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

[GHC]

#10905: Incorrect number of parameters in "role" errors
-------------------------------------+-------------------------------------
        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 Revisions:
-------------------------------------+-------------------------------------

Comment (by 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:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler