Making `Ptr` (and ForeignPtr) have a nominal role would have prevented
recent bugs where it was possible to coerce Vectors between any types.
See: https://github.com/haskell/vector/pull/224
and: https://phabricator.haskell.org/D4941
On Tue, Oct 30, 2018 at 5:57 PM David Feuer
Currently, we have
data Ptr a = Ptr Addr# type role Ptr phantom
This is weird: accidentally coercing a pointer to a different type is very bad. The only reason Ptr has this role is that without it, castPtr and such may not be free or will involve unsafe coercions.
Thankfully, we have enough power to fix this now.
data Addr = Ptr_ Addr#
newtype Ptr a = Ptr_ Addr type role Ptr nominal
pattern Ptr :: Addr# -> Ptr a pattern Ptr a# = Ptr_ (Addr a#)
castPtr :: Ptr a -> Ptr b castPtr (Ptr a) = Ptr a
ptrCoercible :: ((forall a b. Coercible (Ptr a) (Ptr b)) => r) -> r ptrCoercible r = r
ptrCoercion :: Coercion (Ptr a) (Ptr b) ptrCoercion = Coercion
I propose that we do this. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries