[GHC] #10902: Refactor type families in Template Haskell

[GHC]

#10902: Refactor type families in Template Haskell
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              Reporter:  goldfire    |             Owner:
                  Type:  task        |            Status:  new
              Priority:  normal      |         Milestone:  8.0.1
             Component:  Template    |           Version:  7.11
  Haskell                            |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 I see that we now have `OpenTypeFamilyD` and `ClosedTypeFamilyD` in
 Template Haskell. These constructors have quite a lot in common, and I
 think it's worth having a `TypeFamilyHead` datatype that contains the bits
 in common.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10902>
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