[GHC] #10267: Add support for typed holes in Template Haskell

[GHC]

#10267: Add support for typed holes in Template Haskell
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        Reporter:  jstolarek         |                   Owner:  jstolarek
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Template Haskell  |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by jstolarek):

 Yes, the implementation is actually a matter of ~30 minutes of hacking.
 There is one catch though: `HsUnboundVar`, used to represent holes, always
 stores `RdrName`. That becomes a problem in `DsMeta`, where we expect
 `Name`s. My solution: parametrize `HsUnboundVar` with `id` like all the
 other constructors in `HsExpr`. I'm not certain if I got the details right
 - patch will require careful review in several place (I will mark those on
 Phab with inline comments).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10267#comment:3>
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