On Apr 23, 2012, at 4:34 PM, Stefan Holdermans wrote:
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.
Agreed. But I just can't imagine that the other instance is in any way useful. You basically define a function max': max' :: Int -> Int -> Int max' 0 b = b max' a 0 = a max' a b = max a b i.e. max' -2 -1 = -1 max' -2 0 = -2 max' -2 1 = 1 Wouldn't you agree that if you saw this defined in some code, you'd think something is wrong?
*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
And this doesn't bother you? greetings, Sjoerd