[GHC] #7204: Use a class to control FFI marshalling

[GHC]

#7204: Use a class to control FFI marshalling
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Now that #2721/#8165 have been fixed, the technical obstacle of not being
 able to use `GeneralizedNewtypeDeriving` on classes with associated types
 is gone. The only question remaining is if we want to pursue this design.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7204#comment:9>
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