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

[GHC]

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

Comment (by diatchki):

 I thought some more about the OVERLAPPING and OVERLAPPABLE idea, but I
 couldn't come up with examples where I'd want to use that feature.  So,
 for the time being, I'll leave the implementation as is, and document it
 accordingly.

 Related to this: having looked at the code for the instance database, I
 think it would be quite easy to implement "instance chains", which
 eliminates the need for overlapping instances in pretty much all
 situations that I've encountered them.   I outlined the details in ticket
 #9334.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:12>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler