default roles

[Edward Kmett]

Wait, that sounds like it induces bad semantics.

Can't we use that as yet another way to attack the sanctity of Set?

class Ord a => Foo a where
  badInsert :: a -> Set a -> Set a

instance Foo Int where
  badInsert = insert

newtype Bar = Bar Int deriving (Eq,Foo)

instance Ord Bar where
  compare (Bar x) (Bar y) = compare y x

Now you can badInsert into a Set.

If that is still in play then even with all the roles machinery then GND
doesn't pass the restrictions of "SafeHaskell". =(

-Edward


On Thu, Oct 10, 2013 at 9:52 PM, Richard Eisenberg <eir at cis.upenn.edu>wrote:

>
> On Oct 10, 2013, at 1:14 PM, David Menendez wrote:
> > Sure, but if op uses show internally, we get Int's show, not Age's,
> right? That seems correct, in that it's doing what GND is supposed to do,
> but I'll bet it will surprise people.
>
> Yes, you're right. If a method in a subclass uses a superclass method, it
> uses the base type's instance's method, not the newtype's. Very weird, but
> I guess it makes sense in its own way. This does show how GND can create
> instance incoherence even without storing dictionaries in datatypes.
>
> Richard
>
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