Hi Corentin It looks like you are writing the event handler on the server side. If so, the range of events you can handle is fixed to just those you implement handlers for - having an openly extensible event type is useless if this is the case. Ignoring client/server for a moment, a function (State -> State) would be the most "extensible" API you could allow for clients. You don't need to worry about an open set of Event types, a client knows the state exactly and doesn't need extensibility. Client/Server operation won't allow a state transformer API as Haskell can't readily serialize functions. But you can implement a "command language" enumerating state changing operations. The second Quickcheck paper gives a very good example of how to implement such a command language. Testing Monadic Code with QuickCheck (2002) Koen Claessen , John Hughes www.cse.chalmers.se/~rjmh/Papers/QuickCheckST.ps Or Citeseer if you need a PDF: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.9275