[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 diatchki):

 I think that rwbarton's comments above explain quite well why danilo2's
 program is incorrect, and GHC correctly rejects it.  No matter which rule
 we use to ''implement'' the consistency check for the functional-
 dependency, it is crucial that the rule is ''sound''.  No sound rule will
 accept that program, because it violates the ''specification'' for
 functional dependencies.

 As for `DysfunctoinalDependencies`: I have no idea what that means, or how
 it is supposed to work.  Is there a write-up or a spec that explains the
 extension?   I'll refrain from commenting further until I understand the
 concept, but I do think that if it gets implemented, it should use a
 different syntax from FDs.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:18>
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