some algorithms are hard to code in O(n) in Haskell

Hi - does anybody know how to do a term unification in Haskell in O(n), where n is length of the input terms? I know Montanari-Martelli algorithm for this, but I'm unable to code it in Haskell (in O(n)). I would need a destructive update of the directed graph representing terms (to do a substitution) and checking for cycles in directed graphs (these cycles emerge when unifying things like x=f(x) -- their detection is also called occurs-check). I've been trying hard but still have no solution. Maybe I'm not smart enough, maybe I don't know some trick, maybe it's impossible in Haskell. Thanks, _dave_