[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 simonpj):

 > I don't see how a tiny bit of impredicativity just for bottoms can cause
 trouble.

 Impredicativity never causes trouble for the dynamic semantics; after all
 Core is impredicative.  In contrast levity-polymorphism does.

 Impredicativity does cause trouble for type inference -- see zillions of
 papers on the subject. In contrast levity-polymorphism does not.  (Except
 in enforcing the restrictions that levity polyormophism must follow to
 avoid screwing up the dynamic semantics.)

 There is no reason (that I can see) that the restrictions for levity
 polymorophism should be sufficient to avoid all type-
 inference/impredicativity difficulties.  Maybe so, but it would be a
 coincidence.

 There's an engineering issue too.  Richard is about to eliminate `RetTv`.
 But `RetTv` is essential to the implementation that accepts the above
 program. Are we going to leave `RetTv` for this sole (ill-defined)
 purpose?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11431#comment:2>
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