August 2012 - Haskell-Cafe

For consistency; it would be better if the import statement matched the cabal install statement or :m form.
by KC 15 Aug '12

15 Aug '12

:m +Data.Array.Repa.Algorithms.Randomish cabal install repa.algrothms would be more consistent. -- -- Regards, KC

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Debian 6.0.5: 'cabal install curl' problem
by dokondr 14 Aug '12

14 Aug '12

Please help to solve a problem installing curl package ( http://hackage.haskell.org/package/curl/) on Debian 6.0.5. I am running the most recent Debian Haskell platform with GHC 6.12.1. I did: -> cabal update -> cabal install cabal-install It is interesting that in case you do again 'cabal update" you again get a suggestion from cabal to upgrade cabal-install, and this process never ends: """ -> cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install """ Anyway, then I did: -> apt-get install curl -> cabal install curl Cabal fails to install 'curl' because it can not find curl library. Please see detailed cabal output at the end of this message. So my questions: 1) What is missing to install curl package? 2) Should I upgrade Debian GHC 6.12.1 to the latest GHC 7.4.2? Maybe this should solve the problem with 'cabal install'? Thanks! ---------- cabal install output ---------- -> cabal install curl Resolving dependencies... Configuring curl-1.3.7... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... 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 ANSI C... none needed checking how to run the C preprocessor... gcc -E configure: error: curl libraries not found, so curl package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: curl-1.3.7 failed during the configure step. The exception was: ExitFailure 1

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I use "cabal install repa" but then WinGHCi says module Data.Array.Rep.Algorithms.Ramdomish not found.
by KC 14 Aug '12

14 Aug '12

-- -- Regards, KC

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Fixity declaration extension
by Евгений Пермяков 14 Aug '12

14 Aug '12

fixity declaration has form *infix(l|r)? [Digit]* in haskell. I'm pretty sure, that this is not enough for complicated cases. Ideally, fixity declarations should have form *infix(l|r)? [Digit](\.(+|-)[Digit])** , with implied infinitely long repeated (.0) tail. This will allow fine tuning of operator priorities and much easier priority selection. For example, it may be assumed, that bit operations like (.&.) operator have hightest priority and have priorities like 9.0.1 or 9.0.2, anti-lisps like ($) have lowest priority like 0.0.1, control operators have base priority 1.* and logic operations like (&&) have priority of 2.* and it will be possibly to add new operators between or above all (for example) control operators without moving fixity of other ones. Agda2 language supports wide priority range, but still without 'tails' to my knowledge. Is there any haskell-influenced language or experimental syntactic extension that address the issue?

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Lost global Contents of haddock
by Marco Túlio Pimenta Gontijo 14 Aug '12

14 Aug '12

Hi. I just noticed that my global Contents of the haddock documentation of modules ( file:///home/marcot/.cabal/share/doc/index.html ) lost most of the modules. My workflow is fairly straightforward, but I do some --reinstall --force-reinstalls with cabal install. For instance, the link for Prelude is not there, but Prelude.Unicode from base-unicode-symbols is present. I can still access documentation from the package's Contents page ( file:///usr/local/share/doc/ghc/html/libraries/base-4.5.1.0/index.html ). What could have caused this? Is there a way to recover all the links? I'm using Debian Squeeze with ghc-7.4.2 (installed with http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-x86_64-unknown-linux.tar.bz2 ), cabal install version 0.14.0, using version 1.14.0 of the Cabal library. Greetings. -- marcot http://marcot.eti.br/

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overloading integer literals
by Евгений Пермяков 14 Aug '12

14 Aug '12

During development some toy base library I found impossible to use Numeric literals. Quick search showed, that one need both fromInteger in scope (reasonable) and, as I understand, access to Integer type from 'base' package ('base' for clarity later). It is perfectly reasonable if we assume that every module must depend on 'base'. However, with ghc-prim and NoIplicitPrelude it is not necessary this way. One may wish to drop 'base' dependency entirely. Currently, the only workaround I found is to use cast from string literals (that works perfectly okay) or use unboxed literals like 0xBB## (of unboxed type). Both solutions look dirty, especially in pattern guards. Is there any better solution or movement to improve RebindableSyntax facilities ? for example, moving out classes for RebindableSyntax into ghc-prim package and tuning them so all syntax facilities in base could be defined in terms of RebindableSyntax would be great.

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ANNOUNCE: abacate and chuchu
by Marco Túlio Pimenta Gontijo 13 Aug '12

13 Aug '12

Hi. I'm pleased to announce the first release of Abacate and Chuchu. Chuchu is a port of Ruby's Cucumber to Haskell, and Abacate is a parser for the Gherkin language, used by Cucumber. According to their web site, "Cucumber lets software development teams describe how software should behave in plain text. The text is written in a business-readable domain-specific language and serves as documentation, automated tests and development-aid - all rolled into one format." For more information on Cucumber: http://cukes.info/ Example of usage for a stack calculator: calculator.feature: Feature: Division In order to avoid silly mistakes Cashiers must be able to calculate a fraction Scenario: Regular numbers Given that I have entered 3 into the calculator And that I have entered 2 into the calculator When I press divide Then the result should be 1.5 on the screen calculator.hs: import Control.Applicative import Control.Monad.IO.Class import Control.Monad.Trans.State import Test.Chuchu import Test.HUnit type CalculatorT m = StateT [Double] m enterNumber :: Monad m => Double -> CalculatorT m () enterNumber = modify . (:) getDisplay :: Monad m => CalculatorT m Double getDisplay = do ns <- get return $ head $ ns ++ [0] divide :: Monad m => CalculatorT m () divide = do (n1:n2:ns) <- get put $ (n2 / n1) : ns defs :: Chuchu (CalculatorT IO) defs = do Given ("that I have entered" *> number <* "into the calculator") enterNumber When "I press divide" $ const divide Then ("the result should be" *> number <* "on the screen") $ \n -> do d <- getDisplay liftIO $ d @?= n main :: IO () main = chuchuMain defs (`evalStateT` []) Both packages are available at Hackage at http://hackage.haskell.org/package/abacate http://hackage.haskell.org/package/chuchu I have written them after the specification from Felipe Almeida Lessa. Any comments are welcome. -- marcot http://marcot.eti.br/

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Haskell eXchange LDN October 10th // Feat. Simon Peyton Jones!
by Theo 13 Aug '12

13 Aug '12

*Skills Matter are putting on a fantastic 1-day conference coming up on October 10th- the Haskell eXchange. This fast-paced, interactive day will showcase a range of expert-led talks on all things Haskell. The conference will feature a spectrum of leading experts including a keynote from thefather of Haskell, Simon Peyton Jones. Whether you are an experienced Haskell user or a curious observer, join us on October 10th for this unique event. What: Haskell eXchange When: October 10th 2012 Where: The Crypt, London Featuring: Simon Peyton Jones, Simon Marlow, Lennart Augustsson, & Blake Rain Learn more and book online: http://bit.ly/RMSfuK*

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hGetContents Illegal byte sequence / ghc-pkg
by Benjamin Edwards 12 Aug '12

12 Aug '12

Hello café, I have a program that is crashing, and I have no idea why: module Main where import System.Process (readProcessWithExitCode) main :: IO () main = do _ <- readProcessWithExitCode "ghc-pkg" ["describe", "hoopl"] "" putStrLn "Should never get here" this is using the process package from hackage. The program crashes with minimal-test: fd:5: hGetContents: invalid argument (invalid byte sequence) minimal-test: thread blocked indefinitely in an MVar operation inspecting the source of readProcessWithExitCode yields an obvious explanation to the MVar problem, but I don't understand why hGetContents is so offended. For the lazy it is defined as follows: readProcessWithExitCode :: FilePath -- ^ command to run -> [String] -- ^ any arguments -> String -- ^ standard input -> IO (ExitCode,String,String) -- ^ exitcode, stdout, stderr readProcessWithExitCode cmd args input = do (Just inh, Just outh, Just errh, pid) <- createProcess (proc cmd args){ std_in = CreatePipe, std_out = CreatePipe, std_err = CreatePipe } outMVar <- newEmptyMVar -- fork off a thread to start consuming stdout out <- hGetContents outh _ <- forkIO $ C.evaluate (length out) >> putMVar outMVar () -- fork off a thread to start consuming stderr err <- hGetContents errh _ <- forkIO $ C.evaluate (length err) >> putMVar outMVar () -- now write and flush any input when (not (null input)) $ do hPutStr inh input; hFlush inh hClose inh -- done with stdin -- wait on the output takeMVar outMVar takeMVar outMVar hClose outh hClose errh -- wait on the process ex <- waitForProcess pid return (ex, out, err) Now having looked at the source of ghc-pkg it is dumping it's output using putStr and friends, so that should be using my local encoding on the system, right? and so should hGetContents in my program..? Now, for the curious: the reason I care is that this problem has effectively prevented me from using virthualenv. Sadness and woe.

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pattern matching vs if-then-else
by Maarten Faddegon 12 Aug '12

12 Aug '12

Hi there, I am writing a toy compiler in Haskell to further my skills in functional programming. One of the functions I wrote is to determine the iteration count of a loop. I have a number of different test that I want to do and I find myself now testing some of these using pattern matching and some properties using an if-then-else construction. I do not consider this very pretty. My question is: are there guidelines of when to use pattern matching and when to use if-then-else? Snippet of the function I mentioned: ---8<----------------------------------------------------------------------------------- itercount (ForLoop [ ( Assignment update_lcv (Op2 "+" (Content update_lcv') update_expr) ) ] [(Assignment init_lcv init_expr)] (TestStmt (Op2 "<" (Content test_lcv) test_expr)) bodyblock) = if -- All stmts use the same lcv test_lcv == init_lcv && test_lcv == update_lcv && test_lcv == update_lcv' -- And the lcv is not updated in the body && intersect [test_lcv] (blockkills bodyblock) == [] then Just $ simple_itercount init_expr test_expr update_expr else Nothing itercount _ = Nothing ---8<----------------------------------------------------------------------------------- Thanks, Maarten

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