[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 int-e):

 Replying to [comment:25 dreixel]:
 > How about just marking `Typeable` instances for polykinded type
 constructors as Unsafe (if that's possible)?

 Given how notoriously hard it is to control exports of instances, that
 won't be enough. To make things worse, the offending instance used here
 (for {{{Proxy}}}) is derived in {{{Data.Typeable.Internal}}}, which is
 marked {{{Trustworthy}}}. So conscientious users of safe Haskell would not
 be able to trust the base package anymore, rendering safe Haskell all but
 useless.

 I guess the right way to leverage safe Haskell is to mark
 {{{Data.Typeable}}} as unsafe, and explain the situation in a big comment.
 It would still be a pain to track uses of {{{Data.Typeable}}} in
 {{{Trustworthy}}} modules of other libraries (say, lens and its
 dependencies).

 I'm afraid there is no good interim solution here. I favor spj's
 suggestion, but #9111 suggests that some people actually use
 {{{Typeable}}} for {{{Proxy}}}.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:26>
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