Nick Name wrote:
Not if you want to support multiple callbacks for each event. In that case, you need to be able to identify a specific callback, which gets us back to the issue of equality of function types.
Unfortunately, the easiest solution (require the application to provide a FunPtr, as that's what's going to be passed to the lower levels) is also the ugliest.
I am perhaps missing the point.
The point is that if: a) you want multiple callback procedures for a single event, and b) you want to be able to de-register individual callbacks, then there has to be some way of specifying *which* callback procedure you wish to de-register. Given that functions (or plain IO actions, in the case where the callback doesn't take parameters) don't implement Eq, you can't just pass the expression which was passed to the registration function. It doesn't matter whether the underlying toolkit supports multiple callbacks directly or whether you emulate them; the problem is in deciding upon the Haskell interface.
you can create the FunPtr
or else, of course, to be portable, you can assign an unique integer to any registered callback, wich is then used to unregister it.
AFAICT, if you're going to have the registration function return some
kind of entity to enable a specific procedure to be de-registered, you
may as well just return a "canned" IO action.
--
Glynn Clements