Proposal: give Ptr a nominal role

[Carter Schonwald]

woops:

i mis stated my previous email, misread some stuff, please ignore it :)

On Fri, Nov 2, 2018 at 7:25 PM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> my understanding is ANY non phantom role suffices in this case ..
>
> i dont use the coercible class in this context, my main concern is how
> role selection will impact client type classes / GND / etc
>
> On Fri, Nov 2, 2018 at 6:34 AM Matthew Pickering <
> matthewtpickering at gmail.com> wrote:
>
>> 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 <david.feuer at gmail.com>
>> wrote:
>> >
>> > 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.
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