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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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Hackage is down?
by hanjoosten 12 Aug '12

12 Aug '12

Hi, Hackage seems to be down. Is there anyone out here who knows how to get it online again? Thanks! -- View this message in context: http://haskell.1045720.n5.nabble.com/Hackage-is-down-tp5715912.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Fwd: hGetContents Illegal byte sequence / ghc-pkg
by Benjamin Edwards 11 Aug '12

11 Aug '12

Responding to the list.. ---------- Forwarded message ---------- From: Benjamin Edwards <edwards.benj(a)gmail.com> Date: 11 August 2012 17:37 Subject: Re: [Haskell-cafe] hGetContents Illegal byte sequence / ghc-pkg To: David McBride <toad3k(a)gmail.com> Thank you Gents, Most useful. One thing that had escaped me is that of course, even if hGetContents and friends are respecting *my* locale, I have a binary distro and the package database probably isn't in my locale. I will try and find that. Thanks, Ben On 11 August 2012 15:26, David McBride <toad3k(a)gmail.com> wrote: > I had this same problem a couple weeks ago when trying to install > virthualenv and I don't really understand it got into a bad state, but the > way I solved it was by fixing the locale settings on my gentoo machine so > that I'm using UTF8. That just involved a few changes in /etc and then the > problem went away. > > On Sat, Aug 11, 2012 at 7:13 AM, Benjamin Edwards <edwards.benj(a)gmail.com>wrote: > >> 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. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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Re: [Haskell-cafe] Data.Data and OverlappingInstances
by oleg＠okmij.org 11 Aug '12

11 Aug '12

Timo von Holtz wrote: > class Test a where > foo :: Monad m => m a > > instance Num a => Test a where > foo = return 1 > > instance Test Int where > foo = return 2 > > test constr = fromConstrM foo constr I'm afraid the type checker is right. From the type of fromConstrM fromConstrM :: forall m a. (Monad m, Data a) => (forall d. Data d => m d) -> Constr -> m a we see its first argument has the type (forall d. Data d => m d) If instead it had the type (forall d. Test d => m d) we would have no problem. As it is, when you pass 'foo' of the type (Test d, Monad m) => m d as the first argument of fromConstrM, which only assures the Data d constraint on 'd' and _nothing_ else, the compiler checks if it can get rid of (discharge) the constraint Test d. That is, the compiler is forced to choose an instance for Test. But there is not information to do that. Overlapping here is irrelevant. If you had non-overlapping instances > class Test a where > foo :: Monad m => m a > > instance Num a => Test [a] where > foo = return [1] > > instance Test Int where > foo = return 2 > > test constr = fromConstrM foo constr 'test' still causes the problem. The error message now describes the real problem: Could not deduce (Test d) arising from a use of `foo' from the context (Monad m, Data a) bound by the inferred type of test :: (Monad m, Data a) => Constr -> m a at /tmp/d.hs:16:1-36 or from (Data d) bound by a type expected by the context: Data d => m d at /tmp/d.hs:16:15-36 Possible fix: add (Test d) to the context of a type expected by the context: Data d => m d or the inferred type of test :: (Monad m, Data a) => Constr -> m a In the first argument of `fromConstrM', namely `foo' and it recommends the right fix: change the type of fromConstrM to be fromConstrM :: forall m a. (Monad m, Data a) => ( forall d. (Test d, Data d) => m d) -> Constr -> m a That will solve the problem. Alas, fromConstrM is a library function and we are not at liberty to change its type. > Right now I use a "case (typeOf x) of" kind of construct That is precisely the right way to use Data. SYB provides good combinators for building functions (generic producers) of that sort. But you never need unSafeCoerce: gcast is sufficient.

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Re: [Haskell-cafe] CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 11 Aug '12

11 Aug '12

Anthony Clayden wrote: > So three questions in light of the approach of abandoning FunDeps and > therefore not getting interference with overlapping: > A. Does TTypeable need to be so complicated? > B. Is TTypeable needed at all? > C. Does the 'simplistic' version of type equality testing suffer possible > IncoherentInstances? It is important to keep in mind that Type Families (and Data Families) are _strictly_ more expressive than Functional dependencies. For example, there does not seem to be a way to achieve the injectivity of Leibniz equality http://okmij.org/ftp/Computation/extra-polymorphism.html#injectivity without type families (and relaying instead on functional dependencies, implemented with TypeCast or directly). I'd like to be able to write data Foo a = Foo (SeqT a) where SeqT Bool = Integer SeqT a = [a] otherwise (A sequence of Booleans is often better represented as an Integer). A more practical example is http://okmij.org/ftp/Haskell/GMapSpec.hs http://okmij.org/ftp/Haskell/TTypeable/GMapSpecT.hs It is possible to sort of combine overlapping with associated types, but is quite ungainly. It is not possible to have overlapping associated types _at present_. Therefore, at present, TTYpeable is necessary and it has to be implemented as it is. You point out New Axioms. They will change things. I have to repeat my position however, which I have already stated several times. TTypeable needs no new features from the language and it is available now. There is no problem of figuring out how TTypeable interacts with existing features of Haskell since TTypeable is implemented with what we already have. New Axioms add to the burden of checking how this new feature interacts with others. There have been many examples when one feature, excellent by itself, badly interacts with others. (I recall GADT and irrefutable pattern matching.)

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