July 2011 - Beginners

HaXml will not install...
by Sean Charles 04 Jul '11

04 Jul '11

I have just tried to install HaXml after installing htx with no problems, thinking it might have been that I tried ghc-pkg hide hxt and tried again but no luck, that's gotten me out of issues with different version of GLFW but alas not this time. So, can anybody help me out of this one! I've included the output from the install plus the output of running "cabal info base". Thanks Sean. -------- installation output -------- scharles@ubuntu:~/stuff/hackinge/objitsu/trunk/haskell/cimd$ cabal install haxml Resolving dependencies... Configuring HaXml-1.22... Preprocessing library HaXml-1.22... Preprocessing executables for HaXml-1.22... Building HaXml-1.22... src/Text/XML/HaXml/Schema/TypeConversion.hs:8:7: Could not find module `Text.XML.HaXml.Schema.Environment': Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: HaXml-1.22 failed during the building phase. The exception was: ExitFailure 1 scharles@ubuntu:~/stuff/hackinge/objitsu/trunk/haskell/cimd$ cabal install -v haxml Reading available packages... Resolving dependencies... In order, the following would be installed: HaXml-1.22 (new package) Extracting /home/scharles/.cabal/packages/hackage.haskell.org/HaXml/1.22/HaXml-1.22.tar.gz to /tmp/HaXml-1.2215949... Configuring HaXml-1.22... Flags chosen: bytestringinbase=False, splitbase=True Dependency base ==4.2.0.2: using base-4.2.0.2 Dependency bytestring ==0.9.1.7: using bytestring-0.9.1.7 Dependency containers ==0.3.0.0: using containers-0.3.0.0 Dependency filepath ==1.1.0.4: using filepath-1.1.0.4 Dependency polyparse ==1.7: using polyparse-1.7 Dependency pretty ==1.0.1.1: using pretty-1.0.1.1 Dependency random ==1.0.0.2: using random-1.0.0.2 Using Cabal-1.10.2.0 compiled by ghc-6.12 Using compiler: ghc-6.12.3 Using install prefix: /home/scharles/.cabal Binaries installed in: /home/scharles/.cabal/bin Libraries installed in: /home/scharles/.cabal/lib/HaXml-1.22/ghc-6.12.3 Private binaries installed in: /home/scharles/.cabal/libexec Data files installed in: /home/scharles/.cabal/share/HaXml-1.22 Documentation installed in: /home/scharles/.cabal/share/doc/HaXml-1.22 Using alex version 2.3.5 found on system at: /home/scharles/.cabal/bin/alex Using ar found on system at: /usr/bin/ar Using c2hs version 0.16.3 found on system at: /home/scharles/.cabal/bin/c2hs Using cpphs version 1.11 found on system at: /home/scharles/.cabal/bin/cpphs No ffihugs found Using gcc version 4.5.2 found on system at: /usr/bin/gcc Using ghc version 6.12.3 found on system at: /usr/bin/ghc Using ghc-pkg version 6.12.3 found on system at: /usr/bin/ghc-pkg No greencard found Using haddock version 2.6.1 found on system at: /usr/bin/haddock Using happy version 1.18.6 found on system at: /home/scharles/.cabal/bin/happy No hmake found Using hsc2hs version 0.67 found on system at: /usr/bin/hsc2hs No hscolour found No hugs found No jhc found Using ld found on system at: /usr/bin/ld No lhc found No lhc-pkg found No nhc98 found Using pkg-config version 0.25 found on system at: /usr/bin/pkg-config Using ranlib found on system at: /usr/bin/ranlib Using strip found on system at: /usr/bin/strip Using tar found on system at: /bin/tar No uhc found creating dist/build creating dist/build/autogen Preprocessing library HaXml-1.22... Preprocessing executables for HaXml-1.22... Building HaXml-1.22... Building library... creating dist/build /usr/bin/ghc --make -package-name HaXml-1.22 -hide-all-packages -fbuilding-cabal-package -i -idist/build -isrc -idist/build/autogen -Idist/build/autogen -Idist/build -optP-DMYVERSION="\"1.22\"" -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package-id base-4.2.0.2-d09520f27d14dc38c6981fa942393083 -package-id bytestring-0.9.1.7-253db8fad875ae549d30aec0d8553adf -package-id containers-0.3.0.0-7be04792060ce1514c52e2cb13a5c18a -package-id filepath-1.1.0.4-1425f312faf5b8e9f4babf887e252326 -package-id polyparse-1.7-b5af8124e8aeb60817d4c8a8608def30 -package-id pretty-1.0.1.1-b6652d4fb1f996119a76dcaa986e9d70 -package-id random-1.0.0.2-a4bf29bbfd08ec91d43a4c919e9253f8 -O -XCPP -XExistentialQuantification Text.XML.HaXml.Schema.Schema Text.XML.HaXml Text.XML.HaXml.ByteStringPP Text.XML.HaXml.Combinators Text.XML.HaXml.DtdToHaskell.Convert Text.XML.HaXml.DtdToHaskell.Instance Text.XML.HaXml.DtdToHaskell.TypeDef Text.XML.HaXml.Escape Text.XML.HaXml.Html.Generate Text.XML.HaXml.Html.Parse Text.XML.HaXml.Html.ParseLazy Text.XML.HaXml.Html.Pretty Text.XML.HaXml.Lex Text.XML.HaXml.Namespaces Text.XML.HaXml.OneOfN Text.XML.HaXml.Parse Text.XML.HaXml.ParseLazy Text.XML.HaXml.Posn Text.XML.HaXml.Pretty Text.XML.HaXml.SAX Text.XML.HaXml.Schema.Parse Text.XML.HaXml.Schema.XSDTypeModel Text.XML.HaXml.Schema.HaskellTypeModel Text.XML.HaXml.Schema.NameConversion Text.XML.HaXml.Schema.TypeConversion Text.XML.HaXml.Schema.PrettyHaskell Text.XML.HaXml.Schema.PrimitiveTypes Text.XML.HaXml.ShowXmlLazy Text.XML.HaXml.Types Text.XML.HaXml.TypeMapping Text.XML.HaXml.Util Text.XML.HaXml.Validate Text.XML.HaXml.Verbatim Text.XML.HaXml.Wrappers Text.XML.HaXml.XmlContent Text.XML.HaXml.XmlContent.Parser Text.XML.HaXml.XmlContent.Haskell Text.XML.HaXml.Xtract.Combinators Text.XML.HaXml.Xtract.Lex Text.XML.HaXml.Xtract.Parse src/Text/XML/HaXml/Schema/TypeConversion.hs:8:7: Could not find module `Text.XML.HaXml.Schema.Environment': Use -v to see a list of the files searched for. World file is already up to date. cabal: Error: some packages failed to install: HaXml-1.22 failed during the building phase. The exception was: ExitFailure 1 scharles@ubuntu:~/stuff/hackinge/objitsu/trunk/haskell/cimd$ -------- base output -------- scharles@ubuntu:~$ cabal info base * base (library) Synopsis: Basic libraries (backwards-compatibility version) Versions available: 3.0.3.1, 3.0.3.2, (4.0.0.0), (4.1.0.0), (4.2.0.0), (4.2.0.1), (4.2.0.2), (4.3.0.0), (4.3.1.0) Versions installed: 3.0.3.2, (4.2.0.2) Homepage: [ Not specified ] Bug reports: http://hackage.haskell.org/trac/ghc/newticket?component=libraries/base Description: This is a backwards-compatible version of the base package. It depends on a later version of base, and was probably supplied with your compiler when it was installed. License: BSD3 Maintainer: libraries(a)haskell.org Source repo: http://darcs.haskell.org/packages/base3-compat Dependencies: base >=4.0 && <4.3, syb ==0.1.* Documentation: [ Not installed ] Cached: No Modules: Control.Applicative Control.Arrow Control.Category Control.Concurrent Control.Concurrent.Chan Control.Concurrent.MVar Control.Concurrent.QSem Control.Concurrent.QSemN Control.Concurrent.SampleVar Control.Exception Control.Monad Control.Monad.Fix Control.Monad.Instances Control.Monad.ST Control.Monad.ST.Lazy Control.Monad.ST.Strict Data.Bits Data.Bool Data.Char Data.Complex Data.Dynamic Data.Either Data.Eq Data.Fixed Data.Foldable Data.Function Data.Generics Data.Generics.Aliases Data.Generics.Basics Data.Generics.Instances Data.Generics.Schemes Data.Generics.Text Data.Generics.Twins Data.HashTable Data.IORef Data.Int Data.Ix Data.List Data.Maybe Data.Monoid Data.Ord Data.Ratio Data.STRef Data.STRef.Lazy Data.STRef.Strict Data.String Data.Traversable Data.Tuple Data.Typeable Data.Unique Data.Version Data.Word Debug.Trace Foreign Foreign.C Foreign.C.Error Foreign.C.String Foreign.C.Types Foreign.Concurrent Foreign.ForeignPtr Foreign.Marshal Foreign.Marshal.Alloc Foreign.Marshal.Array Foreign.Marshal.Error Foreign.Marshal.Pool Foreign.Marshal.Utils Foreign.Ptr Foreign.StablePtr Foreign.Storable GHC.Arr GHC.Base GHC.Conc GHC.ConsoleHandler GHC.Desugar GHC.Dotnet GHC.Enum GHC.Environment GHC.Err GHC.Exception GHC.Exts GHC.Float GHC.ForeignPtr GHC.Handle GHC.IOscharles@ubuntu:~$ cabal info base * base (library) Synopsis: Basic libraries (backwards-compatibility version) Versions available: 3.0.3.1, 3.0.3.2, (4.0.0.0), (4.1.0.0), (4.2.0.0), (4.2.0.1), (4.2.0.2), (4.3.0.0), (4.3.1.0) Versions installed: 3.0.3.2, (4.2.0.2) Homepage: [ Not specified ] Bug reports: http://hackage.haskell.org/trac/ghc/newticket?component=libraries/base Description: This is a backwards-compatible version of the base package. It depends on a later version of base, and was probably supplied with your compiler when it was installed. License: BSD3 Maintainer: libraries(a)haskell.org Source repo: http://darcs.haskell.org/packages/base3-compat Dependencies: base >=4.0 && <4.3, syb ==0.1.* Documentation: [ Not installed ] Cached: No Modules: Control.Applicative Control.Arrow Control.Category Control.Concurrent Control.Concurrent.Chan Control.Concurrent.MVar Control.Concurrent.QSem Control.Concurrent.QSemN Control.Concurrent.SampleVar Control.Exception Control.Monad Control.Monad.Fix Control.Monad.Instances Control.Monad.ST Control.Monad.ST.Lazy Control.Monad.ST.Strict Data.Bits Data.Bool Data.Char Data.Complex Data.Dynamic Data.Either Data.Eq Data.Fixed Data.Foldable Data.Function Data.Generics Data.Generics.Aliases Data.Generics.Basics Data.Generics.Instances Data.Generics.Schemes Data.Generics.Text Data.Generics.Twins Data.HashTable Data.IORef Data.Int Data.Ix Data.List Data.Maybe Data.Monoid Data.Ord Data.Ratio Data.STRef Data.STRef.Lazy Data.STRef.Strict Data.String Data.Traversable Data.Tuple Data.Typeable Data.Unique Data.Version Data.Word Debug.Trace Foreign Foreign.C Foreign.C.Error Foreign.C.String Foreign.C.Types Foreign.Concurrent Foreign.ForeignPtr Foreign.Marshal Foreign.Marshal.Alloc Foreign.Marshal.Array Foreign.Marshal.Error Foreign.Marshal.Pool Foreign.Marshal.Utils Foreign.Ptr Foreign.StablePtr Foreign.Storable GHC.Arr GHC.Base GHC.Conc GHC.ConsoleHandler GHC.Desugar GHC.Dotnet GHC.Enum GHC.Environment GHC.Err GHC.Exception GHC.Exts GHC.Float GHC.ForeignPtr GHC.Handle GHC.IO GHC.IOBase GHC.Int GHC.List GHC.Num GHC.PArr GHC.Pack GHC.Ptr GHC.Read GHC.Real GHC.ST GHC.STRef GHC.Show GHC.Stable GHC.Storable GHC.TopHandler GHC.Unicode GHC.Weak GHC.Word Numeric Prelude System.CPUTime System.Console.GetOpt System.Environment System.Exit System.IO System.IO.Error System.IO.Unsafe System.Info System.Mem System.Mem.StableName System.Mem.Weak System.Posix.Internals System.Posix.Types System.Timeout Text.ParserCombinators.ReadP Text.ParserCombinators.ReadPrec Text.Printf Text.Read Text.Read.Lex Text.Show Text.Show.Functions Unsafe.Coerce scharles@ubuntu:~$ GHC.IOBase GHC.Int GHC.List GHC.Num GHC.PArr GHC.Pack GHC.Ptr GHC.Read GHC.Real GHC.ST GHC.STRef GHC.Show GHC.Stable GHC.Storable GHC.TopHandler GHC.Unicode GHC.Weak GHC.Word Numeric Prelude System.CPUTime System.Console.GetOpt System.Environment System.Exit System.IO System.IO.Error System.IO.Unsafe System.Info System.Mem System.Mem.StableName System.Mem.Weak System.Posix.Internals System.Posix.Types System.Timeout Text.ParserCombinators.ReadP Text.ParserCombinators.ReadPrec Text.Printf Text.Read Text.Read.Lex Text.Show Text.Show.Functions Unsafe.Coerce scharles@ubuntu:~$

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code explanation
by moowoo9＠fastmail.fm 03 Jul '11

03 Jul '11

I'm a beginner, I'd like someone to help me understand a few lines of code My questions are: 1) Maybe String is an application of the constructor Maybe to the type String. What does it mean? Is there anything equivalent in the C language? 2) Is this a function with three arguments? type T = [Char] type P = (Char, Maybe String) type Delta = ((Maybe Char, Char), Maybe String) fromGtoM :: T -> [P] -> [Delta] 3) is this a usage of the above mentioned function? fromGtoM t p = terminalRules ++ varRules I dont'see why the function has three arguments but then only two are used t p. 4) what does ++ mean ? Sorry, for posting too many questions. Thanks in advance. -- moowoo9(a)fastmail.fm -- http://www.fastmail.fm - Access your email from home and the web

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(no subject)
by Jan Brosius 03 Jul '11

03 Jul '11

Hi, Is there in Haskell a function like unwindprotect in Lisp. Thanks Jan

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First haskell package..
by Mike Meyer 03 Jul '11

03 Jul '11

After much dithering, I'm finally ready to release a first haskell project, eddie. I figured I'd run it past the helpful people here for feedback before making a more official announcement. eddie is a command line tool that lets you use haskell functions to filter files. Functions is meant in the most general sense - any expression that can be evaluated to a type of "String -> String" can be used. The home page is http://eddie.googlecode.com/. There are a couple of pages of examples in the wiki. The 0.1 source tarball is available in downloads, and includes the .cabal file for building. It's not in hackage yet - I'm awaiting a username. <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

1 1

How to process a list that contains "undefined" values?
by Costello, Roger L. 03 Jul '11

03 Jul '11

Hi Folks, How would you find the maximum value in this list: [undefined, 10, undefined, 20] Is it bad practice to create lists that contain undefined values? If yes, what is a better way to express in a list a "no value present/available" value? For example would it be better to express the above list using Maybe: [Nothing, Just 10, Nothing, Just 20] Or perhaps something else? What do you recommend? /Roger

5 7

error handling in libraries
by Michael Xavier 02 Jul '11

02 Jul '11

Hi Beginners, A little while ago I wrote my first library. Internally, it is pretty solid but it makes API calls out to the internet so network problems can be an issue. Additionally, if the API that I'm calling decides to change their format in the future, it is probably not a good idea that my library crash the program that uses it. I'm curious what the current best practice is for handling exceptions in library code. I realize that in self-contained projects, it is much easier to choose whatever you want because it will not directly effect the user, but in a library one must consider what exception handling system the end user is most likely to know. -- Michael Xavier http://www.michaelxavier.net

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mapM etc.
by Dennis Raddle 02 Jul '11

02 Jul '11

It's been about a year since I used Haskell and I'm rusty. I think I knew how to do this once, but I need a reminder. I've got some functions: getDeviceInfo :: Int -> IO DeviceInfo name :: DeviceInfo -> String I want to do something like func :: IO () func = do ds <- mapM getDeviceInfo [0..10] mapM_ (print . name) ds Is there a way to combine 'getDeviceInfo', 'name', and 'print' in one line? Dennis

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Problem with do statement
by Michael Litchard 02 Jul '11

02 Jul '11

I received an error message that might be due to indentation, and not what the error message claims. Here's the error, then the code. Below that I have some comments. ghc --make CreateSession.lhs [2 of 3] Compiling HtmlParsing ( HtmlParsing.lhs, HtmlParsing.o ) HtmlParsing.lhs:81:5: The last statement in a 'do' construct must be an expression: let makeIDPage = do { initial } > generateResourceHtml :: Curl -> String -> String -> FilePath -> IO (Either String String) > generateResourceHtml curl user pass ipFile = do > urlSequence <- popURLrec ipFile > let makeIDPage = do > initial = urlInitial urlSequence > login = urlLogin urlSequence > flash1 = urlFlash1 urlSequence > flash2 = urlFlash2 urlSequence > showWebForwards = urlShowWebForwards urlSequence > quickCreate = urlQuickCreate urlSequence > getResource = urlGetResource urlSequence > curlResp curl $ initial method_GET > curlResp curl $ urlLogin urlSequence $ loginOpts user pass > curlResp curl $ urlFlash1 urlSequence resourceOpts > curlResp curl $ urlFlash2 urlSequence resourceOpts > curlResp curl $ urlShowWebForwards urlSequence resourceOpts > curlResp curl $ urlQuickCreate urlSequence resourceOpts > curlResp curl $ urlGetResource urlSequence resourceOpts > runErrorT makeIDPage It seems to me the first do construct ends with a statement, runErrorT makeIDPage. Also the second do construct ends with a statement as well, > curlResp curl $ urlGetResource urlSequence resourceOpts So I'm either wrong about what the definition of a statement is, or this problem is really about indentation or similar. Clarification?

2 1

OpenGL keyboardMouseCallback ...confused over type
by Sean Charles 02 Jul '11

02 Jul '11

I asked a question the other day about passing application state around an OpenGL program and I've consulted RWH and the inter-web extensively and I think I am on the right lines so far! In my main program, I use Data.IORef like this: gameState <- newIORef $ newGameState and GameState contains all the stuff to do with the game model. Next the confusing bit... to make myself learn things I typed out the full type of the keyboardMouseCallback function: myKeyMouse :: Key -> KeyState -> Modifiers -> Position -> IO () and slotted it into the code like so: keyboardMouseCallback $= Just myKeyMouse In order to be able to pass the "IORef GameState" into the keyboard handler I have to re-type my keyboardMouseCallback function like so: myKeyMouse :: IORef GameState -> Key -> KeyState -> Modifiers -> Position -> IO () and modify the assignment in main to this: keyboardMouseCallback $= Just (myKeyMouse gameState) But I haven't really understood *why* that is the case. The displayCallback is also similarly affected as it is now: myDisplay :: IORef GameState -> IO () ... displayCallback $= (myDisplay gameState) and again I am not sure what magic is happening here to have made it compile and work! My real source of confusion is over my apparent victory over the type system in that I appear to have extended the type signature (is that the correct term?) for both of the callbacks be prepending "IORef GameState" to them and yet it all compiles and works. Is "prepending" the key ? I feel a tingling of Eureka! organ but I am not sure yet. Explanations are welcome as I am sure there is something "important" here that will really advance my understanding of the type system. It smells like curry but I don't think... hang on... the keyboard one is wrapped in the Maybe monad... therefore(?) the function I am handing in can be *any* type so long as they actually match types! Is that it then? I can smell partial application and curry now. HELP! Thanks, Sean.

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version of base in Haskell Platform
by Dennis Raddle 01 Jul '11

01 Jul '11

I just installed the latest Haskell Platform on Windows XP. Then I tried to install the musicxml package, and it says it needs base 3.something. How do I find out what version of base comes with a version of the Haskell Platform? Thanks, Dennis

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