[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:  closed
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
            Resolution:  fixed       |     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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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => fixed


Comment:

 I have finally done the INCOHERENT thing.

 I'll close this ticket.  There are some loose ends which I'll list here:

  *  David, if you want to open a new ticket for OVERLAPS, by all means do
 so, if you think you've got a good proposal with user support.

  * I'd like to make `-XUndecideableInstances` a per-instance pragma,
 instead of a module-wide LANGUAGE flag.  That would be consistent with
 this `OVERLAPPABLE` stuff.  Anyone want to do that?

  * At least one person suggested that there should be a LANGUAGE pragma
 that switches on recognition of the per-instance pragmas.  So `{-#
 OVERLAPPABLE #-}` is only honoured if you have
 `-XHonourOverlappingInstnaces` or something.  Personally I think this is
 too much, but users may differ!

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:36>
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