[GHC] #10281: Type error with Rank-n-types

[GHC]

#10281: Type error with Rank-n-types
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        Reporter:  rhjl              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |  Peirce_eq_LEM.hs
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by rhjl):

 Thank you for your explanations!

 I think that I now understand the issue.  I find your explanation of
 impredicative polymorphism better than the one given in the GHC manual:

   This means that you can call a polymorphic function at a polymorphic
 type, and      parameterise data structures over polymorphic types.

 The hint about `newtype` wrappers also helped me to adjust the module to
 work with `RankNTypes`.  I did not realise that `newtype` wrappers hide
 the polymorphism of the type they wrap.  I think that a remark of this
 sort in the section on rank-n-types of the GHC manual would also help.

 By the way: this issue with impredicative instantiations precludes the use
 of `($)` and `(.)` to get rid of excessive parenthesis in certain
 situations.  Is there a workaround?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10281#comment:6>
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