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Re: [Haskell-cafe] Right way to implement setPixel function
by CK Kashyap 21 Aug '09

21 Aug '09

thank you very much Job. Regards, Kashyap On Thu Aug 20th, 2009 1:13 PM EDT Job Vranish wrote: >Opps: >setPixel = State setPixel' > >should be: >setPixel x y rgb = State $ setPixel' x y rgb > >- Job > >On Thu, Aug 20, 2009 at 1:05 PM, Job Vranish <jvranish(a)gmail.com> wrote: > >> Your setPixel function is almost ready to work in a State monad >> If you modify your setPixel function slightly like so: >> >> setPixel' :: Int -> Int -> Color -> B.ByteString -> ((), B.ByteString) >> setPixel' x y (r,g,b) image = ((), B.concat [beforePixel, pixel, >> afterPixel]) >> >> and then wrap it in the State monad constructor: >> >> setPixel = State setPixel' >> >> then you can do >> >> drawPixels = do >> setPixel 5 10 (200, 0, 0) >> setPixel 20 1 (0, 200, 0) >> setPixel 90 2 (0, 0, 200) >> >> modifiedImage = execState drawPixels originalImage >> >> See! you were already using a monad and didn't even know it! :D >> >> Performance wise, B.concat is O(n), which is very not good for your >> purpose. It copies the whole string and the optimizer won't be able to >> magically make it go away. For something that works in O(1), you will have >> to use something like STArrays instead of bytestrings. >> >> - Job >> >> >> >> On Thu, Aug 20, 2009 at 2:32 AM, CK Kashyap <ck_kashyap(a)yahoo.com> wrote: >> >>> Hi, >>> I had posted a note on line drawing algo with Haskell some time back. Now, >>> I am trying to write a PNM image. >>> >>> import qualified Data.ByteString as B >>> >>> width = 256 >>> height = 256 >>> bytesInImage = width * height * 3 >>> blankImage = B.pack $ take bytesInImage (repeat 0) >>> >>> type Color = (Int,Int,Int) >>> setPixel :: B.ByteString -> Int -> Int -> Color -> B.ByteString >>> setPixel image x y (r,g,b) = B.concat [beforePixel, pixel, afterPixel] >>> where >>> beforePixel = B.take before image >>> afterPixel = B.drop (before+3) image >>> pixel=B.pack [(fromIntegral r),(fromIntegral >>> g),(fromIntegral b)] >>> -- number of bytes before the 3 bytes of >>> -- the pixel at x y >>> before = (y * width * 3) + (x * 3) - 3 >>> >>> main = do >>> putStrLn "P6" >>> putStrLn ( (show width) ++ " " ++ (show height) ) >>> putStrLn "255" >>> -- Set a red pixel at 100 100 >>> B.putStr (setPixel blankImage 100 100 (255,0,0)) >>> >>> >>> Can I please have some review comments on the code above? Would recreating >>> the entire ByteString for each setPixel be an overhead? >>> Also, I am barely beginning to grasp the Monad concept....I was wondering >>> if there could be a monadic style of implementation of this - that could >>> potentially have a series of setPixels inside a do block? >>> >>> Regards, >>> Kashyap >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >>

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Parsec lookahead and <|>
by Martijn van Steenbergen 21 Aug '09

21 Aug '09

Goedemiddag café, Consider the following function, using parsec-3.0.0: > la :: Parsec String () (Maybe Char) > la = lookAhead (optionMaybe anyChar) *Lookahead> parseTest (char 'a' <|> char 'b') "a" 'a' *Lookahead> parseTest (char 'a' <|> char 'b') "b" 'b' *Lookahead> parseTest (la *> char 'a' <|> char 'b') "a" 'a' *Lookahead> parseTest (la *> char 'a' <|> char 'b') "b" parse error at (line 1, column 2): unexpected "b" expecting "a" The first three work fine and as expected, but the fourth example fails where I would expect success. I know <|> won't try the rhs if the lhs consumed input, but lookAhead's documentation promises not to consume any input. Is this a bug in Parsec or am I missing something? Thanks, Martijn.

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Re: Parsec lookahead and <|>
by Daniel Fischer 21 Aug '09

21 Aug '09

Apologies to Christian for the double post, didn't look at the To-field. Am Thursday 20 August 2009 20:21:30 schrieben Sie: > Daniel Fischer wrote: > > Am Donnerstag 20 August 2009 13:44:15 schrieb Martijn van Steenbergen: > >> Goedemiddag café, > >> > >> Consider the following function, using parsec-3.0.0: > >>> la :: Parsec String () (Maybe Char) > >>> la = lookAhead (optionMaybe anyChar) > >> > >> *Lookahead> parseTest (char 'a' <|> char 'b') "a" > >> 'a' > >> *Lookahead> parseTest (char 'a' <|> char 'b') "b" > >> 'b' > >> *Lookahead> parseTest (la *> char 'a' <|> char 'b') "a" > >> 'a' > >> *Lookahead> parseTest (la *> char 'a' <|> char 'b') "b" > >> parse error at (line 1, column 2): > >> unexpected "b" > >> expecting "a" > >> > >> The first three work fine and as expected, but the fourth example fails > >> where I would expect success. I know <|> won't try the rhs if the lhs > >> consumed input, but lookAhead's documentation promises not to consume > >> any input. Is this a bug in Parsec or am I missing something? > > > > Bad bug in Parsec (from the beginning, the same happens in parsec-2), I'd > > say. > > I'd say, its a feature. lookAhead returns whatever its argument returns. > So in this case it returns "Consumed" without consuming. In that case, the comment "lookAhead p parses p without consuming any input." from the parsec-3 docs is highly misleading. But lookAhead returning Consumed is not only counterintuitive, try parseTest (many la) "a" for a nice memory bomb. > > You can always wrap around a "try" to force the alternative: > > parseTest (try (la >> char 'a') <|> char 'b') "b" > > Cheers Christian > > Maybe it should have been: > > la >> (char 'a' <|> char 'b') > > in the first place.

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Tips for deployment?
by Günther Schmidt 20 Aug '09

20 Aug '09

Hi all, my haskell app is getting closer to shipping and what I need to do now is to give product protection some thought. The product is for hospitals and therefore a very limited set of clients. I do not expect anyone to try to "hack" my application but I need a basic protection mechanism in place, if only to check for a valid license key. As I've never done this before I'd be grateful for some suggestions here. Günther

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Right way to implement setPixel function
by CK Kashyap 20 Aug '09

20 Aug '09

Hi, I had posted a note on line drawing algo with Haskell some time back. Now, I am trying to write a PNM image. import qualified Data.ByteString as B width = 256 height = 256 bytesInImage = width * height * 3 blankImage = B.pack $ take bytesInImage (repeat 0) type Color = (Int,Int,Int) setPixel :: B.ByteString -> Int -> Int -> Color -> B.ByteString setPixel image x y (r,g,b) = B.concat [beforePixel, pixel, afterPixel] where beforePixel = B.take before image afterPixel = B.drop (before+3) image pixel=B.pack [(fromIntegral r),(fromIntegral g),(fromIntegral b)] -- number of bytes before the 3 bytes of -- the pixel at x y before = (y * width * 3) + (x * 3) - 3 main = do putStrLn "P6" putStrLn ( (show width) ++ " " ++ (show height) ) putStrLn "255" -- Set a red pixel at 100 100 B.putStr (setPixel blankImage 100 100 (255,0,0)) Can I please have some review comments on the code above? Would recreating the entire ByteString for each setPixel be an overhead? Also, I am barely beginning to grasp the Monad concept....I was wondering if there could be a monadic style of implementation of this - that could potentially have a series of setPixels inside a do block? Regards, Kashyap

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Haddock: Failed to create dependency graph (when adding sections with * or a module heading)
by Jared Updike 20 Aug '09

20 Aug '09

I compiled and installed haddock-2.4.2 from the tarball source. Adding a few simple comments to the code here: https://dl.getdropbox.com/u/143480/doc/DualMap.hs and running haddock $ haddock -h -o doc Data/DualMap.hs Warning: Data.DualMap: could not find link destinations for: Data.Typeable.Typeable2 GHC.Base.Eq GHC.Show.Show GHC.Base.Ord GHC.Base.Bool Data.Set.Set yields: https://dl.getdropbox.com/u/143480/doc/Data-DualMap.html Things look good. (Note that this module only depends on libs that ship with GHC and no other source modules.) However, when I try to add sections (a la http://www.haskell.org/haddock/doc/html/ch03s04.html#id289234 ) in the comments with "-- * test" I get: $ haddock -h -o doc Data/DualMap.hs Data/DualMap.hs:20:0: parse error on input `-- * test' haddock: Failed to create dependency graph I have no idea where to begin getting this to work since this error message only tells me that Haddock.Interface.depanal returned Nothing (according to a grep of the haddock sources) but not how to stop the dependency analysis from failing. Perhaps I need some more command line arguments or references to missing link destinations in GHC/base/containers documentation or some haddock config file? Searching Google yielded plenty of cabal build errors of the same ilk for packages on hackage but nothing about how to fix them. Jared.

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Can't derive Binary for StdGen
by Grigory Sarnitskiy 20 Aug '09

20 Aug '09

Hello! I'm trying to derive Binary for StdGen with DrIFT: module Main where import System.Random import Data.Binary {-!for StdGen derive : Binary !-} data Foo = Foo StdGen StdGen deriving (Show) {-! derive : Binary !-} but I got error "DrIFT: can't find module System/Random" What shall I do?

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generalize RecordPuns and RecordWildCards to work with qualified names?
by Evan Laforge 20 Aug '09

20 Aug '09

Record punning is not all that useful with qualified module names. If I write '(M.Record { M.rec_x })' it says " Qualified variable in pattern" and if I write '(M.Record { rec_x })' it says 'Not in scope: `rec_x''. Could it be this extension be further extended slightly so that 'f (M.Record { M.rec_x })' will desugar to 'f (M.Record { M.rec_x = rec_x })'? Similarly, RecordWildCards could support this too. It seems simple and useful to me... am I missing anything fatally problematic about this? Would anyone else use it?

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gbp sign showing as unknown character by GHC
by Iain Barnett 20 Aug '09

20 Aug '09

Quick question: I've tested this in a couple of different terminals (roxterm and xterm), so I'm fairly sure it's GHC that's the problem. Have I missed a setting? GHCi, version 6.10.4 Prelude> putStrLn "£" � Hugs98 200609-3 Hugs> putStrLn "£" £ I get the same character output from a password generator I've writtern, after compilation with GHC [iainb]$ ./makepass2 50 2 >> testfile.txt [iainb]$ cat testfile.txt H(xW!:maNyxZ;h,IW=Uu4G$ztc>k@Q[g6?y:�TbG&5Nd")+"5+ Iain

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ANNOUNCE: ministg-0.2, an interpreter for STG operational semantics
by Bernie Pope 20 Aug '09

20 Aug '09

I'm pleased to announce the first public release of Ministg. Ministg is an interpreter for a high-level, small-step, operational semantics for the STG machine. The STG machine is the abstract machine at the core of GHC. The operational semantics used in Ministg is taken from the paper "Making a fast curry: push/enter vs. eval/apply for higher-order languages" by Simon Marlow and Simon Peyton Jones. Ministg implements both sets of evaluation rules from the paper. One of the main features of Ministg is the ability to record a trace of the execution steps as a sequence of html files. And example trace can be viewed here: http://www.cs.mu.oz.au/~bjpop/trace/step0.html The example shows the execution of a program which sums a list of three integers, using the well-known space-leaky version of sum. Follow the "next" and "previous" links to step forwards and backwards through the trace. The main reason I wrote Ministg is to explore various extensions to the STG machine. The current release features a simple extension in the form of call-stack tracing, which is strongly influenced by the cost-centre stacks of GHC. I expect it will also be a useful tool for people who are interested in learning more about the STG machine. More detailed information is available from the haskellwiki page: http://www.haskell.org/haskellwiki/Ministg You can download it from hackagedb: http://hackage.haskell.org/package/ministg Or you can get the latest code from patch-tag: darcs get http://patch-tag.com/r/ministg/pullrepo ministg Cheers, Bernie.

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