[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 goldfire):

 I'm not sure I understand comment:42, danilo2.

 The error you have toward the beginning of the comment is what I see when
 I leave out the fun-dep on `PolyMonad`. It doesn't seem related to my
 commented-out instance. Or, does un-commenting out the instance really
 produce that error? That's something strange.

 And, the `MatchMonad` idea just seems to kick the can down the road a bit:
 how could we have my desired instance for `MatchMonad` without
 `DysfunctionalDependencies`? Yes, `f` will type-check, but it won't be
 able to be called.

 Or am I deeply confused?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:43>
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