Re: Proposal: more general unionWith for Data.Map

Johan Tibell schrieb:

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.

Sounds better than my 'unionUpdate'.