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
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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