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

[GHC]

#9858: Typeable instances should be kind-aware
-------------------------------------+-------------------------------------
        Reporter:  dreixel           |                   Owner:
            Type:  bug               |                  Status:  merge
        Priority:  highest           |               Milestone:  7.10.2
       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:                    |  typecheck/should_fail/T9858a,
                                     |  should_run/T9858b
                                     |                Blocking:
                                     |  Differential Revisions:  Phab:D652
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Comment (by oerjan):

 Replying to [comment:98 diatchki]:
 > Well, `=>` is not really a type-constructor, but rather a syntactic
 form.  So I don't think it should ever appear alone, or partially applied,
 in a Haskell program.

 It doesn't have to appear literally alone.  You can already extract a
 partially applied form, although seemingly not the "constructor" itself.

 {{{
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE FlexibleContexts #-}

 import Data.Typeable

 f :: Proxy (a b) -> Proxy a
 f _ = Proxy

 g = f (Proxy :: Proxy (Eq Int => Int))

 h :: Proxy a -> Proxy (a Bool)
 h _ = Proxy

 i = h g
 }}}

 {{{
 *Main> :t g
 g :: Proxy (* -> *) ((->) (Eq Int))
 *Main> :t i
 i :: Proxy * (Eq Int => Bool)
 }}}

 Once you have a partially applied form, you can get `Typeable` instances
 for the parts, and then compose them, as shown in [comment:90 comment 90].

 Interestingly, GHC internally ensures (and I don't know whether this is a
 bug or an intended feature) that if you try to extract the `(=>)`
 *itself*, you get `(->) :: * -> * -> *` instead, and a kind matching
 error.  The same applies to `->` if you try to pattern match away an
 unlifted argument type.  But in both cases, you can pattern match away a
 final *lifted* argument type to get a partially applied `(->)` with an
 argument of strange kind.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:99>
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