On Fri, 13 Jun 2003 19:39:53 +0200
Christian Buschmann
I tried to implement a simple counter with gtk2hs. To increase my counter I defined following event:
onClicked btn $ do si <- textBufferGetStartIter tb ei <- textBufferGetEndIter tb val <- textBufferGetText tb si ei False textBufferSetText tb $ show $ (+1) $ read val
But now I don't want always read the textfield to get my counter-value, I would like to save the state of my increased value and would like to pass it to the next call of the function. How can I do this?
Use an IORef or an MVar eg. (untested code) let initialCounterValue = 0 counter <- newIORef initialCounterValue btn `onClicked` do counterValue <- readIORef counter tb `textBufferSetText` show (counterValue + 1) writeIORef counter (counterValue + 1) or with an MVar: let initialCounterValue = 0 counter <- newMVar initialCounterValue btn `onClicked` do modifyMVar_ counter (\counter -> do tb `textBufferSetText` show (counterValue + 1) return (counterValue + 1)) The second version is threadsafe. There are more elegant techniques that hide the evil imperitive state (the IORef or MVar) but this is the easiest technique to start with. Duncan