[GHC] #9112: data families with representational matching

[GHC]

#9112: data families with representational matching
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        Reporter:  jwlato            |            Owner:
            Type:  feature request   |           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 jwlato):

 Simon, it's possible that suggestion will work, with a few other
 modifications to the way roles are currently handled.  It doesn't work
 with vector/ghc as currently released due to some other methods.  For
 example, if I add `basicUnsafeRead`

 {{{
 class MVectorClass (v :: * -> * -> *) a where
     basicLength :: v s a -> Int
     basicUnsafeRead :: PrimMonad m => v (PrimState m) a -> Int -> m a
 }}}

 It's not possible to coerce `basicUnsafeRead` because of the coercion from
 `m Int` to `m Age`.  There are a number of other methods that also can't
 be coerced, but I think they're all because `m`s type parameter is at a
 nominal role.  If it were possible to express that `m`s parameter need
 only be representationally equal, I think this would work.  (maybe related
 to the current issues with deriving and AMP?)

 I'm not sure what's involved with making that coercion happen, but it
 seems decidely less tricky than representationally-equal data families (I
 have quite a bit more to say about that also, but I'll leave it for
 another venue).  I'm changing the title of this ticket to more closely
 reflect the actual goal, e.g. deriving vector instances.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9112#comment:3>
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