[GHC] #8441: Allow family instances in an hs-boot file

[GHC]

#8441: Allow family instances in an hs-boot file
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        Reporter:  goldfire          |                Owner:  ezyang
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  7.7
      Resolution:                    |             Keywords:  backpack,
                                     |  TypeFamilies, hs-boot
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 ezyang):

 dcoutts points out a use case for type family instances in hsig files
 here:  https://github.com/haskell/bytestring/issues/102

 It should be easier to implement for hsig because we don't need to equate
 any coercions just check that the equality holds.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8441#comment:7>
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