Hi, I've been thinking about two things for some time now: 1. Sharpening my haskell 2. Implementing a MUD server I've been thinking about doing (1) and (2) at the same time but I've run into some problems with data structures. I need to be able to get a list of players from a room (for things like a 'look' command) and I need to be able to find which room contains a given player. I'm still looking at this problem through an imperative lens, so here's some sort-of-C: struct room{ ... struct player * players; ... }; struct player{ ... struct room * room; ... }; From what I can see, I'd use a record type for players and rooms, but I'm not sure how to replicate the pointer effects: keeping each player pointing to a single instance of a consistent world and maintaining the invariant that (given: p :: Player, r :: Room): p `elem` (Room.players r) => (Player.room p == r) This needs to stand up to concurrent modification of a shared world structure, but I think I'll set up the concurrency controls after I get my head around this. -- Jack