[GHC] #9220: type roles for unboxed arrays

[GHC]

#9220: type roles for unboxed arrays
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              Reporter:  rwbarton    |            Owner:  goldfire
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.8.1
  Libraries                          |         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 ekmett):

 Boxed arrays are representational (just by the default roles) though both
 `Array` and `UArray` are currently ''also'' `representational` in the
 first argument. =/

 That is dangerous for much the same reason as relying on the semantics of
 `Storable` not changing to a newtype, it is often the very point that the
 newtype would change such an instance.

 In fact `UArray` is currently even worse and gets inferred as

 {{{
 type role UArray representational phantom -- ack!
 }}}

 because the underlying constructor doesn't mention the element type at
 all.

 We basically just ignored roles in the `array` package when shipping 7.8.x

 --

 It strikes me that we should have

 {{{
 type role Array nominal representational
 type role UArray nominal nominal
 }}}

 The first argument is formed nominal by depending upon the chosen `Ix`
 instance, and in the case of `UArray` the second argument is forced
 nominal by depending upon the chosen `Storable` instance.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9220#comment:24>
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