[GHC] #11431: GHC instantiates levity-polymorphic type variables with foralls

[GHC]

#11431: GHC instantiates levity-polymorphic type variables with foralls
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        Reporter:  gridaphobe        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      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):

 I think the current behavior is fine, if somewhat unexpected. (We should
 document it if we haven't.)

 There is no such thing as a levity-polymorphic value. (If there were, how
 much space would it take up?) Thus, any levity-polymorphic expression must
 be bottom. So I don't see how a tiny bit of impredicativity just for
 bottoms can cause trouble.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11431#comment:1>
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