May 2015 - Beginners - Haskell.org

Couldn't match expected type `(a0 -> [a0]) -> [a0]', with actual type `[a0]'
by Roelof Wobben 23 May '15

23 May '15

Hello, For some reasons my file's are corrupted. I had repair them with success except this one. Here is the code : toDigits number | number <= 0 = [] | otherwise = toDigits' [] number where toDigits' acc 0 = acc toDigits' acc number = ((number `mod` 10 ): acc) (toDigits' (number `mod` 10)) main = print $ toDigits 123 and here is the error: Couldn't match expected type `(a0 -> [a0]) -> [a0]' with actual type `[a0]' The function `(number `mod` 10) : acc' is applied to one argument, but its type `[a0]' has none In the expression: ((number `mod` 10) : acc) (toDigits' (number `mod` 10)) In an equation for toDigits': toDigits' acc number = ((number `mod` 10) : acc) (toDigits' (number `mod` 10)) Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Question regarding CIS 194 Homework Assignment 1
by Vaibhav Goel 20 May '15

20 May '15

My apologies, this is only tangentially related to Haskell and I am not looking for a solution. I am confused with the assignment itself and hence asking here. The assignment (https://www.seas.upenn.edu/~cis194/spring13/hw/01-intro.pdf) says: ====== BEGIN Double the value of every second digit beginning from the right. That is, the last digit is unchanged; the second-to-last digit is dou- bled; the third-to-last digit is unchanged; and so on. For example, [1,3,8,6] becomes [2,3,16,6] Add the digits of the doubled values and the undoubled dig- its from the original number. For example, [2,3,16,6] becomes 2+3+1+6+6 = 18 ======== END Firstly, I am confused as to how the doubled values are being added to the undoubled number in the above example. It looks like only the individual numbers of the doubled values are being added Secondly, later on in the assignment: ======== BEGIN Example : validate 4012888888881881 = True Example : validate 4012888888881882 = False ========= END If we are to follow the algorithm described (double the value of "every second digit" beginning from the right, last digit unchanged", then the above numbers are identical EXCEPT For the last digits (1 and 2) Any help is appreciated in decoding these instructions. Again please do not provide Haskell code, since I want to attempt to write it myself. I am just looking for help with the algorithm. Regards,

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Iterating through lists
by Dananji Liyanage 18 May '15

18 May '15

Hi All, I'm building a 9x9 grid like list, and I want to extract each column of that grid. My input is a list of integers as follows; input = [0, 0, 0, 0, 0, 0, 4, 0, 9, 0, 0, 0, 0, 8, 0, 0, 5, 0, 7, 0, 2, 4, 5, 3, 6, 0, 0, 6, 7, 0, 0, 0, 1, 5, 0, 2, 2, 0, 8, 7, 0, 4, 3, 0, 1, 9, 0, 4, 5, 0, 0, 0, 8, 6, 0, 0, 6, 3, 1, 9, 8, 0, 7, 0, 2, 0, 0, 7, 0, 0, 0, 0, 1, 0, 7, 0, 0, 0, 0, 0, 0] I tried list comprehension with 'take' function in lists as follows; columns xs = [x | x <- (take 1 xs)] : columns (drop 9 xs) --> columns input This extracts only the first column. How I can go for the other columns? -- Regards, Dananji Liyanage

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which ones to use
by Roelof Wobben 18 May '15

18 May '15

Hello, My linux distro has haskell-platform not avaible. But things like ghc and cabal are avaible. Which ones does I need so I can programm in Haskell with SublimeText 3 as editor ? Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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profiling :: memory allocation
by Konstantin Saveljev 18 May '15

18 May '15

Hello, I'm trying to figure out how to reduce the allocation rate in my program. But I'm stuck with the profile report which has the top allocator to be shown like this: COST CENTRE MODULE %time %alloc >>= Internal 20.5 22.6 I do not understand what it actually means? Can anyone explain to me or is there anything I can do so it shows some more information (instead of showing the bind operator >>=) ? Thanks, Konstantin

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How to check the help page of a function?
by Peng Yu 17 May '15

17 May '15

Hi, I can not find how to check the help page of a function. For example, I want to check the help page of "head". How should I do it? (":help" doesn't seem to help.) -- Regards, Peng

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ld: warning: directory not found for option '-L/private/tmp/ghc20150401-90656-rxy9sj/ghc-7.10.1/gmp-static'
by Peng Yu 17 May '15

17 May '15

Hi, I see the following error. Does anybody know what is wrong? Thanks. $ cat hello.hs main = putStrLn "Hello, World!" $ ghc -o hello hello.hs [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... ld: warning: directory not found for option '-L/private/tmp/ghc20150401-90656-rxy9sj/ghc-7.10.1/gmp-static' -- Regards, Peng

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Parser
by Mike Houghton 17 May '15

17 May '15

Hi, I’me writing a simple parser and can test it in GHCI parseTest moveParser "(1,2)->(3,3)” and I get what I expect i.e. Move {from = Location 1 2, to = Location 3 3} I now have a text file (test.txt) with lots of (1,2)->(3,3) (3,6)->(3,9) etc etc and I really can’t see how I get the contents of the text file into the parser!! I have main = do handle <- openFile "test.txt" ReadMode contents <- hGetContents handle !! what goes here!!! - how do I invoke moveParser with contents ?? Thanks Mike

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Building glib
by C W Rose 15 May '15

15 May '15

I've been trying to build the glib package (pretty much any version) for a few days now, with no success. The sequence of results is: =================================================================== cwr@sixpence cabal $ cabal --verbose --global --package-db=/opt/cabal/package.conf.d install glib-0.12.5.4 '/opt/cabal/bin/alex' '--version' '/opt/cabal/bin/cpphs' '--version' '/usr/bin/gcc' '-dumpversion' '/opt/cabal/bin/haddock' '--version' '/opt/cabal/bin/happy' '--version' '/usr/bin/hpc' 'version' looking for tool hsc2hs near compiler in /usr/bin found hsc2hs in /usr/bin/hsc2hs '/usr/bin/hsc2hs' '--version' '/opt/cabal/bin/HsColour' '-version' '/usr/bin/ghc' '-c' '/tmp/6826.c' '-o' '/tmp/6826.o' '/usr/bin/ld' '-x' '-r' '/tmp/6826.o' '-o' '/tmp/6827.o' '/usr/bin/pkg-config' '--version' '/bin/tar' '--help' Reading available packages... Warning: The package list for 'hackage.haskell.org' is 52.1 days old. Run 'cabal update' to get the latest list of available packages. Choosing modular solver. Resolving dependencies... Ready to install glib-0.12.5.4 Waiting for install task to finish... Extracting /opt/cabal/packages/hackage.haskell.org/glib/0.12.5.4/glib-0.12.5.4.tar.gz to /tmp/glib-0.12.5.4-6826... Updating glib.cabal with the latest revision from the index. creating /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup creating /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist creating /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup copy /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/Setup.hs to /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/setup.hs '/usr/bin/ghc' '--make' '-odir' '/tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup' '-hidir' '/tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup' '-i' '-i/tmp/glib-0.12.5.4-6826/glib-0.12.5.4' '-package-db' '/opt/cabal/package.conf.d' '-package-id' 'Cabal-1.22.1.1-98c5a01d59dd973fd7113dbb82e82560' '/tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/setup.hs' '-o' '/tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/setup' '-threaded' [1 of 2] Compiling SetupWrapper ( /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/SetupWrapper.hs, /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/SetupWrapper.o ) /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/SetupWrapper.hs:118:28: Warning: In the use of `configCompiler' (imported from Distribution.Simple.Configure): Deprecated: "'configCompiler' is deprecated. Use 'configCompilerEx' instead." [2 of 2] Compiling Main ( /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/setup.hs, /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/Main.o ) Linking /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/setup ... /tmp/glib-0.12.5.4-6826/glib-0.12.5.4/dist/setup/setup configure --verbose=2 --ghc --prefix=/opt/cabal --bindir=/opt/cabal/bin --libdir=/opt/cabal/lib --libsubdir=glib-0.12.5.4/ghc-7.6.3 --libexecdir=/opt/cabal/libexec --datadir=/opt/cabal/share --datasubdir=glib-0.12.5.4 --docdir=/opt/cabal/share/doc/glib-0.12.5.4 --htmldir=/opt/cabal/share/doc/glib-0.12.5.4/html --haddockdir=/opt/cabal/share/doc/glib-0.12.5.4/html --sysconfdir=/opt/cabal/etc --enable-library-profiling --enable-optimization=2 --global --package-db=/opt/cabal/package.conf.d --flags=closure_signals --dependency=utf8-string=utf8-string-0.3.8-51169ff3d90963aa54c0a13eee5c3a53 --dependency=containers=containers-0.5.0.0-e7e1a93d7945e93a00ce12660e9ff290 --dependency=base=base-4.6.0.1-220b875e77822b0e1bf50c8f35d37dff --disable-tests --exact-configuration --disable-benchmarks [1 of 2] Compiling Gtk2HsSetup ( Gtk2HsSetup.hs, dist/setup-wrapper/Gtk2HsSetup.o ) [2 of 2] Compiling Main ( SetupMain.hs, dist/setup-wrapper/Main.o ) Linking dist/setup-wrapper/setup ... unrecognized option `--sysconfdir=/opt/cabal/etc' unrecognized option `--dependency=utf8-string=utf8-string-0.3.8-51169ff3d90963aa54c0a13eee5c3a53' unrecognized option `--dependency=containers=containers-0.5.0.0-e7e1a93d7945e93a00ce12660e9ff290' unrecognized option `--dependency=base=base-4.6.0.1-220b875e77822b0e1bf50c8f35d37dff' unrecognized option `--exact-configuration' Failed to install glib-0.12.5.4 World file is already up to date. cabal: Error: some packages failed to install: glib-0.12.5.4 failed during the configure step. The exception was: ExitFailure 1 cwr@sixpence cabal $ =================================================================== Moving to the default version of glib, I then get: =================================================================== cwr@sixpence cabal $ cabal --verbose --global --package-db=/opt/cabal/package.conf.d install glib Reading available packages... Choosing modular solver. Resolving dependencies... Ready to install glib-0.13.1.0 Waiting for install task to finish... Extracting /opt/cabal/packages/hackage.haskell.org/glib/0.13.1.0/glib-0.13.1.0.tar.gz to /tmp/glib-0.13.1.0-3703... Updating glib.cabal with the latest revision from the index. creating /tmp/glib-0.13.1.0-3703/glib-0.13.1.0/dist/setup creating /tmp/glib-0.13.1.0-3703/glib-0.13.1.0/dist creating /tmp/glib-0.13.1.0-3703/glib-0.13.1.0/dist/setup Failed to install glib-0.13.1.0 World file is already up to date. cabal: Error: some packages failed to install: glib-0.13.1.0 failed during the configure step. The exception was: user error (The package 'glib' requires Cabal library version -any && >=1.18 but no suitable version is installed.) cwr@sixpence cabal $ cabal --version cabal-install version 1.18.0.2 using version 1.18.1.2 of the Cabal library cwr@sixpence cabal $ =================================================================== And moving to the default version of cabal, I get: =================================================================== cwr@sixpence cabal $ cabal --verbose --global --package-db=/opt/cabal/package.conf.d install glib '/opt/cabal/bin/alex' '--version' '/opt/cabal/bin/cpphs' '--version' '/usr/bin/gcc' '-dumpversion' '/opt/cabal/bin/haddock' '--version' '/opt/cabal/bin/happy' '--version' '/usr/bin/hpc' 'version' looking for tool hsc2hs near compiler in /usr/bin found hsc2hs in /usr/bin/hsc2hs '/usr/bin/hsc2hs' '--version' '/opt/cabal/bin/HsColour' '-version' '/usr/bin/ghc' '-c' '/tmp/4186.c' '-o' '/tmp/4186.o' '/usr/bin/ld' '-x' '-r' '/tmp/4186.o' '-o' '/tmp/4187.o' '/usr/bin/pkg-config' '--version' '/bin/tar' '--help' Reading available packages... Warning: The package list for 'hackage.haskell.org' is 51.4 days old. Run 'cabal update' to get the latest list of available packages. Choosing modular solver. Resolving dependencies... Ready to install glib-0.13.1.0 Waiting for install task to finish... Extracting /opt/cabal/packages/hackage.haskell.org/glib/0.13.1.0/glib-0.13.1.0.tar.gz to /tmp/glib-0.13.1.0-4186... Updating glib.cabal with the latest revision from the index. creating /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup creating /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist creating /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup copy /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/Setup.hs to /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/setup.hs '/usr/bin/ghc' '--make' '-odir' '/tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup' '-hidir' '/tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup' '-i' '-i/tmp/glib-0.13.1.0-4186/glib-0.13.1.0' '-package-db' '/opt/cabal/package.conf.d' '-package-id' 'Cabal-1.22.1.1-98c5a01d59dd973fd7113dbb82e82560' '/tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/setup.hs' '-o' '/tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/setup' '-threaded' [1 of 2] Compiling SetupWrapper ( /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/SetupWrapper.hs, /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/SetupWrapper.o ) [2 of 2] Compiling Main ( /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/setup.hs, /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/Main.o ) Linking /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/setup ... /tmp/glib-0.13.1.0-4186/glib-0.13.1.0/dist/setup/setup configure --verbose=2 --ghc --prefix=/opt/cabal --bindir=/opt/cabal/bin --libdir=/opt/cabal/lib --libsubdir=glib-0.13.1.0/ghc-7.6.3 --libexecdir=/opt/cabal/libexec --datadir=/opt/cabal/share --datasubdir=glib-0.13.1.0 --docdir=/opt/cabal/share/doc/glib-0.13.1.0 --htmldir=/opt/cabal/share/doc/glib-0.13.1.0/html --haddockdir=/opt/cabal/share/doc/glib-0.13.1.0/html --sysconfdir=/opt/cabal/etc --enable-library-profiling --enable-optimization=2 --global --package-db=/opt/cabal/package.conf.d --flags=closure_signals --dependency=utf8-string=utf8-string-0.3.8-51169ff3d90963aa54c0a13eee5c3a53 --dependency=text=text-1.1.1.3-3a54592a9f7d64cef4851163d5553453 --dependency=containers=containers-0.5.0.0-e7e1a93d7945e93a00ce12660e9ff290 --dependency=bytestring=bytestring-0.10.0.2-14ba8002009bf89cdad6038c30eda3fd --dependency=base=base-4.6.0.1-220b875e77822b0e1bf50c8f35d37dff --disable-tests --exact-configuration --disable-benchmarks [1 of 2] Compiling Gtk2HsSetup ( Gtk2HsSetup.hs, dist/setup-wrapper/Gtk2HsSetup.o ) Gtk2HsSetup.hs:167:16: Not in scope: `LBI.getComponentLocalBuildInfo' Failed to install glib-0.13.1.0 Updating world file... cabal: Error: some packages failed to install: glib-0.13.1.0 failed during the configure step. The exception was: ExitFailure 1 cwr@sixpence cabal $ =================================================================== Does anyone have any idea how to fix any or all of these errors? Thanks - Will

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Haskell data generics
by Eduardo 15 May '15

15 May '15

Hello everyone, Here is what I am trying to do: using Data and Typeable generate a representation of any datatype. After looking at some examples this was my attempt: http://lpaste.net/132669. But I can't seem to understand how extQ should work. For instance https://github.com/faylang/fay/blob/master/src/Fay/Convert.hs#L54 https://github.com/lymar/hastache/blob/master/Text/Hastache/Context.hs#L321 All seem to use extQ to "choose" the right way to encode the values(I imagine it's because of the cast inside extQ?). So what am I missing here?

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