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

[GHC]

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

 What about the last bullet point of the ticket description?  The
 relationship between two overlapping instances is not symmetrical. I
 suggested "OVERLAPPABLE" and "OVERLAPPING".  Yes, it's a bit more fiddly,
 but much more explicit. Oh, and I suppose you should be able to have both.
 {{{
 instance {-# OVERLAPPABE #-} C a where
 instance {-# OVERLAPPABLE, OVERLAPPING #-} C [a] where ...
 isntance {-# OVERLAPPING #-} C [Maybe a] where ...
 }}}
 Here the middle one overlaps `C a` and is overlapped by `C [Maybe a]`.

 Or is this over-elaborate?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:6>
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