Re: Proposal: give Ptr a nominal role

woops: i mis stated my previous email, misread some stuff, please ignore it :) On Fri, Nov 2, 2018 at 7:25 PM Carter Schonwald 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@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 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@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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