[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
      Resolution:                    |         Keywords:  proposal
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by heisenbug):

 * cc: ggreif@… (added)


Comment:

 Copying a use-case from #9196:

 My point is that I'd like to have polymorphic (multiparameter)
 constraints, where the param universally abstracted over is a non-`*`
 kind, that is an ADT.

 See:
 {{{
 class Foo a where ...
 data Bar (b :: Bool) x = ...
 instance Foo (Bar True x) where ...
 instance Foo (Bar False x) where ...

 test :: (forall b. Foo (Bar b x)) =>...
 }}}
 Here the `forall` condition is satisfiable, as all `Bool` alternatives are
 covered with `instance` declarations.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:12>
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