[GHC] #9858: Typeable instances should be kind-aware

[GHC]

#9858: Typeable instances should be kind-aware
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        Reporter:  dreixel           |                   Owner:  dreixel
            Type:  bug               |                  Status:  new
        Priority:  highest           |               Milestone:  7.10.1
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by oerjan):

 Replying to [comment:42 goldfire]:
 > What's nice about this one is that it should break no existing code.

 It may be too weird to count as "existing code", but can it handle this?

 {{{
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE DeriveDataTypeable #-}

 import Data.Typeable

 newtype C a b = C () deriving Typeable

 step :: Proxy (a :: k) -> Proxy (C a :: k -> *)
 step Proxy = Proxy

 nest :: Typeable a => Int -> Proxy a -> TypeRep
 nest 0 x = typeRep x
 nest n x = nest (n-1) (step x)

 main = print $ nest 10 (Proxy :: Proxy ())
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:43>
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