[GHC] #9840: Permit empty closed type families

[GHC]

#9840: Permit empty closed type families
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       Reporter:  adamgundry         |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler (Type     |                 Version:  7.8.3
  checker)                           |        Operating System:
       Keywords:                     |  Unknown/Multiple
   Architecture:  Unknown/Multiple   |         Type of failure:
     Difficulty:  Unknown            |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 At the moment, closed type families without any equations fail with a
 parse error. In addition, they cannot be created by TH (see #8028). Would
 it be possible to permit these instead?

 My use case is in my typechecker plugin for units of measure, where I want
 to add new type-level operators without any equational theory (because it
 will be supplied by the plugin) and without users having the ability to
 introduce their own type family instances.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9840>
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