[GHC] #13735: RankNTypes don't work with PatternSynonyms

[GHC]

#13735: RankNTypes don't work with PatternSynonyms
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  invalid           |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:12 Iceland_jack]:
 > `f (B endo) = A (appEndo endo)` works while this doesn't
 >
 > {{{#!hs
 > f :: B -> A
 > f (B (appEndo -> f)) = A f
 > }}}

 That's not surprising. We don't have impredicative types, so `appEndo x`
 always produces a monomorphic result. You have to unpack the `Endo` under
 the `A` constructor.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13735#comment:13>
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