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

[GHC]

#9163: Ptr should have a phantom role
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        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:
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Comment (by simonmar):

 So I've been assuming that `castPtr` was free all this time, and it turns
 out it isn't!  Please make it free.

 We can't change the type of `hPutBuf` and friends without potentially
 breaking code.  There's no better choice than `Ptr a` here, because we
 don't know the type of the underlying data.  Making it `Ptr Word8` would
 just force the caller to use `castPtr` sometimes without any benefit. e.g.
 `Ptr CChar` is also a sensible choice, but it could in reality be
 anything.

 It does look like the two `castPtr`s in `bufWrite` are unnecessary,
 though.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9163#comment:13>
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