- Haskell-Cafe - Haskell.org

fromIntegral not enough?
by Christopher Howard 14 May '13

14 May '13

This is probably a haskell-beginners sort of question, but I usually get about 4x as many responses from cafe, about 10x as fast. I have code like so: code: -------- data Xy a = Xy a a class Coord2 a where coords2 :: Fractional b => a b -> Xy b data CircAppr a b = CircAppr a b b -- number of points, rotation angle, radius deriving (Show) instance Integral a => Coord2 (CircAppr a) where coords2 (CircAppr divns ang rad) = let dAng = 2 * pi / (fromIntegral divns) in let angles = map (* dAng) [0..divns] in undefined -- To be coded... -------- In the instance definition divns is an integral trying to divide a fractional. I hoped wrapping it in fromIntegral would coerce, but apparently not: quote: -------- Could not deduce (Fractional a) arising from a use of `/' from the context (Integral a) bound by the instance declaration at /scratch/cmhoward/pulse-spin/pulse-spin.hs:34:10-42 or from (Fractional b) bound by the type signature for coords2 :: Fractional b => CircAppr a b -> Xy b at /scratch/cmhoward/pulse-spin/pulse-spin.hs:(36,3)-(39,15) Possible fix: add (Fractional a) to the context of the type signature for coords2 :: Fractional b => CircAppr a b -> Xy b or the instance declaration In the expression: 2 * pi / (fromIntegral divns) In an equation for `dAng': dAng = 2 * pi / (fromIntegral divns) In the expression: let dAng = 2 * pi / (fromIntegral divns) in let angles = map (* dAng) [0 .. divns] in undefined -------- So, I'm wondering how I can do what I'm trying to do here, while still keeping my types as generic as possible. -- frigidcode.com

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Problem with mailman at projects.haskell.org
by Tim Docker 13 May '13

13 May '13

Hi, Has anyone noticed problems with the mailman instance running at projects.haskell.org? As best I can see there are no new posts in any of the hosted list archives since mid April. I know that there have been posts made to chart(a)projects.haskell.org in May, and these have neither be distributed or archived. I've not had a response from mailman(a)projects.haskell.org. Tim

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Release Candidates for Haskell Platform 2013.2
by Mark Lentczner 13 May '13

13 May '13

*Some of the release candidates for Haskell Platform 2013.2 are up.* *These are what I expect to simply "re-brand" as the release, unless anyone uncovers some issues.* *If you decide to test these out, please let me know how it goes.* * * The Mac OS X RC2 installers: *32bit: *Haskell Platform 2013.2.0.0 32bit rc2.signed.pkg<http://www.ozonehouse.com/mark/platform/Haskell%20Platform%202013.2.0.0%203…> *64bit: *Haskell Platform 2013.2.0.0 64bit rc2.signed.pkg<http://www.ozonehouse.com/mark/platform/Haskell%20Platform%202013.2.0.0%206…> The source tarball (RC1): haskell-platform-2013.2.0.0-rc1.tar.gz<http://www.ozonehouse.com/mark/platform/haskell-platform-2013.2.0.0-rc1.tar…> *Notable new things in Haskell Platform 2013.2:* - GHC 7.6.3 - Major update to *OpenGL* & *GLUT* - New packages: - *attoparsec* - *case-insensitive* - *hashable* - *unordered-containers* — Mark "mad releaser" Lentczner

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Set monad
by oleg＠okmij.org 13 May '13

13 May '13

The question of Set monad comes up quite regularly, most recently at http://www.ittc.ku.edu/csdlblog/?p=134 Indeed, we cannot make Data.Set.Set to be the instance of Monad type class -- not immediately, that it. That does not mean that there is no Set Monad, a non-determinism monad that returns the set of answers, rather than a list. I mean genuine *monad*, rather than a restricted, generalized, etc. monad. It is surprising that the question of the Set monad still comes up given how simple it is to implement it. Footnote 4 in the ICFP 2009 paper on ``Purely Functional Lazy Non-deterministic Programming'' described the idea, which is probably folklore. Just in case, here is the elaboration, a Set monad transformer. {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} module SetMonad where import qualified Data.Set as S import Control.Monad.Cont -- Since ContT is a bona fide monad transformer, so is SetMonadT r. type SetMonadT r = ContT (S.Set r) -- These are the only two places the Ord constraint shows up instance (Ord r, Monad m) => MonadPlus (SetMonadT r m) where mzero = ContT $ \k -> return S.empty mplus m1 m2 = ContT $ \k -> liftM2 S.union (runContT m1 k) (runContT m2 k) runSet :: (Monad m, Ord r) => SetMonadT r m r -> m (S.Set r) runSet m = runContT m (return . S.singleton) choose :: MonadPlus m => [a] -> m a choose = msum . map return test1 = print =<< runSet (do n1 <- choose [1..5] n2 <- choose [1..5] let n = n1 + n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] -- Values to choose from might be higher-order or actions test1' = print =<< runSet (do n1 <- choose . map return $ [1..5] n2 <- choose . map return $ [1..5] n <- liftM2 (+) n1 n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] test2 = print =<< runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return (i,j,k)) -- fromList [(3,4,5),(4,3,5),(6,8,10),(8,6,10)] test3 = print =<< runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return k) -- fromList [5,10]

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ANN haskdogs-0.3.2
by Sergey Mironov 13 May '13

13 May '13

Hi. I'm pleased to announce haskdogs-0.3.2, a source navigation helper. Haskdogs is a small HSH-based tool which calls hasktags to create tag file for entire haskell project. It takes into account first-level dependencies by recursively scanning imports and adding matching packages to the final tag list. As a result, programmer can use her text editor supporting tags (vim, for example) to jump directly to definition of any function she uses. Note that haskdogs call Unix shell commands like 'test' or 'mkdir' so this tool will likely fail to work on pure Windows platforms. In return, haskdogs is pretty small - only 125 lines long. Changes: * Generate extended tag file by default (hasktags -c -x) * Add -d command line option which allows user to specify additional source directories * Fix no-sources bug Github: https://github.com/ierton/haskdogs See also Vim-hint at https://github.com/ierton/haskdogs#vim-hint Install: cabal install hasktags haskdogs Usage: haskdogs [-d (FILE|'-')] [FLAGS] FLAGS will be passed to hasktags as-is followed by a list of files. Defaults to -c -x. Basically, just run haskdogs in your Haskell project directory Thanks to yihuang for the contributions! Sergey

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Haskell Platform's libstdc++-6.dll may be interfering with other applications
by Andrew Pennebaker 13 May '13

13 May '13

I use a number of different programming languages, so I have Haskell Platform, Strawberry Perl, Node.js, RVM, and Git Bash installed at the same time. I've noticed that compiling packages with C dependencies (e.g. using node-gyp during "npm install node-mhash", or doing "cpan install PAR::Packer") often crashes during the build process with a popup: The procedure entry point __gxx_personality_sj0 could not be located in the dynamic link library libstdc++-6.dll. As Roderich Schupp points out<https://rt.cpan.org/Public/Bug/Display.html?id=84949>, it appears that the different libstdc++-6.dll's are interfering with one another. Haskell Platform comes with one, Vagrant comes with one, GIMP comes with one, MinGW comes with one, and Strawberry Perl comes with two! Objdump seems to indicate that Haskell Platform's DLL is the one to blame. $ find /c/ -name libstdc++-6.dll 2>/dev/null /c/DevKit/mingw/bin/libstdc++-6.dll /c/Program Files/GIMP 2/bin/libstdc++-6.dll /c/Program Files/Haskell Platform/2012.4.0.0/mingw/bin/libstdc++-6.dll /c/strawberry/c/bin/libstdc++-6.dll /c/strawberry/perl/bin/libstdc++-6.dll /c/vagrant/vagrant/embedded/mingw/bin/libstdc++-6.dll $ objdump -ax /c/DevKit/mingw/bin/libstdc++-6.dll | ack gxx_personality [3896] __gxx_personality_sj0 $ objdump -ax /c/Program\ Files/GIMP\ 2/bin/libstdc++-6.dll | ack gxx_personality [3499] __gxx_personality_sj0 $ objdump -ax /c/Program\ Files/Haskell\ Platform/2012.4.0.0/mingw/bin/libstdc++-6.dll | ack gxx_personality [3401] __gxx_personality_v0 $ objdump -ax /c/strawberry/c/bin/libstdc++-6.dll | ack gxx_personality [3510] __gxx_personality_sj0 $ objdump -ax /c/strawberry/perl/bin/libstdc++-6.dll | ack gxx_personality [3510] __gxx_personality_sj0 $ objdump -ax /c/vagrant/vagrant/embedded/mingw/bin/libstdc++-6.dll | ack gxx_personality [3896] __gxx_personality_sj0 Would updating Haskell Platform's libstdc++-6.dll fix this issue? Is there some way to cordon Haskell Platform off from the rest of the system, while still making ghc.exe and friends available on PATH? Here are my system specs, I figure they're highly relevant. $ specs haskell perl node ruby git os Specs: specs 0.4 https://github.com/mcandre/specs#readme cabal --version cabal-install version 0.14.0 using version 1.14.0 of the Cabal library ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.2 ghc-pkg field haskell-platform version version: 2012.4.0.0 cpan --version c:\strawberry\perl\bin/cpan version 1.5902 calling Getopt::Std::getopts (version 1.07 [paranoid]), running under Perl version 5.16.2. [Now continuing due to backward compatibility and excessive paranoia. See 'perldoc Getopt::Std' about $Getopt::Std::STANDARD_HELP_VERSION.] Nothing to install! perl --version This is perl 5, version 16, subversion 2 (v5.16.2) built for MSWin32-x86-multi-thread Copyright 1987-2012, Larry Wall Perl may be copied only under the terms of either the Artistic License or the GNU General Public License, which may be found in the Perl 5 source kit. Complete documentation for Perl, including FAQ lists, should be found on this system using "man perl" or "perldoc perl". If you have access to the Internet, point your browser at http://www.perl.org/, the Perl Home Page. npm --version 1.2.17 node --version v0.10.3 bundle --version Bundler version 1.3.2 gem --version 2.0.3 ruby --version ruby 1.9.3p374 (2013-01-15) [i386-mingw32] git --version git version 1.8.0.msysgit.0 systeminfo | findstr /B /C:"OS Name" /C:"OS Version" OS Name: Microsoft Windows XP Professional OS Version: 5.1.2600 Service Pack 3 Build 2600 -- Cheers, Andrew Pennebaker www.yellosoft.us

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ANN: stm-chans 2.0.0
by wren ng thornton 13 May '13

13 May '13

-------------------------------------------- -- stm-chans 2.0.0 -------------------------------------------- The stm-chans package offers a collection of channel types, similar to Control.Concurrent.STM.{TChan,TQueue} but with additional features. -------------------------------------------- -- Changes (since 1.3.1) -------------------------------------------- * (by kudah) Added TMQueue and TBMQueue, which are like the corresponding T*Chans except using the new T*Queue data types in stm >= 2.4. We use CPP to provide compatibility shims for older versions of stm. -------------------------------------------- -- Long description -------------------------------------------- In particular stm-chans offers the following data types: * Control.Concurrent.STM.TBChan: Bounded FIFO channels. When the channel is full, writers will block/retry. This ensures that the writers do not get too far ahead of the readers, which helps to make sure that memory and cpu resources are used responsibly. * Control.Concurrent.STM.TMChan: Closeable FIFO channels. * Control.Concurrent.STM.TMQueue: Closeable FIFO queues. Like TChan (Maybe a) but with a monotonicity guarantee that once Nothing is returned all future reads will be Nothing as well. * Control.Concurrent.STM.TBMChan: Bounded Closeable FIFO channels. * Control.Concurrent.STM.TBMQueue: Bounded Closeable FIFO queues. Combines the capabilities of TBChan and TMChan. In addition, the stm-chans package offers a (partial) compatibility layer for various changes to the stm package. These include backwards compatibility for the T*Queue types, as well as various API improvements (see ticket #5104 [1]). [1] http://hackage.haskell.org/trac/ghc/ticket/5104 -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/stm-chans Darcs: http://community.haskell.org/~wren/stm-chans Haddock (Darcs version): http://community.haskell.org/~wren/stm-chans/dist/doc/html/stm-chans -- Live well, ~wren

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Stream processing
by oleg＠okmij.org 13 May '13

13 May '13

I'm a bit curious > * be reliable in the presence of async exceptions (solved by conduit, > pipes-safe), > > * hold on to resources only as long as necessary (solved by conduit > and to some degree by pipes-safe), Are you aware of http://okmij.org/ftp/Streams.html#regions which describes both resource deallocation and async signals. Could you tell what you think is deficient in that code? > * ideally also allow upstream communication (solved by pipes and to > some degree by conduit). Are you aware (of, admittedly) old message whose title was specifically ``Sending messages up-and-down the iteratee-enumerator chain'' http://www.haskell.org/pipermail/haskell-cafe/2011-May/091870.html (there were several messages in that thread). Here is the code for those messages http://okmij.org/ftp/Haskell/Iteratee/UpDown.hs

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Google Summer of Code Proposal - Communicating with mobile devices
by Marcos Pividori 13 May '13

13 May '13

Greetings, I am a Computer Science student from Argentina. I am interested in working this summer in a project related to Haskell for the Google Summer of Code. I have been discussing my idea with Michael Snoyman in order to have a clearer idea. Now, I would like to know the community interest in this project. I want to develop a server-side library in Haskell for sending push notifications to devices running different OS, such as Android, iOS, Windows Phone, BlackBerry, and so on. To pass a subject, I have recently worked with Yesod (a Web Framework based in Haskell) developing a server to comunicate with Android-powered devices through Google Cloud Messaging. (It is available: https://github.com/MarcosPividori/Yesod-server-for-GCM – It is a Spanish commented version because it was a project for my University, I will replace it for an English version in the next weeks) To develop this project, I have read a lot about this service and Yesod libraries, and I developed two programs, a server written in Haskell and an Android application for mobile phones. Also, I developed an EDSL to write programs which exchange information with the devices. I would be really grateful if you could give me your opinion about this project and the proposal. I want some feedback in order to know if this would be a useful tool and what you would like to get out of it. *Communicating with mobile devices* * * *Abstract* The aim of this project is to develop a server-side library in Haskell for sending push notifications to devices running different OS, such as Android, iOS, Windows Phone, BlackBerry, and so on. The fact is that every company is developing Push Notification services, and these are very similar. Then, I want to find the fundamental concepts to construct a library which enable to configure the options for the different services and send messages easily. When I say they are very similar, I refer to the fact that they all are asynchronous, best-effort services that offers third-party developers a channel to send data to apps from a cloud service in a power-efficient manner. The most popular are: - Google Cloud Messaging (Android) - Apple Push Notification Service (iPhone / iPad) - Microsoft Push Notification Service (Windows Phone) - BlackBerry Push Service (BlackBerry) - Windows Push Notification Services (Windows 8) - etc. Once we have this libraries, I will investigate the possibility of mainting a "back and forth" communication between a server and mobile devices and I will develop a library to handle this. * * *Motivation and expected benefits* I think this idea would be very useful because it will allow all Haskell developers to open to a new world of mobile devices and to build useful programs/services that interact with them. Pushing data to smartphones provides users with instant access to desired updates as they happen, such as news and weather, sports scores, stock prices and other time-sensitive content. The push services provide an efficient way to quickly push timely information updates to many smartphones at once, in a centrally managed and controlled manner. Generally, you can also be very selective in who you send information to, including individual customers or many customers (multicast). This services minimizes the impact on the smartphones battery life. Instead of actively checking for new data, the applications can remain closed. Once the data is delivered, the application can be launched in the background to process it as needed. This processes offer an alternative to other less efficient methods, such as polling, where a device regularly polls an application server to see if new content is available. The main differences between the services, refer to details as: the maxim payload length, the quality of service, queueing the messages or not, and the time limit for this, the way the messages are handled in the devices, etc. As all the libraries to access to these services are developed in Java, I thought that it would be a good idea to offer an option to Haskell programmers. Taking advantage of the similarity of these services, I could develop a very adaptable library which fits the necessities for each one and at the same time offer an abstraction to the user. *Deliverables.* * * * An API library to build and send messages including: - GCM and a demo Android app. - APN and a demo iOS app. - Microsoft Push Notification Service (Windows Phone) and a demo app. - Documentation for all the code developed. Including the explantation on how to use the server library and how to try the demo apps. * A library to handle a "back and forth" comunication between a server and mobile devices. Tools to mantain a state of the connection and manage with a lot of devices at the same time. A Yesod app example of the use of this library and a demo app for each OS (Android, iOS, Windows Phone, etc) to manage this communication. * Optionally: - an API for communication through BlackBerry Push Service (BlackBerry). - an API for communication through Windows Push Notification Services (Windows 8). *Technical Considerations* In the developing of the APIs for the communication through Push Notifications, I will aim to develop a good abstraction and find the properties in common between the differents services in order to develope an customizable tool but at the same time with a common structure. I want to let the user build messages and send these in a simple way following each protocol. Also, I will abstract the process of registering the devices in the server and let the user manage the different registrations behind a similar abstraccion. To develop a “back and forth” comunication between a server and mobile devices, I will investigate the different possibilities of maintaining a state of the connection. It could be through the use of cookies stored by the clients or maintaining some extra information in the server which would enable it to identify the different connections and provide the appropiate services.

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GPGPU
by Christopher Howard 12 May '13

12 May '13

Has anybody on the list been playing around with OpenCL at all? I'm just starting to look into it - need to get a newer Radeon card, I think - but I'm strongly interested in GPGPU programming. -- frigidcode.com

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