One of mind-opening ideas behind Functional Reactive Programming is to have an event handling function producing BOTH reaction to events AND the next event handling function. Thus an evolving system is represented as a sequence of event handling functions.
For me, learning of Yampa became a crucial poing to get that functions-producing-functions thing properly. There are some nice papers about Yampa. I recommend The Yampa Arcade:
http://www.cs.nott.ac.uk/~nhn/Talks/HW2003-YampaArcade.pdf (slides, PDF)
http://www.cs.nott.ac.uk/~nhn/Publications/hw2003.pdf (full article, PDF)
There is a wiki page on Yampa at Haskell.org
http://www.haskell.org/haskellwiki/Yampa
Original Yampa home page:
http://www.haskell.org/yampa (unfortunately is broken at the moment)
