[GHC] #9123: Need for higher kinded roles

[GHC]

#9123: Need for higher kinded roles
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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 ekmett):

 True, if you keep the existing role rules that works for the concrete
 choice of (,) and State works.

 Consider a parameterized version where we fuse the two parameters into
 arguments of the same type argument, but don't make it concrete.

 e.g.

 {{{
 newtype Foo s p a = Foo (s -> p a s)
 }}}

 and there you get stuck on GND for `a` whether or not you keep the
 existing role-based machinery for lifting coercions.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:16>
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