March 2010 - Beginners

Simple lookahead with Parsec
by Derek Thurn 19 Mar '10

19 Mar '10

Hello. I'm just starting out with Parsec and Haskell, and I want to parse a very simple grammar with it. I think I may not understand how to use try correctly. Here's a simplified version of my grammar: myType = try (primitiveType) <|> arrayType primitiveType = do {reserved "int"; return "primitive"} arrayType = do { primitiveType; symbol "["; symbol "]"; return "array"} Basically, I want to use lookahead to produce the result "array" on inputs like "int[]" and the result "primitive" on inputs like "int". However, no matter what I do with the try function, I am not able to get what I expect. When I run this code using the runLex function described in the Parsec documentation, it chokes on "int[]", saying that the '[' character was unexpected. I've discovered that switching the order of primitiveType and arrayType solves this, which makes sense, but I still expected it to be possible to use try() in this manner to resolve the error with lookahead. Am I doing something wrong? (I've got some boilerplate code form the Parsec documentation defined as well, but I didn't include it for brevity's sake. Reserved and symbol are defined how you'd expect.)

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Getting into a Haskell project?
by Duncan Mortimer 18 Mar '10

18 Mar '10

Hi All, I'm very new to both Haskell, and open source development (well... really, development of any kind...). However, I'm really interested in getting good at both. I was wondering whether anyone has any suggestions about good open source Haskell projects I could cut my teeth on, and any advice in general in getting into this stuff more deeply? Thanks! Duncan

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how long it will take to install Haskell
by love_pku 18 Mar '10

18 Mar '10

Dear Sir or Madam: I met problems when install haskell. Following are the interface after do the command 'make',and it stay at the state 'Linking Setup'for a long time. So I am wondering how long it will take to finish 'linking setup'. #make scripts/build.sh Platform package mtl-1.1.0.2 is already installed. Skipping... Building "/usr/local/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.6.0.3" Linking Setup ...

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about installation of Haskell
by love_pku 18 Mar '10

18 Mar '10

Dear Sir or Madam:: Sorry to trouble you! I am a beginner of Haskell and I just run into problems to install it. Things went smoothly when I do ./configure,but when I do the command 'make',errors appeared. The errors are as follows(red font): $make scripts/build.sh Building "/usr/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal" Linking Setup ... "./Setup" "configure" "--package-db=../../packages/package.conf.inplace" "--prefix=/usr/local" "--with-compiler=/usr/bin/ghc" "--with-hc-pkg=/usr/bin/ghc-pkg" "--with-hsc2hs=/usr/bin/hsc2hs" "--enable-library-profiling" Setup: Unrecognised flags: --package-db=../../packages/package.conf.inplace Error: Configuring the mtl-1.1.0.2 package failed make: *** [build.stamp] Error 2 So I do hope you can give me some suggestions. Thank you very much! Best wishes, Yours, juan

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[Announcement] Fedora 12 rpms for leksah 0.8.0.1 now available
by Lakshmi Narasimhan 18 Mar '10

18 Mar '10

Leksah - 0.8.0.1 rpms for Fedora 12 now available http://leksah.org/download.html About Leksah: *Leksah* is the Haskell IDE of choice. It is written in Haskell, uses Gtk, and runs on Linux, Windows and Mac OS X. Leksah is a practical tool to support the Haskell development process. It requires the Glasgow Haskell Compiler (GHC). *Installation Instructions * *Procedure for x86 platform* Before proceeding to install the leksah rpms, Uninstall leksah-0.6.1, ghc-binary-devel packages, incase you have installed them from the leksah download page. Use the following command yum remove leksah ghc-binary-devel Install these dependencies from the following location http://leksah.org/packages/dependencies-0.8.0.1.fc12.i386.tar.gz Extract the rpms from the tar.gz file. Select the rpms, right click and click Install. To install from command line, change into the extracted directory, become superuser and type in this command for x86 yum --nogpgcheck install ghc-binary-devel-0.5.0.2-1.fc12.i386.rpm ghc-deepseq-devel-1.1.0.0-1.fc12.i386.rpm ghc-ltk-devel-0.8-1.fc12.i386.rpm ghc-binary-shared-devel-0.8-1.fc12.i386.rpm ghc-hslogger-devel-1.0.10-1.fc12.i386.rpm haddock-leksah-2.5.0-1.fc12.i386.rpm Download the leksah rpms from this location http://leksah.org/packages/leksah-0.8.0.1-1.fc12.i386.rpm http://leksah.org/packages/leksah-server-0.8.0.1-1.fc12.i386.rpm Use the same procedure used for installing the dependencies. * Procedure for x86_64 platform* Before proceeding to install the leksah rpms, please install these dependencies from the following location http://leksah.org/packages/dependencies-0.8.0.1.fc12.x86_64.tar.gz Extract the rpms from the tar.gz file. Select the rpms, right click and click Install. To install from command line, change into the extracted directory, become superuser and type in this command yum --nogpgcheck install ghc-binary-devel-0.5.0.2-1.fc12.x86_64.rpm ghc-hslogger-devel-1.0.10-1.fc12.x86_64.rpm ghc-binary-shared-devel-0.8-1.fc12.x86_64.rpm ghc-ltk-devel-0.8-1.fc12.x86_64.rpm ghc-deepseq-devel-1.1.0.0-1.fc12.x86_64.rpm haddock-leksah-2.5.0-1.fc12.x86_64.rpm Download leksah rpms from this location http://leksah.org/packages/leksah-0.8.0.1-1.fc12.x86_64.rpm http://leksah.org/packages/leksah-server-0.8.0.1-1.fc12.x86_64.rpm Use the same procedure used for installing the dependencies *Running leksah *Once installation is completed, leksah menu item appears under Applications->Programming (in GNOME). -- Regards Lakshmi Narasimhan T V

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Strange error during compilation/linking
by Erik Quaeghebeur 17 Mar '10

17 Mar '10

Hi, I get the following error while compiling (I guess it's actually the linking step): $ghc -o cmimc cmimc.hs cmimc.o: In function `rP1_info': (.text+0x32a): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rP3_info': (.text+0x396): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rPh_info': (.text+0x666): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' cmimc.o: In function `rPj_info': (.text+0x6ca): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' cmimc.o: In function `sR5_info': (.text+0x1225): undefined reference to `__stginit_containerszm0zi2zi0zi1_DataziMap_' cmimc.o: In function `rP1_srt': (.data+0x90): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rP3_srt': (.data+0xac): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rPh_srt': (.data+0x15c): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' cmimc.o: In function `rPj_srt': (.data+0x174): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' collect2: ld returned 1 exit status However, running the program with ghci: $ghci cmimc.hs Prelude Main> :set args "testinput.txt" Prelude Main> :trace main gives the correct output... Where might there be a problem? In my installation (Kubuntu i386, ghc6.10.4-1ubunbtu2) perhaps? Please tell me if/which additional info I need to provide. TIA, Erik

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Defining 'words' in terms of 'span'
by Roger Whittaker 17 Mar '10

17 Mar '10

I found some exam papers linked from this page: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/external.html And I have been trying some questions from them. I'm currently baffled by question 2(b) on this one: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/tenta2000-04.ps A word is a sequence of alphabetic characters, which you can recognise using the standard function isAlpha :: Char -> Bool Using span, define a function words :: String -> [String] which finds a list of the words occurring in a string. For example, words "Now is the winter of our discontent!" == ["Now","is","the","winter","of","our","discontent"] words "2+3" == [] words "1 by 1" == ["by"] Can anyone give me a clue how to start? -- ======================== Roger Whittaker roger(a)disruptive.org.uk http://disruptive.org.uk ========================

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about installation of Haskell
by love_pku 17 Mar '10

17 Mar '10

Dear Sir or Madam:: Sorry to trouble you! I am a beginner of Haskell and I just run into problems to install it. So I hope if you can tell me which version of Haskell should I use for Ubuntu 8.04 operating system.. Thank you very much! Best wishes, Yours juan

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generating by mapping
by Szilveszter Juhos 16 Mar '10

16 Mar '10

Hi, I have something like: data Stuff = Stuff { aNum :: Int , anStr :: String } deriving (Show) let first = Stuff 123 "qwe" let second = Stuff 321 "asd" print first print second and works fine. What is the right "map" statement if I want to generate these stuff using lists like: let strs = [ "qwe", "asd", "zxc" ] let nums = [ 123, 321, 345] ? Cheers: Szilva

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Reading in the contents of a variable number of files
by Tim Perry 16 Mar '10

16 Mar '10

I need to read in and then combine a variable number of files. I am trying to figure out how to get the contents of all the files into an array. let fileContentsArray = getData ["dataFile1.txt", "dataFile2.txt"] getData :: [String] -> [String] getData [] = [] getData (f:files) = do fileData <- readFile f fileData : (getData files) I figured I could use readFile to open the files. But, I need to use <- to get the data out of the IO Action and into a value. I thought I could write a function like the following, but I'm getting no-where. I suspect I'm missing something basic about IO in Haskell. Can anybody point me to a good resource.... Thanks, Tim

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