[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   |          Version:  7.8.1
  (other)                            |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rwbarton):

 As I see it "representational" has to do with the representation of
 Haskell values as closures (usually on the heap). That's different from
 the representation of the "underlying value" in an unboxed or storable
 array. For example, if I needed to process packed arrays of 24-bit
 integers, I might create a newtype of Int and write a Storable instance
 for it with `sizeOf _ = 3`. (Admittedly this is a somewhat contrived
 scenario.)

 The same considerations also apply to `Ptr`. (Indeed, a storable array is
 little more than a `Ptr` under the hood.) But `Ptr` is an inherently
 unsafe/unmanaged sort of thing. If you want to know how many items' worth
 of space is allocated in an area pointed to by a `Ptr`, you have to keep
 track of it yourself. Unboxed/storable arrays are supposed to be a safe
 abstraction on top of raw memory areas. That's why I think it's reasonable
 to have different defaults between unboxed/storable arrays and `Ptr`s.
 (And it is in some sense a matter of defaults, since there is still
 `unsafeCoerce` when you happen to know that the memory layouts match up
 properly.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9220#comment:3>
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