[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):

 Replying to [comment:6 rhjl]:
 >  I find your explanation of impredicative polymorphism better than the
 one given in the GHC manual:
 ...
 > I think that a remark of this sort in the section on rank-n-types of the
 GHC manual would also help.

 Could you propose some actual, concrete sentences to add or change in user
 manual?  Then I'll just make the change.  As a bona fide user you are in a
 far better position to draft the words than me.

 > 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?

 Becuase of this `($)` has its own special typing rule in GHC.  But `(.)`
 does not (yet).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10281#comment:7>
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