I assume everybody is on holiday, just a mail to keep the discussion on the core functionality going. So far it seems that we would like the following features: a) mandatory arguments b) read-only arguments c) write-only arguments d) setting a property and getting a result I guess the general outline will be that of Daan's posting. Regarding the unregister callback issue, I thought of the following: Maybe we can have the set function just taking one argument: set button $ on click =: messageBox "hi there" so that we need to use a special mapSet for several properties mapSet button [label =: "Open Dialogs", on click =: messageBox "hi there"] This enables each property we set to return something as well. In case of attaching a callback, we could return the unregister function: unReg <- set button $ on click =: messageBox "hi there" What do you guys think? Axel.