August 2014 - Haskell-Cafe

ANN: Yampa 0.9.6
by Ivan Perez 29 Aug '14

29 Aug '14

Hi Café, I'm happy to announce that a new version of the Arrowized Functional Reactive Programming (FRP) library Yampa has been released. This new version includes a lot of haddock documentation, finally addressing the old issue of having to browse a large collection of papers and the code to find information about the API. We would like to encourage users to submit issues on github if they find this documentation insufficient, unclear or otherwise unsuitable in any way. There are no changes to the API itself, so this version remains backwards compatible. Future releases will seek to improve the documentation and introduce new optimisations as mandated by users' needs. As you may have heard, the current version is now working on Android, so we expect new releases to be published relatively soon. Additionally, Henrik Nilsson will give a tutorial on Declarative Game Programming at PPDP 2014, which will take place in Canterbury (UK) on September 8-10 (http://users-cs.au.dk/danvy/ppdp14/) We would like to encourage everyone to attend (see PPDP web page for instructions, venue, etc.). Links and more: * Hackage package: http://hackage.haskell.org/package/Yampa-0.9.6. * Github project page: https://github.com/ivanperez-keera/Yampa * Haskell Wiki page (examples, papers): http://www.haskell.org/haskellwiki/Yampa * Official announcement: http://fplab.bitbucket.org/posts/2014-08-29-yampa-0_9_6-released.html * (Another) Release announcement: http://keera.co.uk/blog/2014/08/29/new-version-yampa-0-9-6. Cheers Ivan -- Facebook: https://www.facebook.com/keerastudios Twitter: https://twitter.com/KeeraStudios Web: http://www.keera.co.uk

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ANN: HacBerlin - Haskell Hackathon in Berlin, 26-28 Sep 2014
by Stefan Wehr 29 Aug '14

29 Aug '14

Hi everyone, this is just a quick reminder: The Haskell Hackathon in Berlin is coming soon and there are still some places left. Please register now: http://goo.gl/aLfnWu The first keynote is also fixed; it will be given by Andres Löh (http://www.andres-loeh.de/) Thanks, Andres! Where: Berlin, Germany When: Fri 26 - Sun 28 September 2014 Meet in Berlin, discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! For all details, visit our wiki page (http://www.haskell.org/haskellwiki/HacBerlin2014) and make sure to register now! Cheers, Stefan

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Conduit+GHC high memory use for simple Sink
by Bryan Vicknair 28 Aug '14

28 Aug '14

Hello Cafe, First I'd like to thank Michael Snoyman and Gabriel Gonzalez for the work they've done on the conduit and pipes stream processing libraries, and all the accompanying tutorial content. I've been having fun converting a text processing app from C to Haskell. I'm seeing unexpectedly high memory usage in a stream-processing program that uses the conduit library. I've created a example that demonstrates the problem. The program accepts gzip files as arguments, and for each one, classifies each line as either Even or Odd depending on the length, then outputs some result depending on the Sink used. For each gzip file: action :: GzipFilePath -> IO () action (GzipFilePath filePath) = do result <- runResourceT $ CB.sourceFile filePath $$ Zlib.ungzip =$ CB.lines =$ token =$ sink2 putStrLn $ show result The problem is the following Sink, which counts how many even/odd Tokens are seen: type SinkState = (Integer, Integer) sink2 :: (Monad m) => SinkState -> Sink Token m SinkState sink2 state@(!evenCount, !oddCount) = do maybeToken <- await case maybeToken of Nothing -> return state (Just Even) -> sink2 (evenCount + 1, oddCount ) (Just Odd ) -> sink2 (evenCount , oddCount + 1) When I give this program a few gzip files, it uses hundreds of megabytes of resident memory. When I give the same files as input, but use the following simple Sink, it only uses about 8Mb of resident memory: sink1 :: MonadIO m => Sink Token m () sink1 = awaitForever (liftIO . putStrLn . show) At first I thought that sink2 performed so poorly because the addition thunks were being placed onto the heap until the end, so I added some bang patterns to make it strict. That didn't help however. I've done profiling, but I'm unable to figure out exactly what is being added to the heap in sink2 but not sink1, or what is being garbage collected in sink1, but not sink2. The full source is here: https://bitbucket.org/bryanvick/conduit-mem/src/HEAD/hsrc/bin/mem.hs Or you can clone the repo, which contains a cabal file for easy building: git clone git@bitbucket.org:bryanvick/conduit-mem.git cd comduit-mem cabal sandbox init cabal install --only-dependencies cabal build mem ./dist/build/mem/mem [GIVE SOME GZIP FILES HERE] You can change which sink is used in the 'action' function to see the different memory usage.

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Haskell Platform 2014 build problems on OS X
by Nicola Gigante 28 Aug '14

28 Aug '14

Hello! I just want to point to your attention this issue of Homebrew: https://github.com/Homebrew/homebrew/issues/31609 It seems that they’re forced to remove the formula for the Haskell Platform because, it seems, the build process has become extremely complicated and they can’t figure out how to automate it. Is there someone that know enough internals of the platform to help them or to give some advice? Thanks, Nicola

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Call to arms for Haskell students
by Ernesto Rodriguez 28 Aug '14

28 Aug '14

Dear Haskellers, My (previous) university is organizing one of the first university hackathons in Europe. I am participating and would like to have a Haskell team for the hackathon. However, I don't know any Haskellers around or who can travel to Bremen. Let me know if anyone is interested to participate and team up with me. If it helps there are quite inexpensive flights to Bremen. The hackathon is only for students though. Quoted is the formal invite. Best regards, Ernesto It is about time to bring hackathons to European universities. A few > students at Jacobs University are therefore organizing a hackathon that > aims to bring people from different countries together to hack. > jacobsHack! is an international student-run hackathon in Bremen, Germany. > Students will get 24 hours to hack on anything they like and have the > chance to win prizes worth up to 2000€. Additionally they can make the > first step in the direction of working with one of our sponsors Google, > SAP, Microsoft, and Entrepreneur First. > > More information at: > https://jacobshack.com/ > > Live long, #hackstrong > -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

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Haskell Weekly News: Issue 303
by Daniel Santa Cruz 28 Aug '14

28 Aug '14

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lightweight web interface for Haskell?
by Richard Eisenberg 27 Aug '14

27 Aug '14

Hi Cafe, Does anyone know of a website where I can write a few lines of Haskell and have them run? Ideally, there would be a place to write a module and then a place to load the module into GHCi. I seem to recall something that fit my needs at paste.hskll.org, but that site is no longer working. I'm asking because I'll be teaching an undergraduate course this fall in introductory Haskell. There are no prerequisites for the course, and my course roster suggests that I'll have several students with no programming background at all. (This is great, actually -- I love facilitating a new programmer's first steps!) It might be nice to get students off the ground quickly before bogging them down with installation details -- hence the website I'm looking for. Thanks! Richard

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[ANN] Wheb 0.3 release
by Kyle Hanson 27 Aug '14

27 Aug '14

Hello all, I have been working on a web framework that I use for my personal projects and I thought I would share my small progress. I started this project several months ago when I hit a big bump in difficulty going from a simple skeleton project to a more intricate project using alternative frameworks. Lately I've added three main features: WebSockets, Cache and Redis. WebSockets use the same WhebT interface as the standard HTTP handlers allowing you to share functions and middlware accross your application. Cache is a new plugin for simplistic Caching. It provides an interface to cache ByteStrings under Text keys. Like the other plugins, it is backend agnostic so you can write your own backend or use the prebuilt Redis cache (this requires you to configure your redis instance for caching). In addition to Redis acting as a Cache, you can also use it as a database and as a backend for the plugins Auth and Sessions. You can mix Redis with other plugins. For one of my projects I use the MongoDB plugin for auth and storage and Redis as a cache for the queries. Also included are typed web-routes, stateful cookie handling, and some other stuff. Feel free to check it out. I tried to include some documentation and examples that I refer to when I use it. Maybe it will help someone. Its far from perfect, so if you find any problems, let me know. https://hackage.haskell.org/package/Wheb https://github.com/hansonkd/Wheb-Framework Cheers, Kyle Hanson

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Re: [Haskell-cafe] Types depending on tuples.
by Mike Izbicki 26 Aug '14

26 Aug '14

This works just fine for me: class Vector a where toLists :: a -> [Double] instance Vector (Double,Double,Double) where toLists (a,b,c)=[a,b,c] On Tue, Aug 26, 2014 at 1:17 AM, Mike Meyer <mwm(a)mired.org> wrote: > It's there, but in the text: > > class Vector a where > toLists :: a -> [Double] > > > On Tue, Aug 26, 2014 at 2:59 AM, Mike Izbicki <mike(a)izbicki.me> wrote: >> >> You never actually provided the Vector type class, so it's difficult >> to tell what exactly you're trying to do. >> >> On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <mwm(a)mired.org> wrote: >> > I'm doing some geometry in 2 and 3 dimensions. To do this, I've got >> > Vector2d >> > and Vector3d classes, which are basically just wrappers around 2 and >> > 3-tuples of doubles: >> > >> > type Vector2d = (Double, Double) >> > type Vector3d = (Double, Double, Double) >> > >> > I've also got a typeclass Vector so I can do polymorphic things on them: >> > >> > type Vector3d = (Double, Double, Double) >> > >> > (Ok, polymorphic thing). >> > >> > I now have a need to convert lists of Vector's to a list of lists of >> > Doubles. I'd like that to be part of the Vector typeclass, but - well, >> > declaring it is interesting. >> > >> > I can't just add a method "toLists :: a -> [Double]", because (according >> > to >> > ghc), there's no match between the expected type 'Double' and the actual >> > type '[t1]'. >> > >> > Can I declare this typeclass? >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > >

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Fwd: Locking of threads in one OS thread
by Nikolay Amiantov 26 Aug '14

26 Aug '14

On Tue, Aug 26, 2014 at 6:59 PM, Sylvain Henry <hsyl20(a)gmail.com> wrote: > AFAIK "errno" is handled explicitly in the RTS: > https://ghc.haskell.org/trac/ghc/browser/ghc/rts/Schedule.c#L484 > > -Sylvain > Oh, thanks for pointing this out! Now this behaviour makes sense.

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