[GHC] #4347: Asymmetry of (impredicative) instantiation/higher rank types

[GHC]

#4347: Asymmetry of (impredicative) instantiation/higher rank types
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    Reporter:  dolio                    |        Owner:                           
        Type:  bug                      |       Status:  new                      
    Priority:  normal                   |    Milestone:  7.4.1                    
   Component:  Compiler (Type checker)  |      Version:  7.1                      
    Keywords:                           |     Testcase:                           
   Blockedby:                           |   Difficulty:                           
          Os:  Linux                    |     Blocking:                           
Architecture:  x86_64 (amd64)           |      Failure:  GHC rejects valid program
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Comment(by simonpj):

 Impredicativity has ''always'' been a very experimental feature.  Although
 I advertised that we'd remove it altogether, GHC 7 does still support a
 simple form of impredicativity, but it's simpler than 6.12.  For what it's
 worth, the new support is closely modeled on QML
 http://research.microsoft.com/en-us/um/people/crusso/qml/

 Let's be concrete: can you give a small test case, that worked before but
 not now?

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4347#comment:5>
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