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

[GHC]

#9858: Typeable instances should be kind-aware
-------------------------------------+-------------------------------------
        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:
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Yes, SPJ's drastic suggestion is: you polykinded type constructor cannot
 be an instance of `Typeable`, by `deriving`, or `AutoDeriveTypable`, or
 standalone deriving.  Period.

 Here's an alternative proposal.

  * `....deriving( Typeable )` (and `AutoDeriveTypeable`) on a polykinded
 tycon will fail (with a helpful message)

  * But instead you can give any number of mono-kinded standalone instances
 {{{
 deriving instance Typeable (Proxy :: (* -> *) -> *)
 }}}

 Comments on the proposal

  * It's still not ideal because you can never declare "enough" instances
 and if you declare an extra one at some use site, it might conflict with
 someone else's.

  * It requires some modification to GHC, which current rejects such
 standalone deriving instances, precisely because they are not fully
 polymorphic.

  * And the kinds would have to be included in the fingerprint of the
 `TyCon` generated by the standalone derived instance.

 But that seems ok as a holding position.

 The "right" solution is, I think, at the bottom of comment:20.  But NB
 that solution will mean that all these standalone derived instances would
 become illegal, so there is a backward compatibility cost.  The drastic
 solution does not have this problem.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:30>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler