[GHC] #10820: Provide a way to detect what extensions are enabled via TH

[GHC]

#10820: Provide a way to detect what extensions are enabled via TH
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              Reporter:  spinda      |             Owner:
                  Type:  feature     |            Status:  new
  request                            |
              Priority:  normal      |         Milestone:
             Component:  Template    |           Version:  7.10.2
  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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 This would be helpful for providing users with intuitive error messages if
 generated code relies on an extension that the user may not have enabled.

 Sample specification, to get things started:

 {{{
 enabledExts :: Q [Extension]
 isExtEnabled :: Extension -> Q Bool
 data Extension = LiberalTypeSynonyms | RankNTypes | ...
     -- mirroring ExtensionFlag in DynFlags
 }}}

 See #10819 for an example case where this could be of use.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10820>
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