January 2012 - Beginners

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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Imperfect Graham Scan
by Zhi-Qiang Lei 08 Jan '12

08 Jan '12

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

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Parse error in pattern
by Zhi-Qiang Lei 07 Jan '12

07 Jan '12

Hi, I am writing a Graham Scan function. What puzzles me is it cannot be compiled. Does anyone know what is wrong with "scan"? Thanks. ==== compile ==== bogon% ghc GrahamScan.hs -- NORMAL -- [1 of 1] Compiling Main ( GrahamScan.hs, GrahamScan.o ) GrahamScan.hs:30:1: Parse error in pattern: scan ==== compile ==== ==== code ==== data Point a = Point { x :: a, y :: b } instance Ord (Point a) where compare (Point x1 y1) (Point x2 y2) = compare (y1, x1) (y2, x2) data Vector a = Vector { start :: Point a, end :: Point a } cosine :: Vector a -> Ratio a cosine (Vector (Point x1 y1) (Point x2 y2)) = (x2 - x1) / ((x2 - x1) ^ 2 + (y2 - y1) ^ 2) instance Ord (Vector a) where compare a b = compare (f a) (f b) where f = negate . cosine sort' :: [Point a] -> [Point a] sort' xs = minPoint : fmap end sortedVectors where sortedVectors = sort $ map (Vector minPoint) $ delete minPoint xs minPoint = minimum xs ccw :: Point a -> Point a -> Point a -> Bool ccw (Point x1 y1) (Point x2 y2) (Point x3 y3) = (x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1) > 0 scan :: [Point a] -> [Point a] scan p1 : p2 : p3 : xs | ccw p1 p2 p3 = p1 : scan (p2 : p3 : xs) | otherwise = scan (p1 : p3 : xs) scan xs = xs grahamScan :: [Point a] -> [Point a] grahamScan = scan . sort' ==== code ==== Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Programming GTK
by gabre＠caesar.elte.hu 06 Jan '12

06 Jan '12

Hello! I am writing a turtle graphics program in Haskell. Now, I am working on the GUI (Gtk2HS). I am a newbie, I have never programmed event driven systems. I have a Button, and when it is clicked the code written in a a TextView is drawn in a DrawingArea. There are some problems: when "Expose event" happens (I dont really know, if I use the right words, but I mean, when another window is in focus, or another window comes before the turtle graphics window etc.) my drawing disappears (I know, it is the normal behaviour). I would like to redraw it, but I dont know, how. I have a function with events... connectGui gui = do onDestroy (mainWin gui) mainQuit onClicked (btn gui) (drawProgram gui) afterActivateLeaf (quitbtn gui) (widgetDestroy (mainWin gui)) (where gui is a record, containing all the GUI elements, such as the buttons) ...but the drawProgram function draws everything, and if I add the "onExpose" with the same drawing function, everything is redrawn although the Button was never clicked before. So I would like to modify the code: -if the button was already clicked and redraw is needed, then it should be done -if the button was never clicked, then there must be no redraw Could you tell me, how can I do this? Gabre

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Re: [Haskell-beginners] Beginners Digest, Vol 43, Issue 4
by Gregory Guthrie 05 Jan '12

05 Jan '12

Thanks - I did try a cabal install xxx" for the missing packages, a few seemed to succeed, a few failed, and now I get a longer list of overall failures! :-) ... There are problems in package scion-0.1.0.9: dependency "AttoJson-0.5.10-4932e72997280d8c2e35b4aca3328284" doesn't exist dependency "binary-0.5.0.2-72c84eaab351d41e936261d8d5d416f5" doesn't exist dependency "list-tries-0.4.1-a2fcbc51b309eb5aade37b40c6ef166e" doesn't exist There are problems in package bytestring-show-0.3.4: dependency "binary-0.5.0.2-72c84eaab351d41e936261d8d5d416f5" doesn't exist There are problems in package carray-0.1.5: dependency "binary-0.5.0.2-72c84eaab351d41e936261d8d5d416f5" doesn't exist dependency "ix-shapable-0.1.0-68747ed518a0145d47f8095a08e0bcbb" doesn't exist ... The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. warp-0.4.6.3 tagsoup-0.12.3 simple-sendfile-0.2.0 blaze-builder-enumerator-0.2.0.3 scion-0.1.0.10 aeson-0.3.2.11 hashable-1.1.2.1 blaze-textual-0.2.0.4 double-conversion-0.2.0.1 derive-2.5.4 hoogle-4.2.6 warp-0.4.3 wai-0.4.1 simple-sendfile-0.1.2 http-types-0.6.5.1 case-insensitive-0.3.0.1 blaze-builder-enumerator-0.2.0.2 bmp-1.1.2.1 scion-0.1.0.9 bytestring-show-0.3.4 carray-0.1.5 unordered-containers-0.1.4.2 I tried an install (reinstall?) of the things it lists as missing, and generally they report "already installed". For example; >cabal install AttoJson Resolving dependencies... No packages to be installed. All the requested packages are already installed. If you want to reinstall anyway then use the --reinstall flag. (same for binary, list-tries, ix-shapable, ...) >ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3 Some things do seem to resolve after a few iterations through re-installations, but others seem stuck, and overall the process seems like a mess. > What is the remedy (is there a simple remedy?) if "ghc-pkg check" reports something amiss? > > In this case, the simple remedy is to reinstall the packages which the error messages say do > not exist (blaze-builder, hashable, AttoJson) and then reinstall any packages you want to use > which depend on them. > > -Brent

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Re: [Haskell-beginners] WinRT and Haskell
by Antoine Latter 05 Jan '12

05 Jan '12

On Thu, Jan 5, 2012 at 9:55 AM, AbdulSattar Mohammed <codingtales(a)gmail.com> wrote: > WinRT has a concept of projections that expose its API. Microsoft has > implemented projections for Native (C and C++), HTML/Javascript and .NET > (from Miguel de Caza's WinRT demystified > post: http://tirania.org/blog/archive/2011/Sep-15.html) There's no mention > of creating your own projections (or I have seen none). I don't see any > reason why we can't create our own projections. If it's possible a Haskell > projection could really help. > Yep, that's what I was thinking of. I'm pretty sure the C/C++ projections rely on compiler extensions for the MS C/C++ compiler, so I don't think we could use those directly from Haskell. Antoine > > On Thu, Jan 5, 2012 at 8:11 PM, Antoine Latter <aslatter(a)gmail.com> wrote: >> >> On Thu, Jan 5, 2012 at 5:12 AM, AbdulSattar Mohammed >> <codingtales(a)gmail.com> wrote: >> > I suppose this should go into the GUI mailing list, but it is filled >> > with >> > spam. So, WinRT does not depend on the .NET Framework. C++ applications >> > can >> > directly compile to x86 and be able to use WinRT. Do we have a room for >> > Haskell development there? >> > >> >> From what I understand, WinRT is a set of COM libraries - C++ can >> directly compile to it because the MS C++ compiler has special >> extensions to handle the COM resources in the library. >> >> There is a COM library for Haskell: >> >> http://hackage.haskell.org/package/com >> >> But I've never used it. >> >> The API is encoded in a special meta-data format, which is then used >> by C++, .NET and Javascript to create the language-specific APIs - it >> might be possible to generate Haskell bindings to the COM components >> from this metadata. >> >> Antoine > > > > > -- > Warm Regards, > > AbdulSattar Mohammed

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WinRT and Haskell
by AbdulSattar Mohammed 05 Jan '12

05 Jan '12

I suppose this should go into the GUI mailing list, but it is filled with spam. So, WinRT does not depend on the .NET Framework. C++ applications can directly compile to x86 and be able to use WinRT. Do we have a room for Haskell development there? -- Warm Regards, AbdulSattar Mohammed

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