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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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Parse Json tweets using Aeson
by Tushar Jarhad 19 Sep '14

19 Sep '14

{-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE DeriveGeneric #-} import Data.Aeson import GHC.Generics import qualified Data.ByteString.Lazy as B data Tweet = Tweet { id_str :: String, text :: String }deriving Generic instance FromJSON Tweet main = do input <- B.readFile "1.json" let mm = decode input :: Maybe Tweet case mm of Nothing -> print "error parsing JSON" Just m -> (putStrLn.greet) m greet m = (show.id_str) m This program is fine if 1.json file has 1 tweet if 1.json file has multiple tweets then it fails so how we can parse multiple tweets

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How can I replace content of tag in xml file with haxml.
by Dontdie YCH 17 Sep '14

17 Sep '14

I'm trying to replace `CurrentURI` tag's contents using HaXml. from, ``` <?xml version="1.0"?> <s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"> <s:Body> <u:SetAVTransportURI xmlns:u="urn:schemas-upnp-org:service:AVTransport:1"> <InstanceID>0</InstanceID> <CurrentURI>AAAAAAAAAAAA</CurrentURI> <CurrentURIMetaData/> </u:SetAVTransportURI> </s:Body> </s:Envelope> ``` to, ``` <?xml version="1.0"?> <s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"> <s:Body> <u:SetAVTransportURI xmlns:u="urn:schemas-upnp-org:service:AVTransport:1"> <InstanceID>0</InstanceID> <CurrentURI>BBBBBBBBBBBBBB</CurrentURI> <CurrentURIMetaData/> </u:SetAVTransportURI> </s:Body> </s:Envelope> ``` I had look around, hxt, xml-light, and haxml. I found `onContents` function from `Text.XML.HaXml.Wrappers` module and It looks like way to go. ``` λ: xml <- readFile "SetAVTransportURI.xml" :: IO String λ: let doc = xmlParse "dumb" xml λ: doc Document (Prolog (Just (XMLDecl "1.0" Nothing Nothing)) [] Nothing []) [] (Elem (N "s:Envelope") [(N "xmlns:s",http://schemas.xmlsoap.org/soap/envelope/),(N "s:encodingStyle",http://schemas.xmlsoap.org/soap/encoding/)] [CString False "\n " file dumb at line 2 col 125,CElem (Elem (N "s:Body") [] [CString False "\n " file dumb at line 3 col 11,CElem (Elem (N "u:SetAVTransportURI") [(N "xmlns:u",urn:schemas-upnp-org:service:AVTransport:1)] [CString False "\n " file dumb at line 4 col 79,CElem (Elem (N "InstanceID") [] [CString False "0\n " file dumb at line 5 col 19]) file dumb at line 5 col 7,CString False "\n " file dumb at line 6 col 20,CElem (Elem (N "CurrentURI") [] [CString False "\n" file dumb at line 7 col 19,CMisc (Comment " #EXTM3U ") file dumb at line 8 col 1,CMisc (Comment " #EXTINF:5850,\52628\49437\53945\49440\50689\54868 \47609\47329\44284\44053 140909 HDTV x264 720p AC3 \50864\47532\47568 \45433\51020 - nature boy ") file dumb at line 9 col 1,CMisc (Comment " http://192.168.219.2:8200/MediaItems/587.mkv ") file dumb at line 10 col 1,CMisc (Comment " #EXTM3U ") file dumb at line 11 col 1,CMisc (Comment " #EXTINF:6411,The.Adventures.of.Tintin.2011.1080p.BluRay.x264.YIFY ") file dumb at line 12 col 1,CString False "http://192.168.219.2:8200/MediaItems/165.mp4\n " file dumb at line 13 col 1]) file dumb at line 7 col 7,CString False "\n " file dumb at line 14 col 20,CElem (Elem (N "CurrentURIMetaData") [] []) file dumb at line 15 col 7,CString False "\n " file dumb at line 15 col 28]) file dumb at line 4 col 5,CString False "\n " file dumb at line 16 col 27]) file dumb at line 3 col 3,CString False "\n" file dumb at line 17 col 12]) [Comment "\nOrignal header from captured packets.\n\nPOST /smp_18_ HTTP/1.1\nHost: 192.168.219.6:7676\nSOAPAction: \"urn:schemas-upnp-org:service:AVTransport:1#SetAVTransportURI\"\nAccept-Language: en-us;q=1, en;q=0.5\nAccept-Encoding: gzip\nContent-Type: text/xml; charset=\"utf-8\"\nUser-Agent: gupnp-av-cp GUPnP/0.20.12 DLNADOC/1.50\nConnection: Keep-Alive\nContent-Length: 1405\n"] λ: onContent (Text.XML.HaXml.Combinators.deep $ tag "CurrentURI") doc -- this result is not what I want. Document (Prolog (Just (XMLDecl "1.0" Nothing Nothing)) [] Nothing []) [] (Elem (N "CurrentURI") [] [CString False "\n" file dumb at line 7 col 19,CMisc (Comment " #EXTM3U ") file dumb at line 8 col 1,CMisc (Comment " #EXTINF:5850,\52628\49437\53945\49440\50689\54868 \47609\47329\44284\44053 140909 HDTV x264 720p AC3 \50864\47532\47568 \45433\51020 - nature boy ") file dumb at line 9 col 1,CMisc (Comment " http://192.168.219.2:8200/MediaItems/587.mkv ") file dumb at line 10 col 1,CMisc (Comment " #EXTM3U ") file dumb at line 11 col 1,CMisc (Comment " #EXTINF:6411,The.Adventures.of.Tintin.2011.1080p.BluRay.x264.YIFY ") file dumb at line 12 col 1,CString False "http://192.168.219.2:8200/MediaItems/165.mp4\n " file dumb at line 13 col 1]) [Comment "\nOrignal header from captured packets.\n\nPOST /smp_18_ HTTP/1.1\nHost: 192.168.219.6:7676\nSOAPAction: \"urn:schemas-upnp-org:service:AVTransport:1#SetAVTransportURI\"\nAccept-Language: en-us;q=1, en;q=0.5\nAccept-Encoding: gzip\nContent-Type: text/xml; charset=\"utf-8\"\nUser-Agent: gupnp-av-cp GUPnP/0.20.12 DLNADOC/1.50\nConnection: Keep-Alive\nContent-Length: 1405\n"] λ: ``` How can I write filter to replace `CurrentURI` tag? 1. find 'CurrentURI' tag 2. replace content with some string 3. return modified whole document not just specific tag Or, Where can I found tutorial or example for haxml? Yes, I'm reading now haxml document. But needs some overview document. Any advice would be appreciated. Thanks.

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