[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      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Has there been any discussion about GHC generating instances of
 [https://hackage.haskell.org/package/constraints-0.8/docs/Data-
 Constraint.html#t::-61--62- :=>] and
 [https://hackage.haskell.org/package/constraints-0.8/docs/Data-
 Constraint.html#t:Class Class]:

 {{{#!hs
 -- Given
 --   class Foo a => Bar a
 -- GHC should generate
 --   instance Class (Foo a) (Bar a) where
 --     cls :: Bar a :- Foo a
 --     cls = Sub Dict
 class Class b h | h -> b where
   cls :: h :- b
 }}}

 or [https://hackage.haskell.org/package/constraints-0.8/docs/Data-
 Constraint-Lifting.html#t:Lifting Lifting]

 {{{#!hs
 class Lifting p f where
   lifting :: p a :- p (f a)

 class Lifting2 p f where
   lifting2 :: p a :- Lifting p (f a)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:19>
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