[GHC] #10905: Incorrect number of parameters in "role" errors

[GHC]

#10905: Incorrect number of parameters in "role" errors
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:  roles
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
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Comment (by crockeea):

 To me this is a separate issue from the bug I reported. I have no problem
 with the fact that roles were reported for a missing constructor error. If
 anything I think the roles *help* in my example. The ticket as reported is
 really just about when the kind-roles should be printed, as that's the
 part I found confusing.

 Replying to [comment:4 simonpj]:
 > I don't think my reasoning was any deeper than the comment says.  In the
 example in the Description, the roles are a distraction from the main
 point: the constructor isn't in scope.  They just get in the way, and for
 most people will be mysterious and confusing.
 >
 > Do we even want to see these role, or perhaps the roles of type
 constructors we have decomposed on the way to getting this error?
 >
 > To put it another way, can you give an example of where they are useful?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10905#comment:6>
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