September 2014 - Beginners

Different behaviors between GHC 7.6.3 and 7.8.3
by Emilio De Camargo Francesquini 01 Oct '14

01 Oct '14

Hello everyone, I'm quite intrigued about these different behaviors between GHCs 7.6.3 and 7.8.3: GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Integer) Prelude> :force x x = 7 Prelude> print _t1 7 Prelude> GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Num a => a) Prelude> :force x x = _ Prelude> print _t1 ghc: panic! (the 'impossible' happened) (GHC version 7.8.3 for x86_64-unknown-linux): tcTyVarDetails a{tv atm} [tv] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Is it really a bug or is it something I really shouldn't be doing? I've found this (fixed) bug (https://ghc.haskell.org/trac/ghc/ticket/8557) with a similar error output, but it does not seem to be the same case. Thanks Emilio

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How to pass configuration to a function further down the call stack?
by Thomas Koch 30 Sep '14

30 Sep '14

Hi, I wrote my first Haskell 'project', a test suite for my personal SIEVE mail filter script using dovecots 'sieve-test' tool: https://github.com/thkoch2001/sieve-test-hs The file Network/Sieve/Test.hs might actually be interesting as a standalone library. The most important function is assertMailActions :: Mail -> Actions -> IO () 'Actions' is a type that describes the result of running a SIEVE filter over the passed mail, e.g. store in a certain folder, discard, reject, ... Somewhere the external process 'sieve-test' is called. At this point I'd like to make certain things configurable: - the path and name of the executable - the parameters for the test-sieve executable - the name of the sieve script - the place where to store the temporary mail files - whether or not to remove the temporary mail files Does anybody has an idea where I could start to look for a solution? Thank you, Thomas Koch

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Unformatted binary IO
by David Raymond 30 Sep '14

30 Sep '14

I am trying to read and write to/from files or stdout/stdin that have an ascii header followed by unformatted float32 binary data. (The files are created using a C program.) The ascii header first needs to be parsed to understand the structure of the floating point data. I have solved the parsing problem, but getting the unformatted binary float data into a series of float (or double) immutable, unboxed vectors has defeated me so far. The binary package doesn't help as far as I can see, as the binary format it reads and writes has some control information at the beginning that doesn't exist in the format I am reading. This is easy in C, but seems to be hard in Haskell unless I am missing something. Any suggestions? -- David J. Raymond Prof. of Physics New Mexico Tech http://www.physics.nmt.edu/~raymond/index.html

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HXT: encoding problem
by Elias Diem 30 Sep '14

30 Sep '14

Hi guys I have got the following haskell program: ------------------------------------------------------ import Text.XML.HXT.Core main = do xml <- readFile "test_data-small.xml" let doc = readString config xml res <- runX . xshow $ doc >>> getChildren >>> isElem >>> hasName "contacts" >>> deep isText mapM_ putStrLn res config = [ withParseHTML no , withWarnings yes , withInputEncoding utf8 , withOutputEncoding utf8 , withValidate yes ] ------------------------------------------------------ The file 'test_data-small.xml' contains the following data: ------------------------------------------------------ <?xml version='1.0' encoding='UTF-8' ?> <contacts> <person> <name> <firstname>Max</firstname> <lastname>Müller</lastname> </name> </person> </contacts> ------------------------------------------------------ Note the umlaut in the lastname! If I run the program, I get the following error: ------------------------------------------------------ error: UTF-8 encoding error at input position 127: ValueOutOfBounds ------------------------------------------------------ Any help is appreciated. Thanks. -- Greetings Elias

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Is there a way to add package to .cabal when I import some module?
by Dontdie YCH 29 Sep '14

29 Sep '14

Is there a way to add package to .cabal when I import new module?

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aeson json paring
by Miro Karpis 27 Sep '14

27 Sep '14

Hi, I'm trying to run an example from the eason documentation <https://hackage.haskell.org/package/aeson-0.6.1.0/docs/Data-Aeson.html#g:3> : λ> do result <- decode "{\"name\":\"Dave\",\"age\":2}" flip parseMaybe result $ \obj -> do age <- obj .: "age" name <- obj .: "name" return (name ++ ": " ++ show (age*2)) I made a function: jsonTest = do result <- decode "{\"name\":\"Dave\",\"age\":2}" flip parseMaybe result $ \obj -> do age <- obj .: "age" name <- obj .: "name" return (name ++ ": " ++ show (age*2)) which can not compile: hh.hs:35:41: No instance for (Show a0) arising from a use of ‘show’ The type variable ‘a0’ is ambiguous .... ... Please what am I doing wrong? Cheers, Miro

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Using a monadic operation in a list comprehension.
by Derek McLoughlin 25 Sep '14

25 Sep '14

Hi, I'm trying to get Graham Hutton's "Programming with Effects" article (http://www.cs.nott.ac.uk/~gmh/monads) working with Data.Tree instead of the Tree he provides. My code is here: https://gist.github.com/derekmcloughlin/95ab690e0c07c5a7221e The compiler is complaining about the use of lab <- mlabel c in the list comprehension: Couldn't match expected type ‘[Tree (a, Int)]’ with actual type ‘ST (Tree (a, Int))’ This makes sense, but is there any way to execute the monad operation "mlabel" within the list comprehension? I've tried the MonadComprehensions extension but still get an error. Derek.

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fresh install - cabal version confusion
by Miroslav Karpis 23 Sep '14

23 Sep '14

Hi, please can you help me with following? I¹m having some issues with a Haskell upgrade. Before upgrade I have uninstalled Haskell, removed directory + removed .ghc and .cabal files. After I install haskell and run cabal V I get the output below. Confusing is the versions cabal returns: cabal-install version 1.18.0.5 using version 1.18.1.4? I have tried to run also cabal update, what installed 1.20.0.3, but I still got the same output. What does it mean? I¹m running on Mac. ********************************************************************** === Configuration for cabal has been written to /Users/miro/.cabal/config === Executables will be installed in: /Users/miro/Library/Haskell/bin You may wish to place this on your PATH by adding the following line to your ~/.bash_profile: export PATH="$HOME/Library/Haskell/bin:$PATH" === When documentation is built, a master index to all documentation will be placed in: /Users/miro/Library/Haskell/doc/index.html You may wish to bookmark that file once it gets built (after the first cabal install). ********************************************************************** 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 cabal-install version 1.18.0.5 using version 1.18.1.4 of the Cabal library

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what space leak could happen here?
by Dimitri DeFigueiredo 22 Sep '14

22 Sep '14

Hi All, I was looking at the definition of intersperse on http://hackage.haskell.org/package/base-4.7.0.1/docs/src/Data-List.html#int… And found this -- | The 'intersperse' function takes an element and a list and -- \`intersperses\' that element between the elements of the list. -- For example, -- -- > intersperse ',' "abcde" == "a,b,c,d,e" intersperse :: a -> [a] -> [a] intersperse _ [] = [] intersperse sep (x:xs) = x : prependToAll sep xs -- Not exported: -- We want to make every element in the 'intersperse'd list available -- as soon as possible to avoid space leaks. Experiments suggested that -- a separate top-level helper is more efficient than a local worker. prependToAll :: a -> [a] -> [a] prependToAll _ [] = [] prependToAll sep (x:xs) = sep : x : prependToAll sep xs I don't understand why we need to "make every element in the 'intersperse'd list available as soon as possible to avoid space leaks." Could somebody shed some light on what could cause a space leak here? In particular, would this simpler version cause problems? intersperse :: a -> [a] -> [a] intersperse _ [] = [] intersperse _ [x] = [x] intersperse a (x: xs@(y:ys)) = x : a : intersperse a xs Thanks, Dimitri

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Patch source, then pickup compile
by James Toll 20 Sep '14

20 Sep '14

Hi, I’m trying to install Hakyll within a cabal sandbox. The issue I’m running into is that the current version of Pandoc doesn’t build because of this bug: https://github.com/jgm/pandoc/issues/1590 The bug is a simple fix, and is fixed on github, but not in the version on hackage. I’d like to use ‘cabal get’ to download the source, manually fix the source, then compile. The question I have is how do I pick up the installation process after fixing the source? So far this is what I’ve done: $ mkdir hakyll $ cabal update $ cd hakyll $ cabal sandbox init --sandbox . $ cabal install -j hakyll [this is the spot where pandoc fails to install] $ cabal get pandoc-1.13.1 $ cd pandoc-1.13.1/ [patch the source] At this point I need to resume the compilation and installation process. How do I do that? Do I move back up to the parent directory and then issue the 'cabal install -j hakyll’ command again? Thanks, James

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