[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 simonpj):

 When I copy/paste this into my emacs buffer the foralls show up ok but the
 a's and b's (and much else besides) do not.  Could you do it in ASCII
 please?

 Do you think the program needs impredicative polymorphism?  If so, GHC
 just doesn't properly support that, notwithstanding the partially
 implemented `-XImpredicativeTypes`.

 I'm not sure what "it accepts it if the product type is replaced by
 tuples" means.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10281#comment:3>
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