[GHC] #2893: Implement "Quantified contexts" proposal

[GHC]

#2893: Implement "Quantified contexts" proposal
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    Reporter:  porges            |       Owner:                  
        Type:  feature request   |      Status:  new             
    Priority:  normal            |   Milestone:  _|_             
   Component:  Compiler          |     Version:  6.10.1          
    Keywords:  proposal          |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by sjoerd_visscher):

 Edward Kmett created a workaround:
 [http://hackage.haskell.org/packages/archive/constraints/0.3.1/doc/html
 /Data-Constraint-Forall.html Data.Constraint.Forall]

 It works like this: if you have p A and p B, for unexported A and B, then
 you must have forall a. p a. Then unsafeCoerce is used to coerce p A to p
 a.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2893#comment:10>
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