Is union really an appropriate name here? I expect the following to hold:
forall k ∊ (keys m1) ∪ (keys m2) => k ∊ (m1 ∪ m2)
This means that keys must not be deleted by the union operator. Perhaps 'merge' is a better name.
Cheers, Johan
I don't care much about the naming, but note that the analogous property already fails for the generalized intersectionWithKey in the development version.
It is true that forall k ∊ (keys m1) ∩ (keys m2) => k ∊ (m1 ∩ m2) fails, but this expression is not really correct for intersectionWithKey, as it intersects Map k a and Map k b, so Map k a ∩ Map k b is not well defined. Anyway, we could instead of proposed unionWithKey offer functions mergeWith :: Ord k => (Maybe a -> Maybe b -> Maybe c) -> Map k a -> Map k b -> Map k c mergeWithKey :: Ord k => (k -> Maybe a -> Maybe b -> Maybe c) -> Map k a -> Map k b -> Map k c The combining function is executed for every unique key from both maps. All functions unionWith, intersectionWith, differencseWith, proposed unionWith can be expressed using this function. But I am not sure about the performance (using so many Maybes). Cheers, Milan