I need ideally some generalizations of unionWith and unionWithKey, for efficiency matters (i.e. avoiding conversions and traversing the maps more than once). I could use a modification of the code in Map.hs, but then the problem is that the module Map interface does not export the constructors of data Map. So suggestions are welcome. For example, in Map String Integer (sparse representation of monomials) compute the minimum value of all associative pairs with the same key (the gcd); if only one key is present, the absent should be treated as having value 0. So unionWith min xs ys will not work, because unionWith will always apply the identity to the remaining value when one key is missing, whereas it should be sent to 0. So here, one would want: (a -> c) -> (b -> c) -> (a -> b -> c) -> Map k a -> Map k b -> Map k c where the two first functions are applied when the first or second key is missing. Hans