On Tue, May 20, 2008 at 09:15:57AM +0100, Yann Golanski wrote:
To help me learn Haskell, I decided on a simple (AH!) problem: given a list of images, display a random one as my desktop backdrop. After some time, change the image. Simple?
What I actually want to do is a little more specific: Read a list of images (one per line) from a file. Given that a working day is about 8 hours, I want to see all the images during the day. So, the time between changes should be (nbr_of_images) / (8 * 60 * 60) seconds. Of course, if the file changes (I add or remove any number of images) this need to change and be recalculated. Clearly, I want some interaction with a pseudo-random number generator.
Because this is a learning exercise, I want to have a pretty GUI for this. Three buttons: Exit (which quits the application), Reset (re-reads the file whether it changed or not) and Next (display the next image). Then I want a counter and a progress bar telling me when the next change will occur.
1- Get a list out of a file: I managed to do that using the following:
parseImageFile :: FilePath -> IO [String] parseImageFile file = do inpStr <- readFile file return $ filter (/="") (breaks (=='\n') inpStr)
Nice, simple and I understand what it is doing.
2- Get a random element from a list and remove it: Okay, this I understand less well. I looked at the solutions of problems 23 and 20 in http://www.haskell.org/haskellwiki/99_questions so there is a skeleton there. However, my list is IO [String].... Hum, monads.
Any pointers as to how to do that?
3- Wait and do something later.... How, I have no idea how to do that! Help?
One way is System.Concurrent.threadDelay, though there might be another way that integrates more nicely with wxHaskell. Hopefully you can find it in their documentation. http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurren...
4- I guess that progress bars and updating text will be somewhere in the GUI (I chose wxHaskell)... Again, no idea where.
5- How do you call an external program in Haskell? Either xv or Esetroot will do the job of displaying the image. Is there a Haskell native way to do that?
Try the System.Cmd.system function. http://www.haskell.org/ghc/docs/latest/html/libraries/process/System-Cmd.htm...
Once this is done and I have commented to the code, I will be happy to put it onto the wiki as a teaching aid.
Thanks.
-- yann@kierun.org -= H+ =- www.kierun.org PGP: 009D 7287 C4A7 FD4F 1680 06E4 F751 7006 9DE2 6318
Cheers, Spencer Janssen