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 <jvran...@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_kash...@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@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >
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