[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:  spinda
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Template Haskell  |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  th/T18020.hs
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1200
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"c1e25536d67fba33ad6ddae5556115340d99000a/ghc" c1e2553/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c1e25536d67fba33ad6ddae5556115340d99000a"
 Expose enabled language extensions to TH

 This exposes `template-haskell` functions for querying the language
 extensions which are enabled when compiling a module,

 - an `isExtEnabled` function to check whether an extension is enabled
 - an `extsEnabled` function to obtain a full list of enabled extensions

 To avoid code duplication this adds a `GHC.LanguageExtensions` module to
 `ghc-boot` and moves `DynFlags.ExtensionFlag` into it. A happy
 consequence of this is that the ungainly `DynFlags` lost around 500
 lines. Moreover, flags corresponding to language extensions are now
 clearly distinguished from other flags due to the `LangExt.*` prefix.

 Updates haddock submodule.

 This fixes #10820.

 Test Plan: validate

 Reviewers: austin, spinda, hvr, goldfire, alanz

 Reviewed By: goldfire

 Subscribers: mpickering, RyanGlScott, hvr, simonpj, thomie

 Differential Revision: https://phabricator.haskell.org/D1200

 GHC Trac Issues: #10820
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10820#comment:7>
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