[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 danilo2):

 Replying to [comment:39 simonpj]:
 1) Of course - you are right - with `-XScopedTypeVariables` it does
 compile and works well - please forgot me, I overlooked the flag.
 2) Yes this example satisfies the Coverage Condition, but as I described
 before it is just another simplification of some problem and in general we
 got instances which satisfy and which does not the condition.
 3) There is other problem with providing such types by hand - we are
 generating the code and it would be very hard to generate such annotations
 in general.

 @rwbarton: Thank you again for your example - of course it works as you
 described. Still, while it needs some type signatures, they are hard for
 us to generate (while we are generating the Haskell's code). I think
 introducing optional `-XDysfunctionalDependencies` flag even as a local
 flag or using `~>` symbol, as Simon suggested above, would be a great
 thing - allowing to play with type system and creating easier some
 "hackish" things if user wishesh to.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:40>
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