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

[GHC]

#9858: Typeable instances should be kind-aware
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        Reporter:  dreixel           |                   Owner:
            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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Changes (by rwbarton):

 * owner:  dreixel =>
 * status:  merge => new


Comment:

 (Reopening for discussion)

 Sorry for getting here a bit late; but I have some concerns about this
 change as it stands currently.

 Abstractly my main concern is that we've made a change (implicitly
 deriving Typeable everywhere) for the wrong reasons, and concretely my
 main fear, given the timing of this change, is that we'll find the new
 approach is untenable for some reason or other and be forced back into a
 world in which explicit deriving Typeable is required.

 I'm certainly not trying to say that implicitly deriving Typeable is the
 wrong decision; but it seems that it arose as fallout from the issue in
 this ticket and #10000 for technical reasons, and not because we decided
 that we really want it. We will always be able to transition to implicit
 deriving Typeable in the future, but transitioning back to explicit
 deriving Typeable loses compatibility with programs written for an earlier
 version of the compiler that had implicit deriving Typeable.

 So, seeing as we are in the RC phase for 7.10, my recommendation would be:

 - Keep the requirement to explicitly derive Typeable (and keep
 AutoDeriveTypeable, etc.) as in 7.10 RC 2

 - Don't actually generate evidence terms when deriving Typeable. Instead
 mark the type as "Typeable-allowed".

 - When generating evidence in the Typeable solver, check that all of the
 types involved are marked as "Typeable-allowed".

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:64>
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