[GHC] #9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas

[GHC]

#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas
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              Reporter:  simonpj     |            Owner:  diatchki
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 David, I don't think it's feasible to remove `OVERLAPS` at the moment,
 unless you can amass some real support for it.

  * Main reason: people have argued precisely the opposite: that
 `OVERLAPPING` and `OVERLAPPABLE` are too complicated, and they should be
 removed in favour of just `OVERLAPS`.  In principle I like the idea of
 forcing people to be explicit, but I think it's  bridge too far.

  * Tiresome reason: the parser isn't up to parsing two pragmas; currently
 `{-# OVERLAPPING, OVERLAPPABLE #-}` isn't allowed.  Someone would have to
 fix that.

 Of course, if `-XSafe` is on, you can modify the behaviour if you like.
 So if you want to propose something along those lines, go ahead.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:32>
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