January 2012 - Beginners

Another install problem
by Tom Murphy 19 Jan '12

19 Jan '12

Hi, Sorry to copy-paste more install errors, but this one has been confusing me for a while: Summary: "The pkg-config package poppler-glib version >=0.12.4 is required but it could not be found." I don't know how to fix this, as "poppler-glib" is not a package (and not a dependency). Thanks in advance! Tom $ cabal install poppler Resolving dependencies... /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/Gtk2HsSetup.hs:25:0: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/Gtk2HsSetup.hs, /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/dist/setup/Gtk2HsSetup.o ) [2 of 2] Compiling Main ( /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/Setup.hs, /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/dist/setup/Main.o ) Linking /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/dist/setup/setup ... Configuring poppler-0.12.2... setup: The pkg-config package poppler-glib version >=0.12.4 is required but it could not be found. cabal: Error: some packages failed to install: poppler-0.12.2 failed during the configure step. The exception was: ExitFailure 1

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Is there an "unscan" function?
by Jeffrey Thornton 19 Jan '12

19 Jan '12

Hello- Is there standard function in Haskell that effectively does an inverse scan? For example, scanl1 (\ x y -> x+y) [1,2,3,4] == [1,3,6,10]. So is there a very simple built-in way to do this hypothetical example?: unscanl1 (\ x y -> y-x) [1,3,6,10] == [1,2,3,4] Thanks, Jeffrey

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questions about product types
by Tom Doris 18 Jan '12

18 Jan '12

My application is a server written in Haskell that receives uploads of structured data and persists them to disk and redistributes them on demand, kind of a persistent messaging middleware. Anyway, I started out using Haskell record syntax for everything, hit the usual problems with name clashes and ended up with field label names that looked like hungarian notation, everything looked a bit grimy and verbose etc,etc. As an experiment I got rid of the labels, and anywhere I had a product type that had more than one field of the same type (usually these were primitive types I hadn't bothered to create wrapper types for, e.g. Price and Quantity were both Double), I took that to be a "code smell" and created types that more accurately captured the use of the field's value in the program domain. Next I created a class Extractor to provide a standard way to pull individual fields from the product type: class Extract a b where extract :: a -> b e.g.: instance Extract Upload ClientID where extract (Upload c _ _ _ _ _) = c instance Extract Upload RecordPeriod where extract (Upload _ p _ _ _ _) = p So I can pass an Upload to extract and thanks to type inference it will usually be unambiguous, and I can annotate with a type signature to make it clearer what field I'm extracting as I see fit. I was missing record update syntax so I added a class to make it easier to do that: class Transform a b where transform :: a -> (b->b) -> a e.g.: instance Transform Upload ClientID where transform (Upload c p ab ar lr br) f = (Upload (f c) p ab ar lr br) i.e. take a function that updates a contained value and apply it to the appropriate field in an otherwise identical copy of the original. Apart from the verbosity of the instance declarations for every field I was pleased with the result, and the imposed discipline of giving each field a distinct type significantly improved the codebase. My questions are: 1) I presume this approach has been explored and probably there is a well thought through library of helper classes and functions already out there, can someone please point me to it? 2) I don't know much about Haskell generics, but for product types in which all contained values are distinct types, it should be possible to automatically derive instances of Extract, since there's never ambiguity as to which field you want from a container if you've specified the type of value you want, is this possible, and if so does it already exist? 3) In type theory, are product types consisting of distinct member types considered special in any way? i.e. does the concept have a name and what special properties do they have? Thanks for your help. T

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Error in converting List of Lists into PArray ( Parray a )
by mukesh tiwari 18 Jan '12

18 Jan '12

Hello all I am trying to convert List of Lists ( [[(Int , Double )]] ) into PArray ( PArray ( Int , Double )) but getting run time error. This code works fine and print list of PArray ( Int , Double ) but when i put print $ P.fromList ( map P.fromList c ) then i am getting runtime error. It says "Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR". Could some one please tell me how to resolve this issue. Thank you --import ParallelMat import Data.List import System.Environment import Data.Array.Parallel import qualified Data.Array.Parallel.PArray as P processMatrix :: [ [ Double ] ] -> [ [ Double ] ] -> [ ( [ ( Int , Double ) ] , [ ( Int , Double ) ]) ] processMatrix [] [] = [] processMatrix ( x : xs ) ( y : ys ) | ( all ( == 0 ) x ) Prelude.|| ( all ( == 0 ) y ) = processMatrix xs ys | otherwise = ( filter ( \( x , y ) -> y /= 0 ) . zip [ 1..] $ x ,filter ( \( x , y ) -> y /= 0 ) . zip [1..] $ y ) : processMatrix xs ys main = do [ first , second ] <- getArgs a <- readFile first b <- readFile second let a' = transpose . tail . map ( map ( read :: String -> Double ) . words ) . lines $ a b' = tail . map ( map ( read :: String -> Double ) . words ) . lines $ b ( c , d ) = unzip $ processMatrix a' b' print $ ( map P.fromList c ) --print d Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in [fromList<PArray> [(1,1.0),(6,1.0)],fromList<PArray> [(4,11.0),(9,11.0)],fromList<PArray> [(1,4.0),(4,2.0),(6,4.0),(9,2.0)]] Putting print $ P.fromList ( map P.fromList c ) Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR Input file A.in 10 10 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0

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(Basic?) IO question
by Guillaume Basse 18 Jan '12

18 Jan '12

Hello, So I've been trying to print some information in a file. I have two versions of the same function: one of them doesn't work and I don't understand why. Here are the functions: # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # -- This one works as intended when called from ghci renderTrackInfosToFile :: FilePath -> String -> IO [TrackInfos] -> IO () renderTrackInfosToFile filename sep ls = do nls <- ls writeFile filename (intercalate "\n" $ map (renderTrackInfo sep) nls) return () -- This one does nothing when called from ghci renderTrackInfosToFile2 :: FilePath -> String -> [TrackInfos] -> IO () renderTrackInfosToFile2 filename sep ls = writeFile filename (intercalate "\n" $ map (renderTrackInfo sep) ls) renderTrackInfo :: String -> TrackInfos -> String # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # Both functions load in ghci, and the following commands are issued: > -- c :: IO [TrackInfos] > liftM (renderTrackInfosToFile2 "blabla.txt" "|") c -- this command seems to do absolutely nothing > renderTrackInfosToFile "blabla.txt" "|" c -- this one works as intended # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # And this is not the first time I have this sort of problem using liftM. I now that I'm missing something important here, can anyone explain me my mistake? Much thanks, Guillaume Basse

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install problem
by Andy Larocque 18 Jan '12

18 Jan '12

Hi I've been trying to build wxWidgets from source, and downloaded the following straightforward instructions. Everything worked fine until I got to #5 . I got the error : bash: ./configure: No such file or directory; and it seems that the 'configure' command is not available on my system, ..anywhere . (Ubuntu 11). Why is this command not available , and how do I fix this and continue the build?* AL ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Building wxWidgets for use on Linux<http://gmat.ed-pages.com/wiki/tiki-index.php?page=Building+wxWidgets+for+us…> * GMAT's GUI is written using the wxWidgets cross-platform GUI library. GMAT's source code is not Unicode compliant, so you may need to download and install wxWidgets if you want to run GMAT from its GUI. This procedure will take you through the wx build process: 1. Download wxWidgets from the wx site, and uncompress the source code. On Linux, you should download the wxGTK code base. 2. Open a console window 3. Change directories to the folder containing wxWidgets. 4. Setup a build directory, and change directories to that folder: $ mkdir installation $ cd installation 5. Configure the framework. wxWidgets provides many different configuration options that can be previewed using the configure --help option. For example, you may want to turn off some of the graphics file formats (TIFF support, for example), change the location of the library files, or enable debugging. This is the minimal set that I use (though I sometimes omit the optimization option): $ configure --with-opengl --enable-shared --enable-optimise 6. Read the summary information after configuring to be sure that the correct operating system and options were set. 7. Build wxWidgets: $ make 8. Install it: $ make install 9. Run ldconfig on the install folder (/usr/local/lib, unless you changed it in step 5).

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LYAH Control.Monad.Writer tell
by TJ Takei 16 Jan '12

16 Jan '12

Hi I have a trouble to run an example of "Learn Your A Haskell.." Chap 13 below: ======== import Data.Monoid --Don't import Control.Monad.Writer newtype Writer w a = Writer { runWriter :: (a, w) } instance (Monoid w) => Monad (Writer w) where return x = Writer (x, mempty) (Writer (x,v)) >>= f = let (Writer (y, v')) = f x in Writer (y, v `mappend` v') --Define tell tell :: [String] -> Writer [String] Int tell w = Writer (0, w) -- what'sa hell "0" for ???!!! logNumber :: Int -> Writer [String] Int logNumber x = Writer (x, ["Got number: " ++ show x]) multWithLog :: Writer [String] Int multWithLog = do a <- logNumber 3 b <- logNumber 5 tell ["Gonna multiply these two"] return (a*b) main = putStrLn . show $ runWriter multWithLog ======== I changed two places to run it without error: [1] Ambiguity error of Writer, uneless I comment out "import Control.Monad.Writer", and [2] Define tell function My questions are: Why does LYAH sample fail as is? Do the changes above look reasonable? I'm not certain about my "tell". Where is the correct instantiation of "tell" included? Thanks, TJ

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Spoj BWHEELER problem - input problem
by Artur Tadrała 15 Jan '12

15 Jan '12

Hello While learning Haskell I'm trying to solve some simple problems on spoj.ploccasionally. Currently I'm working on: http://www.spoj.pl/problems/BWHEELER/. I figured out how to solve it but I have some problems with reading input (that's my guess) Here is my solution: import Data.List import Data.Array import qualified Data.ByteString.Lazy.Char8 as BS import IO traverse :: Array Int (Char, Int) -> Int -> Int -> String -> String traverse endings n k acc = let (c,i) = endings ! n in if k == 0 then acc else traverse endings i (k-1) (c:acc) solve :: (Int, String) -> String solve (n,w) = let l = length w endings = sort $ zip w [0..] endingsArray = array (0, l) (zip [0..] endings) in reverse $ traverse endingsArray (n-1) l "" parseCases :: [BS.ByteString] -> [(Int, String)] parseCases (l:l':ls) = let n = readInt l w = BS.unpack l' in (n,w):parseCases ls parseCases _ = [] main :: IO () main = do ls <- BS.lines `fmap` (BS.readFile "input.txt") --BS.getContents putStr $ unlines $ map solve $ parseCases ls readInt :: BS.ByteString -> Int readInt x = case BS.readInt x of Just (i,_) -> i Nothing -> error ("Unparsable Int" ++ (show x)) The input.txt file contains following text: 2 bacab 3 rwlb 11 baaabaaaabbbaba 0 When I compile and execute this code i get follwing output: aaaaaa lllll bbb It's different when compared to this in ghci ( this is what I expect): > map solve [(2,"bacab"), (3, "rwlb"), (11,"baaabaaaabbbaba")] ["abcba","rbwl","baaabbbbaaaaaab"] Can you explain me what I'm doing wrong? I appreciate any tips how to improve this code also. Thanks for help! Artur Tadrała

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Installing dph-examples in Mac OS X Version 10.7.2
by mukesh tiwari 15 Jan '12

15 Jan '12

Hello all I am trying to install dph-examples on Mac OS X. First I have Xcode 4.2.1 and during the installation i got this <http://hpaste.org/56445> error Installing library in /Users/mukesh/.cabal/lib/dph-seq-0.5.1.1/ghc-7.2.1Registering dph-seq-0.5.1.1...Downloading dph-examples-0.5.1.2...Configuring dph-examples-0.5.1.2...Building dph-examples-0.5.1.2...Preprocessing executable 'dph-sumsq-seq' for dph-examples-0.5.1.2...[1 of 5] Compiling SumSquaresVectorised ( imaginary/SumSquares/dph/SumSquaresVectorised.hs, dist/build/dph-sumsq-seq/dph-sumsq-seq-tmp/SumSquaresVectorised.o )Error (fd:12: hGetLine: end of file)Warning: Couldn't figure out LLVM version!Make sure you have installed LLVMghc: could not execute: optcabal: Error: some packages failed to install:dph-examples-0.5.1.2 failed during the building phase. The exception was:ExitFailure 1 On stackoverflow<http://stackoverflow.com/questions/8864696/installing-dph-examples-in-mac-o…> , i got suggestion to remove Xcode 4.2 and install Xcode 3.2. After installing Xcode 3.2 , I am still getting this error. Macintosh:~ mukesh$ cabal install dph-examples Resolving dependencies... Configuring dph-examples-0.5.1.2... Building dph-examples-0.5.1.2... Preprocessing executable 'dph-sumsq-seq' for dph-examples-0.5.1.2... [1 of 5] Compiling SumSquaresVectorised ( imaginary/SumSquares/dph/SumSquaresVectorised.hs, dist/build/dph-sumsq-seq/dph-sumsq-seq-tmp/SumSquaresVectorised.o ) Error (fd:11: hGetLine: end of file) Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM ghc: could not execute: opt cabal: Error: some packages failed to install: dph-examples-0.5.1.2 failed during the building phase. The exception was: ExitFailure 1 Macintosh:~ mukesh$ uname -a Darwin Macintosh 11.2.0 Darwin Kernel Version 11.2.0: Tue Aug 9 20:56:15 PDT 2011; root:xnu-1699.24.8~1/RELEASE_I386 i386 Macintosh:~ mukesh$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.2.1 Xcode 3.2 ( 64 bit ) Could some one please tell me how to resolve this issue. Regards Mukesh Tiwari

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(no subject)
by Alia 15 Jan '12

15 Jan '12

http://teachingjet.com/modules/mod_wdbanners/friends.php?help2.php

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