Sjoerd,
This is not just about map, but it also a problem for the Monoid instance. You are basically adding an extra identity element, 0, to the max monoid, which works but is weird.
Still that's how union is typically defined for hybrid sets. It's what happens if want union and empty to behave as generalisations of these concepts for ordinary (multi)sets.
Then why would you want that?
You don't have to. (SignedMultiset a, additiveUnion, empty) gives you the Monoid that you seem to have a preference for. The library supplies it through the Additive wrapper. The point is that you have a choice: different applications may ask for different monoidal structures.
*Data.SignedMultiset> let empty' = insert () $ delete () empty
*Data.SignedMultiset> empty' `union` delete () empty == empty' False
*Data.SignedMultiset> empty `union` delete () empty == delete () empty True
Ah, I missed the check in insertMany.
What about the same with
let empty' = multiply 0 $ delete () empty
*Data.SignedMultiset> let empty' = multiply 0 $ delete () empty *Data.SignedMultiset> empty' `union` delete () empty == empty' True *Data.SignedMultiset> empty `union` delete () empty == delete () empty True Cheers, Stefan