[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 Iavor S. Diatchki <iavor.diatchki@…>):

 In [changeset:"97f499b56c5888740ddb147fb198c28a3c06bac7/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="97f499b56c5888740ddb147fb198c28a3c06bac7"
 Implement OVERLAPPING and OVERLAPPABLE pragmas (see #9242)

 This also removes the short-lived NO_OVERLAP pragama, and renames
 OVERLAP to OVERLAPS.

 An instance may be annotated with one of 4 pragams, to control its
 interaction with other overlapping instances:

   * OVERLAPPABLE:
     this instance is ignored if a more specific candidate exists

   * OVERLAPPING:
     this instance is preferred over more general candidates

   * OVERLAPS:
     both OVERLAPPING and OVERLAPPABLE (i.e., the previous GHC behavior).
     When compiling with -XOverlappingInstances, all instance are OVERLAPS.

   * INCOHERENT:
     same as before (see manual for details).
     When compiling with -XIncoherentInstances, all instances are
 INCOHERENT.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:16>
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