[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):

 I think it'd be better to consider any non-type-variable context as
 exotic, and reject it.  You can always use standalone deriving.

 The trouble is that it's very easy to get contexts like `(C T)`, which
 perhaps might be satisfiable at the call site, but not at the `deriving`
 site, but that is unusual, and the programmer ought to be calling it out
 explicitly.

 GHC originally accepted more exotic `deriving` contexts, and moved step by
 step to the current fairly conservative position.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:34>
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