I was playing with some code for compiling regular expressions to finite state machines and I ran into the following problem. I've solved it, but I'm not terribly happy with my solution and was wondering if someone could come up with a better one. :-) Essentially I have data FSM = State { transitions :: (Map Char FSM) } and transitions' :: Regexp -> Map Char Regexp I want to lift this so that the Regexps become states of the finite state machine (while making sure I set up a loop in the data structure). Tying the knot is the traditional way of doing such things, but we couldn't figure out a way to make it work without the set of keys known in advance because of the strictness of Map in its keys (an association list was suggested, and that would probably work, but it seemed a bit ugly and would be fairly inefficient). In the end what I did was just work out the set of reachable regexps in advance and use a standard tying the knot trick, but it felt vaguely unsatisfactory (and does some repeat work which I felt should be unneccessary). Anyone have a more elegant suggestion? Regards, David