- Beginners - Haskell.org

(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 17 Jan '12

17 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 14 Jan '12

14 Jan '12

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

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error
by Andy Larocque 13 Jan '12

13 Jan '12

I've been trying to run this ( paint.hs) program ,shown below, in ghci - but I get this error I don't know how to correct : Loading package wxcore-0.13.2 ... can't load .so/.DLL for: stdc++ (libstdc++.so: cannot open shared object file: No such file or directory) Does anyone know the fix for it? ( I got the same thing with the other sample programs for wxHaskell) --------------------------------------------------------------------------------- module Main where import Graphics.UI.WXCore import Graphics.UI.WX main :: IO () main = start gui gui :: IO () gui = do f <- frame [text := "Paint demo", fullRepaintOnResize := False] sw <- scrolledWindow f [ on paint := onpaint , virtualSize := sz 500 500, scrollRate := sz 10 10 , fullRepaintOnResize := False] set f [clientSize := sz 150 150, layout := fill $ widget sw] return () where onpaint dc viewArea = do circle dc (pt 200 200) 20 [penKind := PenDash DashDot] arc dc (pt 100 100) 20 90 230 [color := red, penWidth :~ (+1), penKind := PenSolid] ellipticArc dc (rect (pt 20 20) (sz 60 30)) 90 230 [color := blue, penWidth :~ (*2)] c <- get dc color -- set dc [font := fontDefault{ fontFace = "Courier New", fontSize = 16, fontWeight = WeightBold }] set dc [fontFace := "Courier New", fontSize := 16, fontWeight := WeightBold ] drawText dc (show c) (pt 50 50) [] rotatedText dc "rotated text" (pt 80 160) 45 [textColor := green] ----------------------------------

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module "static" data
by Dennis Raddle 10 Jan '12

10 Jan '12

I would like to know how to do something in Haskell. I have a module which needs to load some data from disk in order to run, and I'd like to load it at most once. In C, I would use a "static" variable in the module to store the data. How can I do this in Haskell?

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Re: [Haskell-beginners] Imperfect Graham Scan
by Ray Song 10 Jan '12

10 Jan '12

Typo /counterwise/ - counterclockwise. BTW, the 'cosine's functionalty can be replaced by the outer product. 2012-1-8 下午11:54 於 "Ray Song" <emacsray(a)gmail.com> 寫道： > The 'scan' is flawed. A counterwise angle formed by the first three points > does not guarantee p1's existence in the hull. > 2012-1-8 下午3:32 於 "Zhi-Qiang Lei" <zhiqiang.lei(a)gmail.com> 寫道： > > > > Hi, > > > > The Graham Scan function I wrote, looks like running well. But when I > put it in QuickCheck, it just failed in some case. Anyone can show me some > clues about the problem? Thanks. > > > > When I test it in ghci with some example, it returns the right result. > > *Main> let xs = [Point {x = 1.0, y = 1.0},Point {x = 0.0, y = 4.0},Point > {x = 0.0, y = 6.0},Point {x = 3.0, y = 5.0},Point {x = 4.0, y = 4.0},Point > {x = 4.0, y = 1.0},Point {x = 3.0, y = 3.0},Point {x = 2.0, y = 2.0},Point > {x = 5.0, y = 5.0}] > > *Main> grahamScan xs > > [Point {x = 1.0, y = 1.0},Point {x = 4.0, y = 1.0},Point {x = 5.0, y = > 5.0},Point {x = 0.0, y = 6.0},Point {x = 0.0, y = 4.0}] > > *Main> grahamScan it > > [Point {x = 1.0, y = 1.0},Point {x = 4.0, y = 1.0},Point {x = 5.0, y = > 5.0},Point {x = 0.0, y = 6.0},Point {x = 0.0, y = 4.0}] > > > > However, QuickCheck find some points which can fail it. Could it be a > data type overflow problem? > > > > prop_scan_idempotent xs = not (null xs) ==> (grahamScan . grahamScan) xs > == grahamScan xs > > > > *Main> quickCheck prop_scan_idempotent > > *** Failed! Falsifiable (after 13 tests and 4 shrinks): > > [Point {x = -6.29996952110807, y = -91.37172300100718},Point {x = > 9.353314917365527, y = 64.35532141764591},Point {x = -23.826685687218355, y > = 60.32049750442556},Point {x = -1.4281411275074123, y = > 31.54197550020998},Point {x = -2.911218918860731, y = 15.564623822256719}] > > > > === code === > > module GrahamScan (grahamScan, Point(..)) > > where > > > > import Data.List > > import Data.Ratio > > > > data Point = Point { > > x :: Double, > > y :: Double > > } deriving (Eq, Show) > > > > instance Ord Point where > > compare (Point x1 y1) (Point x2 y2) = compare (y1, x1) (y2, x2) > > > > data Vector = Vector { > > start :: Point, > > end :: Point > > } deriving (Eq) > > > > cosine :: Vector -> Double > > cosine (Vector (Point x1 y1) (Point x2 y2)) = (x2 - x1) / ((x2 - x1) ^ 2 > + (y2 - y1) ^ 2) > > > > instance Ord Vector where > > compare a b = compare (f a) (f b) where > > f = negate . cosine > > > > sort' :: [Point] -> [Point] > > sort' xs = pivot : fmap end sortedVectors where > > sortedVectors = sort . fmap (Vector pivot) . delete pivot $ xs > > pivot = minimum xs > > > > counterClockwise :: Point -> Point -> Point -> Bool > > counterClockwise (Point x1 y1) (Point x2 y2) (Point x3 y3) = (x2 - x1) * > (y3 - y1) > (y2 - y1) * (x3 - x1) > > > > scan :: [Point] -> [Point] > > scan (p1 : p2 : p3 : xs) > > | counterClockwise p1 p2 p3 = p1 : scan (p2 : p3 : xs) > > | otherwise = scan (p1 : p3 : xs) > > scan xs = xs > > > > grahamScan :: [Point] -> [Point] > > grahamScan = scan . sort' . nub > > === code === > > > > > > Best regards, > > Zhi-Qiang Lei > > zhiqiang.lei(a)gmail.com > > > > > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://www.haskell.org/mailman/listinfo/beginners >

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For most geometric algorithms (e.g. Graham Scan) sorting the points & not using trig functions is usually better.
by KC 09 Jan '12

09 Jan '12

Often you sort the points to find a boundary point (extremal point). Using trig functions burns a lot of CPU cycles so if one can use the inner product, etc. so much the better. -- -- Regards, KC

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