I haven't yet, but I saw someone had a simple MUD-like game engine a few years ago.  I don't know if Frag supports network play, but I would assume not.  David Roundy (original author of Darcs) wrote something for bridge years ago, but I don't know if he wrote it in Haskell or if it's still available.

If I had infinite free time it would be a fun project!  Any particular reason why you're interested?

Jason