[GHC] #7918: SrcSpan's associated with expanded quasi-quotes are inconsistent

[GHC]

#7918: SrcSpan's associated with expanded quasi-quotes are inconsistent
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        Reporter:  edsko                 |            Owner:  edsko
            Type:  bug                   |           Status:  new
        Priority:  high                  |        Milestone:  7.8.1
       Component:  Compiler              |          Version:  7.7
      Resolution:                        |         Keywords:
Operating System:  Unknown/Multiple      |     Architecture:
 Type of failure:  None/Unknown          |  Unknown/Multiple
       Test Case:  quasiquotation/T7918  |       Difficulty:  Unknown
        Blocking:                        |       Blocked By:
                                         |  Related Tickets:
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Comment (by simonpj):

 (I wrote this comment yesterday, but it has disappeared. Sigh.)

 Edsko, putting a special case in `rnLExpr` is a bit undesirable because:
  * It makes `rnLExpr` more than a dispactch to `rnExpr`
  * It means that `rnExpr` will never see a `HsQuasiQuoteE`, so it is
 "missing a case"; there's an invariant that the `HsQuasiQuoteE` case never
 occurs
  * It's fragile, because there might be ''other'' calls to `rnExpr` that
 don't satisfy that invariant.

 Here's an alternative.  Replace
 {{{
 rnExpr (HsQuasiQuoteE qq)
   = runQuasiQuoteExpr qq        `thenM` \ (L _ expr') ->
     rnExpr expr'
 }}}
 with
 {{{
 rnExpr (HsQuasiQuoteE qq)
   = runQuasiQuoteExpr qq        `thenM` \ lexpr' ->
     rnExpr (HsPar lexpr')
 }}}
 Here I'm wrapping the returned expression in parens (which is probably a
 reasonable idea if we ever want to display the result of expanding
 quasiquotes), and that makes the types work out ok.  The location on
 `lexpr'` will correctly be handled by the call to `rnLExpr` in the `HsPar`
 case of `rnExpr`.

 Would that work?

 Simon

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7918#comment:10>
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