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

[GHC]

#11534: Allow class associated types to reference functional dependencies
-------------------------------------+-------------------------------------
        Reporter:  ekmett            |                Owner:  (none)
            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:                    |
-------------------------------------+-------------------------------------

Comment (by AntC):

 Replying to [comment:30 dfeuer]:
 >  I think the choice to reuse `->` was a bit fishy from the start.

 The arrow was borrowed straight from database Functional Dependencies. (I
 guess it helped that `->` was already a recognised lexeme.)

 But note that Mark Jones' original proposal was that FunDeps should look
 like this

 {{{
 class Foo a b c d  | {a} -> {b, c}, {b, d} -> {a}  where ...
 }}}

 IOW: the determinant and dependent are sets of parameters (order
 immaterial). That's exactly how they look in database theory. `b d` does
 indeed excite in my brain that is tyvar `b` applied to `d`. (The Haskell
 abbreviated form was also taken over into injective type families, where
 it gives a potential syntactic ambiguity.)

 So the fishiness arose from dropping the set notation braces with
 commalist.

 > I'd have no objection to using some symbol other than `::` if you'd
 prefer.

 Aside from how to bind naming for a FunDep, beware that a Type Familiy's
 parameters are positional whereas a FunDep's are not. IOW this is
 (supposed to be) an entirely equivalent class decl

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

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534#comment:33>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler