[Haskell-cafe] ANN: signed-multiset-0.1

[Stefan Holdermans]

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