Proposal: more general unionWith for Data.Map

2012/1/24 Johan Tibell

On Tue, Jan 24, 2012 at 9:35 AM, Christian Sattler wrote:

...

There are some high-level operations on maps which take two tree traversals only because the interface fails to expose sufficiently general functions. This proposal is concerned with an analogue of unionWithKey of type Ord k => (k -> a -> a -> Maybe a) -> Map k a -> Map k a -> Map k a, with the intended semantics that if a key is present in both maps and the operation applied to the key and corresponding values returns Nothing, the key is deleted from the result.

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.