- Haskell-Cafe - Haskell.org

hmatrix under ghci on x86_64
by Tom Doris 13 Feb '12

13 Feb '12

I'm using ghci + hmatrix and a few other packages as a Haskell based replacement for Matlab, everything works well so far in terms of available functionality. However, I have encountered an issue when running in ghci on x86_64 systems - calls into functions that in turn call gsl functions will result in a bus error, e.g: Prelude> :m +Numeric.Container Prelude Numeric.Container> randomVector 10 Gaussian 10 fromList Bus error (core dumped) I attached gdb and found the bus error was happening in gsl_rng_alloc() but some investigation indicate that the problem is probably due to this bug in ghci: http://hackage.haskell.org/trac/ghc/ticket/2912 which has been marked as a duplicate of http://hackage.haskell.org/trac/ghc/ticket/781 and a recent update to 781 indicates that it won't be addressed until at least v 7.6.1 (781 also references http://hackage.haskell.org/trac/ghc/ticket/3658 and it seems that this is a pretty large piece of work - moving to fully dynamically linked ghci which has been around for a while and pushed back a few times). Does anyone know of a workaround that would allow ghci to use wrapped gsl functionality on x86_64 systems in the meantime? Most linux boxes used by quants are x86_64 now, so this issue will impact many people who would like to use Haskell instead of Matlab. Thanks

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[ANNOUNCE] fb package, bindings to Facebook's API
by Felipe Almeida Lessa 13 Feb '12

13 Feb '12

Hello, everyone! I'm pleased to finally announce the fb package, a Haskell-only package that provides bindings to Facebook's API. This package is sponsored by my employer, which will soon enough be able to become the copyright holder of all of our open source contributions. http://hackage.haskell.org/package/fb It's by no means complete, but we thrive to be as correct as possible and to turn API errors into type errors caught at compile time. Right now the current version is fb 0.7.2. While this is the first announcement, this package is already being actively used by Facebook authentication code on Yesod (yesod-auth-fb package) and internally at my day job's projects. Hope you find it useful! =) Cheers, -- Felipe.

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Re: [Haskell-cafe] ANN: stm-conduit-0.2.1
by John Lato 13 Feb '12

13 Feb '12

> Message: 6 > Date: Sun, 12 Feb 2012 01:47:40 -0500 > From: wren ng thornton <wren(a)freegeek.org> > Subject: Re: [Haskell-cafe] ANN: stm-conduit-0.2.1 > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <4F37608C.3090808(a)freegeek.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 2/9/12 2:29 PM, Felipe Almeida Lessa wrote: >> Your package uses TMChans which AFAIK are unbounded. That means that >> if the writer is faster than the reader, then everything will be kept >> into memory. This means that using TMChans you may no longer say that >> your program uses a constant amount of memory. Actually, you lose a >> lot of your space reasoning since, being concurrent processes, you >> can't guarantee almost anything wrt. progress of the reader. > > Of course, you're free to use TBMChans instead, which are bounded :) This is what I did in iteratee-stm. The stm-chans package is very nice. (and the results with iteratee-stm were very good as well) John L.

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A universal data store interface
by Sergiu Ivanov 13 Feb '12

13 Feb '12

Hello, I am interested in applying for GSoC-2012 with the Haskell community. (I think I've told that to quite a number of people already, sorry for the duplicate.) I'm currently digging my way towards http://hackage.haskell.org/trac/ghc/ticket/4213 . However, instead of focusing on a single project right now, I'd like to diversify a bit and consider at least one more project. I've come over this: http://hackage.haskell.org/trac/summer-of-code/ticket/1605 . It looks quite interesting and also promising in the sense that it is going to be useful to many people. However, this page http://hackage.haskell.org/trac/summer-of-code/report/1 states the priority is undefined for this project, which makes me wonder whether the Haskell community would be interested in having a GSoC student work on the task. If the community is interested in having a student assigned to this task, I'd be glad to submit proposal for this project as well. Sergiu

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ghc-mod v1.10.3
by Kazu Yamamoto 13 Feb '12

13 Feb '12

Hello, I have just released ghc-mod v1.10.3. C-cC-t of this version was powered by Hideyuki Tanaka. The old implementation just show the type of the current expression on Emacs. Howver, the new implementation first show the type of the current expression and the expression is highlighted. If you type C-cC-t again, the region is enlarged to its upper expression, and its type is displayed. # I don't know well but this is called "annot" in the OCaml community Enjoy! --Kazu

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ANN: stm-conduit-0.2.1
by Clark Gaebel 12 Feb '12

12 Feb '12

Hi all, I've just released stm-conduit [1] on Hackage. This package introduces conduits to the wonderful world of concurrency. My package solves the common problem of constant bottleneck switching loaders have. This is when, for example, we stream XML from the disk and then parse the XML in one conduit pipeline. While it streams a file from the disk, the process is clearly IO bound, and while it parses the XML, the process is CPU bound. By putting each task on its own thread, the disk IO doesn't need to wait for the CPU to parse a document before loading the next file. By using stm-based conduits, we have full resource utilization. The way it does this is by creating a source and sink for TChans, letting us stream data between conduits and channels. There are more examples in the docs. Check it out! Regards, - clark [1] http://hackage.haskell.org/package/stm-conduit

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question regarding Data.Array.Accelerate
by Philipp 12 Feb '12

12 Feb '12

Hi, I have read the papers about the accelerate package that provides a high-level interface to nvidia's cuda library and am very intrigued. However, I have some start-up problems. Is this the right place for such questions? In particular, I'm having trouble with lifting and unlifting. From what I understood, I should be able to use unlift to get an Int from an Exp (Plain Int), but when I try, for example, (unlift . lift) (1::Int) :: Int ghci complains with "No instance for (Unlift Int)". I have the latest versions of accelerate (accelerate-0.9.0.1) and ghc (version 7.4.1). I think I'm missing something obvious, so if any of you could point it out to me I would be glad. Thanks, Philipp -- View this message in context: http://haskell.1045720.n5.nabble.com/question-regarding-Data-Array-Accelera… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Calling OpenGL functions from multiple threads
by Clark Gaebel 12 Feb '12

12 Feb '12

Do the OpenGL bindings on Hackage expose anything like glXMakeCurrent [1]? I need to use OpenGL functions from multiple threads (note: not at once, I protect it with an MVar), but that's forbidden in extension-less OpenGL. Thanks, - clark [1] http://www.talisman.org/opengl-1.1/Reference/glXMakeCurrent.html

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Any advices on optimizing an array processing in a pitch tuner
by Dmitry Vyal 12 Feb '12

12 Feb '12

Hello Haskellers, Recently I've revived my old toy project. It's a sound spectrum analyzer with Gtk2hs interface. I decided to rewrite it into a pitch tuner. Basically I'm done, but I've ran into performance difficulties while trying to rise a resolution in a frequency space. The original code used Storable arrays to get data out of fftw, for some processing I convert it to lists, which obviously hinders performance. I've heard there are some modern efficient array processing libraries available. So I hope I can avoid writing all those low-level for loops. Any hints on how to optimize and prettify the code are welcome. Repository is located at https://github.com/akamaus/HTuner Best regards, Dmitry

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Re: [Haskell-cafe] Some thoughts on Type-Directed Name Resolution
by Anthony Clayden 12 Feb '12

12 Feb '12

> > I chose the most available non-ASCII character I could > find. Set the criterion to be "present in most ISO 8-bit > character sets" and there are really only two candidates, > section sign and degrees sign. ... > Brilliant! We'll use degrees sign for function composition (so that it follows the convention in everything except a handful of obscure programming languages). That frees up dot for record.field. And still we can scratch our heads for something to do with section sign. SS as in set? Some smoother syntax for swifter, saner, salubrious setting. Super! AntC

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some question about conduit usage
by Alexander V Vershilov 11 Feb '12

11 Feb '12

Hello, cafe! I have somq questions about proper conduit usage in some usecases, I can't find a good way to solve this problems 1). adding additional source to conduit processing [1] Sometimes I want to run a Source and send use it to produce output e.g. have something like: > runSource :: ResourceT m => a -> Source m b > runSource = undefined > > test1 :: ResourceT m => Conduit a m b > test1 = Conduit push close > where > push = runSource > close = undefined > -- In my first problem I've got > -- push i = case i of Left _ -> oneValue ; Right -> runSource > -- but it can be solved with oneValue = sourceList [oneValue] 2). pushing data back [1] e.g. I have > test2 :: ResourceT Conduit ByteString m b > test2 = Conduit push close > where > conduit i = let (item,lft) = parse i > in -- I want to send lft back to test2 and > -- process item > -- in the other case I want to be able to send data back to > -- previous or arbitrary level, e.g. > test1 $= test2 $= test3 > ^--------| (<<<=) lft 3). split stream to send it to different Sinks, e.g. > sendToAll test3 [sink1,sink2,sink3] 4). send additional data from the other place, e.g. > src $= handle $$ snk > -- state -^ > -- I want to send message into handle to change it's state > -- It can be done with stm-conduit and Either (Left for commands > -- and Right for data), but it seems its overkill I will be apprecated, if smb has any idea how to implement it correnly, or why I shouldn't implement it at all. [1] http://stackoverflow.com/questions/9226780/how-to-add-a-new-source-inside-a… -- Regards, Alexander V Vershilov

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Compile one static Haskell binary for all x86_64 Linux distributions, is it possible?
by Morten Olsen lysgaard 10 Feb '12

10 Feb '12

Hi, I've written a program that converts an XML file from a database into another XML compatible with another database. Quite standard. I'm using the mtl library for State and HaXml for XML parsing. Is it possible to compile this program into a single static binary that works on all Linux computers that run x86_64. My development box is an Arch Linux x86_64 running GHC 7.0.3. I tried with: ghc --make -static -rtsopts=all -optl-pthread -optl-static Program.hs When I copy the binary created over to my x86_64 Ubuntu 11.10 desktop and run it I get: Program: mkTextEncoding: invalid argument (Invalid argument) I've googled around and found out this has to do with iconv, but I've never compiled anything to run on another pc than the development machine, so I've got no experience in fixing these kinds of problems. Any help appreciated. -- Morten

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Error in installing dph-examples on Mac OS X 10.7.3
by mukesh tiwari 10 Feb '12

10 Feb '12

Hello all I am trying to install dph-examples on Mac OS X version 10.7.3 but getting this error <http://hpaste.org/57699>. I am using ghc-7.4.1. Error running clang! you need clang installed to use the LLVM backend cabal: Error: some packages failed to install: dph-examples-0.6.1.3 failed during the building phase. The exception was: ExitFailure 1 I have already installed Xcode 4.2 and it shows Macintosh-0026bb610428:tmp mukesh$ clang clang clang++ Macintosh-0026bb610428:tmp mukesh$ clang -v Apple clang version 3.0 (tags/Apple/clang-211.10.1) (based on LLVM 3.0svn) Target: x86_64-apple-darwin11.3.0 Thread model: posix Macintosh-0026bb610428:tmp mukesh$ clang++ -v Apple clang version 3.0 (tags/Apple/clang-211.10.1) (based on LLVM 3.0svn) Target: x86_64-apple-darwin11.3.0 Thread model: posix Macintosh-0026bb610428:tmp mukesh$ l Display all 113 possibilities? (y or n) Macintosh-0026bb610428:tmp mukesh$ llvm- llvm-ar llvm-config llvm-diff llvm-extract llvm-gcc llvm-link llvm-objdump llvm-rtdyld llvm-tblgen llvm-as llvm-cov llvm-dis llvm-g++ llvm-gcc-4.2 llvm-mc llvm-prof llvm-size llvm-bcanalyzer llvm-cpp-4.2 llvm-dwarfdump llvm-g++-4.2 llvm-ld llvm-nm llvm-ranlib llvm-stub Macintosh-0026bb610428:tmp mukesh$ llvm-g++ -v Using built-in specs. Target: i686-apple-darwin11 Configured with: /private/var/tmp/llvmgcc42/llvmgcc42-2336.1~1/src/configure --disable-checking --enable-werror --prefix=/Developer/usr/llvm-gcc-4.2 --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-prefix=llvm- --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib --build=i686-apple-darwin11 --enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2336.1~1/dst-llvmCore/Developer/usr/local --program-prefix=i686-apple-darwin11- --host=x86_64-apple-darwin11 --target=i686-apple-darwin11 --with-gxx-include-dir=/usr/include/c++/4.2.1 Thread model: posix gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00) Regards Mukesh Tiwari

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Mersenne-random and standard random API
by Yves Parès 10 Feb '12

10 Feb '12

Hi, I've been in the past told that mersenne-random was much better than the standard random package. However, System.Random.Mersenne doesn't follow the general API described in System.Random, MTGen is not an instance of RandomGen. But a sample on System.Random.Mersenne.getStdRandom documentation ( http://hackage.haskell.org/packages/archive/mersenne-random/1.0.0.1/doc/htm…) indicates this: rollDice :: IO Int rollDice = getMTRandom (randomR (1,6)) It looks like wrong documentation (checking of older versions show that getMTRandom have never lived or been exposed by this module), but suggests MTRandom should be an instance of RandomGen (if I assume well that randomR is that of System.Random). So is it possible to use the fast and efficient mersenne generator with the convenient and general random API?

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Re: [Haskell-cafe] [Haskell] Support tools for development with Cabal and Darcs
by Duncan Coutts 10 Feb '12

10 Feb '12

On 10 February 2012 12:06, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: > > I have uploaded three packages to Hackage that shall simplify maintaining > cabal packages under darcs revision control. They simplify tasks like > uploading packages after some tests, cabal version handling, compiling > multiple local packages in the right order, replacing identifiers in > multiple files, renaming modules. > > * http://hackage.haskell.org/package/darcs-scripts > * http://hackage.haskell.org/package/cabal-scripts > * http://hackage.haskell.org/package/cabal-sort Hi Henning, Would you mind giving me a breif explanation of the "compiling multiple local packages in the right order" feature? I'd like to understand what is missing in cabal-install in this respect. Currently with cabal-install you can say: cabal install ./a ./b ./c and it will install the packages in those directories in the right order. (It is also possible to list other install targets including local and remote tarballs, and of course packages from hackage). Duncan

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Fancy REPL
by Heinrich Apfelmus 10 Feb '12

10 Feb '12

Dear list, the Show class is extremely useful for exploring Haskell in a terminal, but sometimes, I just want something fancier. For instance, I'm currently dabbling with sound generation and it is only natural that I want to hear the sound instead of seeing a textual representation. Another example would be graphics, that are simply drawn on screen whenever you evaluate their value. Of course, this is simple to implement with a class class Demonstrable a where demo :: a -> IO () instance Demonstrable Sound where demo = play instance Demonstrable Sound where demo = draw instance Demonstrable GUI where demo = run etc. However, I don't want to reinvent the wheel, small as it may be, hence my question: is there a package on hackage that already defines a class similar to Demonstrable ? Or any other projects in this direction, like, say, a fancy REPL built on wxHaskell? Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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The State of Testing?
by Michael Craig 10 Feb '12

10 Feb '12

I've been picking up Haskell as a side project for the last few months, and I'm now considering publishing some useful code I've written and reused in several small projects. So far I've done relatively little with testing (these have been non-mission-critical applications) but I feel I should get my act together before pushing my work into the common ecosystem. I'm comfortable writing tests in QuickCheck and HUnit and bundling them as optional executables with cabal, but I understand there's a better way. Specifically, I'm looking at the test-framework package and cabal's (newish) test-suite sections. Are these two used together or each to the exclusion of the other? Is there something else I should be using? Cheers, Mike Craig

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: hs-json-rpc 0.0.0.1
by Felipe Almeida Lessa 09 Feb '12

09 Feb '12

[Redirecting to haskell-cafe] Congrats on your first release! While I haven't looked into your package in more depth, I'd suggest taking a look at http-conduit [1]. While I don't know of any benchmarks, it should be faster or at least as fast the HTTP package. It's also used by many people everyday (myself included), which means that it has no major bugs and that bugs are promptly solved. It also has some features that HTTP doesn't. Cheers! [1] http://hackage.haskell.org/package/http-conduit -- Felipe.

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Yesod and dependencies hell
by Никита Тимофеев 09 Feb '12

09 Feb '12

For three days I can't compile dependencies for my project using yesod, yesod-auth, yesod-persistent, persistent-template, persistent-sqlite, persistent. When I varied version I received a variety of broken dependencies: persistent (0.6.* vs 0.7.*), conduit (0.1.* vs 0.2.*), conduit-pool, path-pieces, attoparsec and something else. In this regard, I have a few questions. How to build these packages together? And, Michael, why so bad? -- Тимофеев Н.Д.

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Re: [Haskell-cafe] Is ListT a valid MonadPlus?
by oleg＠okmij.org 09 Feb '12

09 Feb '12

First of all, ListT is not a monad transformer, since it breaks the law of associativity of bind: *Control.Monad.List> let one = (lift $ putStrLn "1") :: ListT IO () *Control.Monad.List> let two = (lift $ putStrLn "2") :: ListT IO () *Control.Monad.List> let choice = return 1 `mplus` return 2 :: ListT IO Int *Extensible Control.Monad.List> runListT $ choice >> (one >> two) 1 2 1 2 [(),()] *Extensible Control.Monad.List> runListT $ (choice >> one) >> two 1 1 2 2 [(),()] > It appears to me that the MonadPlus instance for ListT breaks the > following MonadPlus law > m >> mzero = mzero The precise set of MonadPlus laws isn't actually agreed upon. The above law is the least agreed upon. Incidentally, this law is certainly violated for _any_ back-tracking monad transformer. Consider (lift <any action in base monad>) >> mzero for example (lift (putStrLn "printed")) >> mzero Is it reasonable to expect that printing should be undone? That the paper should be fed back into the printer and the toner lifted? There are back-tracking monad transformers; for example, LogicT.

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Is ListT a valid MonadPlus?
by Richard Wallace 09 Feb '12

09 Feb '12

It appears to me that the MonadPlus instance for ListT breaks the following MonadPlus law m >> mzero = mzero I've tried with every implementation of ListT I could find - the one in mtl, transformers, List, even "ListT done right" and it's alternative. They all seem to violate the above law. Following is a ghci session that shows this with the mtl impl. Prelude> import Control.Monad.List Prelude Control.Monad.List> let nt = ListT Nothing :: ListT Maybe Int Prelude Control.Monad.List> let e = mzero :: ListT Maybe Int Prelude Control.Monad.List> runListT nt Nothing Prelude Control.Monad.List> runListT e Just [] Prelude Control.Monad.List> runListT $ nt >> e Nothing According to the MonadPlus law above, that last line should evaluate to `Just []`. So is ListT a valid MonadPlus? Or am I missing something? If not, can a valid MonadPlus instance for ListT be written? I've tried a few things and come up short. Thanks, Rich

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Haskell Weekly News: Issue 214
by Daniel Santa Cruz 09 Feb '12

09 Feb '12

Welcome to issue 214 the HWN, a newsletter covering stories, questions, and remarks seen around the web during the week of January 29 to February 4, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-214.h… Quotes of the Week * Jafet: [on memoising an O(n) algorithm:] This is like trying to save work in a police investigation by having each detective determine whether another detective has already removed all evidence from his area by walking through it to search for evidence in the area. * CodeWeaver: keep in mind encryption's only as good as how much you trust that the implementors got it right. * cmccann: length [1..10^20] lambdabot: mueval-core: Time limit exceeded cmccann: looks like 10^20 is infinity * elliott: race condition waiting to happen * elliott: o'reilly publishes attoparsec tutorial: exactly the same as their parsec tutorial, but 10^-18th the size * rwbarton: Num works for Float/Double, it's Float/Double that doesn't work for Num * ddarius: Space isn't that big. * Brent Yorgey: > Sometimes, I suspect that Haskell not only makes easier of the hard > things for imperative programming languages but also makes harder of > some easy things. This is true. But usually these are things that you don't *want* to be easy, like shooting yourself in the kidney. (But these features are often described in ways that make them sound great, like "flexible usage of bullet-placement tools".) Inserting arbitrary print statements into the middle of some code is difficult for a reason. Top Reddit Stories * GHC 7.4.1 is out. Here's the release notes Domain: haskell.org, Score: 107, Comments: 59 On Reddit: [1] http://goo.gl/su4jl Original: [2] http://goo.gl/Nstve * Coming soon, to a bookstore near you Domain: yesodweb.com, Score: 82, Comments: 16 On Reddit: [3] http://goo.gl/6gZXg Original: [4] http://goo.gl/zSW1l * ImplicitCAD is a Programmatic CAD Tool/3D Modelling Engine in Haskell. Also my first big Haskell project. Feedback, advice? Domain: github.com, Score: 63, Comments: 23 On Reddit: [5] http://goo.gl/YHMRa Original: [6] http://goo.gl/39rc3 * Using monadic effects to reverse a merge sort Domain: github.com, Score: 42, Comments: 4 On Reddit: [7] http://goo.gl/dVuyp Original: [8] http://goo.gl/jL9qj * Exciting changes coming to conduit 0.2 Domain: yesodweb.com, Score: 38, Comments: 7 On Reddit: [9] http://goo.gl/neyW2 Original: [10] http://goo.gl/1ikN5 * Vectorisation without Replication in Data Parallel Haskell Domain: disciple-devel.blogspot.com, Score: 35, Comments: 4 On Reddit: [11] http://goo.gl/iJ4OD Original: [12] http://goo.gl/uMp4S * If software purity in functional languages is such an awesome idea, why are we still writing Python, Ruby or Java? Domain: self.haskell, Score: 33, Comments: 136 On Reddit: [13] http://goo.gl/F0QCY Original: [14] http://goo.gl/F0QCY * Released Data.Array.Accelerate 0.9.0.0 — the Haskell array library for GPUs Domain: justtesting.org, Score: 32, Comments: 3 On Reddit: [15] http://goo.gl/NqUwp Original: [16] http://goo.gl/JYTQ4 * Fun With -XPolyKinds : Polykinded Folds Domain: typesandotherdistractions.com, Score: 24, Comments: 1 On Reddit: [17] http://goo.gl/daxL8 Original: [18] http://goo.gl/u9gHi * Introduction to using acid-state, IxSet and data-lens as a native Haskell, noSQL, RAM-cloud database Domain: happstack.com, Score: 23, Comments: 21 On Reddit: [19] http://goo.gl/9bliO Original: [20] http://goo.gl/ZGXyW * Can't wait for the Haskell Platform? -- Compiling GHC 7.4.1 from source has never been easier. Domain: blog.jasonknight.us, Score: 21, Comments: 8 On Reddit: [21] http://goo.gl/IKjm6 Original: [22] http://goo.gl/dgMF2 * Generating random functions Domain: mainisusuallyafunction.blogspot.com, Score: 20, Comments: 6 On Reddit: [23] http://goo.gl/DB4f6 Original: [24] http://goo.gl/aOAPo * How to deal with undesired values when designing types Domain: self.haskell, Score: 19, Comments: 28 On Reddit: [25] http://goo.gl/aPCbQ Original: [26] http://goo.gl/aPCbQ * Guarded Pipes or How to write Conduit.List Combinators in Pipes. Domain: pcapriotti.wordpress.com, Score: 17, Comments: 16 On Reddit: [27] http://goo.gl/vo8ip Original: [28] http://goo.gl/mhnfH * UHC JavaScript Backend Project Page (Work in Progress) Domain: uu-computerscience.github.com, Score: 16, Comments: 0 On Reddit: [29] http://goo.gl/42xXs Original: [30] http://goo.gl/Cz8R6 Top StackOverflow Questions * What are the rules about concurrently accessing a persistent database votes: 17, answers: 1 Read on SO: [31] http://goo.gl/m3QhB * Functions don't just have types: They ARE Types. And Kinds. And Sorts. Help put a blown mind back together votes: 15, answers: 2 Read on SO: [32] http://goo.gl/fKjQR * Defining polymorphic constants in Haskell votes: 12, answers: 2 Read on SO: [33] http://goo.gl/3H3ke * Do guarded pipes behave the same as pipes using await? votes: 12, answers: 2 Read on SO: [34] http://goo.gl/Mf3HQ * What is the purpose of the IdentityT transformer? votes: 11, answers: 2 Read on SO: [35] http://goo.gl/L7Syk * call/cc implementation? votes: 10, answers: 5 Read on SO: [36] http://goo.gl/tfLLz * Understanding bind function in Haskell votes: 9, answers: 4 Read on SO: [37] http://goo.gl/amtSd * How to show that a monad is a functor and an applicative functor? votes: 8, answers: 2 Read on SO: [38] http://goo.gl/Cg0EQ * Avoiding lift with Monad Transformers votes: 7, answers: 2 Read on SO: [39] http://goo.gl/fBhCz * Container element types votes: 7, answers: 1 Read on SO: [40] http://goo.gl/UfZnG Until next time, Daniel Santa Cruz References 1. http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-4-1.html 2. http://www.reddit.com/r/haskell/comments/p886g/ghc_741_is_out_heres_the_rel… 3. http://www.yesodweb.com/blog/2012/01/coming-soon 4. http://www.reddit.com/r/haskell/comments/p4hpt/coming_soon_to_a_bookstore_n… 5. https://github.com/colah/ImplicitCAD 6. http://www.reddit.com/r/haskell/comments/p9jzh/implicitcad_is_a_programmati… 7. https://github.com/DanBurton/Blog/blob/master/Literate%20Haskell/recovery.l… 8. http://www.reddit.com/r/haskell/comments/p2kar/using_monadic_effects_to_rev… 9. http://www.yesodweb.com/blog/2012/01/conduit-changes-0-2 10. http://www.reddit.com/r/haskell/comments/p1iu0/exciting_changes_coming_to_c… 11. http://disciple-devel.blogspot.com/2012/02/vectorisation-without-replicatio… 12. http://www.reddit.com/r/haskell/comments/p7f6u/vectorisation_without_replic… 13. http://www.reddit.com/r/haskell/comments/pavb1/if_software_purity_in_functi… 14. http://www.reddit.com/r/haskell/comments/pavb1/if_software_purity_in_functi… 15. http://justtesting.org/released-dataarrayaccelerate-0900-the-haskell 16. http://www.reddit.com/r/haskell/comments/p5sij/released_dataarrayaccelerate… 17. http://www.typesandotherdistractions.com/2012/02/fun-with-xpolykinds-polyki… 18. http://www.reddit.com/r/haskell/comments/p8djh/fun_with_xpolykinds_polykind… 19. http://www.happstack.com/docs/crashcourse/AcidState.html 20. http://www.reddit.com/r/haskell/comments/p75ko/introduction_to_using_acidst… 21. http://blog.jasonknight.us/2012/02/how-to-install-ghc-741-in-ubuntu-from.ht… 22. http://www.reddit.com/r/haskell/comments/pabfi/cant_wait_for_the_haskell_pl… 23. http://mainisusuallyafunction.blogspot.com/2012/02/generating-random-functi… 24. http://www.reddit.com/r/haskell/comments/p89fq/generating_random_functions/ 25. http://www.reddit.com/r/haskell/comments/p5lv0/how_to_deal_with_undesired_v… 26. http://www.reddit.com/r/haskell/comments/p5lv0/how_to_deal_with_undesired_v… 27. http://pcapriotti.wordpress.com/2012/02/02/an-introduction-to-guarded-pipes/ 28. http://www.reddit.com/r/haskell/comments/p8g55/guarded_pipes_or_how_to_writ… 29. http://uu-computerscience.github.com/uhc-js/ 30. http://www.reddit.com/r/haskell/comments/p1nqh/uhc_javascript_backend_proje… 31. http://stackoverflow.com/questions/9055214/what-are-the-rules-about-concurr… 32. http://stackoverflow.com/questions/9074261/functions-dont-just-have-types-t… 33. http://stackoverflow.com/questions/9088821/defining-polymorphic-constants-i… 34. http://stackoverflow.com/questions/9145656/do-guarded-pipes-behave-the-same… 35. http://stackoverflow.com/questions/9122825/what-is-the-purpose-of-the-ident… 36. http://stackoverflow.com/questions/9050725/call-cc-implementation 37. http://stackoverflow.com/questions/9074284/understanding-bind-function-in-h… 38. http://stackoverflow.com/questions/9098247/how-to-show-that-a-monad-is-a-fu… 39. http://stackoverflow.com/questions/9054731/avoiding-lift-with-monad-transfo… 40. http://stackoverflow.com/questions/9078168/container-element-types

1 0

Contributing to http-conduit
by Myles C. Maxfield 08 Feb '12

08 Feb '12

To: Michael Snoyman, author and maintainer of http-conduit CC: haskell-cafe Hello! I am interested in contributing to the http-conduit library. I've been using it for a little while and reading through its source, but have felt that it could be improved with two features: - Allowing the caller to know the final URL that ultimately resulted in the HTTP Source. Because httpRaw is not exported, the caller can't even re-implement the redirect-following code themselves. Ideally, the caller would be able to know not only the final URL, but also the entire chain of URLs that led to the final request. I was thinking that it would be even cooler if the caller could be notified of these redirects as they happen in another thread. There are a couple ways to implement this that I have been thinking about: - A straightforward way would be to add a [W.Ascii] to the type of Response, and getResponse can fill in this extra field. getResponse already knows about the Request so it can tell if the response should be gunzipped. - It would be nice for the caller to be able to know in real time what URLs the request is being redirected to. A possible way to do this would be for the 'http' function to take an extra argument of type (Maybe (Control.Concurrent.Chan W.Ascii)) which httpRaw can push URLs into. If the caller doesn't want to use this variable, they can simply pass Nothing. Otherwise, the caller can create an IO thread which reads the Chan until some termination condition is met (Perhaps this will change the type of the extra argument to (Maybe (Chan (Maybe W.Ascii)))). I like this solution, though I can see how it could be considered too heavyweight. - Making the redirection aware of cookies. There are redirects around the web where the first URL returns a Set-Cookie header and a 3xx code which redirects to another site that expects the cookie that the first HTTP transaction set. I propose to add an (IORef to a Data.Set of Cookies) to the Manager datatype, letting the Manager act as a cookie store as well as a repository of available TCP connections. httpRaw could deal with the cookie store. Network.HTTP.Types does not declare a Cookie datatype, so I would probably be adding one. I would probably take it directly from Network.HTTP.Cookie. I'd be happy to do both of these things, but I'm hoping for your input on how to go about this endeavor. Are these features even good to be pursuing? Should I be going about this entirely differently? Thanks, Myles C. Maxfield P.S. I'm curious about the lack of Network.URI throughout Network.HTTP.Conduit. Is there a particular design decision that led you to use raw ascii strings?

5 57

Loading a texture in OpenGL
by Clark Gaebel 08 Feb '12

08 Feb '12

Using the OpenGL package on Hackage, how do I load a texture from an array? In the red book[1], I see their code using glGenTextures and glBindTexture, but I can't find these in the documentation. Are there different functions I should be calling? [1] http://glprogramming.com/red/chapter09.html

6 14

-XPolyKinds and undefined arguments
by Douglas McClean 08 Feb '12

08 Feb '12

There are all sorts of useful functions that would otherwise require explicit type applications which we work around by passing undefined and ignoring its value. (See http://www.haskell.org/pipermail/libraries/2010-October/014742.html for a prior discussion of why this is, how the use of undefined for something that is perfectly well defined smells, and various suggestions about one day changing it.) Examples: Data.Typeable.typeOf, Foreign.Storable.sizeOf, etc. It seems to me that the new -XPolyKinds extension might provide a good long-term solution to statically insuring that arguments like this are always undefined and are never inspected. If we had a library definition: -- a kind-indexed family of empty types, Proxy :: AnyK -> * data Proxy a then we could write things that resemble type applications such as "sizeOf @Int64". Unlike the current approach, we'd be assured that nobody tries to use the value of such a proxy in any meaningful way, since you can't pattern match it or pass a "Proxy t" in a context expecting a "t". We'd have formal documentation of which parameters are in this value-is-unused-and-only-type-matters category (which might help us erase more often?). This is essentially the same Proxy type as in section 2.5 of the "Giving Haskell a Promotion" paper, except that by making it empty instead of isomorphic to () we end up with a type that only has one value (bottom) instead of two (Proxy and bottom), which should be a desirable property if we are trying to erase uses and forbid pattern matches and such, I think. Adding a syntax extension (or even some TH?) to make something like "@Int64" desugar to "undefined :: Proxy Int64" would eliminate the clutter/smell from the undefineds. Poly kinds allow us to use the same mechanism where the "undefined" parameter is a phantom type at a kind other than *. It seems to work nicely, and it cleaned up some of my code by unifying various proxy types I had made specialized at kinds like Bool. How would you compare this to some of the proposals in the 2010 thread? Would people still prefer their fantasy-Haskell-of-the-future to include full-blown support for explicit type applications instead? -Doug

2 1

FP activities and researchers in Warwickshire
by Ivan Perez 07 Feb '12

07 Feb '12

Hello, I recently moved to Kenilworth, Warwickshire, UK, and I'd like to know if there are meetings, talks, or any FP-related activities going on around here. I contacted somebody at Warwick University but, from what I understood, their Formal Methods group doesn't exist as such any longer and they don't carry out any coordinated, regular events regarding FP anymore. Cheers, Ivan

2 1

Evaluating parallel computations in order of finishing (approximately)
by Victor Miller 07 Feb '12

07 Feb '12

Suppose that we have a list [a] of computations that we want to evaluate in parallel. I would like to have something (probably a monad) which would return the list in order (roughly) of finishing: Say the monad is M. It would be something like the state monad, in that it would be implemented by a state transformer function. In this case the state would the set of computations to be evaluated. we might have a function include :: [a] -> M a () which would say that the monad's responsibility would be to evaluate all the members of a in parallel. We might also have a function strategy :: Strategy -> M a () which would indicate the parallel strategy to be used. The key thing would be function, completed, which produces a list of all the computations to be evaluated as a list roughly in order of completion. That is, if, inside the M monad we finished the do with completed then we would have a value M a [a] which would be the list in order of completion. Since everything is lazy we could ask for the head of the list, and it would be the first value whose computation finished. Does such a thing exist? Victor

3 2

network-2.3.0.10 compiled for ghc 7.4.1 windows
by Alberto G. Corona 07 Feb '12

07 Feb '12

Hi, After some pains, I compiled the package Network. sa_family_t is needed in network.Socket.Internal.asc and MinGW seems that it has no such variable defined. I share here the package with the compiled binaries included: http://skydrive.live.com/?cid=2F229EB748D4CBA3&id=2F229EB748D4CBA3%21105

3 8

Domain Specific Languages and better Error messages
by Victor Miller 07 Feb '12

07 Feb '12

This week, at work, I've been attending a workshop which is concerned with writing (actually rewriting) a domain specific language that we've been using. Although I've only known Haskell for four months, I can see that it's infected my brain, since I kept seeing Haskell solutions to all sorts of problems that were coming up. This DSL was written by two demon programmers in C. They've made many ad-hoc choices which look rather ugly to me. Even worse, they've invented a very primitive type system which would only be checked at run time. This looks particularly bad since the applications that we'd be running are very large parallel computations. I would like to make a try at writing an alternative based on Haskell, even though there's probably little chance that I could get it adopted (since no-one else here is familiar with functional programs, etc.). However, there is one issue that I'd like to address. I was looking at http://donsbot.wordpress.com/2007/03/10/practical-haskell-shell-scripting-w… gave a very nice presentation of a domain specific language for shell scripts. One point that was made that commands were implemented as functions, and privileged commands (i.e. those which needed root privileges) were produced a result with a separate type, so that type checking would give a compile time error if you didn't execute it via the priv combinator. But, the error messages given by ghc would be rather incomprehensible to someone who wasn't familiar with Haskell (and maybe even to someone who was!). Has anyone looked into something that would filter the error messages of put out by GHC and rewrite them into something more domain specific? Victor

1 0

Switching GHC Version
by HASHIMOTO, Yusaku 07 Feb '12

07 Feb '12

Hi, I wrote a simple shell function for switching GHC version on the system. It works only under Mac OSX, and only switch GHCs installed via .pkg installers. It's useful to experiment newer features without worrying breaking environment. GHC_BASE_DIR=/Library/Frameworks/GHC.framework/Versions/ ghcs () { VERSION=$1 sudo $GHC_BASE_DIR/$VERSION/Tools/create-links . /Library/Frameworks / } Usage: ~/work/today 08:21 ghcs 7.4.0.20111219-x86_64 Password: ~/work/today 08:21 ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.0.20111219 ~/work/today 08:21 ghcs 7.4.1-x86_64 ~/work/today 08:22 ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 Now I'm curious of better way to achieve this. This have limitations described above. Even it requires sudo because it modified symbolic links under `/usr`. Suggestions? -nwn

4 4

ANN: exists-0.1
by Gábor Lehel 07 Feb '12

07 Feb '12

There's a common pattern in Haskell of writing: data E where E :: C a => a -> E also written data E = forall a. C a => E a I recently uploaded a package to Hackage which uses the new ConstraintKinds extension to factor this pattern out into an Exists type parameterized on the constraint, and also for an Existential type class which can encompass these kind of types: http://hackage.haskell.org/package/exists My motivation was mostly to play with my new toys, if it turns out to be useful for anything that's a happy and unexpected bonus. Some interesting things I stumbled upon while writing it: - Did you know you can write useful existentials for Functor, Foldable, and Traversable? I sure didn't beforehand. - You can even write them for various Comonad classes, though in their case I don't think it's good for anything because you have no way to run them. - Surprisingly to me, the only * kinded class in the standardish libraries I found which is useful with existentials is Show, the * -> * kinded ones are more numerous. - I don't know if anyone's ever set out what the precise requirements are for a type class method to be useful with existentials. For example, any method which requires two arguments of the same type (the type in the class head) is clearly useless, because if you have two existentials there's no way to tell whether or not their contents were of the same type. I think this holds any time you have more than one value of the type among the method's parameters in any kind of way (even if it's e.g. a single parameter that's a list). If the type-from-the-class-head (is there a word for this?) is used in the method's parameters in a position where it's not the outermost type constructor of a type (i.e. it's a type argument), that's also no good, because there's no way to extract the type from the existential, you can only extract the value. On the other hand, in the method's return type it's fine if there are multiple values of the type-from-the-class-head (or if it's used as a type argument?), because (as long as the method also has an argument of the type) the type to put into the resulting existentials can be deduced to be the same as the one that was in the argument. But if the type-from-the-class-head is used *only* in the return type, then it's difficult to construct an existential out of the return value because the instance to use will be ambiguous. - There are a lot of ways you can write existentials, and the library only captures a small part of them. Multiparameter constraint? No go. More than one constraint? No go (though you can use Control.Constraint.Combine). More than one type/value stored? No go. Anything which doesn't exactly match the patterns data E where E :: C a => a -> E or data E a where E :: C f => f a -> E a? No go. I don't think there's any way to capture all of the possibilities in a finite amount of code. - ConstraintKinds lets you write class aliases as type synonyms, type Stringy a = (Show a, Eq a). The old way to do this is class (Show a, Eq a) => Stringy a; instance (Show a, Eq a) => Stringy a and requires UndecidableInstances. But if the alias has multiple parameters, the old way is still superior, because it can be partially applied where type synonyms can't. This is analogous to the situation with type synonyms versus newtype/data declarations, but interestingly, unlike data and newtypes, the class+instance method doesn't require you to do any manual wrapping and unwrapping, only the declaration itself is different. - One of the advantages FunctionalDependencies has over TypeFamilies is that type signatures using them tend to be more readable and concise than ones which have to write out explicit equality constraints. For example, foo :: MonadState s m => s -> m () is nicer than foo :: (MonadState m, State m ~ s) => s -> m (). But with equality superclass constraints (as of GHC 7.2), it's possible to translate from TF-form to FD-form (but not the reverse, as far as I know): class (MonadStateTF m, s ~ State m) => MonadStateFDish s m; instance (MonadStateTF m, s ~ State m) => MonadStateFDish s m. - PolyKinds only seems to be useful as long as there's no value-level representation of the polykinded type involved (it's only used as a phantom). As soon as you have to write 'a' for kind * and 'f a' for kind * -> *, you have to do the duplication manually. Is this right? - Writing this library really made me want to have a type-level "Ord instance" for constraints, more precisely a type-level is-implied-by operator. The typechecker clearly knows that Eq is-implied-by Ord, for example, and that Foo is-implied-by (Foo :&: Bar), but I have no way to ask it, I can only use (~). I tried implementing this with OverlappingInstances, but it seems to be fundamentally impossible because you really need a transitive case (instance (c :<=: d, d :<=: e) => c :<=: e) but the transitive case can't work. (My best understanding is that it's because the typechecker doesn't work forward, seeing "ah, c :<=: d and d :<=: e, therefore c :<=: e"; rather it works backwards, and sees that "c might be :<=: e, if there's a suitable d", but then it has no idea what to choose for d and goes into a loop.) Filing a feature request is in the plans. Er... </ul>. Cheers, ~g

4 10

ANNOUNCE MFlow-0.0.3 an Application Server for stateful web apps with statically typed widget combinators
by Alberto G. Corona 07 Feb '12

07 Feb '12

Hi Haskellers I I´m happy to announce the first version of MFlow http://hackage.haskell.org/package/MFlow MFlow permits the execution of web applications in a procedural form, that is, with multiple requests-response interactions in a single procedure. MFlow stores the execution state, manage timeouts, shutdown and restart the procedure on demand at the point of execution, even after server stop. These services are transparent for the programmer. The other aspect of MFlow are combinators for the definition of widgets and formlets that can be mixed freely with HTML formatting and produce statically typed values using the "traditional" formlet/applicative approach. There are bindings for the package XHtml, so no need to learn new paradigms/scripting/formatting.. Console and window oriented apps are possible. Out of te box serialization in files permits very rapid prototyping, but any backend can be used . MFlow (MessageFlow) was created initially as the user interface for the Workflow package . Currently is an alpha version. It has only basic authentication but I plan to inprove it for serious applications. I upload this version just in case anyone wants to play with it.

1 1

Exceeding OS limits for simultaneous socket connections
by Rob Stewart 07 Feb '12

07 Feb '12

Hi, I'm experiencing the "accept: resource exhausted (Too many open files)" exception when trying to use sockets in my Haskell program. The situation: - Around a dozen Linux machines running my Haskell program, transmitting thousands of messages to each other, sometimes within a small period of time. - I'm using the Network.Socket.ByteString.Lazy module to send and receive lazy bytestrings - The socket sent to the getMsg is bound and is listening, and is not closed until the program exits. getMsg is called in a loop to receive lazy bytestring from remote nodes. The socket is initialized with: sock <- Network.Socket.socket (addrFamily myAddr) Stream defaultProtocol bindSocket sock (addrAddress myAddr) listen sock 10 Here's the code: sendMsg :: Maybe HostName -> Int -> Lazy.ByteString -> IO () sendMsg dest sckt msg = do result <- try $ withSocketsDo $ do addrinfos <- getAddrInfo Nothing dest (Just (show sckt)) let serveraddr = head addrinfos sock <- socket (addrFamily serveraddr) Stream defaultProtocol connect sock (addrAddress serveraddr) sendAll sock msg sClose sock case result of Left (ex::IOException) -> return () -- permit send failure Right _ -> return () getMsg :: Socket -> IO Lazy.ByteString getMsg sock = do result <- try $ withSocketsDo $ do (conn, addr) <- accept sock getContents conn case result of Left (ex::IOException) -> putStrLn (show ex) >> getMsg sock Right msg -> return msg The current topology is a master/slave setup. For some programs that use these functions above, `sendMsg' is called thousands of times in quick succession on the remote nodes, where the destination of the `sendAll' function is the master node. Here's the maximum number of simultaneous sockets I am permitted to have open on my Linux machines: $ ulimit -n 1024 Indeed, when I experience the "accept: resource exhausted (Too many open files)" exception, I check the number of open sockets, which exceeds 1024, by looking at the contents of the directory: ls -lah /proc/<prod_id>/fd It is within the getContents function that, once the lazy bytestring is fully received, the socket is shutdown http://goo.gl/B6XcV : shutdown sock ShutdownReceive There seems to be no way of limiting the number of permitted connection requests from remote nodes. What I am perhaps looking for is a mailbox implementation on top of sockets, or another way to avoid this error. I am looking to scale up to 100's of nodes, where the possibility of more than 1024 simultaneous socket connections to one node is increased. Merely increasing the ulimit feels like a temporary measure. Part of the dilemma is that the `connect' call in `sendMsg' does not throw an error, despite the fact that it does indeed cause an error on the receiving node, by pushing the number of open connections to the same socket on the master node, beyond the 1024 limit permitted by the OS. Am I missing something? One would have thought such a problem occurs frequently with Haskell web servers and the like.. ? -- Rob Stewart

5 4

a variant of TDNR (was: Re: Some thoughts on Type-Directed Name Resolution)
by Evan Laforge 07 Feb '12

07 Feb '12

I brought up an idea on another thread, but it was a bit off subject and I think it got buried. Anyway, I'm curious to know if there are any obvious problems with this idea, or if it could be pursued further: It could be called Tagged Type Directed Name Resolution. The idea is a # prefix for identifiers. #f is a "type directed function". It requires the type of its argument to be monomorphic, and is desugared to M.f, where M is the module defining the type of 'f's argument. Everything else remains the same. Records wishing the same name must live in separate modules, but field access looks like: (#a . #b) record This works to set fields of a record as well. Let M.f be a function from a record to a lens focused on a certain field of that record. Now you can write: M.a :: M.Record -> Lens M.Record M.A set :: Lens record field -> record -> record val = set (#a 42) record which becomes: set (M.a record) 42 Or composed 'set ((#a.#b) record) 42' becomes 'set ((M.a . N.b) record) 42' An operator with convenient fixity should be able to remove the parens. As long as the compiler can figure out a monomorphic type expected by the input of #a then it shouldn't need type annotations. The thing I'm not sure about is if the ghc typechecker can do this. It would know that '#a' is a function 'alpha -> beta', then need to figure out what alpha is. Then once it has a monomorphic type for alpha, then it has to look up a symbol 'a' in the module defining that type, and only then does it know the type of beta. If #b.#a style composition is to work, then of course '#b' would have to hold off until the return type of '#a' has been resolved. I don't know how hard that would be, or it would even be possible. This idea is less powerful than other proposals because the #-decorated function must be able to statically determine a monomorphic type for its argument. So no structural types. And no records with the same field name in the same module. Personally that doesn't bother me much since I try to keep modules small, but perhaps that could be addressed in an orthogonal nested module feature. The '#' prefix would mess up people using that as an operator. I guess @ is free, right?

1 0

ghc-api Static Semantics?
by Christopher Brown 06 Feb '12

06 Feb '12

Hi, I was wondering if anyone could tell me if it's possible to get an AST from the ghc-api decorated with static-semantics? In particular, I am interested in use and bind locations for all names in the AST together with the module they are bound, etc. Looking through the online docs, there doesn't seem to be a way to do this. Even if I can tell from the AST where a variable is bound that would be enough, if this is by making all names unique and qualified that would be better than nothing. Hope someone can help, Chris.

4 10

Re: [Haskell-cafe] Rewrite this imperative in FP way
by Haisheng Wu 06 Feb '12

06 Feb '12

*d = [sum $ map (a !!) [i | i <- b, j <- c, i + j < 3, i + j == dIndex] | dIndex <- [0..3]] * This is cool. -Simon On Sun, Feb 5, 2012 at 5:07 PM, L Corbijn <aspergesoepje(a)gmail.com> wrote: > On Sun, Feb 5, 2012 at 7:28 AM, Haisheng Wu <freizl(a)gmail.com> wrote: > > a = [1,1,1,1] > > b = [0,1,2,3] > > d = [0,0,0,0] > > > > for i in b: > > for j in c: > > if (i+j)<3: > > d[i+j] += a[i] > > > > My just work implementation in Haskell > > http://hpaste.org/57452 > > > > Another people implementation in Haskell with Monad and it turns out > complex > > and very imperatively. > > http://hpaste.org/57358 > > > > Do you have any cool solution in FP way? > > > > Thanks. > > -Simon > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > There are several ways to make it nicer. > Without assumption on what lists a, b and c contain it can be written as > d = [sum $ map (a !!) [i | i <- b, j <- c, i + j < 3, i + j == dIndex] > | dIndex <- [0..3]] > > With the assumption that b and c are both [0..3] this can be 'improved' to > d = (take 3 . map sum . tail $ inits a) ++ replicate (4 - 3) 0 > This generates the first three values by the complicated expression > and then adds the extra zero's by using the replicate expression. This > works as the value of the i-th element of d is the sum over the first > i elements in a if i < 3 and 0 otherwise. A list of lists with the > first i elements is generated with 'tail $ inits a' which is then > summed and restricted to length 3. > An alternative for this is > d = (take 3 . snd $ mapAccumL (\acc ai -> (acc + ai, acc + ai)) 0 a) > ++ replicate (4 - 3) 0 > Where the summation and tail generating is done in the mapAccumL function. > > Greetings, > Lars > > P.S. Yes the replicate (4-3) 0 can be replaced by [0], but I wanted to > explicitly include the length (4) of the expected list. >

2 1

ANN: Updated OpenGL libraries
by Jason Dagit 06 Feb '12

06 Feb '12

Hello, I'm pleased to anounce a minor bug fix release of the OpenGL libraries. This release was prompted by issues and warnings when compiling with ghc-7.4.1. The following packages have been updated: * OpenGLRaw 1.2.0.0 * OpenGL 2.5.0.0 * GLURaw 1.2.0.0 * GLUT 2.3.0.0 Thanks goes out to: * Corey O'Connor * mgajda (github id) * #haskell-game and #haskell on freenode The changes include: * Removing a data type context * Exposing the GLfoo constructors from OpenGLRaw * Deriving Typeable instead of using the now deprecated mkTyCon * Added Eq constraint, necessary now that Num doesn't have an Eq constraint * Tighten constraints in cabal files * Conservative package version bump (X.Y.Z -> X.(Y+1).0) As usual, please submit pull requests and bug reports on github: https://github.com/haskell-opengl Thank you! Jason

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Rewrite this imperative in FP way
by Haisheng Wu 06 Feb '12

06 Feb '12

a = [1,1,1,1] b = [0,1,2,3] d = [0,0,0,0] for i in b: for j in c: if (i+j)<3: d[i+j] += a[i] My just work implementation in Haskell http://hpaste.org/57452 Another people implementation in Haskell with Monad and it turns out complex and very imperatively. http://hpaste.org/57358 Do you have any cool solution in FP way? Thanks. -Simon

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Problem resolving: Hangman
by Haisheng Wu 06 Feb '12

06 Feb '12

Though the problem I'd ask is not quiet related to haskell, I hope talent people here could provide some suggestions. The problem is not just solve the Hangman problem but solve it in sort of AI way. General idea is : 1. Randomly pick words, e.g. 20 from a predefined word list file. ( http://www.word-list.com/) 2. The application play hangman with each word till either WON or FAIL. NO human being interactive. 3. The expect result is the application be able to win the game at most of time before a certain max wrong guess, e.g. 5 Please let me where is a better place to ask such question and thank you so much in advance. -Simon

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Greetings and Maybe GSoC-2012
by Sergiu Ivanov 06 Feb '12

06 Feb '12

Hello everyone, (long mail ahead) My name is Sergiu Ivanov, I have been trying to wrap my mind around Haskell for 2 years already, but success still feels far away :-) This makes me more and more attached to Haskell though, so I can plainly say: I love this language :-) I guess I should have joined the online Haskell community waaaay earlier; too bad the idea's only struck me now. Therefore, I'd like to hereby greet everyone and wish a lot of good luck and good fun with whatever Haskell-related and non-Haskell-related tasks you may be on! :-) I have graduated Computer Science and I'm now pursuing my MSc in what is officially referred to "Mathematics and Informatics" (sic), but has turned out to be quite heavily biased towards algebra for me. The subject I currently seriously focus on is category theory; I have graduated a course in module theory and I'm now going through quasigroups, lattices, and Gröbner bases. I'm doing some research in computability theory, namely, in P systems (http://en.wikipedia.org/wiki/P_system) but also other computing devices, including some attempts at quantum computer science. My usual IRC nickname is scolobb, and I plan to be hanging out on #haskell quite regularly. I'd be glad to contribute to the community with whatever knowledge and skill assets I'd be able to provide :-) (End of Presentation) I'll turn to the Maybe part of the subject of this letter now. I've seen haskell.org has participated as an org in GSoC 2011. If the community plans to file an application this year as well and if haskell.org is accepted again, I'd be happy to submit an application for some fancy project. I'd enjoy working on something hard, requiring a lot of mental effort to understand, something which would make me feel cleverer when I have finished it :-) I have a fairly broad experience with programming languages and frameworks, so I don't mind committing to http://hackage.haskell.org/trac/summer-of-code/ticket/1592 , http://hackage.haskell.org/trac/summer-of-code/ticket/1547 , or http://hackage.haskell.org/trac/summer-of-code/ticket/1583 . However, among my favourites is this one: http://hackage.haskell.org/trac/summer-of-code/ticket/1582 . This one sounds complicated and mixing a lot of stuff. My experience with LLVM is zero, but I'm willing to learn, of course. You can apprehend my Haskell skills by throwing a look at https://gitorious.org/remoting and https://gitorious.org/psim/haskellengine . The first one is a simple actor-model based remoting framework; that was a project for one of the university projects. The second one is a part of my graduation project, in which I mixed Java, Jython, CUDA and, of course, Haskell. I'm not trying to boast, though: I can easily see the yet unwide horizon of my Haskell knowledge, which I would like to expand as far as possible. As far as I remember from my previous experience with GSoC, potential students are required to solve some simpler tasks to show what they're capable of. I understand that it's quite premature to talk about the summer of code as yet, but I'd be glad to hear of such possible test (or not-so-test) tasks which I could complete. Thank you for reading this much, Sergiu

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Re: [Haskell-cafe] ANN: combinatorics
by Balazs Komuves 05 Feb '12

05 Feb '12

> -------------------------------------------- > -- combinatorics 0.1.0 > -------------------------------------------- > > The combinatorics package offers efficient *exact* computation of common > combinatorial functions like the binomial coefficients and factorial. > (For fast *approximations*, see the math-functions package instead.) > Shameless self-promotion: The combinat package (which deliberately does not try to own the valuable namespace Math.Combinatorics) is a more extensive combinatorics library: http://hackage.haskell.org/package/combinat While the main focus is the generation of combinatorial objects themselves, counting functions and common number sequences like the above are also offered. Even though the binomial and factorial definition in this package are the naive ones, a quick experiment imply that the differences start show themselves around 100,000 factorial, or choosing 50,000 elements out of 100,000, which is probably a rather specialized use case. The primes function in the combinat package is based on an old Cafe thread, and actually seems to be faster than the one in the combinatorics package. In any case, I may switch to the faster algorithms in the future :) Balazs

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Increase GHC stack size?
by Michael Rice 05 Feb '12

05 Feb '12

Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. ====================== Couldn't find much on the man or info pages. Example please, say double it (16000000) for starters. Michael

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Fwd: Rewrite this imperative in FP way
by L Corbijn 05 Feb '12

05 Feb '12

---------- Forwarded message ---------- From: L Corbijn <aspergesoepje(a)gmail.com> Date: Sun, Feb 5, 2012 at 10:07 AM Subject: Re: [Haskell-cafe] Rewrite this imperative in FP way To: Haisheng Wu <freizl(a)gmail.com> On Sun, Feb 5, 2012 at 7:28 AM, Haisheng Wu <freizl(a)gmail.com> wrote: > a = [1,1,1,1] > b = [0,1,2,3] > d = [0,0,0,0] > > for i in b: > for j in c: > if (i+j)<3: > d[i+j] += a[i] > > My just work implementation in Haskell > http://hpaste.org/57452 > > Another people implementation in Haskell with Monad and it turns out complex > and very imperatively. > http://hpaste.org/57358 > > Do you have any cool solution in FP way? > > Thanks. > -Simon > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > Oops forgot to reply to the list, There are several ways to make it nicer. Without assumption on what lists a, b and c contain it can be written as d = [sum $ map (a !!) [i | i <- b, j <- c, i + j < 3, i + j == dIndex] | dIndex <- [0..3]] With the assumption that b and c are both [0..3] this can be 'improved' to d = (take 3 . map sum . tail $ inits a) ++ replicate (4 - 3) 0 This generates the first three values by the complicated expression and then adds the extra zero's by using the replicate expression. This works as the value of the i-th element of d is the sum over the first i elements in a if i < 3 and 0 otherwise. A list of lists with the first i elements is generated with 'tail $ inits a' which is then summed and restricted to length 3. An alternative for this is d = (take 3 . snd $ mapAccumL (\acc ai -> (acc + ai, acc + ai)) 0 a) ++ replicate (4 - 3) 0 Where the summation and tail generating is done in the mapAccumL function. Greetings, Lars P.S. Yes the replicate (4-3) 0 can be replaced by [0], but I wanted to explicitly include the length (4) of the expected list. P.P.S. for those of you wondering what c is, looking at the solutions in hpaste c is probably defined as c = [0..3].

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Conduit experiment: Is this correct?
by Ertugrul Söylemez 04 Feb '12

04 Feb '12

Hello there, I'm trying to build a server for testing the conduit and network-conduit packages. As a contrived example the goal is to pick the first three lines from the client and send them back without the line feeds. After that, I'd like to switch to a simple echo server. This is the code: module Main where import Data.Conduit import Data.Conduit.Binary as Cb import Data.Conduit.List as Cl import Data.Conduit.Network handleClient :: Application handleClient src snk = src $$ do (Cb.lines =$= Cl.isolate 3) =$ snk snk main :: IO () main = runTCPServer (ServerSettings 4000 Nothing) handleClient I'm not sure whether it is correct to use the 'snk' sink multiple times, and intuitively I'd say that this is wrong. What would be the proper way to do this? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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how to print out intermediate results in a recursive function?
by Qi Qi 04 Feb '12

04 Feb '12

Hello, I have a question;how can I print out the intermediate number lists in a mergesort recursive function like the following one. merge [] ys = ys merge xs [] = xs merge (x:xs) (y:ys) = if x <= y then x : merge xs (y:ys) else y : merge (x:xs) ys mergesort [] = [] mergesort [x] = [x] mergesort xs = let (as, bs) = splitAt (length xs `quot` 2) xs in merge (mergesort as) (mergesort bs) main = do print $ mergesort [5,4,3,2,1] In the main function, it only prints out the final number list. But I'd like to print out the number lists in every recursive level. How can I do that? Thanks. -- Qi Qi

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ANN: bytestring-lexing 0.4.0
by wren ng thornton 04 Feb '12

04 Feb '12

-------------------------------------------- -- bytestring-lexing 0.4.0 -------------------------------------------- The bytestring-lexing package offers efficient reading and packing of common types like Double and Integral types. -------------------------------------------- -- Changes (since 0.3.0) -------------------------------------------- * Data.ByteString.Lex.Integral: added the function readDecimal_ :: Integral a => ByteString -> a A variant of readDecimal which does not return the tail of the string, and returns 0 instead of Nothing. This is twice as fast for Int64 on 32-bit systems, but has identical performance to readDecimal for all other types and architectures. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/bytestring-lexing Darcs: http://community.haskell.org/~wren/bytestring-lexing Haddock (Darcs version): http://community.haskell.org/~wren/bytestring-lexing/dist/doc/html/bytestri… -- Live well, ~wren

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Re: [Haskell-cafe] Some thoughts on Type-Directed Name Resolution
by wren ng thornton 03 Feb '12

03 Feb '12

On 2/3/12 6:13 AM, Gábor Lehel wrote: > The first problem is that mixing prefix and postfix function > application within the same line makes it harder to read. When you > read code to try to understand what it does, the direction you like to > go in is "here's some object, first do this to it, then do that to it, > then do this other thing to it, then this fourth thing to produce the > final result". In Haskell code with prefix application, this is easy: > you read it from right to left. In OO-style code using dots, it's even > easier: you read it from left to right. But if you mix the two, it's > much harder than either: you first have to figure out where the > sentence even begins, which is going to be somewhere in the middle, > and then every time the expression switches between prefix and > postfix, you have to figure out where to continue reading. The > algorithm your brain needs to follow is a lot branchier, so to speak. It's just as easy as reading function pointers in C :) -- Live well, ~wren

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TFPIE: Trends in Functional Programming in Education 2012
by Simon Thompson 03 Feb '12

03 Feb '12

[Apologies for multiple postings, Simon.] CALL FOR PAPERS/PRESENTATIONS TFPIE 2012 International Workshop on Trends in Functional Programming in Education 2012 June 11 2012 University of St Andrews, Scotland http://www.cs.ru.nl/P.Achten/TFPIE_2012/TFPIE_2012_home.html The first International Workshop on Trends in Functional Programming in Education, TFPIE 2012, will be co-located with TFP 2012 at the University of St Andrews in Scotland. The goal of TFPIE is to gather researchers, professors, teachers, and all professionals that use or are interested in the use of functional programming in education. TFPIE aims to be a venue where novel ideas, classroom-tested ideas, and work in progress on the use of functional programming in education are discussed. The one-day workshop will foster a spirit of open discussion by having a review process for publication after the workshop. The program chairs of TFPIE 2012 will screen submissions to ensure that all presentations are within scope and are of interest to participants. Potential presenters are invited to submit an extended abstract (4-6 pages) or an article (up to 16 pages). The authors of all accepted presentations will have their preprints and their slides made available on the workshop's website/wiki. Any visitors to the TFPIE 2012 website/wiki will be able to add comments. This includes presenters who may respond to comments and questions as well as provide pointers to improvements and follow-up work. After the workshop, the program committee will review, using prevailing academic standards, the articles accepted for presentation to select the best for publication in Electronic Proceedings in Theoretical Computer Science (EPTCS). Articles rejected for presentation and all extended abstracts will not be formally reviewed by the PC. TOPICS OF INTEREST TFPIE 2012 welcomes submissions describing practical techniques used in the classroom, tools used and/or developed, and any creative use of functional programming (FP) to aid education in or outside Computer Science. Topics of interest include, but are not limited to: FP and beginning CS students FP in Artificial Intelligence FP in Robotics FP and Music Advanced FP for undergraduates FP in graduate education Engaging students in research using FP FP in Programming Languages FP in the high school curriculum FP as a stepping stone to other CS topics FP and Philosophy If you are not sure if your work is appropriate for TFPIE 2012, please contact the PC chairs by e-mail at: tfpie2012(a)cs.ru.nl . Program Committee Peter Achten, Radboud University Nijmegen Jost Berthold, University of Copenhagen Marc Feeley, University of Montreal Ralf Hinze, University of Oxford Shriram Krishnamurthi, Brown University Michel Mauny, ENSTA Paris Tech James McKinna, UK Marco T. Morazan, Seton Hall University Rinus Plasmeijer, Radboud University Nijmegen Simon Thompson, University of Kent Important Dates May 20 submission of abstract or article May 25 notification of acceptance June 11 TFPIE July 6 submission of formal paper September 10 notification of acceptance October 1 camera-ready paper Venue The University of St Andrews is Scotland's first university and the third oldest in the English-speaking world, founded in 1413. Over six centuries it has established a reputation as one of Europe's leading and most distinctive centers for teaching and research. St Andrews is situated on the east coast of Fife, Scotland, UK. The town is approximately 50 miles north-east of Edinburgh, 14 miles south-east of Dundee, 78 miles south of Aberdeen, and 82 miles east of Glasgow making it easily accessible by any means of transportation. Help on traveling to St Andrews can be found at: http://www.st-andrews.ac.uk/visiting/GettingtoStAndrews/ . Questions? If you have any questions, do not hesitate to contact us at: tfpie2012(a)cs.ru.nl . Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Again, version conflicting problem with cabal-install
by Magicloud Magiclouds 03 Feb '12

03 Feb '12

Hello, As I recalled, ghc works in staticly link mode. So after one library is compiled, all its build dependencies are useless. Lost, changed, wheresoever, it does not matter. Then why the problem of version conflicting exists? By version conflicting I mean like following. This way, A is not installable by cabal. A needs B 0.1 A needs C 0.1 B needs C0.2 -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Some thoughts on Type-Directed Name Resolution
by Steve Horne 03 Feb '12

03 Feb '12

There's a proposal at the moment to add support for TDNR to Haskell - to leverage "the power of the dot" (e.g. for intellisense). http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResoluti… I approve of the goal, but I'd like to suggest a different approach. My basic idea is stolen from Bertrand Meyer (Object-Oriented Software Construction, second edition). Basically, a class *is* both a module and a type. Quote... Classes as modules Object orientation is primarily an architectural technique: its major effect is on the modular structure of software systems. The key role here is again played by classes. A class describes not just a type of objects but also a modular unit. In a pure object-oriented approach: Classes should be the only modules. By the logic of equivalence relations, we can conclude that a type *is* a module. Only I'd adapt that a little. In C++, the following operators can all be used to access the "module" for some type or value... * :: Scope resolution * . Member dereference * -> Member dereference via a pointer * .* Member-pointer dereference * ->* Member-pointer dereference via a pointer In C++, a type and an instance each have their own modules. A (smart) pointer has its own module, separate from the module for the type it points to. And member-pointers exist because sometimes there's a need to reference a member without knowing or (yet) caring which instance. We already have member pointers - the functions that map an instance to the field value. It would make some sense if these could be placed in a module associated with the type (not the instance). When an instance is created of a type, that can effectively (without run-time overhead) create a new module associated with the new instance. This will contain the same field-access functions, but with the instance parameter already curried in. So there's no real need for any new meaning of the . operator - it's just access to names within a module. And there's no need for a new mechanism for accessing fields - only for a way to place them in that module scope, and a little sugar that gives us the same field-access function but with the instance parameter already curried in. Once we have these modules containing compiler-generated field-access functions, though, it makes some sense to allow additional functions (and perhaps types) to be added within that types module explicitly by the programmer. It may also make sense to allow functions to be explicitly defined which will be added to the instance-modules and support the prefix-instance-parameter sugar. Finally, as with C++, when dealing with IORef and similar, it make make sense to have a separate -> operator (spelled differently, of course). Or it could use the standard dot. C++ and D disagree in this (in C++, the smart pointer has its own module separate from the pointed-at instance - in D, there is no -> or equivalent). As an aside, Ada has already gone through a related transition. The original Ada 83 had variant records, but no "true classes". In Ada 95, "tagged types" were added which were like variant records, but which supported inheritance and run-time dispatch. The discriminant is replaced by a "tag" which is presumably implemented as a virtual table pointer. However, functions and procedures weren't members. The typical call of a "method" would be... packagename.procedure_name ( instance_arg, other_args ); Ada 2005 added some workarounds to allow conventional OOP call notation. See section 1.3 of the Ada 2005 rationale for details. However, it all feels a bit kludgy. In particular, the procedures and functions still aren't members - there are just some special rules for when they can be used as if they were. I've not actually used Ada 2005, but I'd bet some confusion can result from that. Personally, I think Meyer was at least partly right - if types (and instances) are modules, the kludge-factor is much lower. C++ actually doesn't get this quite right IMO (you can access static class members through the instance objects, for example, not just through the classes), but C++ classes *do* act mostly like modules and that is a very useful trait - particularly within the declarative sublanguage (templates etc).

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