On Monday 11 October 2010 18:19:24, Tom Hobbs wrote:

Hi all,

I playing around with simple cell automaton (Conway's Game Of Life). I've got working code which prints everything out nicely to the terminal, I'm trying to wrap everything in a nice GUI using Glade and Gtk2Hs.

In order to create my world, I have a function defined as;

makeWorld :: Int -> [(Int,Int)]

Which accepts an Int, representing the size of the world, then then outs the world itself - note the world is always assumed to be square.  The list contains pairs such that the first element of the pair is the index (such as for an array) and the second is the value at that position.  For right or wrong, that's how my (working) Game Of Life has been implemented.  The problem comes in the Gtk2Hs code...

main = do   initGUI   Just xml <- xmlNew "gameOfLife.glade"   window <- xmlGetWidget xml castToWindow "mainWindow"   onDestroy window mainQuit   quitButton <- xmlGetWidget xml castToButton "quitButton"   onClicked quitButton $ do     widgetDestroy window   setWorldSize <- xmlGetWidget xml castToButton "setWorldSize"   sizeBox <- xmlGetWidget xml castToEntry "worldSize"   onClicked setWorldSize $ do     canvas <- xmlGetWidget xml castToDrawingArea "world"     let size = (read (get sizeBox entryText) :: Int)     cellworld <- makeWorld size     printWorld cellworld   widgetShowAll window   mainGUI

The above function does not compile under GHC because;

gameOfLife.hs:27:20: Couldn't match expected type `String' against inferred type `IO String' In the first argument of `read', namely `(get sizeBox entryText)' In the expression: (read (get sizeBox entryText) :: Int) In the definition of `size': size = (read (get sizeBox entryText) :: Int)

get sizeBox entryText is an IO-action which, when executed gets the entry-text from sizeBox. The type is IO String. What you want is the entry-text, hence the result of the IO-action, so you have to run that action and bind its result to a name, for example text <- get sizeBox entryText let size = read text or, on one line size <- fmap read (get sizeBox entryText)

gameOfLife.hs:28:2: Couldn't match expected type `[(Int, Int)]' against inferred type `IO (Int, Int)' In a stmt of a 'do' expression: cellworld <- makeWorld size In the second argument of `($)', namely `do { canvas <- xmlGetWidget xml castToDrawingArea "world"; let size = (read (get sizeBox entryText) :: Int); cellworld <- makeWorld size; printWorld cellworld }' In a stmt of a 'do' expression: onClicked setWorldSize $ do { canvas <- xmlGetWidget xml castToDrawingArea "world"; let size = (read (get sizeBox entryText) :: Int); cellworld <- makeWorld size; printWorld cellworld }

makeWorld size, on the other hand is not an IO-action but a list (of (Int,Int) pairs), so you can't use "<-" to bind it to a name in an IO- block, here you must use let: let cellworld = makeWorld size print cellWorld

It seems to be complaining about the call to my makeWorld function, although I do not understand why that should be. The line number which GHC complains about does correspond to the "cellworld <- makeWorld size" line in my main function.

I've not yet worked out how to draw my world in the GUI, the "printWorld" function mentioned above just spills the world out to the termainal for now.

Does anyone have any ideas?

Many thanks,

Tom