Proposal: give Ptr a nominal role

[Carter Schonwald]

https://ghc.haskell.org/trac/ghc/ticket/9163 seems to talk about *why* Ptr
being phantom actually has a pretty measurable impact on code. (cast ptr
becomes a noop and this has a measurable impact on a number of application
measures)

On Fri, Nov 2, 2018 at 7:27 PM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> 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.
>>> > _______________________________________________
>>> > Libraries mailing list
>>> > Libraries at haskell.org
>>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20181102/0819e616/attachment.html>