Left-bias and non-structural equality.

[Jean-Philippe Bernardy]

On 1/4/06, Christian Maeder <maeder at tzi.de> wrote:
> Adrian Hey wrote:
> > So I vote we drop the left/right biasing distinction. This way lies
> > madness (as they say:-)
>
> I would not like to drop biasing distinction, as I don't think this
> costs too much. With biasing, maps could be implemented via "biased
> sets" (Set (MapEntry a b)):
>
> data MapEntry a b = a := b
>
> instance Eq/Ord a => Eq/Ord (MapEntry a b) where
>     compare (a1 := _) (a2 := _) = compare a1 a2

I would discourage such a use. This is exaclty why we provide Maps, after all.

> > If there's some good reason why we should care then the corresponding
> > type should not be an instance of Ord. So what should be done in cases
> > like this? It's tempting to think we should add HOF versions like
> > this..
> >
> > insertUsing :: (a -> a -> COrdering a) -> a -> Set a -> Set a
>
> It may also be possible to pass an order to "empty" and use it further
> on. The major problem is if varying orders are used ie. for "union".
>
> By good, bad or earlier design Data.Set/Map relies on Ord instances and
> I'm quite content with it.

A "good" solution would require dependent types or its type-classes
emulation, but I think this is overkill.

Cheers,
JP.