Sjoerd,
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?
If max' is supposed to implement the maximum of two nonzero values, I wouldn't be the slightest bit concerned. Seriously: if this is what people have agreed on to be a sensible semantics for hybrid sets, I am fine implementing it like this.
*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?
Of course it does; it pinpoints a bug in multiply. It's fixed now: *Data.SignedMultiset> let empty' = multiply 0 $ delete () empty *Data.SignedMultiset> empty' `union` delete () empty == empty' False *Data.SignedMultiset> empty `union` delete () empty == delete () empty True Cheers, Stefan