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

[GHC]

#9163: Ptr should have a phantom role
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  7.10.1
       Component:  Compiler          |              Version:  7.8.2
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  roles/should_compile/Roles2
      Blocked By:                    |             Blocking:
 Related Tickets:  #9164             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 goldfire, this gets back to what we discussed in Philly. What we *really*
 want is for `Ptr` to be defined with a phantom argument, but exported with
 a representational one.

 We can work around the trouble today using a newtype:

 {{{#!hs
 data Addr = Addr Addr#
 newtype Ptr a = Ptr_ Addr
 type role Ptr representational

 pattern Ptr :: Addr# -> Ptr a
 pattern Ptr addr# = Ptr_ (Addr addr#)

 castPtr :: Ptr a -> Ptr b
 castPtr = coerce

 ptrCoercion :: Coercion (Ptr a) (Ptr b)
 ptrCoercion = Coercion
 }}}

 The only trouble I see is that the `Generic` representation will change. I
 kind of doubt anyone's relying on the details of that representation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9163#comment:38>
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