> Designers of FiniteMap and Set (it's me again), > > I wish "keysFM" would yield "Set key" rather than "[key]"! > > Could such an implementation take constant time (by simply ignoring > "elts")? IIRC a Set key is simply a FiniteMap key key. So what you require is simply a mapFM (\k v -> k). Correct me if I'm wrong... --KW 8-)