November 2011 - Haskell-Cafe

ST not strict enough?
by Jason Dusek 21 Nov '11

21 Nov '11

Hi All, I'm having some trouble with memory usage in rebuilding a ByteString with some sequences escaped. I thought I'd try vectors. However, it seems that even a relatively simple function, one that places all the bytes of a ByteString in to a vector, uses a great deal of memory. I've pulled this function out into a mini-module for testing: https://github.com/solidsnack/arx/blob/ST/Rebuild.hs#L37 On a one megabyte input, it peaks at ~38M of memory: :; dd if=/dev/zero bs=1M count=1 | ./rebuildprof +RTS -s ... 38,724,208 bytes maximum residency (5 sample(s)) 1,983,720 bytes maximum slop 76 MB total memory in use (0 MB lost due to fragmentation) ... %GC time 61.3% (61.5% elapsed) A heap profile by type, with -hy, shows a linear rise in ST items -- up to ~26M -- and then a linear decrease. It would stand to reason that, with sufficient strictness, the memory allocated to ST would stay constant and small. Should I be annotating my functions with strictness, for the vector reference, for example? Should I be using STUArrays, instead? -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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Job Opportunity at Parallel Scientific
by Peter Braam 21 Nov '11

21 Nov '11

Parallel Haskell Programmers Parallel Scientific, LLC is a Boulder, CO based early stage, but funded startup company working in the area of scalable parallelization for scientific and large data computing. We are implementing radically new software tools for the creation and optimization of parallel programs benefiting applications and leveraging modern systems architecture. We build on our mathematical knowledge, cutting edge programming languages and our understanding of systems software and hardware. We are currently working with the Haskell development team and major HPC laboratories world wide on libraries and compiler extensions for parallel programming. Parallel Scientific was founded by Peter Braam in 2010. Peter formerly taught mathematics and computer science at Oxford and Carnegie Mellon. Then he contributed file systems to Linux and invented Lustre (which provides storage to 9 of the top 10 systems in the world). He ran several successful startups, and Parallel Scientific is run by an very experienced management team and board. Successful candidates can in some cases work remotely and will work in a modern virtual environment. We provide training in advanced processes for software design and implementation and domain specific knowledge. Required skills: - Very strong background in computer science or mathematics - Experience with Haskell - Knowledge of systems programming and operating systems functionality - Knowledge of system architectures, such as high performance networking, memory architectures, multi and manycore CPUs and GPGPUs - Experience with performance tuning of parallel or concurrent algorithms - Experience designing and implementing concurrent or parallel programs Experience in one or more of the following areas is desirable: - Experience with Haskell compiler technology - In depth knowledge of core Haskell libraries for parallel programming (NDP, REPA etc) - Experience in the area of middleware algorithms for data flow programming, graphs, cloud based data analytics, or sparse matrices - Domain specific knowledge in scientific programming areas related to irregular and sparse problems, e.g. data analytics using graph analysis, genomics, tightly connected numerical analysis - Experience with performance tuning for parallel applications on multi core systems (e.g. with the Intel toolkit), for GPGPU's using Cuda/OpenCL or with MPI/OpenMP on clusters of SMPs To apply, please send a resume to jobs(a)parsci.com.

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Interpreter with Cont
by Tim Baumgartner 21 Nov '11

21 Nov '11

Hi all, I wrote a simple interpreter that can be run in the console: data Interaction a b = Exit b | Output b (Interaction a b) | Input (a -> Interaction a b) runConsole :: Interaction String String -> IO () runConsole (Exit b) = putStrLn $ "Finished. Result: " ++ b runConsole (Output s cont) = putStrLn s >> runConsole cont runConsole (Input f) = putStr "> " >> getLine >>= runConsole . f interpreter :: Int -> Interaction String String interpreter i = interaction where interaction = Input input input "exit" = Exit (show i) input "inc" = Output "ok" $ interpreter (i+1) input "show" = Output (show i) interaction input "hello"= Output "Hello World!" interaction input s = Output ("Whas's '" ++ s ++ "' ?") interaction main = runConsole . Output "Known commands: show, inc, hello, exit" $ interpreter 5 I have not yet gained a good understanding of the continuation monad, but I wonder if it could be used here. What would a clean solution look like? Perhaps there are other things that need to be changed as well? Regards, Tim

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Superset of Haddock and Markdown
by Johan Tibell 21 Nov '11

21 Nov '11

Hi all, I spent some time today documenting a library and the experience left me wanting a better markup language. In particular, Haddock lacks: * markup for bold text: bold text works better than italics for emphasis on computer monitors. * hyperlinks with anchor texts: having the actual URL rendered inline with text hurts readability. Could Haddock markup be extended to also include some Markdown features? The new features could be hidden behind a flag so old documentation doesn't get unwanted markup (e.g. if it uses *...* to not mean bold). P.S. This good make a good weekend hack that shouldn't be too difficult. -- Johan

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Lifted Spine View
by bob zhang 21 Nov '11

21 Nov '11

Hi, all I tried to follow the program of the paper "Scrap your boilerpolate" Revolutions. Unfortunately, I found the program in the section lifted spine view does not compile in my GHC, could anybody point out where I am wrong? Many Thanks My code is posted herehttp://hpaste.org/54357

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Deduce problem.
by Magicloud Magiclouds 21 Nov '11

21 Nov '11

Hi, Consider I have declarations like this: class (ClassA a) => ClassC a where from :: (ClassB b) => a -> [b] to :: (ClassB c) => a -> [c] data H = ... instance ClassB H where ... data Test = Test { m :: H } instance ClassA Test where ... instance ClassC Test where from = m to = m Well, I got "could not deduce" error here at "from = m" and "to = m". `c' is a rigid type variable bound by the type signature for to :: ClassB c => Test -> [c]. Referring to some similar questions on internet, I should remove the (ClassB c) thing. Is this the only solution? -- 竹密岂妨流水过山高哪阻野云飞

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thread blocked indefinitely in an STM transaction
by Eric Wong 20 Nov '11

20 Nov '11

Hi everyone, I'm writing an API server using the snap framework with the new snaplet infrastructure. In order to connect with the database, I'm using snaplet-hdbc, HDBC, HDBC-mysql. (I'm using InnoDB for storage engine so it can process transaction). Every query is wrapped in the function withTransaction' so I don't have to commit or rollback by hand. If there's a SqlError raised, I will catch it, log the error and return an error message to the client side. But when running the program, if there's a DB error, the process will be blocked and there's an error message: " thread blocked indefinitely in an STM transaction". There's no STM related code in my code. Do you have any idea on this error and how to process this? Thanks. Eric.

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os.path.expanduser analogue
by Ben Gamari 20 Nov '11

20 Nov '11

On the whole, the filepath package does an excellent job of providing basic path manipulation tools, one weakness is the inability to resolve "~/..." style POSIX paths. Python implements this with os.path.expanduser. Perhaps a similar function might be helpful in filepath? Cheers, - Ben Possible (but untested) implementation expandUser :: FilePath -> IO FilePath expandUser p = if "~/" `isPrefixOf` p then do u <- getLoginName return $ u ++ drop 2 p else return p

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Visi.Pro -- My Next Thing
by David Pollak 20 Nov '11

20 Nov '11

Howdy Haskeller, I've launch a new, Haskell powered, adventure: Visi.Pro, Cloud Computing for the Rest of Us. Visi.Pro will offer a HyperCard-like development environment that will empower normal people to build and run beautiful, interactive apps on their iPad and seamlessly integrate the Cloud including real-time data feeds and destinations. More about the journey that led me to Visi.Pro at http://blog.visi.io/ and more about the product vision at http://visi.pro (http://visi.pro/) I am totally stoked about where this journey will lead... it's going to be amazingly exciting! I'm also looking forward to taking advantage of Haskell awesomeness as I build Visi.Pro and Visi.io out. Haskell has been a wonderful journey for me so far. The quality in design and implementation of the language, the runtime, and the libraries is amazing. I am standing on the shoulders of giants by building Visi.Pro on Haskell. Thanks! David -- @dpp

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Re: [Haskell-cafe] [Haskell-beginners] Int V.S. Word32
by Haisheng Wu 20 Nov '11

20 Nov '11

Hmm... I think I made a little confusion so I put my finding here: http://haisgwu.info/posts/2011-11-20-euler-problem-14.html I do got stack overflow thus need several compile opts to fix it. Not sure if it is what you mean by "You get overflow using 32-bit types here." -Haisheng On Sat, Nov 19, 2011 at 10:49 PM, Daniel Fischer < daniel.is.fischer(a)googlemail.com> wrote: > On Saturday 19 November 2011, 09:09:50, Haisheng Wu wrote: > > Hello, > > I got great performance difference for the following code if I used > > type `Int` rather than `Data.Word.Word32`. > > Anyone can help to explain why such difference? > > Short answer: GHC optimises Int calculations far better than Word32 > calculations, rather, it optimises more calculations for Int than for > Word32. It also optimises more calculations for Word than for Word32, but > not as many as for Int. > > The reason is that Int (and Word) are (at least expected to be) far more > often used, so the effort has gone to these types primarily. Work is being > done to get the fixed-width types on par, but it's not around the corner > yet. > > You can try using Word instead of Word32, that's likely to be faster. > > But > > > Thanks a lot. > > > > -Simon > > > > module Main where > > import Data.Word > > > > main :: IO () > > main = print $ p14 > > > > p14 = maximum [ (startChain n 0, n) | n <- [2..1000000] ] > > You get overflow using 32-bit types here. > > > > > startChain :: Word32 -> Int -> Int > > startChain 1 count = count + 1 > > startChain n count = startChain (intTransform n) (count+1) > > > > intTransform :: Word32 -> Word32 > > intTransform n > > > > | even n = n `div` 2 > > | otherwise = 3 * n + 1 > >

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