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

[GHC]

#11534: Allow class associated types to reference functional dependencies
-------------------------------------+-------------------------------------
        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 dfeuer):

 If people want to go forward with desugaring fundeps to type families
 (sounds great to me), I think it would be lovely to add naming syntax:

 {{{#!hs
 class Foo a b c d | (Q, R :: a -> b c), (S :: b d -> a)
 }}}

 would desugar to

 {{{#!hs
 class (b ~ Q a, c ~ R a, a ~ S b d) => Foo a b c d where
   type family Q a
   type family R a
   type family S b d
 }}}

 To work with classes that lack named fundeps, some nasty names could be
 made available.

 {{{#!hs
 class Foo a b c d | a -> b c, b d -> a
 }}}

 could desugar to use type families with (exposed, but unlikely) names like
 `Foo__1_DET_2#`, `Foo__1_DET_3#`, and `Foo__2_4_DET_1#`, naming the
 positions that determine other positions.

 It might be good to add dysfunctional "soft dependencies" at the same
 time, with `~>` syntax and vague semantics to be pinned down later (the
 idea being that soft dependencies would only guide inference, as best they
 could, without being relied upon for type checking).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534#comment:27>
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