[Haskell-cafe] Map unionWith generalization

[Hans Aberg]

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