Hello Michael I think I put my point forward with too little nuance, my point wasn't that existentials aren't useful[*] (I did call them inert rather then useless...), but that they aren't the rosetta stone for getting to flexible designs. They do solve the original posters problem of putting objects different types in a list, but with the expense that they need heavy use of classes afterwards to be able to manipulate them. For the original problem, dynamic typing with Data.Typeable would probably be a better solution as it avoids annotating the existential Obj type with every class the simulation needs. Best wishes Stephen [*] I think with just existentials you can get dynamic types - Data.Typeable itself use a couple of other tricks (unsafe coerce), see "Typing Dynamic Typing" Arthur I. Baars and S. Doaitse Swierstra or "A Lightweight Implementation of Generics and Dynamics" James Cheney and Ralf Hinze.