Hi Haskell'ers, I'm trying, more as a first excercise in Haskell than anything else, to code a simulation framework in Haskell. A simulation is a bunch of simulation state consisting of constituents (e.g. physical entities like a ball or properties like temperature), on which agents (e.g. simulated humans) in the system can do activities. Activities have a certain duration, and define how the simulation progresses. The simulator framework should just define the basics to maintain a normal representation of time and of cause and effect, i.e. make sure that the state evolves through the application of the activities in the right order. In a later step I will add "laws", which transform activities when they overlap or conflict (e.g. two drive activities leading to collision). I've run into the following problem. I defined a class that allows to define activities on constituents: class Activity a c where start :: c -> a -> Time --start of the activity (this isn't actually dependent on c, I guess) end :: c -> a -> Time --end of the activity delta :: a -> Time -> c -> c --how the constituent is changed at the given time Two parameter type class because some activities are only applicable to certain constituents: instance Activity Paint Ball where instance Activity Move Ball where instance Activity Paint Wall where but you can't move a wall for example, so no instance for Move Wall. My question is: How can I now represent the state of the simulation (i.e. all activites on all constituents). E.g. a list of activities won't do since the list is heterogeneous (i.e. [Paint Ball White, Move Ball (2,0)]) I know about existentials, but I'm at a loss at how to implement the "wrapper" datatype that is exemplified on http://www.haskell.org/hawiki/ExistentialTypes since my class has two parameters, in fact I'm at a loss at how to use existentials here completely. Then, how could I go back from the "general" lists (or whatever datatype) of [a]'s and [c]'s, to a list of [([a],c)] of all activities a that are applicable to a certain constituent c? I can't seem to be able to use the Typeable class for example, since this can not "cast" to typeclasses, only to concrete types (I think...). What I initially liked about this idea is that it can be encoded in the typesystem which activities apply to which consituents, but now it seems like I will have to encode it in the simulation framework more directly (i.e. giving each consituent a String name to encode its "type"). More straightforward ways of modelling this problem (avoiding multiple type class parameters and existentials :) )are also welcome. thanks for any pointers, Kurt