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

[GHC]

#2893: Implement "Quantified contexts" proposal
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        Reporter:  porges            |                Owner:  (none)
            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      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:12 heisenbug]:
 > 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.

 Actually, the `forall` condition is ''not'' satisfiable. The smaller
 problem is that stuck type families like `Any` can inhabit `Bool` without
 being `'True` or `'False`. The bigger problem is that Haskell's "logic" is
 inherently constructive. The claim that for any type `b :: Bool` there
 exists some instance `Foo b x` does not translate to a way to actually get
 the dictionary representing that instance. So I think your beautiful dream
 is, sadly, almost certainly unrealizable.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:24>
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