[GHC] #11142: Type-level skolem capture leads to core lint error

[GHC]

#11142: Type-level skolem capture leads to core lint error
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        Reporter:  goldfire          |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 That's true -- but we're not talking about kind generalization here,
 because the user is asking for it (albeit a bit indirectly). So: all kind
 variables that GHC invents are always quantified at the top level, but
 previously, users could specify kind variables to be quantified below top
 level.

 If that ability has been removed, this is a real loss of expressiveness
 that will bite. However, it's all easily recovered with the capabilities
 of my branch, where you can write kind quantification directly. So perhaps
 it all works out. But there's a bad migration story, because there will be
 no way to write higher-rank kind quantification that works in both GHC
 7.10 and GHC 8.0.

 In any case, we need to clearly document the new behavior in the release
 notes.

 Is it worth trying to resurrect the old behavior on your branch, just to
 ease migration?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11142#comment:5>
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