[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):

 Moved from #5927.

 Use case from [http://ozark.hendrix.edu/~yorgey/pub/twisted.pdf How to
 Twist Pointers without Breaking Them]:

 > {{{#!hs
 > class Monoid m => Action m a where
 >  act :: m -> a -> a
 > }}}
 > ''[...]''
 >
 > We first formalise the action of monoids on functors. Intuitively, for a
 monoid `m` to act on the functor `f`, it should act in a uniform way on
 all the types `f a`.  Therefore, we would like to assert a constraint
 something like `forall a. Action m (f a)`. Unfortunately, Haskell does not
 allow the use of universally quantified constraints and hence we need a
 new type class.
 >
 > {{{#!hs
 > class (Monoid m, Functor f) => ActionF m f where
 >   act' :: m -> f a -> f a
 > }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:17>
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