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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 21 Nov '11

21 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 21 Nov '11

21 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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Problems with installing the correct version of "process". Help?
by Blaine 20 Nov '11

20 Nov '11

Hi everyone. I'm new to haskell (venturing over from python land); i can't figure this one problem out. This has been incredibly difficult to solve, and it is quite discouraging! i'm trying to get berp up and running ( https://github.com/bjpop/berp/) berp-libs installed fine, berp-compiler is hanging up: >From what I can tell, the only hangup I have is that the "process" package depends on the wrong version of "directory". [blaine@macbook:~/Dropbox/src/berp/compiler Tue Nov 15] 75$ cabal configure Resolving dependencies... Configuring berpcompiler-0.0.3... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. *package process-1.0.1.3 requires directory-1.0.1.0* package cpphs-1.13.2 requires directory-1.1.0.1 package berplibs-0.0.3 requires directory-1.1.0.1 package berpcompiler-0.0.3 requires directory-1.1.0.1 OK so I need to rebuild process so it uses the newest version of directory like the rest of them (I have no idea if this is the right thing to do, but this is what i did with cpphs and it worked). But when I try and build process, it can't find a header on my file that does exist. I also did successfully install from source package manually (using these instructions<http://www.haskell.org/haskellwiki/Cabal/How_to_install_a_Cabal_package>on the package downloaded from here <http://hackage.haskell.org/package/process-1.0.1.3>, which worked fine, but didn't seem update the dependency to rely on the newest version of directory (aka still can't configure berp). I don't know where to go from here. Please note: I have installed from source package, so even if I fix this cabal issue I'm not sure it will fix the dependency issue (but who knows, maybe it will). I'm having a hard time scaring up internet resources on where to proceed. What am I doing wrong? Thanks so much in advance, Blaine Reference: 22$ uname -a Linux macbook 2.6.32-34-generic #77-Ubuntu SMP Tue Sep 13 19:39:17 UTC 2011 x86_64 GNU/Linux (Here I am trying to force include the directory where the missing header file lives. It does exist but cabal won't have it) [blaine@macbook:~/Dropbox/src/berp/compiler Tue Nov 08] 92$ sudo cabal --extra-include-dirs=/usr/lib/ghc-6.12.1/process-1.0.1.2/include/ install process Resolving dependencies... Configuring process-1.1.0.0... configure: WARNING: unrecognized options: --with-compiler checking for gcc... gcc checking whether the C compiler works... yes checking for C compiler default output file name... a.out checking for suffix of executables... checking whether we are cross compiling... no checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking how to run the C preprocessor... gcc -E checking for grep that handles long lines and -e... /bin/grep checking for egrep... /bin/grep -E checking for ANSI C header files... yes checking for sys/types.h... yes checking for sys/stat.h... yes checking for stdlib.h... yes checking for string.h... yes checking for memory.h... yes checking for strings.h... yes checking for inttypes.h... yes checking for stdint.h... yes checking for unistd.h... yes checking for pid_t... yes checking vfork.h usability... no checking vfork.h presence... no checking for vfork.h... no checking for fork... yes checking for vfork... yes checking for working fork... yes checking for working vfork... (cached) yes checking signal.h usability... yes checking signal.h presence... yes checking for signal.h... yes checking sys/wait.h usability... yes checking sys/wait.h presence... yes checking for sys/wait.h... yes checking fcntl.h usability... yes checking fcntl.h presence... yes checking for fcntl.h... yes checking for setitimer,... no checking for sysconf... yes checking value of SIG_DFL... -1 checking value of SIG_IGN... -1 configure: creating ./config.status config.status: creating include/HsProcessConfig.h config.status: include/HsProcessConfig.h is unchanged configure: WARNING: unrecognized options: --with-compiler cabal: Missing dependency on a foreign library: ** Missing header file: runProcess.h* This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: process-1.1.0.0 failed during the configure step. The exception was: ExitFailure 1 [blaine@macbook:~/Dropbox/src/berp/compiler Tue Nov 08] 93$ ls /usr/lib/ghc-6.12.1/process-1.0.1.2/include/ HsProcessConfig.h *runProcess.h* Blaine

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ANNOUNCE: SmallCheck 0.5
by Roman Cheplyaka 19 Nov '11

19 Nov '11

Following the recent discussion on Haskell-Cafe, I've taken over maintainership of SmallCheck. This is to announce a new minor release, SmallCheck 0.5. It fixes build on GHC 7.2 and makes a few cosmetic changes. http://hackage.haskell.org/package/smallcheck-0.5 In the future there will be more releases that hopefully will make SmallCheck more convenient to use. Feel free to submit bugs, wishes and patches through github: https://github.com/feuerbach/smallcheck -- Roman I. Cheplyaka :: http://ro-che.info/

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Const vs Constant types
by Denis Moskvin 19 Nov '11

19 Nov '11

Is there any rationale for coexistence of newtype Const a b = Const { getConst :: a } from Control.Applicative and newtype Constant a b = Constant { getConstant :: a } from Data.Functor.Constant (transformers package)? Denis Moskvin

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Documenting strictness properties for Data.Map.Strict
by Johan Tibell 19 Nov '11

19 Nov '11

Hi all, Data.Map is getting split into Data.Map.Lazy and Data.Map.Strict (with Data.Map re-exporting the lazy API). I want to better document the strictness properties of the two new modules. Right now the documentation for Data.Map.Strict reads: Strictness properties ===================== * All functions are strict in both key and value arguments. Examples: insertWith (+) k undefined m == undefined delete undefined m == undefined * Keys and values are evaluated to WHNF before they are stored in the map. Examples: map (\ v -> undefined) == undefined mapKeys (\ k -> undefined) == undefined I'm not entirely happy with this formulation. I'm looking for something that's clear (i.e. precise and concise, without leaving out important information), assuming that the reader already knows how lazy evaluation works at a high level. Ideas? Cheers, Johan

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