| World.hs:42:0: | No instances for (Eq (a (M.Map String Player)), | Eq (a (M.Map ItemId Item)), | Eq (a (M.Map PlayerId Player)), | Eq (a (M.Map RoomId Room)), | Eq (a RoomId)) | arising from the 'deriving' clause of a data type declaration | at World.hs:(42,0)-(50,14) Yes, automatic 'deriving' in GHC became a little more restrictive in GHC 6.8 (I think), for very good reasons. I think that's what your problem is. The darcs repo http://cdsmith.twu.net/demos/mud seems to be offline today, so I can't look, but I think that's highly likely. The solution is invariably to add a manual instance declaration, as you have done in your subsequent message. Simon