[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):

 Good trick w/ transitivity!

 You've won me over on the simpler rule. =)

 That leaves these:

 {{{
 instance Representational f => Representational (Compose f)
 instance Representational p => Representational (WrappedArrow p)
 instance Representational (p a) => Representational (WrappedArrow p a)
 }}}

 ...and if I understand correctly, the form we're talking about also gets
 stuck whenever an earlier argument occurs 'under' a later argument, e.g.
 the `s` when we try to define `instance Representational StateT` occurs
 under the `m` but everything else should work.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:20>
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