[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):

 Ah, I had misunderstood what the wiki page said, then.  Now this gives me
 an idea for a expanded test case.

 If my picture of how such a solver would work is now more correct, then I
 think by expanding the kinds in my example a bit, we can get a case where
 the solver would need not just to ''build'' new kind reps, but also to
 take the kind rep inside its provided `Typeable` instance further apart.
 And for good measure let's include a data kind as well:

 {{{
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE AutoDeriveTypeable #-}

 import Data.Typeable

 newtype C a b = C ()

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

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

 main = print $ nest 10 (Proxy :: Proxy (C () :: [()] -> *))
 }}}

 Assuming you would want to support this kind of code.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:46>
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