[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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Changes (by adamgundry):

 * cc: adamgundry (added)


Comment:

 Thanks for the report!

 I think that both errors are genuine, because both definitions require
 impredicative instantiations to typecheck, not just `RankNTypes`. An
 impredicative instantiation is where a type variable needs to be filled in
 with a polymorphic type. For example, in `peirce_eq_lem`, the use of `and`
 needs `a = Peirce -> LEM, b = LEM -> Peirce` and both `LEM` and `Peirce`
 expand to polytypes. If you inline `and` then the definition is accepted
 by 7.10.1 with just `RankNTypes`.

 In general, you will probably find such definitions easier to work with if
 you use `newtype` wrappers instead of polymorphic `type` synonyms. That
 will give you more control over exactly where polymorphism is introduced
 and eliminated.

 I agree that the error messages could be better, but it's hard to see how
 to improve them in general.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10281#comment:5>
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