hey David, heres a simpler version (attached below), that I think
accomplishes the same goal.
One issue, that would need to be evaluated empirically: how many type class
instances would break from this change? Would any?
is it Storable instances that are an issue, or things that use storable?
what safety is gained vs what is impacted? (i guess i mostly just wanna
understand what would be broken by this change, a lot of code in the wild
uses pointers for systems integrations)
newtype Ptr a = Ptr Addr#
type role Ptr nominal
castPtr :: Ptr a -> Ptr b
castPtr = unsafeCoerce
ptrCoercible
:: ((forall a b. Coercible (Ptr a) (Ptr b)) => r)
-> r
ptrCoercible r = r
ptrCoercion :: Coercion (Ptr a) (Ptr b)
ptrCoercion = Coercion
On Tue, Oct 30, 2018 at 1:57 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