[GHC] #12778: Expose variables bound in quotations to reify

[GHC]

#12778: Expose variables bound in quotations to reify
-------------------------------------+-------------------------------------
        Reporter:                    |                Owner:
  facundo.dominguez                  |
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:  template-
                                     |  haskell reify
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by facundo.dominguez):

 The proposal is incomplete. For it to work, we would need to extend `Exp`,
 `Pat` and `Type` with constructors mimicking what `HsSpliced` does in the
 GHC AST.

 This is ok for producing code with splices inside brackets, but what about
 pattern mathing `Exp` values?

 To be comprehensive of all cases, the following code
 {{{
 case e of
   TupE [LitE _, LitE _] -> ...
   _                     -> ...
 }}}
 would need to be rewritten
 {{{
 case e of
   TupE [             LitE _,              LitE _] -> ...
   TupE [             LitE _, SplicedE _ (LitE _)] -> ...
   TupE [SplicedE _ (LitE _),              LitE _] -> ...
   TupE [SplicedE _ (LitE _), SplicedE _ (LitE _)] -> ...
   _                     -> ...
 }}}

 It could be alleviated with view patterns like
 {{{
 case e of
   TupE [ dropSplicedE -> (LitE _), dropSplicedE -> (LitE _)] -> ...
   _                     -> ...
  where
   dropSplicedE (SplicedE _ e) = e
   dropSplicedE e              = e
 }}}

 but is it tolerable?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12778#comment:3>
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