[GHC] #10524: PolyKinds doesn't interact well with DeriveFunctor

[GHC]

#10524: PolyKinds doesn't interact well with DeriveFunctor
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        Reporter:  RyanGlScott       |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by RyanGlScott):

 Oh, right. So in both GHC 7.10 and 7.11, the derived kinds for the type
 variables in {{{(Functor f, Functor g) => Functor (Compose f g)}}} are
 incorrect, but GHC 7.11 defers the errors until later.

 In that case, can this can of problem be fixed? Would {{{deriving}}}
 clauses always be able to infer the correct kinds, or would it be
 necessary in some cases to specify the kinds in a standalone
 {{{deriving}}} statement, e.g.,

 {{{#!hs
 deriving instance (Functor (f :: * -> *), Functor (g :: * -> *)) =>
 Functor (Compose f g)
 }}}

 (The kind signatures wouldn't be needed here due to the explicit
 {{{Functor}}} constraint, but you get the idea.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10524#comment:4>
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