[GHC] #8678: Derivin `Functor` complains about existential type

[GHC]

#8678: Derivin `Functor` complains about existential type
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        Reporter:  heisenbug         |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by heisenbug):

 Replying to [comment:1 carter]:
 > where does the variable {{{n}}} get introduced in {{{Standard :: a ->
 NonStandard (S n) a}}} ? That seems existential

 No, it is a variable that appears in the type of the result, so it is not
 existential.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8678#comment:2>
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