[GHC] #9123: Need for higher kinded roles

[GHC]

#9123: Need for higher kinded roles
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              Reporter:  simonpj     |            Owner:  goldfire
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.12.1
             Component:  Compiler    |          Version:  7.8.2
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Project (more
       Type of failure:              |  than a week)
  None/Unknown                       |       Blocked By:
             Test Case:              |  Related Tickets:
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Replying to [comment:31 goldfire]:

 > If, instead, we quantified over the leftover implication, we'd be done,
 with an inferred implication constraint.

 Not a good plan.  I'm sure you can't be saying
  * Reject a constraint `C a Int` as "exotic"
  * Accept a constraint `forall a. C a Int` as fine

 When would an implication be "exotic"?

 I think it'd be much better to ask the user to use standalone deriving and
 write the constraint; but that's going to be jolly scary for users who
 just want `deriving( Monad )`.  Your point that it only affects monad
 transformers is a good one though.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:32>
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