[GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")

[GHC]

#8634: Relax functional dependency coherence check ("liberal coverage condition")
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              Reporter:  danilo2     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:  7.10.1
              Priority:  high        |           Version:  7.7
             Component:  Compiler    |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D69    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #1241,      |
  #2247, #8356, #9103, #9227         |
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Comment (by simonpj):

 Incidentally, on the Phab comment stream, danilo2 says: I'm very attracted
 to the idea to be able to throw away liberage coverage condition - in some
 places it could give interesting effects (like IncoherentInstances). But
 in opposite to IncoherentInstances, I've got a real-life code, which is
 right now working using DysfunctionalDependencies and is working great. I
 and some people from the company I'm working in were thinking for a long
 time if we can replace a very special type class instance with other
 mechanisms and we came to a conclusion, that using this extension would be
 life saver for us - and this is the reason I started looking into ghc and
 implementing it. I would be very happy If you agree with me and would
 allow this extension to exist within GHC.

 I'm adding the comment in here by way of further motivation.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:16>
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