deficiencies of cabal (eg inability to uninstall, quarrels between cabal and apt etc) OS: Debian/testing System haskell: 7.0.4 My question on beginners for a little more context: http://www.haskell.org/pipermail/beginners/2011-November/008943.html capri: http://www.haskell.org/haskellwiki/Capri virtualenv: https://github.com/Paczesiowa/virthualenv cabal-dev: http://corp.galois.com/blog/2010/12/20/cabal-dev-sandboxed-development-buil… cab: http://www.mew.org/~kazu/proj/cab/en/

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Send SMTP email with plain text authentication
by Justin Greene 08 Nov '11

08 Nov '11

Hi All, I am attempting to send email to an SMTP server that uses plain text authentication. So far I have tried both the SMTPClient ( http://hackage.haskell.org/package/SMTPClient-1.0.4) and HaskellNet ( http://hackage.haskell.org/package/HaskellNet) The SMTPClient library works fine for connecting to a non-restricted local SMTP server, but doesn't seem to have any ability to authenticate, so unless it gets updated I believe it's out of the picture (unless I'm wrong about it not being able to authenticate). The HaskellNet SMTP bindings are having problems connecting to my local postfix server even without authentication involved. I get a "connect: does not exist (Connection refused)" error. This seems odd to me since the SMTPClient code connects to my local server just fine, so I don't believe this is a postfix configuration issue (I could be wrong here of course). Here is the code I am currently using: http://hpaste.org/53764 Any help is appreciated. Thanks,

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Putting constraints on "internal" type variables in GADTs
by Anupam Jain 08 Nov '11

08 Nov '11

Hi all, I was trying to do something very simple with GADTs when I ran into this problem - -- My datatype data T o where Only ∷ o → T o TT ∷ T o1 → (o1 → o2) → T o2 -- Show instance for debugging instance Show o ⇒ Show (T o) where show (Only o) = "Only " ⊕ (show o) show (TT t1 f) = "TT (" ⊕ (show t1) ⊕ ")" When I try to compile this I get the following - Could not deduce (Show o1) arising from a use of `show' from the context (Show o) While I understand why I get this error, I have no idea how to fix it! I cannot put a Show constraint on o1 because that variable is not exposed in the type of the expression. I can work around this by changing my data type declaration to include Show constraints but I don't want to restrict my data type to only Showable things just so I could have a "Show" instance for debugging - Only ∷ Show o ⇒ o → T o TT ∷ (Show o1, Show o2) ⇒ T o1 → (o1 → o2) → T o2 What else can I do to declare a Show instance for my datatype? Thanks, Anupam Jain

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Project Euler Problem 357 in Haskell
by mukesh tiwari 08 Nov '11

08 Nov '11

Hello all Being a Haskell enthusiastic , first I tried to solve this problem in Haskell but it running for almost 10 minutes on my computer but not getting the answer. A similar C++ program outputs the answer almost instant so could some one please tell me how to improve this Haskell program. import Control.Monad.ST import Data.Array.ST import Data.Array.Unboxed import Control.Monad prime :: Int -> UArray Int Bool prime n = runSTUArray $ do arr <- newArray ( 2 , n ) True :: ST s ( STUArray s Int Bool ) forM_ ( takeWhile ( \x -> x*x <= n ) [ 2 .. n ] ) $ \i -> do ai <- readArray arr i when ( ai ) $ forM_ [ i^2 , i^2 + i .. n ] $ \j -> do writeArray arr j False return arr pList :: UArray Int Bool pList = prime $ 10 ^ 8 divPrime :: Int -> Bool divPrime n = all ( \d -> if mod n d == 0 then pList ! ( d + div n d ) else True ) $ [ 1 .. truncate . sqrt . fromIntegral $ n ] main = putStrLn . show . sum $ [ if and [ pList ! i , divPrime . pred $ i ] then pred i else 0 | i <- [ 2 .. 10 ^ 8 ] ] C++ program which outputs the answer almost instant. #include<cstdio> #include<iostream> #include<vector> #define Lim 100000001 using namespace std; bool prime [Lim]; vector<int> v ; void isPrime () { for( int i = 2 ; i * i <= Lim ; i++) if ( !prime [i]) for ( int j = i * i ; j <= Lim ; j += i ) prime [j] = 1 ; for( int i = 2 ; i <= Lim ; i++) if ( ! prime[i] ) v.push_back( i ) ; //cout<<v.size()<<endl; //for(int i=0;i<10;i++) cout<<v[i]<<" ";cout<<endl; } int main() { isPrime(); int n = v.size(); long long sum = 0; for(int i = 0 ; i < n ; i ++) { int k = v[i]-1; bool f = 0; for(int i = 1 ; i*i<= k ; i++) if ( k % i == 0 && prime[ i + ( k / i ) ] ) { f=1 ; break ; } if ( !f ) sum += k; } cout<<sum<<endl; } Regards Mukesh Tiwari

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Re: [Haskell-cafe] cabal install: Could not find module `Text.XML.HXT.Arrow'
by dokondr 08 Nov '11

08 Nov '11

On Tue, Nov 8, 2011 at 2:31 PM, Audun Skaugen <audunskaugen(a)gmail.com>wrote: > Erik Hesselink wrote: > >> On Tue, Nov 8, 2011 at 12:16, Ivan Lazar Miljenovic >> <ivan.miljenovic(a)gmail.com> wrote: >> >> >>> On 8 November 2011 22:10, Erik Hesselink <hesselink(a)gmail.com> wrote: >>> >>> >>>> This is because hSimpleDB doesn't specify version ranges on its >>>> dependencies, when it should. Since hxt changed its module structure >>>> going from 9.0 to 9.1, hSimpleDB doesn't build against 9.0. >>>> >>>> You can try to build it by adding '--constraint=hxt==9.0.\*' after >>>> your cabal-install command. You can also ask the author to add version >>>> ranges to the package. >>>> >>>> >>> Is the escape needed if you're using single quotes? >>> >>> >> >> I don't know. I always escape *s in shell commands to be sure the >> shell doesn't expand them, but in most of the cases, it probably works >> without them. It will likely even work without quotes and without the >> escape, unless you have files matching the pattern. >> >> > The single quote doesn't expand anything, so you don't need to escape the > *. Actually, this will result in the backslash also being passed to cabal, > so I would be surprised if your command works at all. > No matter how you escape *, you get the same error: Preprocessing library hSimpleDB-0.3... Building hSimpleDB-0.3... src/Network/AWS/Authentication.hs:47:7: Could not find module `Text.XML.HXT.Arrow':

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cabal install: Could not find module `Text.XML.HXT.Arrow'
by dokondr 08 Nov '11

08 Nov '11

Hi, On Mac OSX, ghc-6.12.3, I have successfully installed the 'hxt' package: http://hackage.haskell.org/package/hxt-8.5.2 Registering hxt-9.1.4... Installing library in /Users/user/.cabal/lib/hxt-9.1.4/ghc-6.12.3 Now when I try to install hSimpleDB ( http://hackage.haskell.org/package/hSimpleDB) I get the following error: cabal install hSimpleDB ... Registering HTTP-4000.0.9... Installing library in /Users/user/.cabal/lib/HTTP-4000.0.9/ghc-6.12.3 Registering HTTP-4000.0.9... Configuring hSimpleDB-0.3... Preprocessing library hSimpleDB-0.3... Building hSimpleDB-0.3... src/Network/AWS/Authentication.hs:47:7: Could not find module `Text.XML.HXT.Arrow': Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: hSimpleDB-0.3 failed during the building phase. The exception was: ExitFailure 1 Any ideas how to solve this? Thanks! Dmitri

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Quicksort in Haskell
by yrazes 08 Nov '11

08 Nov '11

module Main (main) where quicksort [] = [] quicksort (s:xs) = quicksort [x|x <- xs,x < s] ++ [s] ++ quicksort [x|x <- xs,x >= s] main = putStr "Ingrese la lista\n" [xs] <- getLine

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zlib build failure on recent GHC
by Ben Gamari 07 Nov '11

07 Nov '11

With GHC 1ece7b27a11c6947f0ae3a11703e22b7065a6b6c zlib fails to build, apparently due to Safe Haskell (bug 5610 [1]). The error is specifically, $ cabal install zlib Resolving dependencies... Configuring zlib-0.5.3.1... Preprocessing library zlib-0.5.3.1... Building zlib-0.5.3.1... [1 of 5] Compiling Codec.Compression.Zlib.Stream ( dist/build/Codec/Compression/Zlib/Stream.hs, dist/build/Codec/Compression/Zlib/Stream.o ) Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable result type in foreign declaration: IO CInt Safe Haskell is on, all FFI imports must be in the IO monad When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt ... This is a little strange since, a) It's not clear why Safe Haskell is enabled b) The declarations in question seem to be valid Does this seem like a compiler issue to you? Cheers, - Ben [1] http://hackage.haskell.org/trac/ghc/ticket/5610

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FFI / enums
by Johannes Waldmann 07 Nov '11

07 Nov '11

Does this work with "ghc -XForeignFunctionInterface" ? http://en.wikibooks.org/wiki/Haskell/FFI#Enumerations I am getting a syntax error right after "#{". But then, the description is for "hsc2hs" (not ghc). And it mentions macros, while C meanwhile has "real" enums. Well, at leat one can write "enum match { foo, bar }". The FFI Addendum does not mention enums at all. So - what's the agreeable way to do this? - Thanks, J.W.

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Message
by Goutam Tmv 07 Nov '11

07 Nov '11

Would you ever see yourself write a web application like Twitter or Facebook in Haskell?

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[ANNOUNCE] CoreErlang-0.0.2 + maintainer change
by Alex Kropivny 07 Nov '11

07 Nov '11

Hello, With the authors blessing, I'm taking over maintenance for http://hackage.haskell.org/package/CoreErlang, a parser/printer for an intermediate language for the Erlang VM. Changes in 0.0.2: - fixed float parse bug - added Data,Typeable instances to AST Package is rather esoteric, but it's nice for static source analysis - and when someone decides they want to compile a new language to the ERTS, the package is ready... Regards, Alex Kropivny

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Time zones and IO
by Ben Gamari 07 Nov '11

07 Nov '11

Recently I've been working on an iCalendar parser implementation with support for recurrence rules. For the recurrence logic, I've been relying on Chris Heller's excellent time-recurrence package. Unfortunately, it seems we have both encountered great difficulty in cleanly handling time zones. For example, as seen in the case of the CalendarTime LocalTime instance[1], Chris does the following, instance CalendarTimeConvertible LocalTime where toCalendarTime (LocalTime day t) = CalendarTime (fromEnum $ todSec t) (todMin t) (todHour t) d (toEnum m) y weekDay yearDay tz where (y, m, d, weekDay, yearDay) = dayInfo day tz = unsafePerformIO getCurrentTimeZone Ouch. Unfortunately, I have been entirely unable to come up with any better way to deal with timezones in a referentially transparent way. Passing the current timezone as an argument seems heavy-handed and reveals the leakiness of the CalendarTimeConvertible abstraction. Short of making toCalendarTime an action, can anyone see a suitable way of refactoring CalendarTimeConvertible to avoid unsafe IO? Perhaps a TimeZoneInfo monad is in order, exposing lookup of arbitrary (through timezone-olson) as well the current timezone? It seems like this would be inconvenient and overkill, however. Ideas? - Ben [1] https://github.com/hellertime/time-recurrence/blob/master/src/Data/Time/Cal…

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Is it possible to get the information of instances of a type?
by Magicloud Magiclouds 07 Nov '11

07 Nov '11

Hi, If this was in ruby or other languages that support reflection, it won't be a question. But in Haskell, could I write a code to list the classes that a type instanced? TemplateHaskell as well. -- 竹密岂妨流水过山高哪阻野云飞

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memorize function with number parameterized types in GHC
by Bin Jin 07 Nov '11

07 Nov '11

Hi, everyone I'm recently trying to implement the Montgomery reduction algorithm[1] in Haskell, the code can be found on my Github page[2]. After doing some benchmark testing I found that the library works rather slow. With the help of `trace` function from Debug.Trace, I found that GHC is not magical enough to memorize values with the same type(well, it's actually dynamically generated number parameterized type). I used binary representation to handle all positive numbers in type system. > data One = One > data D0 a = D0 a > data D1 a = D1 a > class PostiveN a where > p2num :: (Num b, Bits b) => a -> b > instance PostiveN One ... > instance PostiveN a => PostiveN (D0 a) ... > instance PostiveN a => PostiveN (D1 a) ... 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. So my question is, how to force GHC memorizing this kind of functions? [1]: http://en.wikipedia.org/wiki/Montgomery_reduction [2]: https://github.com/bjin/montg-reduce Regards, Bin

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Re: [Haskell-cafe] memorize function with number parameterized types in GHC
by DavidA 06 Nov '11

06 Nov '11

Bin Jin <bjin1990 <at> gmail.com> writes: > > > In order to represent Z/nZ, regular way need to store two integers, > and a data constructer. With number parameterized types, newtype+Integer > can be used, it's much more efficient. For this project, montg reduce > require to calculate a key for each modulus, but the modulus won't be > changed within Num typeclass. It's better to fix it in type system. > On Nov 7, 2011 1:38 AM, "DavidA" <polyomino <at> f2s.com> wrote: > What's the purpose of all the type trickery? > Why not just implement the algorithm using Integer? > _______________________________________________ > Haskell-Cafe mailing listHaskell-Cafe <at> haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe <at> haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > Ok, but even if you want to represent the inputs and outputs to be in Z/nZ, that doesn't mean that you have to work in Z/nZ while performing the algorithm. I think it would probably be faster to convert from Z/nZ to Integer, do the algorithm, and then convert the result back to Z/nZ. Anyway, good luck, and if you succeed in getting it fast enough, I hope you'll make the results available somewhere.

1 0

Cabal install problem
by Victor Miller 06 Nov '11

06 Nov '11

Lately whenever I try to install a cabal package it fails with the following error message: Could not find module `Control.Monad.State': Perhaps you haven't installed the profiling libraries for package `mtl-2.0.1.0'? Use -v to see a list of the files searched for. The strange thing is that I can load Control.Monad.State from ghci. How do I fix this -- i.e. where should I look?

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[ANNOUNCE] cereal-0.3.4.0
by Trevor Elliott 06 Nov '11

06 Nov '11

Hi Everyone, I'm happy to announce version 0.3.4.0 of the cereal serialization library. This release includes a patch from Adam Foltzer to provide support for IEEE754 encoded Float/Double values. This functionality is exposed through Serialize instances for both types, and specialized get/put functions in the new Data.Serialize.IEEE754 module. Happy hacking! --trevor

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How do you describe systems in general with Haskell?
by Grigory Sarnitskiy 06 Nov '11

06 Nov '11

If you are to describe a system, which consists of several subsystems, how do you approach the problem? What types, classes, functions whatever do you introduce? I guess it is a common problem, is there a general method? Just to describe, not to solve (though if the description implies the solution then it is wonderful). Obviously this is not just a haskell-specific problem, but I think there might be people who are aware of the best current solution (category theory?). For a dummy example, how would you describe the system from a fox, goose and bag of beans puzzle (wolf, goat and cabbage)?

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New CoreErlang maintainer + patch
by Alex Kropivny 05 Nov '11

05 Nov '11

Hello, I've got a patch [1] for the CoreErlang package. It fixes a parsing bug and adds Typeable+Data instances to the syntax tree. I was unable to contact the developers/maintainers via provided email. (No response in ~3 weeks.) The package hasn't been touched for three years, since first uploaded. I would like to take over maintenance of the package, and upload the fix ASAP. Regards, Alex Kropivny [1] https://github.com/amtal/CoreErlang/tree/0.0.2

1 0

Is generic information dumpable?
by Magicloud Magiclouds 04 Nov '11

04 Nov '11

Hi, I am learning the new generic feature of ghc. I'd have to say, it is harder than template to enter. When I started to learn template, ghc -ddump-slices really helped. So I wonder if I could get the representation generated when deriving Generic. I think that would be a great help. -- 竹密岂妨流水过山高哪阻野云飞

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ANN: websockets 0.4.0.0
by Jasper Van der Jeugt 04 Nov '11

04 Nov '11

Hello all, I'm pleased to announce the release of websockets [1] 0.4.0.0 today. You can grab it on Hackage [2]. This update brings an (almost completely rewritten) library which has support for multiple versions of the protocol, meaning you can choose to write backwards-compatible applications, or use the latest and greatest, all using a common API. All feedback is obviously welcome, I'd be glad to hear what you guys think. For the next version, client-side support is planned, so the library user can write clients in addition to servers. If there's any other feature you would like to see, feel free to let me know. [1]: http://jaspervdj.be/websockets/ [2]: http://hackage.haskell.org/package/websockets Cheers, Jasper

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 99, Issue 9
by yrazes 04 Nov '11

04 Nov '11

Hi This is a very simple basic case, but I have a little trouble with this... I hope somebody can help me main :: IO() main = return :: f tipos :: Int -> Int -> Int -> Float tipos a b c f = b / a * c where (a,b,c) = (8,3,15) Julita On Thu, Nov 3, 2011 at 11:10 PM, <haskell-cafe-request(a)haskell.org> wrote: > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Re: How to speedup generically parsing sum types? > (Jos? Pedro Magalh?es) > 2. Re: How to speedup generically parsing sum types? (Bas van Dijk) > 3. Re: The type class wilderness + Separating instances and > implementations into separate packages (Ryan Newton) > 4. Re: How to speedup generically parsing sum types? (Bas van Dijk) > 5. Re: The type class wilderness + Separating instances and > implementations into separate packages (Bas van Dijk) > 6. Re: How to speedup generically parsing sum types? (Claus Reinke) > 7. Re: How to speedup generically parsing sum types? (Bas van Dijk) > 8. [ANNOUNCE] cereal-0.3.4.0 (Trevor Elliott) > 9. System calls and Haskell threads (Andreas Voellmy) > 10. Re: System calls and Haskell threads (Johan Tibell) > 11. Is generic information dumpable? (Magicloud Magiclouds) > 12. Re: Is generic information dumpable? (Jos? Pedro Magalh?es) > 13. Re: How to speedup generically parsing sum types? > (Twan van Laarhoven) > 14. compiler construction (Timo von Holtz) > 15. Re: System calls and Haskell threads (David Barbour) > 16. Re: System calls and Haskell threads (David Barbour) > 17. Re: How to speedup generically parsing sum types? (Bas van Dijk) > 18. Re: compiler construction (Sean Leather) > 19. Re: compiler construction (Thomas Schilling) > 20. Mailing lists at haskell.org (Giovanni Tirloni) > 21. Re: Mailing lists at haskell.org (Bas van Dijk) > 22. Re: Is generic information dumpable? (Bas van Dijk) > 23. Re: Is generic information dumpable? (Jos? Pedro Magalh?es) > 24. Re: Is generic information dumpable? (Bas van Dijk) > 25. Re: Message (Ryan Newton) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 3 Nov 2011 11:49:17 +0000 > From: Jos? Pedro Magalh?es <jpm(a)cs.uu.nl> > Subject: Re: [Haskell-cafe] How to speedup generically parsing sum > types? > To: Bas van Dijk <v.dijk.bas(a)gmail.com> > Cc: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: > <CA+ZFbn_N27z_OABxHr1DPGRE+4m-t1BkK=-W+bDLvp66UC7G4Q(a)mail.gmail.com > > > Content-Type: text/plain; charset="iso-8859-1" > > Hi Bas, > > First of all, thanks for these numbers. I have previously compared the > performance of GP libs [1] and your results confirm what I would expect, > except for that last one, BigSum/fromJSON/generic. > > It's good that you're using INLINE pragmas on the generic function already. > What I would also try: > - Compile with -O2 and -fno-spec-constr-count (this last one is > particularly important) > - Add {-# INLINE [1] #-} pragmas to the to/from methods of your Generic > instances. > > To add these INLINE pragmas you will have to give your own instances of > Generic (so you can't derive them). I'd suggest you get hold of them with > -ddump-deriv, copy-paste and add the pragmas, just for testing purposes. > The phase is important: you first want to make sure you inline the generic > function definition, and only then the from/to. > > Please keep me posted on the effects of these suggestions. In particular, > if the INLINE pragmas on the from/to methods are essential, I'll be happy > to add them to the derived instances. > > > Cheers, > Pedro > > [1] http://dreixel.net/research/pdf/ogie.pdf > > 2011/11/3 Bas van Dijk <v.dijk.bas(a)gmail.com> > > > Hello, > > > > I recently added default generic implementations of toJSON and > > parseJSON to the aeson package. Now I'm optimizing them. Here are some > > benchmark results that compare: > > > > * th: toJSON and fromJSON generated by template-haskell. Can be > > compared to hand-written code. Should be the fastest of all. > > > > * syb: toJSON and fromJSON from the Data.Aeson.Generic module. Uses > > the Data type class. > > > > * generic: my toJSON and fromJSON using GHC Generics. > > > > The benchmark itself can be found here: > > > > > https://github.com/basvandijk/aeson/blob/optimizations/benchmarks/AesonComp… > > > > toJSON > > ====== > > > > D/toJSON/th 3.631734 us > > D/toJSON/syb 32.66679 us > > D/toJSON/generic 3.371868 us > > > > BigRecord/toJSON/th 8.982990 us > > BigRecord/toJSON/syb 48.90737 us > > BigRecord/toJSON/generic 8.971597 us > > > > BigProduct/toJSON/th 1.578259 us > > BigProduct/toJSON/syb 29.21153 us > > BigProduct/toJSON/generic 1.623115 us > > > > BigSum/toJSON/th 51.81214 ns > > BigSum/toJSON/syb 1.256708 us > > BigSum/toJSON/generic 71.32851 ns > > > > > > fromJSON > > ======== > > > > D/fromJSON/th 7.017204 us > > D/fromJSON/syb 23.46567 us > > D/fromJSON/generic 7.968974 us > > > > BigRecord/fromJSON/th 8.513789 us > > BigRecord/fromJSON/syb 36.64501 us > > BigRecord/fromJSON/generic 10.07809 us > > > > BigProduct/fromJSON/th 2.430677 us > > BigProduct/fromJSON/syb 17.97764 us > > BigProduct/fromJSON/generic 2.201130 us > > > > BigSum/fromJSON/th 414.8699 ns > > BigSum/fromJSON/syb 4.113170 us > > BigSum/fromJSON/generic 13.62614 us !!! > > > > > > As can be seen, in most cases the GHC Generics implementation is much > > faster than SYB and just as fast as TH. I'm impressed by how well GHC > > optimizes the code! > > > > Unfortunately the last benchmark, generically parsing a big sum type, > > is much slower. The code for parsing sums, which can be found here: > > > > > > > https://github.com/basvandijk/aeson/blob/optimizations/Data/Aeson/Types/Int… > > > > is basically this: > > > > > > instance (GFromSum a, GFromSum b) => GFromJSON (a :+: b) where > > gParseJSON (Object (M.toList -> [keyVal])) = gParseSum keyVal > > gParseJSON v = typeMismatch "sum (:+:)" v > > {-# INLINE gParseJSON #-} > > > > > > class GFromSum f where > > gParseSum :: Pair -> Parser (f a) > > > > instance (GFromSum a, GFromSum b) => GFromSum (a :+: b) where > > gParseSum keyVal = (L1 <$> gParseSum keyVal) <|> > > (R1 <$> gParseSum keyVal) > > {-# INLINE gParseSum #-} > > > > instance (Constructor c, GFromJSON a, ConsFromJSON a) => > > GFromSum (C1 c a) where > > gParseSum (key, value) > > | key == pack (conName (undefined :: t c a p)) = > > gParseJSON value > > | otherwise = notFound $ unpack key > > {-# INLINE gParseSum #-} > > > > > > notFound :: String -> Parser a > > notFound key = fail $ "The key \"" ++ key ++ "\" was not found" > > {-# INLINE notFound #-} > > > > > > Any idea how to make it faster? > > > > Regards, > > > > Bas > > >

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Mailing lists at haskell.org
by Giovanni Tirloni 03 Nov '11

03 Nov '11

Hi, Does anyone know what's the procedure to create a new mailing list at haskell.org for a forming user group? Thank you, -- Giovanni

2 1

compiler construction
by Timo von Holtz 03 Nov '11

03 Nov '11

Hi, I study computer science in Kiel, Germany and I want to study abroad. Now I look for Universities, which offer compiler construction, since I need that course, preferably in the UK, Ireland, Australia or New Zealand. Ideally it would be in Haskell of course. Greetings Timo

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How to speedup generically parsing sum types?
by Bas van Dijk 03 Nov '11

03 Nov '11

Hello, I recently added default generic implementations of toJSON and parseJSON to the aeson package. Now I'm optimizing them. Here are some benchmark results that compare: * th: toJSON and fromJSON generated by template-haskell. Can be compared to hand-written code. Should be the fastest of all. * syb: toJSON and fromJSON from the Data.Aeson.Generic module. Uses the Data type class. * generic: my toJSON and fromJSON using GHC Generics. The benchmark itself can be found here: https://github.com/basvandijk/aeson/blob/optimizations/benchmarks/AesonComp… toJSON ====== D/toJSON/th 3.631734 us D/toJSON/syb 32.66679 us D/toJSON/generic 3.371868 us BigRecord/toJSON/th 8.982990 us BigRecord/toJSON/syb 48.90737 us BigRecord/toJSON/generic 8.971597 us BigProduct/toJSON/th 1.578259 us BigProduct/toJSON/syb 29.21153 us BigProduct/toJSON/generic 1.623115 us BigSum/toJSON/th 51.81214 ns BigSum/toJSON/syb 1.256708 us BigSum/toJSON/generic 71.32851 ns fromJSON ======== D/fromJSON/th 7.017204 us D/fromJSON/syb 23.46567 us D/fromJSON/generic 7.968974 us BigRecord/fromJSON/th 8.513789 us BigRecord/fromJSON/syb 36.64501 us BigRecord/fromJSON/generic 10.07809 us BigProduct/fromJSON/th 2.430677 us BigProduct/fromJSON/syb 17.97764 us BigProduct/fromJSON/generic 2.201130 us BigSum/fromJSON/th 414.8699 ns BigSum/fromJSON/syb 4.113170 us BigSum/fromJSON/generic 13.62614 us !!! As can be seen, in most cases the GHC Generics implementation is much faster than SYB and just as fast as TH. I'm impressed by how well GHC optimizes the code! Unfortunately the last benchmark, generically parsing a big sum type, is much slower. The code for parsing sums, which can be found here: https://github.com/basvandijk/aeson/blob/optimizations/Data/Aeson/Types/Int… is basically this: instance (GFromSum a, GFromSum b) => GFromJSON (a :+: b) where gParseJSON (Object (M.toList -> [keyVal])) = gParseSum keyVal gParseJSON v = typeMismatch "sum (:+:)" v {-# INLINE gParseJSON #-} class GFromSum f where gParseSum :: Pair -> Parser (f a) instance (GFromSum a, GFromSum b) => GFromSum (a :+: b) where gParseSum keyVal = (L1 <$> gParseSum keyVal) <|> (R1 <$> gParseSum keyVal) {-# INLINE gParseSum #-} instance (Constructor c, GFromJSON a, ConsFromJSON a) => GFromSum (C1 c a) where gParseSum (key, value) | key == pack (conName (undefined :: t c a p)) = gParseJSON value | otherwise = notFound $ unpack key {-# INLINE gParseSum #-} notFound :: String -> Parser a notFound key = fail $ "The key \"" ++ key ++ "\" was not found" {-# INLINE notFound #-} Any idea how to make it faster? Regards, Bas

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System calls and Haskell threads
by Andreas Voellmy 03 Nov '11

03 Nov '11

I just read Kazu Yamamoto's article on a high performance web server in the latest Monad.Reader, and I came across a statement that doesn't sound correct to me. He says: "When a user thread issues a system call, a context switch occurs. This means that all Haskell user threads stop, and instead the kernel is given the CPU time. " Is this right? I thought that when a system call is made by a Haskell thread being run by a particular worker thread on a CPU, other runnable Haskell threads in the run queues of the HECs for other CPUs can continue running concurrently (provided we've run our Haskell program with multiple CPUs using the -Nx RTS argument). That's what I understood from the discussion of foreign calls in "Runtime Support for Multicore Haskell". Andreas

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The type class wilderness + Separating instances and implementations into separate packages
by Ryan Newton 03 Nov '11

03 Nov '11

What is the right interface for a queue? What is the right interface for a random number generator? I don't know, but in both cases you will find many packages on hackage offering different takes on the matter. In fact, there is a wilderness of alternative interfaces. We've had various discussions on this list about the number of alternative packages. I'm fine with lots of packages, but I think it would be great if not every package introduced a new interface as well as a new implementation. If we could agree as a community on common interfaces to use for some basics, that would probably go a long way towards taming the type class wilderness. People have mentioned this problem before with respect to "Collections" generally. One basic part of reaching such a goal is separating interface from implementation. I ran into the following problems just in the last 24 hours. In both cases I wanted to use a type class, but didn't want to depend on the whole package it lived in: - I wanted to use the Benchmarkable class in Criterion in my package. (Criterion deserving to be a "standard" package.) But I can't get that typeclass without depending on the whole Criterion package, which has several dependencies. And in fact on the machine I was on at the time some of those dependencies were broken, so I decided not to use Benchmarkable. - I wanted to use, or at least support, an existing class for Queues. I found the following: http://hackage.haskell.org/packages/archive/queuelike/1.0.9/doc/html/Data-M… I have no idea who uses this package. But because this package (like most packages) includes the implementation along with the interface it introduces additional dependency liabilities. In this case it failed to build on GHC 7.2 so I quickly gave up on supporting that TypeClass. How can we enumerate packages that at least purport to provide standard interfaces that you should both use and pick up to implement? On a Wiki page? -Ryan P.S. I'm working on mutable concurrent Deques right now and am trying to follow my own prescription above by creating an abstract interface in a separate package. See below if you would like to offer feedback on that interface. -------------------------------------- My ultimate goal is an "abstract-deque" parameterizable interface that abstracts over bounded/unbounded, concurrent/non-concurrent, single, 1.5, and double-ended queues, which would include both, say, Michael & Scott linked queues and the Chase-Lev work-stealing deques. An aggregated queue package could use type families to dispatch to the best available queue implementation for a given configuration. I've got a couple drafts of how this might work. They're in different branches here: https://github.com/rrnewton/haskell-lockfree-queue/blob/master/AbstractDequ… https://github.com/rrnewton/haskell-lockfree-queue/blob/one-type-class/Abst… One of them uses an associated data type family, the other an unassociated one. The type family has a bunch (six) phantom type parameters, and the unassociated one allows hiding those parameters at the expense of introducing more type classes. MegaQueue.hs will be used to aggregate together different queue implementations. The end result is that someone can create a new queue by setting all the switches on the type, as follows: test = do q :: Deque NT T SingleEnd SingleEnd Bound Safe Int <- newQ pushL q 33 x <- tryPopR q print x return q With those settings, requiring only single-ended rather than double-ended queues, the above code can use the LinkedQueue (Michael and Scott) implementation included in that repo. That little test, by the way, segfaults for me on Mac OS under GHC 7.2.1 even WITHOUT using casMutVar# (It's using Data.CAS.Fake presently.) I'll file a bug report after I sanity check it some more. Disregarding that... the big problem in this case is the* inability to create overlapping type family instances. * In this case what we dearly WANT to do is specialize some subset of the 64-mode configuration space and then have a "fallback". You can take a look at my struggling in MegaDeque. Both of my approaches require specifying explicitly the modes that you don't cover. Worse, we may want to specialize on element type as well. For example, for simple scalar types (or even Storable types) it may be desirable to use something foreign, like TBB queues. But in that case, there's no way to enumerate the types NOT specialized. As far as I know there is no way for type families to accomplish this (specialize, say "Int", and have a fallback for everything else). In general, is there a recognized work-around for this? For example, is this a place where using functional dependencies instead might do the trick? Also, there are some software-engineering issues here with respect to* where to put the instances*. It would be nice to put the instances for DequeClass inside the individual implementations (like LinkedQueue.hs). But that leads to problems because it's really the job of MegaQueue.hs to figure out how to spread implementations over the configuration-space. And once you put the instances in LinkedQueue.hs there is of course *no way to qualify or hide them upon import....* Feedback welcome. Cheers, -Ryan

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Re: [Haskell-cafe] Tell cabal-install to generate only shared libraries
by Yves Parès 03 Nov '11

03 Nov '11

Apparently, disabling library vanilla causes GHC not to generate the ".hi" files : For instance when 'cabal install quickcheck --disable-library-vanilla': Registering QuickCheck-2.4.1.1... cabal: QuickCheck-2.4.1.1: file Test/QuickCheck/All.hi is missing (use --force to override) QuickCheck-2.4.1.1: file Test/QuickCheck/Function.hi is missing (use --force to override) [...] override) QuickCheck-2.4.1.1: file Test/QuickCheck/State.hi is missing (use --force to override) QuickCheck-2.4.1.1: file Test/QuickCheck/Exception.hi is missing (use --force to override) QuickCheck-2.4.1.1: cannot find libHSQuickCheck-2.4.1.1.a on library path (use --force to override) 2011/11/3 Ivan Perez <ivanperezdominguez(a)gmail.com> > 2011/11/2 Yves Parès <limestrael(a)gmail.com>: > > 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? > > --disable-library-vanilla > > Do not build ordinary libraries. This is useful in conjunction > with --enable-library-profiling > to build only profiling libraries, rather than profiling and > ordinary libraries. > > Haven't tried it myself. Maybe it works in combination with > --enable-shared? > > From: http://haskell.org/ghc/docs/7.0.4/html/Cabal/builders.html > > > > > 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? > > The following two trac tickets mention a separate flag, > named --enable-executable-dynamic: > > http://hackage.haskell.org/trac/hackage/ticket/600 > http://hackage.haskell.org/trac/hackage/ticket/869 > > If that doesn't work, you can probably pass some argument to ghc with > --with-ghc-option > > Cheers, > Ivan > > > > > Thanks. > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > >

2 1

ghc 7.2.1 Generics problem
by Magicloud Magiclouds 03 Nov '11

03 Nov '11

Hi, I do not know why, my ghc 7.2.1 does not seem to support DeriveRepresentable. I compiled the ghc 7.2.1 myself by ghc 7.0.4. All options default. $ ghc Types/TopTalkerRecord.hs Types/TopTalkerRecord.hs:2:14: Unsupported extension: DeriveRepresentable $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.2.1 -- 竹密岂妨流水过山高哪阻野云飞

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Ridiculously slow FFI, or cairo binding?
by Eugene Kirpichov 03 Nov '11

03 Nov '11

Hello, I've got two very simple programs that draw a very simple picture using cairo, doing a couple hundred thousand of cairo calls. One program is in C++. The other is in Haskell and uses the cairo library bindings. The C++ program completes in a fraction of a second, the Haskell program takes about 7-8 seconds to run. They produce exactly the same output. What could be at fault here? Why are the cairo bindings working so slow? (I suppose there isn't too much cairo-specific stuff here, perhaps it's a general FFI question?) #include "cairo.h" int main() { cairo_surface_t *surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 1024, 768); cairo_t *cr = cairo_create(surface); cairo_set_source_rgb(cr, 0, 255, 0); for(int x = 0; x < 1024; x += 2) for(int y = 0; y < 768; y += 2) { cairo_rectangle(cr, x, y, 1, 1); cairo_fill(cr); } cairo_surface_write_to_png(surface, "picture.png"); return 0; } module Main where import qualified Graphics.Rendering.Cairo as C import Control.Monad main = C.withImageSurface C.FormatARGB32 1024 768 $ \s -> do C.renderWith s $ do C.setSourceRGBA 0 255 0 255 forM_ [0,2..1024] $ \x -> do forM_ [0,2..768] $ \y -> do C.rectangle x y 1 1 C.fill C.surfaceWriteToPNG s "picture.png" -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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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 02 Nov '11

02 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 02 Nov '11

02 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 02 Nov '11

02 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 01 Nov '11

01 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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