[GHC] #9163: Ptr should have a phantom role

[GHC]

#9163: Ptr should have a phantom role
-------------------------------------+------------------------------------
        Reporter:  simonpj           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by rwbarton):

 +1 from me on making `Ptr`'s role phantom. But especially I don't think it
 should be ''exactly'' representational; either phantom or nominal must be
 better. A `Ptr a` is not actually a pointer to a Haskell heap object of
 type `a`, for any type `a`. Neither the representation of the `Ptr a`
 itself nor the representation of the thing-being-pointed-to depends on the
 ''representation'' of `a` specifically. I might define a `newtype
 BigEndianInt = BigEndianInt Int` and write a `Storable` instance that uses
 a big-endian format when `peek`ing or `poke`ing. Then it isn't any more
 valid to cast from `Ptr Int` to `Ptr BigEndianInt` than it is to cast from
 `Ptr Int` to `Ptr Double`.

 Similar comments probably apply to Austin's hypothetical example of an
 inferred phantom role. If the library author does not want `Bar` to have a
 phantom role, they probably do not want it to have a representational role
 either. But it's hard to say without a more fledged-out example.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9163#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler