[GHC] #13731: DeriveFunctor and friends don't understand type families

[GHC]

#13731: DeriveFunctor and friends don't understand type families
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        Reporter:  spacekitteh       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
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 Iceland_jack):

 That instance becomes writable if we mark `ExtensionType` injective

 {{{#!hs
 data Test :: Type -> Type -> Type where
   Foo    :: a -> Test ext a
   Extend :: ExtensionType ext a -> Test ext a

 type family ExtensionType a = (res :: Type -> Type)  | res -> a
 data ListExtension
 type instance ExtensionType ListExtension = []

 instance Functor (ExtensionType ext) => Functor (Test ext) where
   fmap :: (a -> a') -> (Test ext a -> Test ext a')
   fmap f = \case
     Foo    a  -> Foo (f a)
     Extend ex -> Extend (fmap f ex)
 }}}

 We need to bump the arity of `ExtensionType` down to one, given that we
 don't have #10832.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13731#comment:3>
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