DData revision

[JP Bernardy]

--- Ketil Malde <ketil+haskell at ii.uib.no> wrote:

> I think you're saying we might need a bag to hold
> items where we want
> to group things according to (equivalence on) some
> arbitrary key?
> Perhaps we could have the bag datatype parametrised
> over a
> user-supplied ordering, so that
> 
>         Set a  
> 
> would be equivalent to a
> 
>         Bag compare a (with compare :: a -> a ->
> Ordering)
> 
> The bag could then hold regular (Eq-based) Sets of
> items.
> 
> Would this make sense?  Would it be useful?

I'm not sure that I understand either :)

Let me give an example...

> type MyString = MyString String 
>
> instance Eq MyString where
>   (MyString x) == (MyString y) = length x == length
y 
>
>  ms = MyString


> Bag.toString $ Bag.fromString $ [ms "x", ms "y"]

should return the original list, [ms "x", ms "y"].

> MultiSet.toString $ MultiSet.fromString $ [ms "x",
ms "y"]

will return [ms "x", ms "x"].

That is why I choose to keep the "MultiSet" name. 
If Eq is defined as structural equality, MultiSet and
Bag are equivalent.

Cheers,
JP.



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