[GHC] #10331: Accept HsSyn in splices and generate it in quotes (ghc-api)

[GHC]

#10331: Accept HsSyn in splices and generate it in quotes (ghc-api)
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              Reporter:  ezyang      |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  low         |           Version:  7.11
             Component:  Template    |  Operating System:  Unknown/Multiple
  Haskell                            |   Type of failure:  None/Unknown
              Keywords:              |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 I propose a language pragma `GhcTemplateHaskell`, which, when enabled,
 causes Template Haskell splices to accept `HsSyn` return values, and
 causes TH quotes to return `HsSyn` trees. Users of `GhcTemplateHaskell`
 would stop using the `template-haskell` library and just directly
 manipulate the frontend ASTs using the `ghc-api`.

 Given our ghc-api track record, you would probably not be able to write
 code using this without having it break every GHC release.  However,
 clients who are willing to track GHC development would reap the following
 benefits:

 1. All syntax in Haskell would be supported in quotes; the AST would never
 lag behind. Additionally, you could use the ghc-api parser to parse
 Haskell code to be returned in a splice. (#2041)

 2. Locations from quoting would be preserved. (Problem 2 of #10330)

 I don't think this would be difficult to implement, but it definitely
 could be a ball of wax. I broke this proposal off from #2041.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10331>
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