[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):

 Martin, thanks.  Yes, as comment:14 specifies (fairly precisely I
 thought), `DysFunctionalDependencies` means relaxing the coverage
 condition altogether.

 Let me stress once more that, apparently contrary to your second bullet,
 dropping coverage does not threaten type soundness.  I'm not quite sure
 what you mean by "type safety".  I did actually go back to our JFP paper
 "Understanding functional dependencies using constraint handling rules" to
 see if I could find a concrete example of the problems that might arise,
 but all I could find I was the unsupported claim that "all is lost" if we
 don't have confluence.

 I ''believe'' (but I have no example that demonstrates) that dropping the
 Coverage Condition might mean that a small change in the program makes a
 large difference in whether the type inference algorithm succeeds.  But it
 would be good to have some concrete cases.  One way to gather some might
 be to allow it, and see whether our users start complaining about
 unpredicatable inference.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:25>
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