Nick wrote (snipped)
The true problem here is that once you got an Event, you never uninstall the callback, even if you just take the first n events and then stop using the stream. This can be really expensive; do you use finalizers to uninstall the callback?
Well there are two approaches: (1) you provide a more complicated function which in addition to the new event, returns an IO () action which unbinds it. This is what HTk does, in general, though for many simple examples it is of course not required. (2) in many but not all cases it is possible to work out that the stream can be automatically deregistered. For example, as well as finalizers you can also forget the stream when the object is destroyed. Of course this isn't anything particular to do with events; a callback model also has to have a way of deinstalling the callback.