WDYT about using RULES to rewrite to nubOrd if an Ord context is available, as John Meacham mentioned?
John: you said you were uneasy about changing the complexity of an algorithm using RULES, but this is exactly what list fusion does (albeit for space, not time).
Indeed. and I am a little uneasy about that too :) not that I don't think it is an excellent idea and reap the benefits of fast list operations.. O(n^2) to O(n log n) just feels like a bigger jump to me for some reason. I think in the end, the RULES are a good idea, but I personally try to make which one I am using explicit when possible. Hence making my ideas as to the time/space requirements for an operation to both the compiler and a future reader of the code
Using RULES in this way could be a pessimization. I can't imagine a nubOrd that would ever be as performant as nub on very small data sets (two or three elements). --Lane