gtk2hs

[Duncan Coutts]

On Fri, 13 Jun 2003 19:39:53 +0200
Christian Buschmann <white_russian@gmx.net> wrote:

> 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