[GHC] #11534: Allow class associated types to reference functional dependencies

[GHC]

#11534: Allow class associated types to reference functional dependencies
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        Reporter:  ekmett            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  FunctionalDependencies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Let's expand your example and see.

 {{{#!hs
 class C a b | a -> b where
   type Bar a
   type Bar a = b

 class D a b | a -> b

 instance D Int Bool
 instance D a b => C [a] [b]
 }}}

 If we have a functional dependency on `D` from `a -> b`, that secretly
 builds some type family for us, we use behind the scenes, right?

 Let's say that `D_ab`  is the "hidden" type family implied by the
 functional dependency from `a -> b` then the expansion of

 {{{#!hs
 instance D a b => C [a] [b]
 }}}

 would be

 {{{#!hs
 instance D a b => C [a] [b] where
   type Bar [a] = [D_ab a]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534#comment:2>
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