[GHC] #9220: type roles for unboxed arrays

[GHC]

#9220: type roles for unboxed arrays
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       Reporter:  rwbarton           |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  libraries (other)  |           Version:  7.8.1
       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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 {{{
 rwbarton at morphism:~$ ghci
 GHCi, version 7.8.1: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 Prelude> :i Data.Array.Unboxed.UArray
 type role Data.Array.Base.UArray representational phantom
 data Data.Array.Base.UArray i e
   = Data.Array.Base.UArray !i
                            !i
                            {-# UNPACK #-} !Int
                            GHC.Prim.ByteArray#
         -- Defined in ‘Data.Array.Base’
 -- [instances trimmed]
 Prelude> :i Data.Array.IO.IOUArray
 type role Data.Array.IO.Internals.IOUArray representational phantom
 newtype Data.Array.IO.Internals.IOUArray i e
   = Data.Array.IO.Internals.IOUArray (Data.Array.Base.STUArray
                                         GHC.Prim.RealWorld i e)
         -- Defined in ‘Data.Array.IO.Internals’
 Prelude> :i Data.Array.ST.STUArray
 type role Data.Array.Base.STUArray nominal representational phantom
 data Data.Array.Base.STUArray s i e
   = Data.Array.Base.STUArray !i
                              !i
                              {-# UNPACK #-} !Int
                              (GHC.Prim.MutableByteArray# s)
         -- Defined in ‘Data.Array.Base’
 Prelude> :i Data.Array.Storable.StorableArray
 type role Data.Array.Storable.Internals.StorableArray representational
 phantom
 data Data.Array.Storable.Internals.StorableArray i e
   = Data.Array.Storable.Internals.StorableArray !i
                                                 !i
                                                 Int
 !(GHC.ForeignPtr.ForeignPtr e)
         -- Defined in ‘Data.Array.Storable.Internals’
 }}}

 These phantom roles for the element types let me create an unboxed array
 of one type (like Word8), cast it with `coerce` to another type (like
 Word64) and read outside the bounds of the array. I think they should all
 be nominal, since a newtype of an existing type could have a totally
 unrelated MArray or Storable instance.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9220>
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