Lemmih wrote:
On 12/4/06, Thorsten Seitz <thorsten.seitz@web.de> wrote:
Hello,
I'm very intrigued by the concepts of STM but I'm having difficulties to see how a TVar based model can be used by a GUI application (e.g. Gtk2hs):
For example, I'd like to update the GUI (e.g. changing widget sensitivity) upon changes in the model. But changing the sensitivity of a widget is an IO operation which cannot be initiated atomically by a model change which is an STM operation. Therefore the GUI change cannot be executed atomically together with the model change. This means for example that I can still press a button although the model has changed such that the button action is not allowed anymore. All I can do is check the condition of the model when the button is pressed and then ignore the button press which is not very satisfying for the user.
Is it simply not appropriate to use TVar based models in a GUI application? Maybe STM should be more used like a service for MVar based GUI models?
How about making a list of IO actions to be executed when the transaction commits?
That onCommit list and its execution can be done by using the slightly modified/enhanced/wrapped AdvSTM monad. A complex example that also can queue actions on retry is at http://haskell.org/haskellwiki/?title=New_monads/MonadAdvSTM#Helper_Thread_C... But for just onCommit the simpler code at http://haskell.org/haskellwiki/New_monads/MonadAdvSTM#Just_onCommit should work. Cheers, Chris